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Ferromagnetic transition in EuS-PbS multilayers
Abstract
The magnetic properties of multilayers of ferromagnetic EuS intercalated with diamagnetic PbS were studied as a function of the EuS layer thickness (varying from 2 to 200 ML). The critical temperature TC of the paramagnet-ferromagnet phase transition was determined from magnetization vs temperature measurements and was found to depend on the underlying substrate [KCl (100) vs BaF2 (111)] as well as on the thickness of the EuS layer. For thick layers (dEuS≈200 ML), which mimic semibulk EuS, the TC values were found shifted with respect to the bulk EuS (about 1 K up for layers grown on KCl and about 3 K down for layers grown on BaF2). This effect is attributed to stress resulting mainly from the difference of thermal expansion coefficients between the substrate and the structure. For thin layers (dEuS<10 ML), a systematic reduction of TC with decreasing EuS layer thickness was observed. This behavior is discussed from two points of view: (a) the reduction of the average number of magnetic neighbors because of the increasing role of the interface for the thin layers, and (b) the three-dimensional/two-dimensional (3D/2D) crossover from a 3D Heisenberg-type ferromagnet to a 2D XY or Ising-like system. The dependence of the magnetic anisotropy on the EuS layer thickness was studied by ferromagnetic resonance measurements. The energy of magnetic anisotropy can be well described as a sum of a thickness-independent (volume) part and a 1/dEuS (surface) term. We found that EuS layers with dEuS>2 Å magnetize in the plane of the structure.
... EuS and PbS crystallize in the same cubic (rock salt) crystal structure and the lattice mismatch between these two crystals is only 0.5%. Since PbS is a typical diamagnet, magnetically the EuS—PbS multilayers are all-semiconducting ferromagnet—diamagnet nanostructures [23] . EuS—PbS multilayers form PbS multiple quantum wells with fundamental electronic transitions in the infrared [24, 25]. ...
... For EuS, the magnetic interactions are important only for nearest (z1 ) and next nearest (z2 ) magnetic neighbors. Since the number of magnetic neighbors varies for ions located in different layers, the average number of neighbors depends on the number n of EuS monolayers in the layer, as well as on the growth direction, and on the intermixing profile at the interface [23]. The final expression for the Curie temperature is TC( n) = TC ( b ) " ( 1 — c / n ) , w h e r e c is a numerical parameter [23]. ...
... Since the number of magnetic neighbors varies for ions located in different layers, the average number of neighbors depends on the number n of EuS monolayers in the layer, as well as on the growth direction, and on the intermixing profile at the interface [23]. The final expression for the Curie temperature is TC( n) = TC ( b ) " ( 1 — c / n ) , w h e r e c is a numerical parameter [23]. The application of other concepts (finite size scaling or exact calculations for the Ising systems) to the analysis of the thickness dependence of the Curie temperature in EuS—PbS multilayers is discussed in [23]. ...
Ferromagnetic semiconductor structures such as superlattices or trilayers form a new class of magnetic systems composed entirely of semiconductor materials. The examples are Ga1-xMnxAs-AlGaAs with the ferromagnetic layer of GaMnAs semimagnetic (diluted magnetic) semiconductor or EuS-PbS with the ferromagnetic member (EuS) of the family of europium chalcogenides. We discuss the spectrum of perspective ferromagnetic semiconductor materials, the effect of size and stress on magnetic properties of ultrathin semiconductor ferromagnetic layers, and the effect of interlayer exchange in all-semiconductor systems.
... [41][42][43][44] The Curie temperature of the bulk EuS is about 16.6 K. [45] With decreasing the layer thickness, EuS would transform from a 3D Heisenberg-type ferromagnet to an Ising-like system and its Curie temperature would be reduced. [46] The Curie temperature of bilayer EuS is reduced to be about 9 K [46] and the estimated value from Ising model is about 12.5 K. Therefore, the Monte Carlo simulations based on Ising model were used to describe the thermal dynamics of magnetism. The Metropolis algorithm on a 16 Â 16 superlattice was used. ...
... [41][42][43][44] The Curie temperature of the bulk EuS is about 16.6 K. [45] With decreasing the layer thickness, EuS would transform from a 3D Heisenberg-type ferromagnet to an Ising-like system and its Curie temperature would be reduced. [46] The Curie temperature of bilayer EuS is reduced to be about 9 K [46] and the estimated value from Ising model is about 12.5 K. Therefore, the Monte Carlo simulations based on Ising model were used to describe the thermal dynamics of magnetism. The Metropolis algorithm on a 16 Â 16 superlattice was used. ...
The heterojunction composed of ferromagnetic insulator (EuS) and topological insulator (Bi2Se3) is interesting due to several unusual behaviors produced by the interfacial effects. Previous experiments suggest a largely enhanced Curie temperature up to room temperature and perpendicular magnetic anisotropy at the interface [F. Katmis et al., Nature 533, 513 (2016).]. However, a recent experiment indicated that room‐temperature ferromagnetic behavior is absent, and only in‐plane anisotropy is observed [A. I. Figueroa et al., PRL 125, 226801 (2020).]. To understand the puzzling observations in EuS/Bi2Se3, we construct a Se‐missing interfacial system and study the magnetic anisotropy energy and the Curie temperature based on density functional calculations. The results suggest that the absence of Se atoms would greatly enhance the perpendicular magnetic anisotropy and the estimated Curie temperature based on Ising model can be up to room temperature. Therefore, the puzzling observations may be concerned with the details of the interfacial system. This article is protected by copyright. All rights reserved.
... Schiller und Nolting [33] sagen theoretisch einen magnetischen Oberflächenzustand voraus, der spekulieren lässt, dass die Temperaturabhängigkeit unterhalb von T C zu einen Oberflächen-Isolator-Halbmetall-Übergang in EuO-Filmen führt, der von einem riesigen Magnetowiderstandseffekt begleitet wird. Auch das magnetische Verhalten von Filmen findet, wie die Dickenabhängigkeit der Curie-Temperatur in [3,34,35] ...
... Hier ist ihr Vorhandensein wegen des Mermin-Wagner-Theorems [77] essentiell für das Erreichen endlicher Magnetisierungen in Filmen. Die berechnete Lagen-, Dicken-, und Temperaturabhängigkeiten der Filmmagnetisierung geben ein konsistentes Bild, welches das "finite size scaling" beim Übergang von zwei zu drei Dimensionen einschließt und sehr gut mit experimentellen Befunden [3,34] bzw. [35] übereinstimmt. ...
Diese Arbeit beschäftigt sich mit theoretischen Untersuchung von Oberflächenzuständen in ferromagnetischen Halbleitern. Einleitend wird ein analytisches "tight-binding"-Modell zur Beschreibung der Oberflächenzustände. Es liefert Aussagen zur Existenz von Oberflächenzuständen, zu deren spektralen Gewicht und Position bezüglich der Energie. Das Kondogitter-Modell wird verwendet, um Korrelations- und Temperatureffekte sowie Oberflächenzustände zu beschreiben. Dies erfolgt zunächst für sc-(100)-Modellfilme im Rahmen des sf-Modells. Die temperaturabhängigen der Oberflächenzustände zeigen abhängig vom Ort in der Brillouinzone sowie den "hopping"-Parametern in der Oberfläche sowohl Stoner-artiges als auch "spin-mixing"-Verhalten. Mit wachsender Temperatur werden Lebensdauereffekte in den Spektren sichtbar. Das Kondogitter-Modell (KLM) wird auf die Mehrbandsituation zum df-Modell verallgemeinert, um eine Beschreibung der Prototypen für magnetische Halbleiter EuS und EuO zu erreichen. Durch die Kombination von LDA-Bandstrukturrechnungen mit Vielteilchenrechnungen zum Multiband-KLM ist es gelungen, die ausgeprägte Temperaturabhängigkeit des unbesetzten 5d-Leitungsbandes und das Verhalten der Oberflächenzustände in EuS- und EuO-Filmen realistisch zu beschreiben. Der exakte Grenzfall des Kondogitter-Modells, das magnetischen Polaron (T=0), ermöglicht Kombination von ab-initio-Bandstrukturrechnungen und der Vielteilchentheorie ohne das Auftreten von Doppelzählungen relevanter Wechselwirkungen. Sowohl in EuO als auch in EuS temperaturabhängige Oberflächenzustände beobachtet werden, die im Fall von EuS jedoch schwerer nachzuweisen sind, da sie im Energiebereich des Volumenbandes auftreten. Die für EuS und EuO berechneten Rotverschiebungen sowie die dickenabhängige Magnetisierung von EuS stimmen hervorragend mit experimentellen Befunden überein. Eine Vielzahl von Korrelationseffekten ist mit wachsender Temperatur in den Spektren der unbesetzten Europium-5d-Bänder zu beobachten.
... Numerous experimental studies suggest the significant influence of a nanoparticle (thickness of ultrathin films) size on the critical temperature of the magnetic phase transition. For example, the size effect was studied in ultrathin magnetic films [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18], among them there are epitaxial films of Ni, Ni/Re, Ni/W [4,9,11] and polycrystalline films of Ni∕SiO 2 [1], antiferromagnetic films of CoO, NiO [3,5,7,16], nanocomposite films Fe 2 O 3 , Pr 0.5 Sr 0.5 MnO 3 , MnP [17,18] and Ni 50 Fe 50 , NiO∕CoO [5,10] alloys. There were materials with different magnetic order in those experiments [19][20][21][22][23][24][25][26][27][28][29]. ...
We present a simple approach (the mean spin method) to investigate the influence of the size of nanoparticles (thickness of ultrathin films) and the concentration of ≪\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ll$$\end{document}magnetic atoms≫\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gg$$\end{document} on the critical temperature and magnetic susceptibility in the magnetic phase transition region. The critical spin-spin correlation indices for nanosized magnets are calculated using the scaling relation. It is shown that the critical concentration pC\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$p_C$$\end{document} of the transition from an ordered to an unordered state increases as the characteristic size of nanoparticles (thickness of ultrathin films) decreases. The results obtained by the mean spin method are in qualitative agreement with the Monte Carlo simulation and experimental data.
... A·m −1 ), indicating the ferromagnetic property. The transition temperature of dependent magnetization is detected from the normalized magnetic moment as a function of temperature using the low-field method, [24] as shown in Fig. 2(b). The ferromagnetic transition is observed in M-T curve, where, above 35 K, the EuS film shows a Curie-Weisstype paramagnetism. ...
EuS is one of typical ferromagnetic semiconductor using as spin filter in spintronic devices, and the doped one could be a good spin injector. Herein, we fabricate a spin-functional tunnel junction by epitaxially growing the ferromagnetic EuS film on Nb-doped SrTiO 3 . The improvement of Curie temperature up to 35 K is associated with indirect exchange through additional charge carriers at the interface of EuS/Nb:STO junction. Its magnetic field controlled current–voltage curves indicate the large magnetoresistance (MR) effect in EuS barriers as a highly spin-polarized injector. The negative MR is up to 60% in 10-nm EuS/Nb:STO at 4 T and 30 K. The MR is enhanced with increasing thickness of EuS barrier. The large negative MR effect over a wide temperature range makes this junction into a potential candidate for spintronic devices.
... Numerous experimental studies suggest the significant influence of a nanoparticle (thickness of ultrathin films) size on the critical temperature of the magnetic phase transition. For example, the size effect was studied in ultrathin magnetic films [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18], among them there are epitaxial films of Ni, Ni/Re, Ni/W [4,9,11] and polycrystalline films of Ni∕SiO 2 [1], antiferromagnetic films of CoO, NiO [3,5,7,16], nanocomposite films Fe 2 O 3 , Pr 0.5 Sr 0.5 MnO 3 , MnP [17,18] and Ni 50 Fe 50 , NiO∕CoO [5,10] alloys. There were materials with different magnetic order in those experiments [19][20][21][22][23][24][25][26][27][28][29]. ...
... Several semiconductor materials (e.g., PbS, SrS, and YbSe) provide excellent lattice matching to ferromagnetic EuS and permit the growth of high crystal quality, epitaxial ferromagnetic-nonmagnetic multilayers composed of semiconductor materials only [2]. In the best studied EuSPbS heterostructures the nonmagnetic PbS layer also crystallizes in the rock-salt structure with the lattice parameter a 0 = 0.5936 nm [3,4]. PbS is a narrow gap (0.3 eV) semiconductor that in EuSPbS multilayers serves as a nonmagnetic quantum well with ferromagnetic EuS barriers [4]. ...
Temperature and magnetic field dependence of magnetization of EuS-SrS multilayers grown epitaxially on KCl (001) substrate is experimentally studied by superconducting magnetometry technique. In these lattice-matched semiconductor heterostructures EuS layers are ferromagnetic quantum wells whereas SrS layers are nonmagnetic spacer barriers. The multilayers composed of EuS layers with thickness 3.5-5 nm and SrS layers (thickness 0.5- 10 nm) exhibit ferromagnetic transition at 17 K. In the multilayers with ultrathin SrS spacers (0.5-1 nm) a nonmonotonic temperature dependence of magnetization as well as a characteristic switching in magnetic hysteresis loops is observed. These experimental findings are explained considering antiferromagnetic interlayer coupling between ferromagnetic EuS layers via nonmagnetic SrS spacers. The strength of this coupling is determined based on model magnetization calculations.
... sions of the film and substrate. 24 At the same time, very thin films will exhibit slightly lower T C due to dimensionality reduction. 25 However, although good electric insulation (ρ ∼ 10 4 Ω·cm) was obtained in high-quality single crystals, difficulties in material fabrication lead to disorder and unintentional doping, which may drastically reduce the resistivity to as low as ρ ∼ 10 −2 Ω·cm. ...
High-quality thin films of the ferromagnetic-insulator europium(II) sulfide
(EuS) were fabricated by pulsed laser deposition on Al2O3 (0001) and Si (100)
substrates. A single orientation was obtained with the [100] planes parallel to
the substrates, with atomic-scale smoothness indicates a near-ideal surface
topography. The films exhibit uniform ferromagnetism below 15.9 K, with a
substantial component of the magnetization perpendicular to the plane of the
films. Optimization of the growth condition also yielded truly insulating films
with immeasurably large resistance. This combination of magnetic and electric
properties open the gate for novel devices that require a true ferromagnetic
insulator.
Diluted magnetic semiconductors (DMS) are interesting because of the interplay between the electronic and magnetic subsystems. We describe selected magnetic properties of IV‐VI diluted magnetic semiconductors, looking at the similarities and differences between magnetic properties of II‐VI, IV‐VI, and III‐V DMS. We focus on the influence of the crystalline and electronic structure of the material on its magnetic properties, especially on the exchange interactions among magnetic ions. We describe methods of determination of the exchange parameters by using different experimental techniques, such as measurements of magnetic susceptibility, magnetization, and specific heat. We follow the development in the material technology from bulk crystals to thin films and nanostructures. This article is protected by copyright. All rights reserved.
An analytical approach is developed for the energy levels in Group-IV–VI narrow-gap semiconductor quantum wells in a parallel magnetic field. Asymptotic closed-form expressions are obtained. Numerical calculations are done for the energy spectrum of the EuS/PbS/EuS quantum well in the magnetic field of an arbitrary strength. The effect of an anisotropy of the electron valleys is also discussed.
This document is part of Subvolume A 'Growth and Structuring' of Volume 34 'Semiconductor Quantum Structures' of Landolt-Börnstein - Group III 'Condensed Matter'. It contains growth techniques and material systems for quantum wells and superlattices. Parent documents: SpringerMaterial s Volume III/ 34A Introduction to semiconductor quantum structures
The diffusion intermixing of layers during annealing of EuS-based epitaxial superlattice nanostructures was studied by X-ray diffraction technique. The interdiffusion coefficients for EuS-PbS, EuS-PbSe, EuS-SrS superlattices were determined basing on change of the intensity of near-Bragg reflection satellites in X-ray diffraction pattern. There are no layer intermixing in EuS-PbTe superlattice was observed.
The temperature, magnetic field, and laser excitation power dependences of photoluminescence (PL) were studied in 5×(EuS (5.5 nm)-PbS (17.5 nm)) semiconductor ferromagnetic multilayers grown epitaxially by high vacuum deposition on a BaF2 (111) substrate. In EuS-PbS heterostructures, ferromagnetic layers of EuS form electron barriers for both electrons and holes in nonmagnetic quantum wells of PbS. PL was observed in the near infrared due to electronic transitions in PbS quantum wells with narrow energy gaps. Measurements carried out at 4.2 and 77 K (i.e., below and above the Curie temperature of EuS layers, which is about 14 K) showed characteristic PL spectra consisting of one or two lines with a strongly non-linear response upon increasing the YAG laser excitation power. Below the Curie temperature, the application of a weak magnetic field of 200 Oe results in a change of the PL intensity as well as a small red shift in the PL energy of about 1 meV. These observations are discussed in terms of the model taking into account the magnetization-dependent height of the EuS potential barrier for electrons in a PbS quantum well.
Ferromagnetic resonance (FMR) study of magnetic anisotropy is presented for
thin layers of IV-VI diluted magnetic semiconductor Ge1-xMn xTe with x=0.14
grown by molecular beam epitaxy (MBE) on KCl (001) substrate with a thin PbTe
buffer. Analysis of the angular dependence of the FMR resonant field reveals
that an easy magnetization axis is located near to the normal to the layer
plane and is controlled by two crystal distortions present in these
rhombohedral Ge1-xMnxTe layers: the ferroelectric distortion with the relative
shift of cation and anion sub-lattices along the [111] crystal direction and
the biaxial in-plane, compressive strain due to thermal mismatch.
This Chapter contains the experimental facts about size effects in nanoferroics. They include ferroelectric, ferroelastic, magnetic and multiferroic nanostructured materials. The main peculiar feature of nanoferroics is the geometric confinement originating from their surfaces and interfaces. This is in contrast to the ordinary bulk ferroics, where the sample surface plays a minor role. In particular, in nanoferroics, the surface generates the physical properties gradients in the normal (to the surface) direction. This fact yields strong size effects and spatial inhomogeneity of the nanoferroics properties, which should be taken into account to get their adequate physical description. We report and analyze an extensive collection of experimental results regarding nanoferroics symmetry, lattice constants, dielectric response, magnetic susceptibility, polarization and hysteresis loops, magnetization and coercive field, heat capacity, soft mode and optical properties.
Modern rapid development of nanotechnology imposes an increased interest on studies of subtleties of fabrication technology and physical properties of nanosized materials. Although the physical properties of any material crushed down to nanometer size are interesting by themselves, the most intriguing are, in our view, the properties of so-called ferroic (i.e. ferroelectric, ferroelastic, ferromagnetic or multiferroic combining different kind of orderings) substances. The point is, that generally speaking, a nanosized particle of any material would have the properties absent in corresponding bulk sample due to excessive role of a surface. Such properties are strongly dependent on the particle shape (and boundary conditions on its surface) and are due to the effects of geometrical confinement, which have been well studied, e.g., for thin films. At the same time, any bulk ferroic obligatorily has a phase transition, where its long-range ordering appears. The simplest common examples are temperature phase transitions in ferroelectrics (ferromagnets) where spontaneous polarization (magnetization) emerges. More complicated ferroics may have many different kinds of phases, both pure like ferroelectric, ferroelastic and/or ferromagnetic (so-called primary ferroics) and their mixture (secondary ferroics). In the vicinity of phase transition points the ferroics have anomalous properties, which, for nanosized samples, are superimposed on the above effects of geometrical confinement. It can be easily imagined that interplay of these effects is “disastrous” - many weak effects related to phase transitions in a bulk sample, can be overamplified in a nanosized one. Conversely, many effects, absent in a bulk ferroic, may appear in a sample consisting of one or several nanosized particles. Latter effects are extremely important for possible nanotechnological applications of ferroics. Our present monograph is devoted to the comprehensive studies of physical properties of nanosized ferroics which we call nanoferroics. The results of such studies, generating the knowledge of physical properties of nanoferroics, permit to improve substantially the corresponding fabrication technology, which has also been discussed in the present book.
The book consists of 5 chapters. The first chapter has introductory character. It contains the general definitions and classification of primary and secondary ferroics with the listing of different ferroic materials. For the sake of generality, we discuss briefly the properties of spin, dipole and quadrupolar glasses as well as superparaelectric, superparamagnetic, and relaxor phases. The multiferroics (or secondary ferroics) like ferromagnetic ferroelectrics have also been considered. In these substances, one can control the magnetic properties by the application of electric fields and conversely.
The main results of experimental studies of size effects in nanoferroics are presented in second chapter. In particular, we collect the extensive experimental data about size effects in nanoparticles and thin films of ferroelectrics, ferroelastics and magnetically ordered ferroics. The data have been collected for different nanoparticles geometries like spherical and cylindrical as well as for nanowires, nanotubes and nanopills. As for nanosizes the local properties play a decisive role, we pay attention to the results of electron spin resonant measurements, which are sensitive to the local properties. To obtain the reliable information about the physical properties of the entire nanostructure, the above local methods should be augmented by other experimental techniques like dielectric, magnetic and optical methods. We hope that our collection of available experimental data will give the idea about both local and average static and dynamic properties of nanostructures.
In the third chapter, we discuss the existing theoretical approaches to describe the size effects in nanoferroics. Based on Landau-Ginzburg-Devonshire (LGD) formalism, augmented by corresponding surface terms and boundary conditions, we present the comprehensive theoretical approach suitable for nanoferroics and compare its results with experiment. In many cases, the solution of corresponding Euler-Lagrange equation, obtained by the minimization of LGD free energy functional can be done analytically. This permits to derive analytical (at least asymptotic) expressions for physical properties of nanoparticles of different morphology as well as for thin films on different substrates. The important factor of nanoparticle sizes distribution has also been taken into consideration. The above LGD formalism permits to consider the interesting question about the influence of domain structure on the physical properties of thin ferroelectric (ferromagnetic) films and multilayers near phase transition temperature. The specific analysis has been performed for ferro - paraelectric multilayers and very good coincidence with experimental data has been achieved. We also consider an important question about the behavior of so-called superparamagnetic ensembles of nanoparticles. We show that weak intergranular interaction in such system leads either to superferromagnetic or super spin glass behavior.
The fourth chapter is devoted to the properties, which are specific to nanosized ferroics and absent in corresponding bulk samples. As we have discussed above, such specific properties are generated by the effects of geometrical confinement in nanosized ferroics. In other words, they are due to dominant surface influence. We present vast experimental evidence on these effects, which have recently been revealed for the films and nanoparticles. Also, the comprehensive theoretical analysis of the available experimental data is presented. In this chapter, more attention has been paid to multiferroics, where ferroelectric and ferromagnetic long-range orders coexist. Latter substances have been considered in the form of thin films on substrate or nanowires. Detailed symmetry consideration of the physical properties with respect to effects of geometrical confinement has been presented. The particular attention has been paid for such interesting phenomenon as so-called spontaneous flexoeffect which occurs only in nanoferroics and is absent in bulk samples. The direct flexoeffect is the appearance of order parameter (polarization, magnetization or elastic stress) in primary ferroics in response to inhomogeneous mechanical impact, i.e. strain gradient. The origin of spontaneous flexoeffect in nanosized ferroics is the mechanical strain gradient which appears spontaneously due to above effects of geometrical confinement.
The fifth chapter presents detailed review of technological aspects of ferroic nanoparticles fabrication. We present the detailed information about methods of chemical synthesis of above nanoparticles. Among them are hydrothermal, sol-gel and coprecipitation methods. We also present the method of unstable compounds decomposition. The combined synthesis methods have also been discussed. Namely, we consider mechanochemical, sonochemical and template synthesis methods. The main idea of these methods is to control the dispersity and agglomeration degree of nanoparticles by inspection of nucleation and growth of a new phase. Self-assembly and self-organization of ferroic nanoparticles as well as composites formation on their base by means of colloidal processes have also been considered.
Our monograph is written on the base of results of our original investigations and those of other authors. Also, the material of the monograph has been widely presented in the form of lectures in Kiev Taras Shevchenko National University, Kiev Polytechnic University (Ukraine), Opole University, Wroclaw Polytechnic University (Poland) as well as many other universities from Ukraine, Russia, Poland, Czech Republic, France, Germany, Italy, Slovenia, USA, Canada, Japan, South Korea and Egypt.
We are indebted to many our colleagues for long-lasting collaboration on subjects which are relevant to our present book. Among them are prof. V.V. Skorokhod, drs. O.O. Vasilkiv, V.V. Laguta and E.A. Eliseev (Ukraine). We are also grateful to profs. Yu. D. Tretyakov (Russia), R. Newnman and C. Randall (USA), R. Blinc (Slovenia) for collaboration and fruitful discussions of physics, chemistry and technology of nanoferroics.
We are also grateful to L. Yurchenko for her invaluable help with the manuscript preparation.
Antiferromagnetic interlayer exchange coupling in semiconductor EuS-PbS-EuS ferromagnetic trilayers grown on PbS (001) substrates with ultrathin (0.6-1.2 nm) nonmagnetic PbS spacers is studied by SQUID magnetometry and model calculations. Analysis of the experimentally observed magnetic field and temperature dependence of the magnetization of EuS-PbS structures reveals a rapid decrease in the interlayer coupling energy with increasing temperature indicating a temperature dependence of the microscopic coupling mechanism acting in these all-semiconductor ferromagnetic/nonmagnetic multilayers.
The interlayer coupling between feitomagnetic EuS layers separated by spacer layers of diamagnetic insulators, YbSe and SrS, is studied within a 3D tight-binding model. The dependencies of the coupling strength on the energy structure of the spacer, on strains resulting from the lattice mismatch between the superlattice constituents, as well as on an applied hydrostatic pressure and lattice deformations, are presented. The sign and the range of the obtained coupling agree with the behavior of magnetic correlations observed recently in neutron reflectivity spectra of EuS/YbSe superlattices.
Magnetic and structural properties of EuS/Co multilayers were studied by magnetic optical Kerr effect and SQUID magnetometry techniques and by X-ray diffraction method. The multilayers containing monocrystalline, ferromagnetic EuS layer (thickness 35-55 Å) and metallic Co layer (thickness 40-250 Å), were grown on KCl (001) and BaF2 (111) substrates using high vacuum deposition technique employing electron guns for Co and EuS. All investigated EuS/Co multilayers exhibit ferromagnetic properties at room temperature due to Co layer with the ferromagnetic transition in EuS layer clearly marked upon cooling below 16 K. In EuS/Co/EuS trilayers grown on KCl substrate the antiferromagnetic alignment of magnetization vectors of Co and EuS layers was experimentally observed as a characteristic low field plateau on magnetization hysteresis loops and a decrease in multilayer magnetization below 16 K. In Co/EuS bilayers the characteristic temperature dependent shift of magnetization loops was found due to exchange bias effect attributed to the CoO/Co interface formed by the oxidation of the top Co layer.
Antiferromagnetic interlayer coupling between ferromagnetic layers of EuS via nonmagnetic PbS spacer layer was experimentally studied in EuS-PbS wedge multilayers grown on KCl (001) substrates with EuS thickness of 6 nm and PbS thickness varying in the wedges in the range 0.3-6 nm (i.e. n=1-20 monolayers). Measurements of magnetic hysteresis loops of EuS-PbS multilayers performed in the temperature range 5-30 K by superconducting (SQUID) and magneto-optical magnetometers revealed a rapid increase in saturation magnetic field in multilayers with PbS spacer thinner than about 1.5 nm. It shows a monotonic increase in interlayer coupling strength with a decreasing PbS spacer thickness, in qualitative agreement with 1/2n dependence predicted theoretically for semiconductor magnetic superlattices.
Magnetic and structural properties of EuS-SrS semiconductor multilayers were studied by SQUID and magneto-optical Kerr effect magnetometry techniques and by X-ray diffraction method. The multilayers composed of monocrystalline, lattice matched ferromagnetic EuS layers (thickness 35-50 Å) and nonmagnetic SrS spacer layers (thickness 45-100 Å) were grown epitaxially on KCl (001) substrates with PbS buffer layer. Ferromagnetic transition in EuS-SrS multilayers was found at the Curie temperature Tc=17 K. The multilayers exhibit only weak in-plane magnetic anisotropy with [110] easy magnetization axis. Coercive field of EuS-SrS multilayers shows a linear increase with decreasing temperature. Magneto-optical mapping of magnetic hysteresis loops of the multilayers revealed good spatial homogeneity of their magnetic properties.
Current-voltage characteristics and temperature dependence of differential conductance were studied in lithographically patterned (lateral dimensions from 10 × 10 μm2 to 100 × 100 μm2) ferromagnetic EuS-PbS-EuS microstructures. Below the ferromagnetic transition temperature a 4% decrease in the structure conductance was observed for mutual antiferromagnetic orientation of magnetization vectors of ferromagnetic EuS layers.
Interlayer coupling was experimentally studied in semiconductor EuS–PbS ferromagnetic superlattice wedge structures grown on KCl (001) substrates with the wedges covering the semiconductor nonmagnetic PbS spacer layer thickness from 0.3 to 6nm. Structural parameters of the wedges were examined by X-ray diffraction analysis of EuS–PbS superlattice period. Measurements of magnetic hysteresis loops of EuS–PbS structures were performed by both SQUID (for small terminal parts of the wedge) and MOKE (magneto-optical analysis along the wedge) magnetometry. A strong decrease of magnetic remanence and an increase of saturation field observed for EuS–PbS structures with the PbS spacer thickness decreasing below about 1.5nm is discussed in terms of the influence of antiferromagnetic interlayer coupling.
We present evidence of the influence of the substrate (GaAs and CdZnTe)
on the magnetic properties and on the surface structure of (001)
zinc-blende MnTe grown by molecular beam epitaxy. High-resolution X-ray
diffraction and atomic force microscopy were employed in the
characterisation of the MnTe structure. The temperature dependence of
collective spin excitations (magnons) was determined via Raman
scattering and subsequently analysed in order to study selected magnetic
properties. Differences in both the Néel temperature and
magneto-elastic coupling between the MnTe layer and the given substrate
were demonstrated, and mechanisms contributing to these effects are
discussed.
Electron tunnelling in double junctions with ferromagnetic barriers and nonmagnetic electrodes is analysed in the sequential and coherent limit of electron tunnelling. The free-electron-like one band model is used. The tunnelling current and its spin polarisation, as well as tunnel magnetoresistance (TMR) are determined. The spin accumulation in the central electrode, leading to a nonvanishing TMR effect, is taken into account and analysed in the sequential limit. In the coherent limit the influence of resonant states on the results obtained is analysed. The conditions leading to an enhancement of TMR and negative differential resistance are discussed. The influence of the parameters of the junction on the results obtained is also investigated.
Magnetic properties of semiconductor EuS(t)-PbS(d)-EuS(t) ferromagnetic trilayers (t = 30 divided by 300 Angstrom and d = 7.5 divided by 70 Angstrom) grown on n-type monocrystalline PbS (100) substrate were studied by SQUID magnetometry and ferromagnetic resonance technique yielding, in particular, the dependence of the ferromagnetic Curie temperature on the thickness of the EuS layer. Structural parameters of layers were examined by X-ray powder diffraction analysis. A high structural quality of the substrate and the multilayer was verified by the measurements of the X-ray rocking curve width indicating the values of the order of 100 arcsec and by atomic force microscopy revealing the presence on the cleft PbS surface regions practically flat in the atomic scale over the area of 1 x 0.1 mum(2).
Kerr magnetometry was employed to study the temperature dependence of magnetization and magnetic hysteresis loops in ferromagnetic EuS-PbS semiconductor multilayers in the temperature range T = 3-35 K at low magnetic fields H ≤ 150 -Oe. For EuS-PbS/KCl(100) structures with ultrathin non-magnetic PbS spacer of 1 nm, we observed a maximum on the temperature dependence of magnetization at low fields H ≤ 30 Oe. For higher fields, we found for these structures a regular mean-field-like increase in magnetization with decreasing temperature. The same regular behavior was also found for EuS-PbS/KCl structures with thicker PbS spacer, as well as for all EuS-PbS/BaF2(111) multilayers independently of spacer thickness. For qualitative interpretation of these findings, we consider two magnetic contributions to the total energy of EuS-PbS multilayers: the Zeeman energy and the antiferromagnetic interlayer exchange coupling between ferromagnetic EuS layers via diamagnetic PbS spacer.
The electron energy spectrum, the energy Em of the exchange coupling between magnetic layers, and the relative polarization β of the electron spins in semiconductor structures with two ferromagnetic barriers and nonmagnetic layers acting as potential wells for electrons are considered. For the example of EuS/PbS(001) structures it is shown that in the case of Fermi statistics Em is a sign-varying oscillatory function of the width a of the potential well between barriers, and with increasing electron density n0 in the wells and increasing thickness d of the nonmagnetic sublayers between the barrier and substrate, the extrema of Em are shifted to smaller a and their amplitudes rapidly increase. As the temperature is lowered from the Curie point, the energy Em, depending on a,n0, and d, can increase (in modulus) monotonically or nonmonotonically, change sign from positive to negative, or change sign twice. The polarization β decreases with increasing a,n0, and d, undergoing sharp jumps when Em changes sign. For Boltzmann statistics only a ferromagnetic orientation of the barrier magnetizations (Em
The sign of the superexchange coupling J2 between next-nearest neighboring Eu2+ magnetic moments in EuO is a matter subject to debate. We have obtained evidence that this coupling is of antiferromagnetic nature (J2<0). EuO thin films grown at different temperatures suggest that lattice expansion results in enhancement of TC as clearly observed in stoichiometric EuO films grown on CaF2 substrates. Resonant photoemission spectroscopy provides compelling evidence of strong hybridization between O 2p and Eu 5d6s6p weighted bands, suggesting that strong superexchange may be mediated by oxygen, thus consistent with the observed antiferromagnetic behavior between the next-nearest neighboring Eu atoms via nearest neighbor oxygen in EuO.
The 1.5Q/1 H type of the misfit layer compound {(EuS)1.15}1.5NbS2 has been studied by X-ray photoelectron spectroscopy. Eu 3d, 4d, 4f and 5p spectra are measured with Al and MgKα radiation. Their intensities are analyzed, taking the escape depth of photoelectrons emitted from the core levels into account. The results indicate that valence transfer of nearly-half Eu2+ ions to Eu3+ happens in the outermost surface layer and a reduced Eu–S distance brings about surface rearrangement. It is confirmed that Eu2+ and Eu3+ coexist in a bulk, in consistency with bond valence calculations and Mössbauer spectra. The XPS spectra are well interpreted in terms of the multiplet structures of Eu2+ and Eu3+.
An overview of neutron scattering studies of ferromagnetic and antiferromagnetic all-semiconductor superlattices is presented. Diffraction experiments on MnTe/CdTe, MnTe/ZnTe and EuTe/PbTe superlattices show pronounced correlations between the MnTe and EuTe layers across the non-magnetic spacers, even though these layers are antiferromagnetic and the systems are nearly-insulating. Current theory status of these systems is discussed. Diffractometry and reflectometry data from EuS/PbS superlattices reveal pronounced antiferromagnetic coupling between the ferromagnetic EuS block. First polarized neutron reflectometry data from superlattices prepared of a novel ferromagnetic "spintronics" material, Ga(Mn)As, are also presented.
The magnetization of single, ultrathin MnTe layers embedded in nonmagnetic quantum wells is studied by magneto-optical spectroscopy as well as by numerical simulations. It is shown to be proportional to the Zeeman splitting and thus it can be directly deduced from the magneto-optical experiments. The inverse of the experimentally determined magnetization measured as a function of temperature clearly demonstrates deviations from Curie-Weiss behavior due to the antiferromagnetic coupling between the Mn ions. By fitting this temperature dependence, an approximate Mn diffusion profile is obtained for each sample. The fitting procedure takes into account the antiferromagnetic coupling between the Mn ions as well as the exchange interactions between the Mn ions and the photoexcited electrons. For this purpose we have numerically solved the two-dimensional Ising model by a Monte Carlo method giving the magnetization of two-dimensional layers as a function of magnetic field, temperature, and Mn concentration.
The magnetism of ferromagnetic metal/ferromagnetic semiconductor PbS/EuS/Fe/PbS thin films has been studied. The presence of antiferromagnetic alignment of neighboring ferromagnetic layers in direct contact with each other was observed. The films also showed an exchange bias effect which appeared due to the formation (likely during film deposition) of antiferromagnetic FeS at the Fe/PbS interface. Antiferromagnetic interfacial exchange coupling between Fe and FeS was also observed. It was shown that the exchange anisotropy could be controlled by the remnant state of Fe magnetization during the cooling process, that in turn could be tuned by an external cooling field. The influence of exchange anisotropy and exchange coupling effects on the field and temperature dependencies of the film magnetization was examined.
The magnetic properties of the antiferromagnetic (AFM) EuTe epitaxial layers and the short period EuTe/PbTe superlattices (SLs), grown by molecular-beam epitaxy on (111) BaF2 substrates, were studied by magnetization and neutron-diffraction measurements. Considerable changes of the Néel temperature as a function of the EuTe layer thickness as well as of the strain state were found. A mean-field model, taking into account the variation of the exchange constants with the strain-induced lattice distortions and the nearest-neighbor environment of Eu atoms, was developed to explain the observed TN changes in a wide range of samples. Pronounced interlayer magnetic correlations have been revealed by neutron diffraction in EuTe/PbTe SL’s with PbTe spacer thickness of up to 60 Å. The observed diffraction spectra were analyzed, in a kinematical approximation, assuming partial interlayer correlations characterized by an appropriate correlation parameter. The formation of interlayer correlations between the AFM EuTe layers across the nonmagnetic PbTe spacer was explained within the framework of a tight-binding model. In this model, the interlayer coupling stems from the dependence of the total electronic energy of the EuTe/PbTe SL on the spin configurations in the adjacent EuTe layers. The influence of the EuTe and PbTe layer thickness fluctuations, inherent in the epitaxial growth process, on the magnetic properties and interlayer coupling is discussed.
We present the temperature- and layer-dependent electronic structure of a 20-layer EuS(100) film using a combination of first-principles and model calculations, the latter based on the ferromagnetic Kondo lattice. The calculated thickness-dependent Curie temperature agrees very well with experimental data. The projected 5d band structure is at finite temperatures strongly influenced by damping effects due to spin-exchange processes. Spin-split unoccupied 5d-surface states are found with a Stoner-like collapsing for increasing temperature towards the Curie point and with an exponential decay of spectral weight with increasing distance from the surface.
Magnetotransmission studies of EuS/PbS multiquantum wells in magnetic fields up to 200 T and in the temperature range from 5 to 300 K are reported. A series of transitions are observed, which we interpret as cyclotron resonance transitions, i.e., the transitions between the lowest magnetic subbands. With this identification, the positions of observed resonances are satisfactorily described by theory when the quantum well width is larger than 100 Å. For narrower quantum wells, however, the discrepancy between theory and experimental measurements is significant. Possible explanations for this discrepancy are discussed.
Superconducting and structural properties of superconducting semiconducting multilayers are investigated. These layered systems are obtained by epitaxial growth of the isomorphic monochalcogenides of Pb, Sn, and rare-earth elements on a KCl substrate. Some of these compounds are narrow-gap semiconductors (PbTe, PbS, PbSe, SnTe). Layered structures containing one or two narrow-gap semiconductors have a metallic type of conductivity and a transition to a superconducting state at temperatures in the range of 2.5–6 K. Structures containing only wide-gap semiconductors (YbS, EuS, EuSe) do not demonstrate such properties. All superconducting layered systems are type-II superconductors. The critical magnetic fields and the resistive behavior in the mixed state reveal features characteristic of other layered superconductors. However, data obtained in magnetic fields testify that the period of the superstructure corresponds to half of that obtained from x-ray-diffractometry investigations. This is evidence that the superconducting layers in these samples are confined to the interfaces between two semiconductors. Electron microscopy studies reveal that in the case of epitaxial growth the interfaces contain regular grids of misfit dislocations covering all the interface area. These samples appear to undergo a superconducting transition if they have a metallic type of conductivity in the normal state. Samples with island-type dislocation grids only reveal partial superconducting transitions. The correlations between the appearance of superconductivity and the presence of dislocations, which have been found experimentally, lead to the conclusion that the normal metallic conductivity as well as the superconductivity are induced by the elastic deformation fields created by the misfit dislocation grids. A theoretical model is proposed for the description of the narrow-gap semiconductor metallization, which is due to a band inversion effect and the appearance of electron- or hole-type inversion layers near the interfaces. For different combinations of the semiconductors, such inversion layers in the superlattices can have different shapes and topology. In particular, they can form multiply connected periodic nets having a repetition period coinciding with that of the dislocation grids. Numerical estimates show that such a scenario for the appearance of superconductivity is quite likely. It is shown that the new type of metallic and superconducting nanoscale two-dimensional structures with unusual properties may be obtained from monochalcogenide semiconductors.
We propose a spin-valve device consisting of a nonmagnetic semiconductor quantum well, sandwiched between ferromagnetic semiconductor layers that act as barriers. The total conductance through such a trilayer depends on the relative magnetization of the two ferromagnetic-barrier layers which act as “spin filters.” With respect to practical realization, EuS/PbS heterostructures may be a suitable candidate. The magnetoresistance should exceed 100% for a wide range of the thicknesses of both the quantum well and the ferromagnetic barriers. From a fundamental physics point of view, the device may not only give insight into the spin lifetimes of the nonmagnetic layer, but the strong spin accumulation taking place in the quantum well may lead to novel optical and nuclear magnetic resonance properties. © 2002 American Institute of Physics.
Thin EuS films of 5.5 and 11 nm have been grown at 300 K on epitaxial Co(111) and Cu(111) surfaces on Si(111) substrates. The EuS layers show volumetric reflections in the RHEED pattern indicating that the EuS is deposited in form of small crystallites on the Co or Cu surface. The magneto-optic properties are investigated by polar Kerr spectroscopy at 300 K and 10 K. At 10 K, a clear signature of the intra-Eu 4f-5d transition is observed at 1.8 eV and at 4.2 eV. Below 1 eV, the magneto-optic contribution of the Co layer is dominant. The influence of the Co or Cu layer on the magnetic ordering of the EuS layer is studied by measuring the Kerr spectra as a function of temperature. The layered structures exhibit an in-plane magnetic anisotropy. Estimates of the paramagnetic EuS contribution to the spectrum at 300 K suggest only a weak coupling between the EuS film and the Co layer. © 2002 American Institute of Physics.
All-semiconducting EuS/PbS/EuS trilayers that show antiferromagnetic coupling were studied by superconducting quantum interference device magnetometry. We analyzed our measurements with a modified Stoner–Wohlfarth model from which the interlayer exchange energy and anisotropy were extracted based on the switching field from antiparallel to parallel alignment of the EuS layers and the zero-field susceptibility, respectively. Magnetic moment versus temperature curves were simulated by taking into account Brillouin type temperature dependence of the saturation magnetization of EuS. Despite their simplicity, the simulated curves show good qualitative agreement with the measurements when strong temperature dependence of interlayer coupling is assumed. © 2004 American Institute of Physics.
The fabrication, morphology, and magnetic properties of EuSe nanoislands were analyzed using molecular-beam-epitaxy (MBE) and magnetic x-ray circular dichroism. An appropriate template was used for deposition in order to obtain well-defined EuSe nanoislands. Atomic force microscopy (AFM) and grazing incidence small-angle x-ray scattering (GISAXS) at an x-ray energy of 8 keV were also used for characterizing the samples. A precursor state to a magnetic phase transformation was indicated by the EuSe islands that formed a regular hexagonal pattern by showing coupling between Eu spins at low temperatures.
EuS nanoparticles were synthesized by solution-phase thermolysis of the diethylammonium salt of the anionic europium dithiocarbamate complex, [Eu(S2CNEt2)4]−. Oleylamine and triphenylphosphine were used as surfactants to prevent nanoparticle agglomeration and stabilize particle growth. By varying the synthetic parameters such as reaction temperature, heating time, and the [surfactant]:[precursor] ratio, nanoparticles of different sizes were obtained. The size-dependent magnetic properties of these nanoparticles were studied, and it was observed that a decrease in the ferromagnetic ordering temperature occurs with decreasing particle size.
The spin-dependent interactions in diluted magnetic semiconductors are reviewed. The non-trivial dependence of the p-d and the d-d exchange interactions, which rule the magnetic properties of these materials, on the electronic configuration of the magnetic ions is presented. Depending on the occupation numbers in the ionic d shell and the position of the d level with respect to the bands, both the kinetic exchange and the superexchange can lead to antiferromagnetic, ferromagnetic Kondo-like interactions or even orbital dependent, sensitive to Jahn-Teller distortion couplings. Finally, the interactions explaining the ferromagnetism in III-V and p-II-VI with Mn ions, the magnetic properties in semiconductor quantum wells and the interlayer spin correlations in magnetic/non-magnetic semiconductor superlattices are discussed.
Magnetic properties of ultrathin EuS films with strain effects taken into account are studied within the framework of the Green function formalism. Standard RPA and more general procedures employed by Callen are used for decoupling of higher order functions generated by exchange and anisotropy terms, respectively. Parameters of the model such as exchange integrals in strained structures and anisotropy constants are estimated on the basis of experimental data obtained with use of SQUID and FMR techniques. Strong influence of stress on spin-wave dispersion relations, especially on high-energy modes, is found. A shift of curves representing magnon DOS towards higher energies is obtained in the presence of stress. All these modifications lead to a slower decrease of the magnetization with increasing temperature and to an enhancement of the critical temperature. The results are consistent with experimental data. Investigations of the course of the magnetization curves for various film thickness also confirm experimental results.
Based on the s–d model using a Green's function technique it is shown that the influence of the substrate induces strong changing of various statical and dynamical properties due to different exchange interactions between the ferromagnetic thin film and the substrate. The increase of the phase transition temperature and the magnetization is attributed to the tensile stress effect induced by the substrate whereas the reverse is true for the compressive stress effect. The damping in thin films is greater compared to the bulk case taking into account substrate effects. This shows that substrate stress is an important parameter for thin films. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
EuS–PbS ferromagnetic multilayers constitute all-semiconductor ferromagnetic-nonmagnetic structures with magnetic layers of EuS (a model nonmetallic ferromagnet) and diamagnetic layers of PbS (IV–VI narrow gap semiconductor compound). The ferromagnetic transition temperature in EuS–PbS multilayers depends on the thickness of the magnetic layer as well as on thermal strain. For ultrathin PbS spacer layers one observes in EuS–PbS–EuS trilayers and superlattices pronounced magnetic effects related to the antiferromagnetic interlayer coupling between ferromagnetic layers of EuS via nonmagnetic semiconductor PbS spacer. These effects are discussed for EuS–PbS structures grown epitaxially on various substrates: insulating KCl(100) and BaF2(111) as well as conducting n-PbS(100) monocrystals.
We report results of magnetization study of EuS/PbS superstructures with different thicknesses of magnetic and nonmagnetic layers. Reduction of ferromagnetic phase transition temperature was found with decreasing EuS thickness. Reasonable description of this effect is obtained within the model based on the mean field approximation.
Investigations of the photoluminescence of PbS-EuS superlattices deposited on (111)BaF2 substrates are presented. Quantum-size and deformation effects in photoluminescence spectra are observed. The strain-induced gap shift and valence-band offset is determined from experimental results. A strong stimulated photoluminescence with relatively low threshold was observed. It was found that the photocarriers generated in EuS barrier strongly affect the population of PbS subbands.
Magnetic resonance investigations of ultra-thin antiferromagnetic EuTe layers show a specific behaviour in the quasi-2D antiferromagnetic ordering: (i) an anisotropy of the critical broadening, (ii) a substantial increase in the Néel temperature and (iii) an anisotropic shift of the resonance frequency which diverges at the Néel point. The results show that exchange coupling is stronger in quasi-2D than in 3D antiferromagnetic samples and that correlation of spin chains aligned in the perpendicular direction occurs already well above the Néel point.
In this paper the results of ferromagnetic resonance study of sputtered Co/Pt multilayers, with a fixed Co layer thickness of 15 Å, are presented. For these multilayers with small Pt layer thickness, in addition to a uniform resonance mode, a spin wave resonance mode is found to exist, which confirms the existence of an interlayer coupling between the neighboring Co layers. Simultaneously, the effective magnetization 4piMeff and the resonance linewidth DeltaH are found to change correlatively with varying Pt layer thickness. This phenomenon can be explained as a result of an interplay between the interlayer coupling and the low-dimensional effect.
We present experimental and theoretical investigations on the magnetic anisotropy of Co ultrathin films sandwiched by Au. Ferromagnetic resonance experiments revealed the presence of a large perpendicular surface anisotropy that makes the easy magnetization direction become perpendicular to the film plane for Co thicknesses lower than 11 Å, as is observed in magnetization measurements. In order to explain this surface anisotropy, we propose various models, taking into account the imperfections of the films. For thicknesses below 11 Å, there is a large increase of the coercive field with decreasing thickness. This effect is tentatively interpreted in a model of propagating Bloch walls, where the interfacial roughness plays an important role.
We have studied the magnetoresistance of (001)Fe/(001)Cr superlattices prepared by molecular-beam epitaxy. A huge magnetoresistance is found in superlattices with thin Cr layers: For example, with tCr=9 Å, at T=4.2 K, the resistivity is lowered by almost a factor of 2 in a magnetic field of 2 T. We ascribe this giant magnetoresistance to spin-dependent transmission of the conduction electrons between Fe layers through Cr layers.
We have explored the relationship between epitaxial growth and magnetism in the case of ultrathin fcc-Co films by means of a multitechnique approach. For high-quality fcc-Co films less than 3 monolayers thick the Curie temperatures TC are dramatically lower than TBC of bulk fcc-Co and decease distinctly with film thickness. In contrast to previous claims, TC of a single monolayer appears to be far below 300 K. Substrate topology is found to strongly influence the structural perfection of the films which in turn determines their magnetic properties (Curie temperature, coercive field, and anisotropies).
The magnetic interactions in superlattices formed by alternating layers of magnetic and nonmagnetic semiconductors with common anions were studied theoretically. Within a one-dimensional tight-binding model by minimising the total electronic energy we show the existence of an efficient long range mechanism of magnetic correlations between the neighbouring magnetic layers in such superlattices. The cases of magnetic ions in the barriers (e.g., EuTe/PbTe) and in the wells, with the order within the magnetic layers being either ferromagnetic or antiferromagnetic, have been considered and compared with the paramagnetic case. In the case of antiferromagnetic ordering within magnetic layers we have found that for even and odd numbers of magnetic monolayers different magnetic superlattices are energetically favourable.
A solid solution of about 6 mol % SnO2 in
α-Fe2O3 has been prepared by mechanical
alloying of α- Fe2O3 and SnO2
powder blends. This result demonstrates that high energy ball milling
can be used to prepare metastable oxide solid solutions with an extended
range of compositions in the immiscible ceramic oxide system. X-ray
diffraction and Möautssbauer spectroscopy investigations show that
mechanical milling of α-Fe2O3 amd
SnO2 involves alloying on an atomic scale and that true solid
solution formation occurs. We suggest that the high defect concentration
and the chemical enthalpy of
Fe3+-O2--Sn4+ interfaces between
nanostructured α-Fe2O3 and SnO2
regions may serve as a driving force for the formation of a solid
solution in the immiscible ceramic system.
Critical phenomena in films of finite thickness are considered. A detailed scaling theory, with allowance for distinct exponents λ and θ=1/ν for the critical-point shift and rounding, respectively, is confirmed by exact calculations on d-dimensional ferromagnetic spherical models and ideal Bose fluids with various boundary conditions. Isingmodel results and existing data on real helium films are consistent with the theory.
Magnetic and transport properties in the magnetic semiconductor EuTe and composite film EuTe/Fe were investigated by using high magnetic fields. Faraday rotation of EuTe in liquid helium temperature suggests a magnetic polarization around the optical 5d-excitation. The stacking effect in EuTe/Fe was observed in the temperature dependence of conductivity and magneto-resistance.
The magnetization of (100) EuTe/PbTe superlattices and of (100) EuTe film was studied. For thin EuTe layers (several monolayers) both the Néel temperature TN and the field Hc necessary to destroy the antiferromagnetic order decrease with decreasing thickness. No transition is found for a single monolayer. Information concerning the EuTe/PbTe interface is obtained from TN using a simple model. Below TN and at low fields the spins are in the (100) plane of the layer, which is explained by calculations of the dipole-dipole anisotropy.
EuTe has been shown by neutron diffraction to undergo at 7.8°K a transition to anti-ferromagnetism. The antiferromagnetic ordering is of the second kind with the spins lying within the (111) planes. The temperature dependence of the magnetic scattering was analyzed by adding a biquadratic exchange interaction to the usual bilinear term.
Specific heat measurements on EuSe from 0.5 to 6°K are reported. An anomaly at 2.8°K was found.
The magnetic interlayer coupling of Fe3O4 across NiO is studied using Fe3O4/NiO/Fe3O4 trilayers epitaxially grown on (001) MgO substrates. For NiO thicknesses between 0.7 and 5 nm, the magnetic moments of the two Fe3O4 layers are directed perpendicularly with respect to each other. The 90 degrees coupling strength is determined to be 0.35 +/- 0.08 mJ/m(2) for a 1.4-nm-thick NiO spacer. The 90 degrees coupling can be understood from the effect of an antiferromagnetic spacer in the presence of interface roughness.
Using neutron diffraction at very high pressures (up to 20.5 GPa), we studied magnetic interactions in the model system EuX ( X = S, Se, Te) in a wide range of lattice constants. Surprisingly, when interatomic distances decrease, the Curie temperatures increase very rapidly and in a different way for each compound. The results were used to test models of an indirect f-d exchange in magnetic semiconductors. The observed behavior cannot be explained by considering only the overlap between Eu2+ d orbitals, pointing out a strong influence of the 2p shells, neglected before, on the first neighbor magnetic interaction.
We report results on the temperature dependence of the susceptibilities of a set of MBE-grown short-period EuTe/PbTe antiferromagnetic superlattices having different EuTe layer thicknesses. In-plane and orthogonal susceptibilities have been measured and display a strong anisotropy at low temperature, confirming the occurrence of a magnetic phase transition in the thicker samples, as seen also in neutron-diffraction studies. We suggest that dipolar interactions stabilize antiferromagnetic long-range order in an otherwise isotropic system and we present numerical and analytical results for the low-temperature orthogonal susceptibility.
The luminescence of very thin (up to 4 nm) films of PbS and EuS-PbS superlattices with PbS potential-well and EuS barrier thicknesses in the 1-20-nm range is studied. It is shown that the PbS luminescence band at 80 K may shift by 240-440 MeV as a function of the thickness of the PbS layers and their deformation by the KCl substrate and EuS layers.
EuS single crystalline films were grown epitaxially on silicon (111), (100) and (110) crystal substrate planes. By Rutherford backscattering techniques they are shown to achieve the stoichiometric composition and crystal perfection expected with molecular beam epitaxy. The angular dependence of the remagnetization process in the film plane confirms the expected crystal symmetry and magnetic anisotropy. The substantial, but different, reductions in the Curie temperatures of EuS-(111), -(110) and -(100) films by up to 15% have been attributed to the largely different thermal expansion coefficients of Si and EuS.
Some divalent europium compounds (EuS, EuSe, EuTe) are made by formerly
used procedures, and their magnetic properties are examined and compared with EuO
characteristics. At 4.2 K, the sulfide and selenide are ferromagnetic and the
telluride is paramagnetic. Graphs display various temperature and magnetic
variations. (D.C.W.)
Phase transitions in Ising models with tree surfaces are studied from various points of view, including a phenomenological Landau theory, high-temperature series expansions, and a scaling theory for thermodynamic quantities and correlation functions. In the presence of a surface a number of new critical exponents must be defined. These arise because of the existence of "surface" terms in the thermodynamic functions, and because of the anisotropy of space and lack of translational symmetry introduced by the surface. The need for these new critical exponents already appears in the phenomenological theory, which is discussed in detail and related to the microscopic mean-field approximation. The essential new parameter appearing in this theory is an extrapolation length λ which enters the boundary condition on the magnetization at the surface. For magnetic systems this length is of the order of the interaction range, in contrast to superconductors, where it is usually much larger. In order to go beyond the mean-field theory, high-temperature series expansions are carried out for the Ising half-space, to tenth order in two dimensions and to eighth order in three dimensions. A scaling theory is developed both for thermodynamic functions and for spin correlations near the surface, and relations are found among the exponents of the half-space. Both the scaling theory and the numerical calculations are compared with the exact solution of the Ising half-plane (two dimensions) by McCoy and Wu, and agreement is found wherever the theory is applicable. In analogy to the bulk situation, the scaling theory is found to agree with mean-field theory in four dimensions. The prediction of the present work which is most easily accessible to experiment is the temperature dependence of the magnetization at the surface, with critical exponent estimated to be β1=2/3. The mean-field result, β1=1, seems to agree more closely with presently available experiment, and more work is needed to clarify the situation.
Inelastic-neutron-scattering methods have been used to measure the spin-wave spectrum in EuO and EuS over the entire Brillouin zone. The samples were polycrystalline powders enriched in 153Eu. Defining the interaction between pairs of spins to be -2JnmS⃗n·S⃗m, we obtained for EuO, J1/kB=0.606±0.008 °K and J2/kB=0.119±0.015 °K; for EuS, J1/kB=0.236±0.009 °K and J2/kB=-0.118±0.011 °K. The measured Curie temperatures for EuO, 69.15 ± 0.05°K, and for EuS, 16.57 ± 0.01 °K, are in excellent agreement with calculated values obtained from series expansions relating Tc to the exchange constants. In EuO, the neutron scattering measurements determine J1 and J2 separately. Although separate values for the exchange constants were reported in earlier experiments, we show that in fact only the spin-wave stiffness constant of EuO, which is proportional to J1+J2, was actually determined. In EuS, our values of J1 and J2 agree with a reanalysis of the specific-heat data.
The magnetic properties of both insulating and conducting single crystals of EuTe were measured at temperatures 0.4≤T≤273 K in magnetic fields 0≤H<150 kOe. The insulating EuTe showed the behavior expected for an Heisenberg antiferromagnet and is believed to be representative of pure EuTe. For this insulating material we obtained a Néel temperature TN=(9.6±0.1) K and a saturation magnetic moment σs=(132±3)emu/g(6.6μB/Euion). In the interval 10TN<T<28TN, the insulating material obeyed the Curie-Weiss law with a Curie-Weiss constant C=(27.8±0.3)×10-3 (emu K/g Oe), and a paramagnetic Curie temperature θ=(-1±2) K. Below TN, the differential susceptibility dσ/dH in the insulating sample showed the following features: (i) At low fields, dσ/dH first increased with H, then passed through a broad maximum near 1 kOe, and finally assumed the constant value χ⊥=1.6×10-3 emu/g Oe. This low-field behavior corresponds to the rotation of spins in the {111} planes (analogous to spin flopping). (ii) At higher fields, in the canted phase, dσ/dH increased with H. (iii) The canted-to-paramagnetic transition was marked by a λ peak in dσ/dH. (iv) The transition field Hc(T) was linear with T3/2 in the interval 2≤T≤4.2 K, but a small deviation from the theoretical T3/2 law was observed below 2 K. At T=0, Hc(0)=(72.2±1) kOe, which leads to an intersublattice exchange field HE(0)≅36 kOe. The values of Hc(0) and θ for the insulating material yield the nearest-neighbor and next-nearest-neighbor exchange constants J1/k=+(0.10±0.03) K and J2/k=-(0.215±0.03) K, respectively. The results in the conducting single crystals (which were n type) showed that the following magnetic properties were influenced by the presence of the charge carriers: (a) At T≲2TN, the zero-field differential susceptibility in the conducting samples was an order of magnitude larger than in the insulating sample. (b) The low-field behavior of σ at temperatures well below TN suggested the existence of a spontaneous magnetic moment at H=0. (c) The λ anomaly in dσ/dH, at Hc, was reduced, or disappeared entirely. (d) The saturation moment σs was slightly higher than for the insulating sample. (e) The paramagnetic Curie temperature θ was several degrees higher than for the insulating material. The presence of charge carriers had little or no effect on TN, and only a small influence on the phase boundary Hc(T). Some of the effects of the charge carriers can be explained qualitatively by de Gennes's model for the double-exchange interaction in antiferromagnets.
The susceptibility of simple-cubic-lattice ferromagnetic Ising films of n two-dimensional lattice layers is studied by extrapolation of high-temperature series expansions (to eleventh and twelfth orders) for n=3,4,…,14 layers with periodic boundary conditions, and n=3,4,…,10 layers with free-surface conditions. The corresponding surface susceptibility series for sc, bcc, and fcc lattices are analyzed in the light of universality hypotheses. On the basis of finite-size scaling theory explicit scaling functions are constructed describing the crossover from two-dimensional to three-dimensional critical behavior in terms of the ratio n/ξ(T), where ξ∼ΔT-ν is the bulk correlation length.
The critical behavior of the surface properties of magnets is discussed. A homogeneity assumption for the free energy, involving a new exponent φ1 to scale the surface field, is introduced. New exponent relations are derived for the surface exponents. These are satisfied for the two-dimensional Ising model, the spherical model, and mean-field theory. The existing estimates for the three-dimensional Ising model, however, appear to be possibly inconsistent.
The title of these lectures reflects the intent more strongly than the content of them. For while we are particularly interested in how the s-f interaction appears differently in magnetic semiconductors than in metals, in which it has been studied extensively, it seemed necessary after the first lecture on the metals to develop the theory of magnetic semiconductors and their optical properties in greater detail before going on to treat the transport properties of these materials in which the s-f exchange interaction is very important. Thus, the greater portion of the lectures is on the whole theory of magnetic semiconductors and the s-f exchange is treated primarily at the beginning and end of the series.
Magnetism has been a topic of study since the ancient times of science. Judging by the number of papers appearing each year, the group of scientists concerned with the properties of magnetic materials is vastly expanding. The reason for the great interest in this field is not only the wide application which these materials have found in technology, but also the chance to contribute to the understanding of basic physics. The term “magnetic material” in this article refers to a substance which spontaneously exhibits magnetic order below a certain temperature. This magnetic order can be of the ferro-, ferri-, antiferro-, or metamagnetic kind. However, within these magnetic structures ferromagnetism appears to be the most important one. Ferromagnetism is defined by the spontaneous parallel alignment of atomic magnetic moments over relatively large regions of a solid. Magnetic order is limited to compounds which contain elements with incompletely filled electron shells, as for example, the transition elements.
The authors review the experimental investigations carried out on IV-VI artificially structured materials are summarised. The problems related to the determination of band offsets are discussed and special emphasis is given to strain effects and the consequences of interdiffusion. Deviations from the bulk magnetic behaviour reported in dilute magnetic IV-VI quantum well structures are presented. The unique structural properties of layered PbGeTe/PbTe which exhibit a stabilisation of the cubic phase are discussed.
Neutron diffraction studies of MBE-grown (111) superlattices are reported. The EuTe layers exhibit an AF structure consisting of antiferromagnetically coupled (111) spin sheets. The data reveal interlayer magnetic coupling accross PbTe spacers with suprisingly large thicknesses (up to 55 Å), despite the low carrier concentration which rules out any significant RKKY interactions.
The magnetization, susceptibilityX and specific heatC of epitaxially grown EuS/SrS multilayers measured at low temperatures is reported.C changes with decreasing thickness of the individual EuS and SrS layers, i.e. decreasing modulation length, from the behavior of nearly pure EuS(CT
3/2) to a behavior resembling that of spin-glass samples of EuxSr1–xS (approximatelyC T). For multilayers with increasing thickness ratior =d
M/dN (M=EuS,N=SrS),C decreases towards pure EuS, thus showing a transition from spin-glass to ferromagnet. The paramagnetic Curie temperature obtained fromX is reduced with respect to that of pure EuS for layers with small EuS layer thickness. All these effects can be explained by the formation of EuxSr1–xS intrrmediate diffusion layers of a fewnm thickness during the evaporation process. Reduction of the substrate temperature results in a much smaller diffusion layer.
A theory of interlayer exchange coupling is presented. A detailed and comprehensive discussion of the various aspects of the problem is given. The interlayer exchange coupling is described in terms of quantum interferences due to confinement in ultrathin layers. This approach provides both a physically transparent picture of the coupling mechanism, and a suitable scheme for discussing the case of a realistic system. This is illustrated for the Co/Cu/Co(001) system. The cases of metallic and insulating spacers are treated in a unified manner by introducing the concept of the complex Fermi surface.
Ferromagnetism has been found in several divalent europium compounds. Some of these materials are particularly simple in both crystal and magnetic structure and are ideal for experimental and theoretical study. The materials we have investigated can be divided into three groups, europium chalcogenides, europium halogens, and europium silicates. Magnetic information on these compounds are summarized and theoretical results reviewed.
The europium chalcogenide series all have the rocksalt structure, the oxide, sulfide, and selenide being ferromagnetic, and it is on these materials that the most work has been done. This is especially true of EuS where detailed moment, specific heat, and nuclear magnetic resonance measurements are available. The oretical analysis indicates that there is positive exchange between nearest-neighbor europium atoms which decreases with increasing atom spacings. There is also a weaker negative superexchange via the anions in these compounds which is responsible for antiferromagnetism in EuTe. The EuTe magnetic ordering has been studied by neutron diffraction at Brookhaven and the sublattice magnetization-temperature behavior is found to be somewhat anomalous. EuSe is ferromagnetic but reveals low-field magnetization curves that have an unusual shape. In view of this, EuS seems to be the simplest of the group so that fundamental parameters obtained from analyzing the data are probably the most reliable.
In the halogen group, EuI2 appears to be ferromagnetic while EuF2 is apparently antiferromagnetic. The latter behavior is somewhat unexpected from interatomic distance considerations which hold for the chalcogenides. Of three silicates prepared, two were ferromagnetic. The most interesting was Eu2SiO4 which was obtained as transparent single crystals. Magneto-optical studies show this compound to have a Verdet constant higher than any other previously reported.
The spin wave excitations of an <sup>153</sup>EuS single crystalline sample have been measured by inelastic neutron scattering through the entire 1. Brillouin zone with the momentum Q parallel to the main symmetry directions 〈100〉, 〈110〉, and 〈111〉, and for temperatures ranging from well below (1.3 K) to well above (36 K) the Curie temperature (T c = 16.6 K). Least square fits of the low‐temperature spin wave dispersion curves to the Holstein‐Primakoff theory yielded the exchange energy constants J r of the Heisenberg operator up to the fifth (r = 5) Eu‐Eu‐neighbour interactions as follows: J 1 /k B = 0.22(1±3)K, J 2 /k B = -0.10(0±4)K, J 3 /k B = 0.00(6±2)K, J 4 /k B = -0.00(7±2)K, and J 5 /k B = -0.00(4±2)K. They correspond with Θ p = 21.1±0.2 K and also with the theoretically expected ratio of T c /Θ p . The temperature dependence of the spin wave energies, which has been measured for large‐q spin‐waves up to T/T c = 1.5, does not agree with the spin‐wave renormalization theory. This EuS‐result differs from EuO, where the spin‐wave renormalization was reported to agree with theory. The variation of the J r with magnetic dilution is discussed by considering related results on the Eu x Sr 1-x S system.
Superexchange mechanisms, which are mostly responsible for the nnn exchange constant I 2 in Eu chalcogenides, are investigated in detail. In contrast with the usual 3d transition metal compounds, the Kramers-Anderson mechanism is estimated to be one order of magnitude too small to explain the experimental results due to a small 4f → 2p transfer energy. The mechanism by which a p electron is transferred to a 5d state through the d-f exchange interaction gives the correct order of magnitude for I 2 , with a negative sign, even though it is a sixth-order perturbation. The cross term between the above two mechanisms is shown to be nearly as important as the second mechanism and may have a positive sign. The indirect exchange mechanisms, in which the anion p level has no important role, are responsible for the nn exchange constant I 1 . The phonon-assisted mechanism proposed by Smit is estimated to be more than one order of magnitude smaller than the experimental value. The d-f mixing term is proved to be responsible for I 1 , in good agreement with experiment.
We have observed a large enhancement of the Curie temperature of undoped EuS(100) epitaxial films, dependent on the growth conditions. The study performed allows us to claim that such an enhancement is associated with indirect exchange through additional charge carriers that are created in the film due to non-stoichiometry caused by dislocations.
The result of recent nuclear resonance, spin resonance, and neutron scattering experiments on Eu X compounds (X = O, S, Se, Te) are summarized and reviewed. Emphasis is laid on recent NMR and Mössbauer effect studies of transferred and supertransferred hyperfine interactions, which can be directly related to the different individual exchange interactions between Eu-pairs. The two-parameter approximation of the Heisenberg operator, limited to effective nearest and next nearest neighbour interactions, J̄1 and J̄2, accounts for different experimental results. The results of nuclear resonance studies, however, provide experimental evidence that the J̄1,2 are not representative for the real individual exchange integrals J1,2. The interactions are shown to have a longer range and an unexpected temperature dependence.
The magnetic coupling of Fe/EuS (100) epitaxial bilayers has been investigated. Evidence of antiferromagnetic coupling has been found, both when the samples are cooled below the Curie temperature of the EuS layer, and in the hysteresis loops at low temperatures. The coupling strength has been measured as a function of temperature.
Since the discovery of antiferromagnetic type interlayer coupling in 1986 and of ‘Giant Magnetoresistance’ in 1988. numerous systems have been investigated. Here we give a critical review of the research on these phenomena and illustrate the development with some results from our groups in Orsay and Juelich.
We report the magnetic susceptibility as a function of temperature (4<T<200 K) and magnetic field (1000<H<7000 Oe) of EuTe/PbTe superlattices grown by molecular-beam epitaxy. Three superlattices were used, consisting of four EuTe monolayers alternating with four PbTe monolayers (EuTe)4/(PbTe)4, (EuTe)2/(PbTe)6, and (EuTe)1/(PbTe)3 monolayers. The magnetic field was applied both in the plane of the layers and perpendicularly to it. Only the superlattice with four Eu monolayers shows an antiferromagnetic phase transition, at a nearly isotropic Néel temperature TN=8.50.7 K comparable to that of bulk EuTe. In the paramagnetic regime, the susceptibility follows a Curie-Weiss behavior, with an anisotropic Curie-Weiss temperature FTHETA. This was explained with use of molecular-field theory by considering the influence of the strain in the superlattice layers on the Eu-Eu nearest-neighbor and next-nearest-neighbor interactions: FTHETA depends on both, while TN depends only on the latter, which is less strain dependent. The (EuTe)4/(PbTe)4 and the (EuTe)2/(PbTe)6 superlattices have roughly the same values for FTHETA, but the latter has no phase transition, presumably because the Eu atoms do not have all six of their nearest neighbors.
The electrical resistivity of Fe-Cr-Fe layers with antiferromagnetic interlayer exchange increases when the magnetizations of the Fe layers are aligned antiparallel. The effect is much stronger than the usual anisotropic magnetoresistance and further increases in structures with more than two Fe layers. It can be explained in terms of spin-flip scattering of conduction electrons caused by the antiparallel alignment of the magnetization.
The nanostructure and local strain of [111] fcc Co/Cu multilayers are studied by means of nuclear magnetic resonance. The atomic topology of the interface can be deduced from the NMR spectrum and the local strains from the shift in the hyperfine fields. The results show that the Co/Cu interface is a mixed monolayer and that the Co layers, including the interface, have uniform strain inversely proportional to the Co thickness (within experimental error) with the proportionality constant depending on the Cu thickness.
We investigated exchange coupling of Fe layers across Au and Cr interlayers by means of light scattering from spin waves. For Au interlayers we find a continuous decrease of this coupling to zero as the Au thickness is increased from 0 to ~=20 Å. For Cr interlayers of prpoer thickness we find antiferromagnetic coupling of the Fe layers. In small external fields such double layers order antiparallel with their magnetization perpendicular to the external field, in analogy to the spin-flop phase of antiferromagnets.
Thin Fe films on Au(100)-including one monolayer-exhibit long-range ferromagnetic order. The temperature dependence of the magnetization, as measured with spin-polarized low-energy electron diffraction and spin-polarized secondary electron emission, shows a second-order phase transition at a thickness-dependent Curie temperature. The critical exponent beta for 1-3 monolayer-thick films is 0.22+/-0.05, independently of film thickness.