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Colossal magnetoresistance magnetic tunnel junctions grown by molecular-beam epitaxy
Abstract
Using molecular-beam-epitaxy growth techniques, we have synthesized ferromagnet/insulator/ferromagnet trilayer heterostructures with the “colossal” magnetoresistance material La1−xSrxMnO3 as the ferromagnet. These trilayer films were fabricated into magnetic tunnel junctions which exhibit magnetoresistance ΔR/R(H) of as much as 450% in 200 Oe applied field at 14 K, and which persists up to ∼250 K. In situ reflection high-energy electron diffraction (RHEED) allows us to correlate the quality of the epitaxial growth with the magnetoresistive properties. Samples which showed signs of disorder in RHEED also exhibit disorder effects in low-temperature transport and have smaller magnetoresistance which vanishes at lower temperatures. © 2000 American Institute of Physics.
... LSMO films around x = 0.3 doping have also been explored for potential device applications like magnetic tunnel junctions and spin valves [14][15][16], given that the double exchange interaction suggests a nearly spin polarized half metal [14,15,17]. Here we systematically explore the doping dependence of correlated electronic and magnetic orderings in strained epitaxial thin films of La 1−x Sr x MnO 3 . ...
... LSMO films around x = 0.3 doping have also been explored for potential device applications like magnetic tunnel junctions and spin valves [14][15][16], given that the double exchange interaction suggests a nearly spin polarized half metal [14,15,17]. Here we systematically explore the doping dependence of correlated electronic and magnetic orderings in strained epitaxial thin films of La 1−x Sr x MnO 3 . ...
... The data and the fit indicates an M Sat value of ∼3.7μ B /Mn. This measured M Sat value is consistent with what is expected for the x = 0.33 sample based on the double exchange interaction assuming a nearly 100% spin polarization [14,15]. Figure 3 shows a magnetization versus field measurement made at 5 K. ...
We have investigated the collective electronic and magnetic orderings of a series of La 1−x Sr x MnO 3 thin films grown epitaxially strained to (001) oriented strontium titanate substrates as a function of doping, x, for 0 ≤ x ≤ 0.4. We find that the ground states of these crystalline thin films are, in general, consistent with that observed in bulk crystals and thin film samples synthesized under a multitude of techniques. Our systematic study, however, reveal subtle features in the temperature dependent electronic transport and magnetization measurements, which presumably arise due to Jahn-Teller type distortions in the lattice for particular doping levels. For the parent compound LaMnO 3 (x = 0), we report evidence of a strain-induced ferromagnetic ordering in contrast to the antiferromagnetic ground state found in bulk crystals.
... 1,2,7,8 These studies have not only focused on the basic science of the mixed-valent manganites but also on possible applications based on the high degree of spin polarization of these materials as in spin valves and magnetic tunnel junction devices. [9][10][11][12] In thin film form, the electronic properties of these manganite phases can also be tuned using epitaxial strain imparted by the choice of substrate. [13][14][15][16] Moreover, with advances to deposition techniques, especially molecular beam epitaxy, 17 there has been an increased interest to construct ordered superlattices [18][19][20][21] where the doping is achieved by sandwiching an atomic layer where 2+ ions occupy the A-site in between layers where 3+ ions occupy the A-site. ...
... This value can be obtained by using the standard Block T 3/2 Law 8,23 and is consistent with nearly 100% spin polarization due to the double exchange interaction for a sample with x = 0.33. 9,10 Both the resistivity [ Fig. 1(a)] and magnetization [ Fig. 1(c)] measurements for the x = 0.50 sample show several similar features at high temperatures to the x = 0.33 system. Specifically, a transition from insulator to metal can be seen in the resistivity data ∼300 K coincident with a paramagnetic to ferromagnetic transition. ...
We have investigated the electronic and magnetic ground state of La 1− x Sr x MnO 3 thin films, especially around doping x = 0.50 in comparison to the well studied system of x = 0.33 and the low doped system of x = 0.04. The films discussed here were grown by molecular-beam epitaxy, epitaxially strained to (001) oriented strontium titanate substrates. Apart from resistivity and magnetization measurements probing the electronic and magnetic ground state, we also study the temperature dependence of the coercive field of these samples. Our measurements reveal that the coercive field increases as the doping is changed from x = 0.33. The coercive field at 5 K for the x = 0.50 sample is H c = 0.0450 T compared to H c = 0.0080 T for the x = 0.33 sample. The temperature dependent coercive field measurements on the x = 0.50 sample show a dramatic cusp around 100 K that is coincident with more subtle features observed in both magnetization and resistivity data at this temperature.
... 6,7 For this reason, the electron-and spin-transport properties can be greatly affected, resulting in a scaling problem of potential devices and degraded device properties, for example, in tunneling magnetoresistance (TMR). 8 The intrinsic mechanism of interfacial reconstruction is typically thought of in term of a polar catastrophe model. [9][10][11]13 The existence of a polar discontinuity at LSMO/STO interfaces causes a buildup of electric field and charge redistribution, leading to an interfacial region with reduced Mn valence. ...
... 10,11 The transition may contribute to the temperature-dependent electron-and spin-transport properties through the LSMO/STO interface and performance of tunneling devices. 8 In this study, we first obtain a direct and quantitative mapping of the octahedral geometry across LSMO/STO interfaces at the atomic scale by utilizing utilizing spherical aberration corrected scanning transmission electron microscopy (STEM) and negative spherical aberration imaging (NCSI) techniques. 20 Theoretical calculations based on density functional theory (DFT) reveal the evolution of the electronic structure due to octahedral reconstruction. ...
Interface coupling between complex oxides offers unique possibilities to tailor materials properties and stabilize novel ground states. Understanding the structural reconstruction of the corner-shared octahedral framework and the charge redistribution are crucial for controlling interfacial properties in oxide electronics. Here, we study the interfacial oxygen octahedral behavior in La0.7Sr0.3MnO3/SrTiO3 heterostructure, by directly imaging the oxygen octahedra at the atomic-scale and extracting the structural parameters. We combine these experimental results with electronic structure calculations to elucidate the effect of reconstructed MnO6 octahedral geometry on increased interfacial magnetization and conductivity. The Mn valence profiles near the interface is quantitatively analyzed and compared at variant temperature, revealing the insulating nature of interfacial manganite with reduced Mn valence. This study suggests a pathway to manipulate the interfacial properties and creation of new ground states in complex oxide heterostructures by tuning competing structural and electronic parameters.
... Theoretical and experimental values of the spin polarization obtained by different techniques vary over a wide range. [570][571][572][573][574][575][576][577] Local spin-density approximation (LSDA) band calculation for La 0.67 Ca 0.33 MnO 3 predicted a spin polarization of only 36% at the Fermi level. 570 At the same time, Park et al. 573 reported pure half-metallic behavior of La 0.7 Sr 0.3 MnO 3 well below the Curie temperature. ...
... A mixture of oxygen + 5% ozone was used as an oxidizing agent. Atomic layer-by-layer molecular beam epitaxy (ALL-MBE), developed by Eckstein and Bozovic 689 for high-temperature superconductors and other complex oxides, has been applied to the growth of LSMO 577,664 and LCMO 665,667 thin films as well. With this method, layers with atomically flat surfaces and abrupt interfaces can be produced by deposition of one atomic layer at a time. ...
We review and critique the recent developments on multifunctional oxide materials, which are gaining a good deal of interest. Recongnizing that this is a vast area, the focus of this treatment is mainly on high-κ dielectric, ferroelectric, magnetic, and multiferroic materials. Also, we consider ferrimagnetic oxides in the context of the new, rapidly developing field of negative-index metamaterials. This review is motivated by the recent resurgence of interest in complex oxides owing to their coupling of electrical, magnetic, thermal, mechanical, and optical properties, which make them suitable for a wide variety of applications, including heat, motion, electric, and magnetic sensors; tunable and compact microwave passive components; surface acoustic wave devices; nonlinear optics; and nonvolatile memory, and pave the way for designing multifunctional devices and unique applications in spintronics and negative refraction-index media. For most of the materials treated here, structural and physical properties, preparation methods accompanied by particulars of synthesis of thin films, devices based on them, and some projections into their future applications are discussed.
... The significant properties of the member of this perovskite family are the giant magnetoresistance (MR) [6], magnetocaloric effect (MCE) at < 20 kOe [3,4], charge and orbital ordering. These characteristics make the above family an interesting candidate in spintronic devices such as magnetic tunnel junctions, Schottky devices and magnetoelectric devices [7][8][9]. ...
We synthesize a high quality Sr-doped lanthanum manganite using solid state reaction route to investigate the various properties for device applications. The crystal structure of the synthesized perovskite was studied by X-ray diffraction (XRD) pattern and also compared with the crystallographic data obtained from the simulation calculations. The magnetization as a function of applied magnetic field and temperature of La0.6Sr0.4MnO3 exhibits ferromagnetic metal phase with the Curie temperature of 361 K. The electrical resistivity with temperature unexpectedly shows semiconducting behavior due to the intergrain effects. On the other hand, the energy dispersion studied by first principles calculations based on density functional theory (DFT) demonstrates metallic conduction in conformity with the available experimental results. No energy gap in the absorption spectrum done by UV–Visible spectrophotometer of this manganite also confirms the nature of identical conductivity. Finally, a quasi-harmonic Debye model was employed to calculate the thermal characteristics like Debye temperature, specific heat capacities, volume expansion coefficient, etc. in this LSMO perovskite.
... LSMO is one of the most popular manganese oxides with a cubic-type perovskite structure (Fig. 1e) [8] and has been widely investigated as a spintronics material due to its half-metallic nature and so-called colossal magnetoresistance. [9] , [10], [11] Much interest has been shown in the underlying basic physics of such intriguing properties, and recent studies have focused on further modi cation of the magnetic properties of LSMO. For example, only 5% Ru substitution at the B sites of LSMO [Refs. ...
The search for new high-Tc superconductors has commonly focused on layered perovskite compounds with isoelectronic or isostructural properties similar to those of cuprates.1, 2 For example, in 2019, a family of 3d nickel-based superconductors (Nd, Sr)NiO2 was discovered by Hwang et al.³More recently, Ca2RuO4, a 4d transition metal oxide, received attention as a superconductor with Tc = 64 K in coexistence with ferromagnetism in nanofilm single crystals.⁴ Even the 5d transition metal oxide Sr2IrO4 was thoroughly studied as a possible high-Tc superconductor.⁵ While no zero-resistance state has yet been observed, surface-electron-doped Sr2IrO4 has shown a spectroscopic signature consistent with a superconducting gap.⁶ However, despite such intensive exploration of analogous structures, no materials other than cuprates or single-layer FeSe/SrTiO3 [Ref. 7] have been reported to exhibit high-Tc superconductivity above 100 K under ambient pressure.
In this study, we report observations of possible new high-Tc superconductivity in a non-layered perovskite, Ir-substituted (La, Sr)MnO3 (LSMIO) thin films. LSMIO samples with Ir compositions of approximately 10% were confirmed to exhibit the zero-resistance state with onset temperatures exceeding 100 K. Furthermore, their magnetic field response was observed to be characteristic of conventional superconductors. All these results strongly indicated that LSMIO showed the most promise as a new high-Tc superconductor among perovskite oxides. The present discovery will boost interest in new theoretical frameworks for unconventional but possible superconductivity in LSMIO, opening a frontier for research on high-Tc superconductors.
... 3c and 4d. Still, for both states, the non-linear I-V curves are similar (Fig. 4d) and the shape of the differential conductance curves (Fig. 4e) is compatible with tunneling conduction, 46 ruling out a drastic change of the conduction mechanism as probed by the investigated range of applied voltage. ...
Multiferroic tunnel junctions (MFTJs) with Hf0.5Zr0.5O2 barriers are reported to show both tunneling magnetoresistance effect (TMR) and tunneling electroresistance effect (TER), displaying four resistance states by magnetic and electric field switching. Here we show that, under electric field cycling of large enough magnitude, the TER can reach values as large as 106%. Moreover, concomitant with this TER enhancement, the devices develop electrical control of spin polarization, with sign reversal of the TMR effect. Currently, this intermediate state exists for a limited number of cycles and understanding the origin of these phenomena is key to improve its stability. The experiments presented here point to the magneto-ionic effect as the origin of the large TER and strong magneto-electric coupling, showing that ferroelectric polarization switching of the tunnel barrier is not the main contribution.
... Advances in epitaxial synthesis provide a fine control over interfaces that is necessary to realize novel magnetic systems hitherto unseen in TMOs, such as spontaneous magnetic reversal (SMR) and exchange spring (12)(13)(14). La 2/3 Sr 1/3 MnO 3 is a material with interesting magnetic properties, such as high Curie temperature (15) and nearly perfect spin polarization (16), used for the realization of magnetic tunnel junctions (17), magnetoelectric devices (18), and spin injection into cuprate superconductors (19) as well as a platform for studying spin-dependent transport in organic materials (20). In this material, like other oxides, the interface-induced magnetic properties are usually discussed in terms of charge transfer (21)(22)(23)(24), while the atomic-scale role of interface structure and intermixture are not explicitly investigated (22,25). ...
Significance
Transition metal oxide interfaces have shown extraordinary promise in the quest to design materials with custom electronic, magnetic, and optical properties. In rare cases, interfaces exhibit electronic and magnetic properties that are radically different from those of the components. An example is the emergent two-dimensional electron gas between two insulators. In this work, we show that the interface of a nonmagnetic oxide substrate and a ferromagnetic metallic thin film possesses a local antiferromagnetic coupling which controls the reversal of the entire film’s ferromagnetic ordering. Electron microscopy and quantum calculations elucidate the atomic-scale origin of the observed phenomena, and demonstrate that local nonstoichiometry and structure are the key factors in interfacial phenomena.
... Advances in epitaxial synthesis provide a fine control over interfaces that is necessary to realize novel magnetic systems hitherto unseen in TMOs, such as spontaneous magnetic reversal (SMR) and exchange spring (12)(13)(14). La 2/3 Sr 1/3 MnO 3 is a material with interesting magnetic properties, such as high Curie temperature (15) and nearly perfect spin polarization (16), used for the realization of magnetic tunnel junctions (17), magnetoelectric devices (18), and spin injection into cuprate superconductors (19) as well as a platform for studying spin-dependent transport in organic materials (20). In this material, like other oxides, the interface-induced magnetic properties are usually discussed in terms of charge transfer (21)(22)(23)(24), while the atomic-scale role of interface structure and intermixture are not explicitly investigated (22,25). ...
Understanding oxide interface-induced effects requires controlled epitaxial growth of films on well-defined substrate surfaces. While conventional film growth on ex situ prepared substrates has proven to be a successful route, the choices of appropriate substrates with atomically defined surfaces are limited. Here, by depositing La2/3Sr1/3MnO3 on Sr2RuO4, we present an alternative method of growing oxide thin films on in situ cleaved surfaces of layered-structured substrates. Cleaving Sr2RuO4 at low temperature in ultrahigh vacuum exposes an atomically flat, solely SrO-terminated surface with up to micrometer-scale terraces. The deposition of La2/3Sr1/3MnO3 spontaneously diminishes the surface RuO6 in-plane rotational distortion of the substrate and results in a cubic-like perovskite film structure with (La/Sr)-O layer termination. The interface is atomically sharp without obvious deviation of lattice spacing and chemical valence, except in the first unit cell where Ru-Mn intermixing is observed. These results demonstrate that film growth on a cleaved substrate can be an alternative route to obtain well-defined interfaces and in addition increase the availability of substrates for future oxide films.
... Using this technique, trilayer heterostructure La 1−x Sr x MnO 3 as the ferromagnet and CaTiO 3 as insulating layer was deposited. The deposited structure exhibits magnetoresistance ∆R/R(H)∆R/R(H) of as much as 450% in 200 Oe applied field at 14 K, and which persists up to ∼250 K [46]. Mia et al. deposited Fe/MgO/Fe structure using MBE method. ...
Magnetic tunnel junctions are promising candidates for the spintronic devices. They are not only important for the futuristic memory devices but are also interesting for their fundamental effects like tunneling magnetoresistance and spin-transfer torque. Thus, attempts are made to develop good quality junctions. Since, the fabrication process is the crucial step to decide the device performance. Therefore, this chapter focuses on the methodology developed for fabrication of magnetic tunnel junctions. To fabricate these devices, first, various layers of magnetic tunnel junction are grown using any method of deposition like molecular beam epitaxy, RF sputtering, e-beam evaporation and ion beam sputtering methods. Then, these grown structures are fabricated to devices of the size of few nanometers by using lithography techniques. Another approach of designing these devices is use of pseudo-masking/metal masking which is free from lithography process. However, mask formation still requires lithography to fabricate devices of smaller size.
... The ZBA is often explained by the presence of defects in the barrier or at the interface, which induce states inside the barrier and thus additional conducting channels. 16,17 The vanishing ZBA is strong evi- dence for a high quality LSMO/Nb:STO interface and/or of the Nb:STO barrier, compared to the undoped-STO case. The temperature dependence of the magnetoresistance is similar to that obtained with an undoped STO barrier ( Fig. 2(b)). ...
... Advances in epitaxial synthesis provide a fine control over interfaces that is necessary to realize novel magnetic systems hitherto unseen in TMOs, such as spontaneous magnetic reversal (SMR) and exchange spring (12)(13)(14). La 2/3 Sr 1/3 MnO 3 is a material with interesting magnetic properties, such as high Curie temperature (15) and nearly perfect spin polarization (16), used for the realization of magnetic tunnel junctions (17), magnetoelectric devices (18), and spin injection into cuprate superconductors (19) as well as a platform for studying spin-dependent transport in organic materials (20). In this material, like other oxides, the interface-induced magnetic properties are usually discussed in terms of charge transfer (21)(22)(23)(24), while the atomic-scale role of interface structure and intermixture are not explicitly investigated (22,25). ...
Interfacial Coupling and Polarization of Perovskite ABO3 Heterostructures - Volume 23 Issue S1 - Lijun Wu, Zhen Wang, Bangmin Zhang, Liping Yu, G.M. Chow, Jing Tao, Myung-Geun Han, Hangwen Guo, Lina Chen, E.W. Plummer, Jiandi Zhang, Yimei Zhu
... The ZBA is often explained by the presence of defects in the barrier or at the interface, which induce states inside the barrier and thus additional conducting channels. 16,17 The vanishing ZBA is strong evidence for a high quality LSMO/Nb:STO interface and/or of the Nb:STO barrier, compared to the undoped-STO case. The temperature dependence of the magnetoresistance is similar to that obtained with an undoped STO barrier ( Fig. 2(b)). ...
All-oxide magnetic tunnel junctions with a semiconducting barrier, formed by the half-metallic ferromagnet La0.7Sr0.3MnO3 and n-type semiconductor SrTi0.8Nb0.2O3, were designed, fabricated, and investigated in terms of their magneto-transport properties as a function of applied bias and temperature. We found that the use of the heavily Nb-doped SrTiO3 as a barrier results in significant improvement in the reproducibility of results, i.e., of large tunnel magnetoresistance(TMR) ratios, and a spectral noise density reduced by three orders of magnitude at low temperature. We attribute this finding to a considerably decreased amount of point defects in SrTi0.8Nb0.2O3, especially oxygen vacancies, compared with the conventional insulating SrTiO3 barrier.
... The multiferroic tunneling junction (MFTJ) is usually fabricated by sandwiching the ferroelectric layer (FE layer) with two ferromagnetic layers (FM 1 and FM 2 layers), where the dominant transport in FE layer is direct tunneling conduction [1,2]. In MFTJ, as the magnetization of FM 1 and FM 2 layers is switched between antiparallel state and parallel state, the tunneling magnetoresistance (TMR) occurs due to the spin valve effect [3][4][5][6]; on the other hand, as the polarization of FE layer is switched between downward state and upward state, the tunneling electroresistance (TER) occurs due to the asymmetry barrier effect [7,8]. The coexistence of TMR and TER in MFTJ results obviously electric switch of magnetoresistance (MR), which is potential for the four-state storage device, electric-write magnetic-read storage device, and so on [1,2]. ...
In this work, the Pb(Zr0.2Ti0.8)O3/(La0.67Ca0.33)MnO3heterostructure film is deposited on the Pt/Ti/SiO2/Si
wafer. The dominant transport is the inelastic hopping conduction. Due to the interaction between
ferroelectric domain and magnetic polaron, film still exhibits weak ferromagnetism above the Curie
temperature. Under lower bias voltage, the non-zero sequential magnetoresistance occurs on the
magnetic granular junction. As soon as bias voltage exceeds the coercive voltage, the ferroelectric domain
is aligned, consequently the magnetoresistance tends to vanish. Such electric switch of magnetoresistance
is potential for the electric-write magnetic-read storage device.
... The multiferroic tunneling junction (MFTJ) is usually fabricated by sandwiching the ferroelectric layer (FE layer) with two ferromagnetic layers (FM 1 and FM 2 layers), where the dominant transport in FE layer is direct tunneling conduction [1,2]. In MFTJ, as the magnetization of FM 1 and FM 2 layers is switched between antiparallel state and parallel state, the tunneling magnetoresistance (TMR) occurs due to the spin valve effect [3][4][5][6]; on the other hand, as the polarization of FE layer is switched between downward state and upward state, the tunneling electroresistance (TER) occurs due to the asymmetry barrier effect [7,8]. The coexistence of TMR and TER in MFTJ results obviously electric switch of magnetoresistance (MR), which is potential for the four-state storage device, electric-write magnetic-read storage device, and so on [1,2]. ...
In this work, the Pb(Zr0.2Ti0.8)O3/(La0.67Ca0.33)MnO3 heterostructure film is deposited on the Pt/Ti/SiO2/Si wafer. The dominant transport is the inelastic hopping conduction. Due to the interaction between ferroelectric domain and magnetic polaron, film still exhibits weak ferromagnetism above the Curie temperature. Under lower bias voltage, the non-zero sequential magnetoresistance occurs on the magnetic granular junction. As soon as bias voltage exceeds the coercive voltage, the ferroelectric domain is aligned, consequently the magnetoresistance tends to vanish. Such electric switch of magnetoresistance is potential for the electric-write magnetic-read storage device.
... However, the requirement of two independently controlled ferromagnetic electrodes and spin-coherent tunneling imposes some technical challenges, such as the uniformity of the magnetic properties of the electrodes, the insulating barrier uniformity or the thermal stability, for the implementation of MTJ-based devices that, in spite of the work already done, are not fully resolved yet. 2 Different attempts have been made to overcome these problems such as, for instance, increasing the magnitude of the tunneling magnetoresistance (TMR) response by using half-metallic materials as electrodes. 3,4 Even though TMR values of several hundred percent have been achieved in some cases 5-8 by using those materials, they become vanishing small well below room temperature, 5,6,9,10 therefore severely hampering technological applications. To overcome these challenges requires exploring new possibilities. ...
The magnetotransport properties of La2/3Sr1/3MnO3(LSMO)/ LaAlO3(LAO)/Pt tunnelingjunctions have been analyzed as a function of temperature and magnetic field. The junctions exhibit magnetoresistance (MR) values of about 37%, at H=90 kOe at low temperature. However, the temperature dependence of MR indicates a clear distinct origin than that of conventional colossal MR. In addition, tunnelinganisotropic MR (TAMR) values around 4% are found at low temperature and its angular dependence reflects the expected uniaxial anisotropy. The use of TAMR response could be an alternative of much easier technological implementation than conventional MTJs since only one magnetic electrode is required, thus opening the door to the implementation of more versatile devices. However, further studies are required in order to improve the strong temperature dependence at the present stage.
... Examples are magnetic tunnel junctions [3][4][5][6], Schottky devices [7][8][9] and magnetoelectric devices [10][11][12][13][14][15]. Furthermore, LSMO is also used to investigate other materials, e.g. by spin-injection into cuprate superconductors [16][17][18][19], to probe spin polarization at the BaTiO 3 /Fe interface [20] and to study spin-dependent transport in organic materials [21]. ...
In this paper, an overview of the fabrication and properties of high-quality La 0.67 Sr 0.33 MnO 3 (LSMO) thin films is given. A high-quality LSMO film combines a smooth surface morphology with a large magnetization and a small residual resistivity, while avoiding precipitates and surface segregation. In the literature, typically only a few of these issues are adressed. We therefore present a thorough characterization of our films, which were grown by pulsed laser deposition. The films were characterized with reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, magnetization and transport measurements, x-ray photoelectron spectroscopy and scanning transmission electron microscopy. The films have a saturation magnetization of 4.0 µ B /Mn, a Curie temperature of 350 K and a residual resistivity of 60 µµ cm. These results indicate that high-quality films, combining both large magnetization and small residual resistivity, were realized. A comparison between different samples presented in the literature shows that focussing on a single property is insufficient for the optimization of the deposition process. For high-quality films, all properties have to be adressed. For LSMO devices, the thin-film quality is crucial for the device performance. Therefore, this research is important for the application of LSMO in devices.
... This finding may be useful to verify symmetry selective filtering for MTJs based on new Heusler alloys, and may also open a route for probing the EEI in correlated systems with electronic bands of different symmetries, e.g., SrRuO 3 where "the degree and importance of correlation are still issues" [60], and La 1−x Sr x MnO 3 as well as other perovskite oxides where metal-insulator transition and interface effects are still not fully understood [44,61,62]. Thus, similar ZBA experiments on SrRuO 3 -based and La 1−x Sr x MnO 3 -based MTJs [63][64][65][66] can also be conducted. ...
... This in combination with their high Curie temperature has made them interesting candidates for spintronics applications [3,4,5,6,7], where the electron's spin degree of freedom is used as well as its charge [8]. In particular there has been an effort put into understanding and designing high magnetoresistance (MR) all oxide tunnel junctions, often using SrTiO 3 (STO) as barrier material [4,9,10,11]. ...
STM based magnetotransport measurements of epitaxial La0.7Sr0.3MnO3 31 nm thick films with and without an internal LaMnO3 layer (0-3.1 nm thick) grown on Nb doped SrTiO3 are presented. The measurements reveal two types of low field magnetoresistance (LFMR) with a magnitude of ~0.1-1.5%. One LFMR contribution is identified as a conventional grain boundary/domain wall scattering through the symmetric I-V characteristics, high dependence on tip placements and insensitivity to introduction of LaMnO3 layers. The other contribution originates from the reverse biased Nb doped SrTiO3 interface and the interface layer of La0.7Sr0.3MnO3. Both LFMR contributions display a field dependence indicative of a higher coercivity (~200 Oe) than the bulk film. LaMnO3 layers are found to reduce the rectifying properties of the junctions, and sub-micron lateral patterning by electron beam lithography enhances the diodic properties, in accordance with a proposed transport model based on the locality of the injected current.
... In particular La 0.7 Sr 0.3 MnO 3 (LSMO) has been regarded as a prototypical model system for magnanites, being one of the highest T C manganites that can be grown. However, it seems as use of the manganites in devices is obstructed by change in properties when interfaced to other materials [5,2,6,7]. Accordingly, understanding the surface and interface properties through the study of the thickness dependence magnetic and transport properties have been the focus of several studies. Such studies report on reduced T C and enlarged coercivity with decreasing thickness, while indicating the existence of inactive and electrically insulating interface layers of thin films [8,9,10] grown on SrTiO 3 (001), these studies report that very thin layers 3-4 unit cells films (uc) are nonmagnetic, and that they become conductive and magnetic at a thickness of 6-8 uc. ...
We present a comprehensive study of the thickness dependence of static and magneto-dynamic magnetic properties of La0.7Sr0.3MnO3. Epitaxial pulsed laser deposited La0.7Sr0.3MnO3/SrTiO3(001) thin films in the range from 3 unit cell (uc) to 40 uc (1.2–16 nm) have been investigated through ferromagnetic resonance (FMR) spectroscopy and SQUID magnetometry at variable temperature. Magnetodynamically, three different thickness, d , regimes are identified: 20 uc ≲d≲d uc where the system is bulk like, a transition region 8 uc ≤d≲20≤d≲20 uc where the FMR linewidth and the position depend on thickness and d=6 uc which displays significantly altered magnetodynamic properties, while still displaying bulk magnetization. Magnetization and FMR measurements are consistent with a nonmagnetic volume corresponding to ~4 uc. We observe a reduction of Curie temperature (TC) with decreasing thickness, which is coherent with a mean field model description. The reduced ordering temperature also accounts for the thickness dependence of the magnetic anisotropy constants and resonance fields. The damping of the system is strongly thickness dependent, and is for thin films dominated by thickness dependent anisotropies, yielding both a strong two-magnon scattering close to Tc and a low temperature broadening. For the bulk like samples a large part of the broadening can be linked to spread in magnetic anisotropies attributed to crystal imperfections/domain boundaries of the bulk like film.
... Advances in epitaxial synthesis provide a fine control over interfaces that is necessary to realize novel magnetic systems hitherto unseen in TMOs, such as spontaneous magnetic reversal (SMR) and exchange spring (12)(13)(14). La 2/3 Sr 1/3 MnO 3 is a material with interesting magnetic properties, such as high Curie temperature (15) and nearly perfect spin polarization (16), used for the realization of magnetic tunnel junctions (17), magnetoelectric devices (18), and spin injection into cuprate superconductors (19) as well as a platform for studying spin-dependent transport in organic materials (20). In this material, like other oxides, the interface-induced magnetic properties are usually discussed in terms of charge transfer (21)(22)(23)(24), while the atomic-scale role of interface structure and intermixture are not explicitly investigated (22,25). ...
SrTiO3(STO) is one of the most common substractes used for
epitaxial film growth of oxide materials. However, the structure and
stoichiometry of STO are still unclear even thought they may dictate the
interface properties. We have studied the STO(001) surface structure by
Low Energy Electron Diffraction(LEED) and angle-resolved X-ray
Photoelectron Spectroscopy (AR-XPS). The unreconstructued p(1x1)
STO(001) surface terminated with TiO2 layer is obtained by
simple chemical etching which is always accompanied by oxygen
deficiency. Both LEED and AR-XPS results suggest that the surface has a
considerable oxygen deficiency, but LEED I(V) analysis indicates the
existence of more oxygen vacancies than that from AP-XPS results. The
structureal analysis indicates a surface polarization due to a surface
buckling combined with oxygen deficiency, which is comsistent with
recent surface x-ray scattering results [1] with important implications
on surface ferroelectric phenomena in STO. The annealing effect of the
as-etched surface in oxygen atmosphere on surface oxgen deficiency as
well as reconstruction will also be disussed.[4pt] [1] R. Herger et al.,
Phys. Rev. Lett. 98, 076102(2007).
... Moreover, in the inset to Figure 1a, the oscillations in the intensity of the RHEED reflected spot during the deposition process are shown. Each of these oscillations is related to the deposition of an atomic monolayer, allowing to control in real time the growth rate [11]. The final thickness of the samples was varied by changing the total deposition time. ...
We have fabricated by an in situ process (La1-x
Srx
)y
MnO3 epitaxial thin films by Molecular Beam Epitaxy using a low partial pressure (
$\cong $
2 x 10-5 torr) of O2 + 5% ozone. Reflected High Energy Electron Diffraction analysis has been performed during the growth process to check the structural properties of the films. The samples have not been subjected to any in situ or ex situ post deposition annealing procedure. Thin films on different substrates (SrTiO3, LaAlO3, NdGaO3) and with different thickness have been fabricated to compare the transport properties and investigate the effects of the epitaxial strain. For compositions around x = 0.3, Metal-Insulator (MI) transitions at temperature as high as T
MI
= 340 K have been observed in thin films few nanometers thick. Resistivity versus temperature curves measured on samples deposited in the same run onto different substrates, have shown clear effects related to the epitaxial strain. These results are very promising for a deeper understanding of the physical mechanisms at work in manganites and in view of the future fabrication of manganite-based heterostructures for electronic applications.
... Coupling different oxides has always raised interest in the scientific community. While interfacial effects in p-n type semiconducting oxide heterojunctions have been studied for decades [1], unexpected superconducting [2] and magnetic [3] properties have been found in oxide film heterostructures recently. Rapidly growing attention is being directed towards the investigation of ionic conductivity in oxide film heterostructures because of their possible deployment in solid-state ionic devices. ...
Rapidly growing attention is being directed to the investigation of ionic conductivity in oxide film heterostructures. The main reason for this interest arises from interfacial phenomena in these heterostructures and their applications. Recent results revealed that heterophase interfaces have faster ionic conduction pathways than the bulk or homophase interfaces. This finding can open attractive opportunities in the field of micro-ionic devices. The influence of the interfaces on the conduction properties of heterostructures is becoming increasingly important with the miniaturization of solid-state devices, which leads to an enhanced interface density at the expense of the bulk. This review aims to describe the main evidence of interfacial phenomena in ion-conducting film heterostructures, highlighting the fundamental and technological relevance and offering guidelines to understanding the interface conduction mechanisms in these structures.
This work presents an examination and unification of fragmented data on spin polarization in half-metallic, ferrimagnetic oxides. It also includes well understood ferromagnetic metals for comparison. The temperature and disorder dependencies of the spin polarization are evaluated. Both the temperature dependence of the tunnel magnetoresistance and, for the very first time, its temperature coefficient are calculated based on the simplified Julliére model. The tunnel magnetoresistance in the magnetic tunnel junctions deteriorates due to the temperature dependence of the spin polarization the lower the Curie temperature is. As a result, magnetic tunnel junctions—consisting of ferromagnetic oxides with a Curie temperature not far above room temperature—are not promising for room temperature applications. Additionally, ferrimagnetic oxides possessing a Curie temperature below 650 K are not suitable for room temperature applications because of an unacceptable temperature coefficient exceeding −2%.
The electron can carry various information due to the fact that its charge and spin of another electron potentially offer devices with a greater diversity of functionality. With the development of the spintronic, novel magnetotransport phenomena occur due to the anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) composed of a nonmagnetic highly conductive metal inserted in two ferromagnetic layers colossal magnetoresistance (CMR) mostly composed of manganese‐based perovskite oxides, tunneling magnetoresistance (TMR) consisting of ferromagnets/insulator/ferromagnets, and quantum anomalous Holzer effect (QAHE). The summarization of the main fabrication methods of composition and microstructure defined magnetic thin films for these four quantum effects and their novel electron transport control is crucial due to their unique signal sensing applications for the integrated information technology.
The ferromagnetic metal La2/3Sr1/3MnO3 (LSMO) is an important correlated oxide material for oxide spintronics. However, the existence of a dead layer not only degrades device performance but also hinders the ultimate miniaturization of devices. Intense research has been made to eliminate the dead layer and understand its origin. Here, we have investigated the dead layer of (110)-oriented LSMO films on SrTiO3 (STO) substrates in which there is no polar discontinuity across the interface, in contrast to the (001)-oriented LSMO/STO interface. Our results reveal an 8 unit cell (u.c.) dead layer for (110) LSMO. Angle resolved x-ray photoemission demonstrating Sr segregation at the surface. Additionally, the broken symmetry at LSMO/vacuum interfaces also degrades the transport property. By introducing a capping layer, the film conductivity is enhanced. Finally, an ultrathin limit dead layer of 5 u.c. (=1.36 nm) has been obtained by introducing both the LaMnO3 capping layer and the buffer layer to compensate the excess holes at both LSMO/STO and Vacuum/LSMO interfaces and also to reduce the structural distortion at the top surface of LSMO. Our work paves the way toward eliminating the LSMO dead layer for spintronics application and provides a route to engineer the intriguing physical properties of oxide heterointerfaces.
Transition metal oxide thin films and heterostructures have attracted remarkable and continuous attention due to the incredible variety of electronic and magnetic properties. A significant challenge remains, to directly measure the electronic properties of the buried interface. Rotational second-harmonic generation (RSHG) is a powerful nonlinear optical technique for probing electronic symmetry originating from broken symmetry at the interface. In this article, we characterize the novel electronic properties at the buried interface due to the broken symmetry, using epitaxially grown high-quality La2/3Sr1/3MnO3 (LSMO) thin films on SrTiO3 (STO) (001) substrates. The RSHG data of LSMO/STO(001) heterostructures show a combination of isotropic and 4-fold patterns. The isotropic symmetry in the RSHG data originates from the LSMO surface. The unusual 4-fold anisotropy originates from the buried interface, indicating that the interface symmetry cannot be higher than C2. A possible structural model is presented, involving a tilt of the octahedra.
The multiferroic properties of mixed valence perovskites such as lanthanum strontium manganese oxide (La¬0.7Sr0.3MnO3) demonstrate a unique dependence on oxygen concentration, thickness, strain and orientation. To better understand the role of each variable, a systematic study has been performed. In this study, epitaxial growth of LSMO (110) thin films with thicknesses ~15nm are reported on epitaxial magnesium oxide (111) buffered Al2O3 (0001) substrates. Four LSMO films with changing oxygen concentration have been investigated. The oxygen content in the films was controlled by varying the oxygen partial pressure from 1x10-4 Torr to 1x10-1 Torr during deposition and subsequent cooldown. X-ray diffraction established the out-of-plane and in-plane plane matching to be (111)MgO//(0001)Al2O3 and <11 ̅0>MgO // <101 ̅0>Al2O3 for the buffer layer with the substrate, and an out-of-plane lattice matching of (110)LSMO//(111)MgO for the LSMO layer. For the case of the LSMO growth on MgO, a novel growth mode has been demonstrated showing that three in-plane matching variants are present: i) <1¯1 0>LSMO // <1¯1 0>MgO, ii) <1¯1 0>LSMO // <10¯1>MgO and iii) <1¯1 0>LSMO // <0¯1 1>MgO. The atomic resolution STEM images were taken of the interfaces that showed a thin, ~2 monolayer intermixed phase while HAADF cross-section images revealed 4/5 plane matching between the film and the buffer and similar domain sizes between different samples. Magnetic properties were measured for all films and the gradual decrease in saturation magnetization is reported with decreasing oxygen partial pressure during growth. A systematic increase in the interplanar spacing was observed by X-ray diffraction of the films with lower oxygen concentration indicating the decrease in the lattice constant in the plane due to the point defects. Samples demonstrated an insulating behavior for samples grown under low oxygen partial pressure and semiconducting behavior for the highest oxygen partial pressures. Magnetotransport measurements showed ~36.2% decrease in electrical resistivity with an applied magnetic field of 10 Tesla at 50 Kelvin and ~1.3% at room temperature for the highly oxygenated sample.
(La0.7Sr0.3MnO3)0.67: (CuO)0.33 (LSMO: CuO) nanocomposite thin films were deposited on SrTiO3 (001), LaAlO3 (001) and MgO (001) substrates by pulsed laser deposition and their microstructure, magnetic and magnetoresistance properties were investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that LSMO: CuO films grow as highly textured self-assembled vertically aligned nanocomposite (VAN), with a systematic domain structure and strain tuning effect based on the substrate type and laser deposition frequency. A record high low-field magnetoresistance (LFMR) value of ~80% has been achieved in LSMO: CuO grown on LaAlO3 (001) substrate under high frequency. Detailed analysis indicates that both the strain state and the phase boundary effect play a significant role in governing the overall LFMR behavior.
We have studied La2/3Sr1/3MnO3 thin films grown on (3×1)-reconstructed SrTiO3 (110) substrates. Films with thicknesses less than the critical thickness of θc≅8 unit cells are insulating in the measured temperature (T) range (2–400 K). However, films with thicknesses slightly over θc exhibit reentrant nonmetallic behavior at low temperatures in addition to the normally observed metal-insulator transition at higher temperatures. In contrast, the magnetization does not show signs of low-T transitions. Such reentrance of a low-T nonmetallic phase is affected by the film thickness as well as the density of oxygen vacancies. The electrical resistivity analysis reveals that localization effects are responsible for the reentrant nonmetallic behavior, which is enhanced with reduced film thickness.
This chapter focuses on the magnetic properties of functional oxide thin films and heterostructures and their interplay with transport and ferroelectric properties that are very appealing for advanced and multifunctional spintronic devices. First, the interconnection between charge, polarization, and magnetization in oxides is introduced theoretically. The possibility to control the structures by electric fields, magnetic fields, and currents is thus raised and is shown to lead to a large range of applications from magnetic sensors to memories and microwave devices, for example.
It is well known that the magnetic state of the La0.7Sr0.3MnO3 (LSMO) thin-film heterostructure is strongly correlated with the lattice, spin, orbital, and charge states, since these influence the electric and magnetic transport properties even on the unit-cell level. Therefore, understanding the material's magnetic properties on the nanoscale is important for the development of novel applications. The recently developed electron magnetic-circular dichroism (EMCD) technique allows the determination of atomic site-specific magnetic information via the use of transmitted electrons; however, its sensitivity is not high enough to quantitatively acquire magnetic information in many weak magnetism systems. Here, we utilized a dynamical diffraction-effect assisted EMCD technique to quantitatively determine the spin and orbital
magnetic moment of LSMO/SrTiO3
thin films on the nanometer scale using a transmission electron microscope. Further, data processing was optimized to enhance the intensity of the EMCD signals for manganese, which have very weak magnetism at room temperature. High signal-to-noise ratio and accurate quantitative magnetic measurement are eventually achieved. Finally, the spin magnetic moments (0.73 ± 0.26 μB) are derived, and we also limited the ratio of the orbital to spin magnetic moment within an interval of (−0.03, 0.01). Our results not only present the nanoscale magnetic parameters of LSMO/SrTiO3, but also demonstrate how the measurement limit of the spin (or orbital)
magnetic moment can be achieved, which is via the developed dynamical diffraction effect assisted EMCD technique.
Giant tunneling magnetoresistance effect (TMR) as large as 1280% at 4.2K and 158% at 300K was observed in Zn-0.41 Fe2.59O4-alpha -Fe2O3 polycrystalline sample. The Zn0.41Fe2.59O4 grains are separated by insulating alpha -Fe2O3 thin layer boundaries. The pattern of nanostructure has been verified by TEM and HREM and the thickness of alpha -Fe2O3 boundary is about 6-7mn. The huge TMR is attributed to the high spin-polarization of Zn0.41Fe2.59O4 grains and insulating antiferromagnetic alpha -Fe2O3 thin layer. The ZnxFe3O4 ferrite is a new type half-metallic material with a huge TMR at room temperature is interesting to further study in future.
Tunnel magnetoresistance junctions with the structure of SrTiO3-sub/La-0.7 Sr-0.3 MnO3 (100nm)/La-0.96 Sr-0.04 MnO3 (5nm)/ La0.7Sr0.3MnO3 (100nm)/Ir-22 Mn-78 (15nm)/Ni-79 Fe-21 (5nm)/Pt (20nm) were micro-fabricated with lithography and Ar ion etching. The first three epitaxial layers of La0.7Sr0.3 MnO3 (100nm)/La-0.96 Sr-0.(04) MnO3 (5nm)/La-0.7 Sr-0.3 MnO3 (100nm) were deposited on single crystal (001) SrTiO3-substrate by magnetron sputtering and heat treatment, then the spinning and top conducting layers of Ir22Mn78(15nm)/Ni79Fe21(5nm)/Pt(20nm) were continuously fabricated on top the three epitaxial layers by normal magnetron sputtering. The colossal TMR ratio of 3270% was observed at 4.2K under the external magnetic field of 8 T. The polarization value of 97% for the La0.7Sr0.3MnO3 is well consistent with the theoretical value of 100% The present results directly indicate the half-metallic ferromagnetism in La0.7Sr0.3MnO3. Tire high TMR value can possibly be attributed to the good interface due to the perfect barrier.
Magnetotransport properties of La2/3Sr1/3MnO3/LaAlO3/Pt tunnel junctions have been thoroughly analyzed, as a function of temperature and magnetic field, to test the suitability of LaAlO3 for insulating barriers and spin injection processes. The insulating behavior of LaAlO3 maintained down to 1–2 nm (corresponding to 4–5 unit cells) renders this material useful as tunnel barrier. The temperature dependence of the junction resistance, R(T), down to 200 K confirms direct tunneling as the dominant conduction channel. The barrier parameters of the junctions, φ0 and s, are estimated using Simmons' model in the intermediate voltage range. The energy of the barrier was estimated to be φ0 ∼ 0.4 eV at room temperature. The dependence of R(T) and φ0 on the magnetic field shows an anisotropic
tunneling
magnetoresistance of ∼4% at low T when changing the direction of the magnetization with respect to the current flow.
Spin polarization and electronic structure of Co90Fe10 were investigated through tunneling magnetoresistance (TMR) effect measurements on the Mn80Ir20/Ni80Fe20/Co90Fe10/SrTiO3/La0.7Sr0.3MnO3 junction. This junction was fabricated with half-metallic ferromagneto-LSMO and has a spin-valve type structure. An inverse TMR of 14 % was observed at 4.2 K. As the spin polarization of the LSMO is positive, the inverse TMR observed in this junction indicates a negative polarization for Co90Fe10 at the interface. The bias dependence of TMR is asymmetric to the applied bias. The band structure calculated with the LMTO-CPA method showed that spin polarization for Co90Fe10 is negative at the Fermi level.
We provide conclusive experimental evidence that zero bias anomaly in the
differential resistance of magnetic tunnel junctions (MTJs) is due to
electron-electron interaction (EEI), clarifying a long standing issue. Magnon
effect is excluded by measuring at low temperatures down to 0.2 K and with
reduced AC measurement voltages down to 0.06 mV. The normalized change of
conductance is proportional to $\ln{(eV/k_{B}T)}$, consistent with the
Altshuler-Aronov theory of tunneling with EEI but inconsistent with magnetic
impurity scattering. The slope of the $\ln{(eV/k_{B}T)}$ dependence is symmetry
dependent: the slopes for P and AP states are different for coherent tunnel
junctions with symmetry filtering, while nearly the same for those without
symmetry filtering (amorphous barriers). This observation may be helpful for
probing and separating electron Bloch states of different symmetries in other
correlated systems.
We grew La1-xSrxMnO3 films by MBE
carefully controlling the rates of La, Sr and Mn, using substrate
temperature of about 700°C and low partial pressure
(≅2×10-5Torr) of O2+5% Ozone. RHEED
analysis were performed to check in situ the structural properties of
the films. Samples on different substrates and with different thickness
have been fabricated to investigate the effects of the epitaxial strain
on the magnetotransport properties.
Uniform La0.6Sr0.4MnO3 (LSMO) nanotubes of an average diameter 180 nm were synthesized by a modified sol–gel method employing nanochannel porous anodic alumina templates. The nanotubes were characterized chemically and structurally by XRD, SEM, EDX, and TEM. Postannealed (700 °C for 1 h hour) nanotubes were found to be polycrystalline from XRD and SAED studies. To get further insight into the nanotube structure, HRTEM studies were done, which revealed that obtained LSMO nanotubes were structurally constituted with nanoparticles of 3–12 nm size. These constituent nanoparticles were randomly aligned and self-knitted to build the nanotube wall. Investigation of magnetic properties at this structured nanoscale revealed remarkable irreversibility between the zero field cooling (ZFC) and field cooling (FC) magnetization curves accompanied with a peak in the ZFC curve indicating spin-glass-like behavior. Structural defects and compositional variations at surfaces and grain-boundaries of constituent nanoparticles might be responsible for this anomalous magnetic behavior.
La0.6Sr0.4MnO3/polyaniline nanocomposites were successfully synthesized via in-situ polymerization of aniline, with ammonium persulfate as oxidant and NaDBS as surfactant. The spectroscopic characterization indicated that the La0.6Sr0.4MnO3 nanoparticles were dispersed in the matrix of polyaniline. The polyaniline continuously deposited on the surface of La0.6Sr0.4MnO3 nanoparticles. Many patches of small cloud-like structures were formed in the nanocomposites. It was found that the dielectric loss of electric dipole polarization produced by PANI and the magnetic loss produced by LSMO can be combined in the composite to improve microwave absorption performance. In addition, the characteristics of microwave absorption properties of the nanocomposites can be adjusted by controlling the compositions of the nanocomposites.
A tunneling-type magnetoresistance (MR) as large as 158% is observed at T=300 K in a polycrystalline Zn{sub 0.41} Fe{sub 2.59} O{sub 4} sample, in which the Zn{sub 0.41} Fe{sub 2.59} O{sub 4} grains are separated by insulating {alpha}-Fe{sub 2}O{sub 3} boundaries. The huge room-temperature MR is attributed to the high spin polarization of Zn{sub 0.41} Fe{sub 2.59} O{sub 4} grains and antiferromagnetic correlations between magnetic domains on both sides of the insulating {alpha}-Fe{sub 2}O{sub 3} boundary. The MR exhibits strong temperature dependence below 100K and its magnitude is enhanced to reach 1280% at 4.2K, which may arise from the Coulomb blockade effect.
We report a procedure to fabricate a tri-layer La0.67Sr0.33MnO3/La0.85Sr0.15MnO3/La0.67Sr0.33MnO3 tunneling magnetoresistance (TMR) device deposited on an SrTiO3 substrate. The devices have been characterized by high-resolution transmission electron microscopy (TEM), tunneling current versus voltage, and magnetoresistance (MR). The TMR phenomenon has been observed up to 320K. A procedure to further optimize this device has been proposed.
The magnetic properties of iron-doped cobalt ferrite (Co${}_{1$-${}x}$Fe${}_{2+x}$O${}_{4}$) (001) thin films grown epitaxially on MgO (001) substrates are investigated by superconducting quantum interference device magnetometry and soft x-ray magnetic linear and circular dichroisms. All Co${}_{1$-${}x}$Fe${}_{2+x}$O${}_{4}$ (0.01 $\leqslant${} $x$ $\leqslant${} 0.63) samples have out-of-plane magnetic easy axes and large coercive fields, unlike Fe${}_{3}$O${}_{4}$, due to a large Co${}^{2+}$ orbital moment. The magnetic moments for those samples are significantly reduced from their bulk values; however, as $x$ increases, the magnetic moments tend nearer to their bulk values and increase more rapidly as $x$ approaches 1. This reduction in magnetic moment is attributed to spin canting among the Co${}^{2+}$ cations, owing to a small in-plane tensile strain in the film and to an increased antiferromagnetic alignment among all the cations caused by a partially inverse spinel cubic structure and the likely presence of antiphase boundaries. Our results show that small changes in stoichiometry can lead to significant changes in the magnetic moment of Co${}_{1$-${}x}$Fe${}_{2+x}$O${}_{4}$, especially at large values of $x$.
We have studied the effect of biaxial strain on thin films of (001) La0.7Sr0.3MnO3. We deposited films by reactive molecular-beam epitaxy on different single crystalline substrates, varying the substrate-induced biaxial strain from -2.3% to +3.2%. Magnetization and electrical transport measurements reveal that the dependence of the Curie temperature on biaxial strain is in very good agreement with the theoretical predictions of Millis et al. [J. Appl. Phys. 83, 1588 (1998)]. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3213346]
Oxides have become a key ingredient for new concepts of electronic devices. To a large extent, this is due to the profusion of new physics and novel functionalities arising from ultrathin oxide films and at oxide interfaces. We present here a perspective on selected topics within this vast field and focus on two main issues. The first part of this review is dedicated to the use of ultrathin films of insulating oxides as barriers for tunnel junctions. In addition to dielectric non-magnetic epitaxial barriers, which can produce tunneling magnetoresistances in excess of a few hundred percent, we pay special attention to the possibility of exploiting the multifunctional character of some oxides in order to realize ‘active’ tunnel barriers. In these, the conductance across the barrier is not only controlled by the bias voltage and/or the electrodes magnetic state, but also depends on the barrier ferroic state. Some examples include spin-filtering effects using ferro- and ferrimagnetic oxides, and the possibility of realizing hysteretic, multi-state junctions using ferroelectric barriers. The second part of this review is devoted to novel states appearing at oxide interfaces. Often completely different from those of the corresponding bulk materials, they bring about novel functionalities to be exploited in spintronics and electronics architectures. We review the main mechanisms responsible for these new properties (such as magnetic coupling, charge transfer and proximity effects) and summarize some of the most paradigmatic phenomena. These include the formation of high-mobility two-dimensional electron gases at the interface between insulators, the emergence of superconductivity (or ferromagnetism) at the interface between non-superconducting (or non-ferromagnetic) materials, the observation of magnetoelectric effects at magnetic/ferroelectric interfaces or the effects of the interplay and competing interactions at all-oxide ferromagnetic/superconducting interfaces. Finally, we link up the two reviewed research fields and emphasize that the tunneling geometry is particularly suited to probe novel interface effects at oxide barrier/electrode interfaces. We close by giving some directions toward tunneling devices exploiting novel oxide interfacial phenomena.
Electronic properties of polarity-discontinued heterointerfaces between two perovskite band insulators, LaAlO3 and SrTiO3, have been investigated. Various heterointerfaces with AlO2-LaO/TiO2-SrO and LaO-AlO2/SrO-TiO2 termination including the mixtures of these two domains are fabricated in a single experimental run. This is accomplished by pulsed laser depositions of prescribed coverage (thetaSrO) of SrO atomic layer on TiO2-terminated SrTiO3 (001) surface and successive LaAlO3 layer, where the processes are regulated in an atomic scale by in situ reflection high-energy electron diffraction. Increasing thetaSrO from 0 to 1, extra electrons density at the heterointerfaces decreases from a value, close to 0.5 electrons at one Ti site, to zero, while low temperature mobility remains above 100 cm2/V\cdots. Our result implies a way to control carrier density in polarity-discontinued heterointerfaces.
Magnetoresistance of spin-dependent tunnel junctions has been studied using a high-quality epitaxial Fe3O4 film. The bottom magnetic electrodes of epitaxial Fe3O4 were grown onto the TiN-buffered (110) surface of MgO single-crystal substrates, and trilayer junctions of Fe3O4/AlOx/CoFe mesa were fabricated by sequential sputtering and Ar ion etching. The junctions showed the magnetoresistance (MR) ratio of more than 10% at room temperature with butterfly-like hysteresis which arose from the different coercive fields between Fe3O4 and CoFe when the field was applied along the easy axis of the epitaxial Fe3O4 layer. The MR ratio remained almost constant against the temperature down to nearly 100 K. Below 100 K, the decrease of MR and the increase of junction resistance were observed, which may be related to the Verwey transition that inevitably occurs in the characteristic of high-quality Fe3O4 samples.
The electronic and magnetic properties of perovskite superlattices composed of five unit-cell layers of ferromagnetic metal La0.6Sr0.4MnO3 and nonmagnetic band insulator SrTiO3 with varying layer thickness have been systematically investigated. The superlattices have well-defined periodic stacking with no sign of interlayer diffusion. The spin canting at the interfaces in La0.6Sr0.4MnO3 layers manifests itself as a suppressed magnetization and huge magnetoresistance subsisting to low temperature. The transport properties show a clear crossover from insulating to metallic perhaps by an increase in the interlayer electron hopping through SrTiO3 layers when the SrTiO3 layer thickness is reduced. This crossover is also discerned in the infrared optical spectra as the filling of absorption in the low-energy region.
Insulator-metal phenomena depending on band filling (doping degree), temperature, and external magnetic field have been investigated for prototypical double-exchange ferromagnets, namely, crystals of La1-xSrxMnO3 (0≤x≤0.6). The electronic phase diagram in the plane of the temperature vs nominal hole concentration (x) has been deduced from the magnetic and electrical measurements on the melt-grown crystals. Around the ferromagnetic transition temperature TC, large negative magnetoresistance was observed. Irrespective of temperature, reduction of the resistivity is scaled with the field-induced magnetization (M) as -Δρ/ρ=C(M/Ms)2 for M/Ms≲0.3, where Ms is the saturated magnetization. The coefficient C strongly depends on x, i.e., C≊4 near the compositional insulator-metal phase boundary (xc∼0.17), but decreases down to ≊1 for x>=0.4, indicating the critical change of the electronic state.
We report on the magnetoresistive properties of all-oxide tunnel spin valves in which
electrodes of mixed valence manganites (La0.7Sr0.3MnO3) are separated
by a thin insulating layer. The structures were prepared by Pulsed Laser Deposition and
several tunnel barriers were used including PrBa2(CuGa)3O7, CeO2 and
SrTiO3. The latter gives the largest magnetoresistive effect where the
trilayer's resistance is increased by a factor of 5.5 at 50 Gauss and 4.2 K. Analysis
of the temperature variation of the barrier resistivity shows that the dramatic loss
of the magnetoresistive effect above 150 K is due to a reduced oxygen content of the
interface between La0.7Sr0.3MnO3 and SrTiO3. Solving this problem
should lead to similar results at room temperature.
Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Physics, 1994. Includes bibliographical references.
Thin film trilayer junction of La$%_{0.67}$Sr$_{0.33}$MnO$_3$ - SrTiO$_3$ - La$_{0.67}$Sr$_{0.33}$MnO$_3$ shows a factor of 9.7 change in resistance, in a magnetic field around 100 Oe at 14K. The junction magnetoresistance is bias and temperature dependent. The energy scales associated with bias and temperature dependence are an order of magnitude apart. The same set of energies also determine the bias and temperature dependence of the differential conductance of the junction. We discuss these results in terms of metallic cluster inclusions at the junction-barrier interface. Comment: 3 pages, 4 figures
Interaction effects in disordered Fermi systems are considered in the metallic regime. In two dimensions, logarithmic corrections are obtained for conductivity, density of states, specific heat, and Hall constant. These results are compared with a recent theory of localization as well as some experiments.
This paper discusses some effects of mobile electrons in some antiferromagnetic lattices. It is shown that these electrons (or holes) always give rise to a distortion of the ground state spin arrangement, since electron transfer lowers the energy by a term of first order in the distortion angles. In the most typical cases this results in: (a) a nonzero spontaneous moment in low fields; (b) a lack of saturation in high fields; (c) simultaneous occurrence of "ferromagnetic" and "antiferromagnetic" lines in neutron diffraction patterns; (d) both ferromagnetic and antiferromagnetic branches in the spin wave spectra. Some of these properties have indeed been observed in compounds of mixed valency such as the manganites with low Mn4+ content. Similar considerations apply at finite temperatures, at least for the (most widespread) case where only the bottom of the carrier band is occupied at all temperatures of interest. The free energy is computed by a variational procedure, using simple carrier wave functions and an extension of the molecular field approximation. It is found that the canted arrangements are stable up to a well-defined temperature T1. Above T1 the system is either antiferromagnetic or ferromagnetic, depending upon the relative amount of mobile electrons. This behavior is not qualitatively modified when the carriers which are responsible for double exchange fall into bound states around impurity ions of opposite charge. Such bound states, however, will give rise to local inhomogeneities in the spin distortion, and to diffuse magnetic peaks in the neutron diffraction pattern. The possibility of observing these peaks and of eliminating the spurious spin-wave scattering is discussed in an Appendix.
The surface-boundary magnetization (M{sub SB}) for a manganese perovskite La{sub 0.7}Sr {sub 0.3}MnO {sub 3} has been investigated using spin-resolved photoemission spectroscopy, which has an {approximately}5 {Angstrom} probing depth. M{sub SB} shows the full moment at very low temperature but decays much faster than the bulk magnetization (M{sub B}) upon heating. This result provides direct insight into various novel properties at grain and surface boundaries observed in the polycrystalline samples and junctions of manganese perovskites. {copyright} {ital 1998} {ital The American Physical Society }
A number of different compounds, such as those derived from LaMnO3, have recently been shown to exhibit very large changes (up to 106%) in electrical resistance when a magnetic field is applied1–4—a phenomenon known as colossal magnetoresistance (CMR). But magnetic fields of several tesla are typically required to obtain such a large magnetoresistive effect, thus limiting the potential for applications. Nevertheless the complex and intimate link between magnetic structure, crystallographic structure and electrical resistivity in CMR materials, in addition to being of fundamental scientific interest, appears to provide some scope for engineering a more sensitive magnetoresistive response. Here we elucidate the effect of specific structural defects on the CMR behaviour of the compound La0.7Ca0.3MnO3. We have made thin film devices that isolate the contribution of a single grain boundary that was introduced into an epitaxial film of the material by growing it on a bicrystal substrate. These devices display sharp resistance switching in magnetic fields orders of magnitude less than those normally associated with CMR. These results both provide insight into the role of grain boundaries, and demonstrate the potential for developing sub-micrometre magnetic field sensors based on the CMR effect.
The thermal J-V characteristic for a tunnel junction is derived in terms of a generalized theory. The resulting functional form of the equations is similar to that of Stratton; however, in the present formulation, the physical parameters of the junction appear explicitly, and their effect upon the thermal characteristic is readily appreciated. In Stratton's work, the physical constants appear in the integrand of integral that can be solved only numerically.
The theory is applied to symmetric and asymmetric junctions. For the symmetric case, it is shown that, at a given temperature, the percentage change Ĵ in the high-temperature thermal component of current from the low-temperature value increases initially with increasing voltage bias up to a maximum peak, and thereafter decreases rapidly. The voltage bias at which the component of thermal current maxima occurs is equal to the interfacial barrier height and, as such, permits what is probably the most accurate method of barrier height determination. Similar results are obtained for the asymmetric barrier; however, in this case, Ĵ depends upon the polarity of the voltage bias for V>φ1, and two Ĵ maxima occur at voltages corresponding to the two distinct interfacial barrier heights φ1 and φ2.
A formula is derived for the electric tunnel effect through a potential barrier of arbitrary shape existing in a thin insulating film. The formula is applied to a rectangular barrier with and without image forces. In the image force problem, the true image potential is considered and compared to the approximate parabolic solution derived by Holm and Kirschstein. The anomalies associated with Holm's expression for the intermediate voltage characteristic are resolved. The effect of the dielectric constant of the insulating film is discussed in detail, and it is shown that this constant affects the temperature dependence of the J‐V characteristic of a tunnel junction.
We have fabricated ferromagnet-insulator-ferromagnet junctions using a ramp-edge geometry based on (La <sub> 0.7 </sub> Sr <sub> 0.3 </sub> )MnO <sub> 3 </sub> ferromagnetic electrodes and a SrTiO <sub> 3 </sub> insulator. Pulsed laser deposition was used to deposit the multilayer thin films and the devices were patterned using photolithography and ion milling. As expected from the spin-dependent tunneling, the junction magnetoresistance is dependent on the relative orientation of the magnetization in the electrodes. A junction magnetoresistance (JMR) as large as 30% is observed at low temperatures and low fields. In addition, we have found that JMR is reduced at high temperatures (T≫100 K ) and decreases monotonically with increasing field at high fields (0.5 T ≪H≪1 T ). Possible causes for these are also discussed. © 1998 American Institute of Physics.
Magnetic tunneling junctions are fabricated from epitaxially grown La <sub> 0.8 </sub> Sr <sub> 0.2 </sub> MnO <sub> 3 </sub> /SrTiO <sub> 3 </sub> /La <sub> 0.8 </sub> Sr <sub> 0.2 </sub> MnO <sub> 3 </sub> trilayers. A large tunneling magnetoresistance of 150% is observed for a junction with a thin barrier layer (1.6 nm) under a low switching field (≪10 Oe) at 5 K. A small tunneling magnetoresistance is observed even at 270 K, which is close to the ferromagnetic Curie temperature (290 K) of the La <sub> 0.8 </sub> Sr <sub> 0.2 </sub> MnO <sub> 3 </sub> film. The large magnetoresistance and high operating temperature are attributed to the sufficiently thin and uniform barrier layer of SrTiO <sub> 3 </sub>. © 1999 American Institute of Physics.
The magnetoresistance of step-edge structures in La <sub> 0.7 </sub> Ca <sub> 0.3 </sub> MnO <sub> 3 </sub> films was investigated. Step-edge arrays with 200 steps of height 140–200 nm and step separation 20 μm along [110] were fabricated on LaAlO <sub> 3 </sub> substrates by chemically assisted ion-beam etching. Thin La <sub> 0.7 </sub> Ca <sub> 0.3 </sub> MnO <sub> 3 </sub> films were deposited on the structured substrates by pulsed-laser deposition. Measurements of the large low-field magnetoresistance, the dynamic conductance, and the anisotropic magnetoresistance lead to the proposal of a model of spin-polarized tunneling in a ferromagnet/spin-glass/ferromagnet geometry. © 1999 American Institute of Physics.
We report on the fabrication of ferromagnet–insulator–ferromagnet junction devices using a ramp-edge geometry based on (La <sub> 0.7 </sub> Sr <sub> 0.3 </sub> )MnO <sub> 3 </sub> ferromagnetic electrodes and a SrTiO <sub> 3 </sub> insulator. The maximum junction magnetoresistance (JMR) as large as 23% is observed below 300 Oe at low temperatures (T≪100 K ). Our ramp-edge junctions exhibit JMR of 6% at 200 K with a field less than 100 Oe. The device performance at room temperature is believed to be limited by both the nearly equivalent coercive fields in the electrodes and the magnetization process, rather than by the insulating barrier. © 1998 American Institute of Physics.
We report significant improvement in the magnetoresistive (MR) response of perovskite manganites for small applied fields by the use of heterostructures with soft ferromagnets. At room temperature, a 5900‐fold enhancement of the MR response at 10 Oe in La 2/3 Ca 1/3 MnO 3 has been achieved using (Mn,Zn)Fe 2 O 4 as the soft ferromagnet. By utilizing soft ferromagnets with various magnetic properties, this technique allows for enormous flexibility in ‘‘designing’’ a desired MR response for the manganites, particularly in low fields. © 1996 American Institute of Physics.
FeGeCo junctions conductance G(V) is studied when mean magnetizations of the two ferromagnetic film are parrallel or antiparallel. Conductance measurement, in these two cases, is related to the spin polarizations of the conduction electrons.
During the past four years, researchers made significant advances in fabricating magnetic tunnel junctions with reproducible magnetic and magnetotransport properties. Important developments include optimization of oxidation processes, discovery of new class of magnetic tunnel junctions, combination of spin dependent tunneling with the Coulomb blockade effect, and a better theoretical understanding of the I–V characteristics of magnetic tunnel junctions. These developments make them promising candidates for magnetic random access memories.
A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance
of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77
kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device
applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are
discussed.