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Transition Metal-Doped Sb2Te3 and Bi2Te3 Diluted Magnetic Semiconductors.
Abstract
Motivated by the discovery of the ferromagnetic order in V-doped Sb2Te3 and Fe-doped Bi2Te3 bulk crystals, the study of the transition metal (TM) doped tetradymite-type semiconductors was extended to their thin film forms with the hope of significantly increasing the content of TM and thus enhancing the Curie temperature. High quality Sb2-xVxTe3 (x up to 0.35) and Sb2-xCrxTe3 (x up to 0.59) epitaxial films have been successfully prepared on sapphire (0001) substrates by non-equilibrium MBE growth technique, and their structural, transport and magnetic properties have been determined. Magnetization studies, Arrott plot analyses, and anomalous Hall effect measurements indicate that long range magnetic order persists to temperatures of at least 177K (in the case of Sb2-xVxTe3) and 190K (for Sb2-xCrxTe3). The observed carrier mediated, p-type favored ferromagnetic behavior supports a scenario of RKKY interaction with mean-field approximation. Bi2-xFexTe3 samples have also been prepared (for x up to 0.46), and they displayed n-type behavior and no signature of spontaneous magnetic ordering down to 2K. Trilayer structures with a normal Sb2Te3 layer sandwiched between two ferromagnetic Sb2-xCrxTe3 layers were fabricated in order to explore the influence of the interlayer exchange coupling mechanism (IEC). Such coupling was found to be present for small thickness of the spacer layer at low temperatures. Transition metal-doped tetradymite-type semiconductors represent a new class of diluted magnetic semiconductors that are octahedrally coordinated and that possess highly anisotropic structural and magnetic properties. Ph.D. Applied Physics University of Michigan, Horace H. Rackham School of Graduate Studies http://deepblue.lib.umich.edu/bitstream/2027.42/57593/2/yjchien_1.pdf
... Transport measurements on V-BST thin films Curie temperature. For another film, the Curie temperature was determined from the kink in ρ xx (T ), which is more often observed when the resistance of the sample is recorded during the cooldown procedure [138]. The results are shown in Fig. 4.5(d). ...
... However, it is clear that increasing the doping concentration leads to a larger Curie temperature. While the value of the Curie temperature is comparable to those found in Ref. [138], these values are larger than those reported in Ref. [112]. The difference between the referenced data may lie in the film thicknesses which differ over an order of magnitude. ...
In this thesis we search for the topological magnetoelectric effect. We first review the theoretical aspects of axion electrodynamics in topological matter. It was found that for such materials, an additional term can be added to the Lagrangian density, which leads to the modification of Maxwell's equations. These modifications arise due to spatial or temporal changes in the axion angle, which is non-zero in topological materials. Several experiments have been proposed to measure the resulting topological magnetoelectric effects, of which we consider three experiments which are related to electronic properties. Two experiments, the search for the magnetic monopole and the interference experiment for fractional charge, are related to a spatial gradient in the axion angle, while the chiral magnetic effect relates to temporal changes in the axion angle of Weyl semimetals. In the rest of this thesis, we use thin films, deposited by molecular beam epitaxy (MBE), and exfoliated flakes of topological materials to study the experiments that were proposed above. For the magnetic monopole experiment, we fabricate top gate samples of magnetically doped topological insulator films. We measure the magnetic response as a function of applied gate voltage via a SQUID sensor in a dilution refrigerator. We find that heat dissipation in the sample plays a major role at ultra low temperatures. The signature of fractional charges is measured in an interference experiment where we make use of a Josephson junction array that is fabricated on both topological and reference samples. While no topological signature was observed in the measurements, we find a good agreement between the experimental results and numerical simulations. For the chiral magnetic effect, we used exfoliated flakes of a Dirac semimetal, of which we fabricated non-local nanostructures to measure the diffusion length of the chiral charge polarization. We find that the relaxation rate of the chiral charge is slower than the Ohmic relaxation rate.
... The materials Bi 2 Se 3 , Bi 2 Te 3 and Sb 2 Te 3 have been theoretically predicted and experimentally observed to be topological insulators with a large bulk band gap [43][44][45][46][47][48][49][50][51][52] . Recent experimental progress has shown that well-controlled layer-by-layer MBE thin film growth can be achieved 45,172 , and various magnetic transition metal elements (such as Ti, V, Cr, Fe) can be substituted into the Bi 2 Se 3 family of parent compounds with observable ferromagnetism even above 100K [173][174][175][176][177] . ...
... From first-principles calculations, we find that the insulating magnetic ground state discussed above can indeed be obtained by a proper choice of magnetic dopants. Experiments [173][174][175] and theoretical calculations 139,182 show that the magnetic dopants will mostly substitute for Bi ions, which has a nominal 3+ valence state. In order to avoid introducing free carriers into the parent material when we dope the system, it is natural to choose those transition metal elements that also have a stable 3+ chemical state, such as Ti, V, Cr and Fe. ...
Over a long period of exploration, the successful observation of quantized
version of anomalous Hall effect (AHE) in thin film of magnetically-doped
topological insulator completed a quantum Hall trio---quantum Hall effect
(QHE), quantum spin Hall effect (QSHE), and quantum anomalous Hall effect
(QAHE). On the theoretical front, it was understood that intrinsic AHE is
related to Berry curvature and U(1) gauge field in momentum space. This
understanding established connection between the QAHE and the topological
properties of electronic structures characterized by the Chern number. With the
time reversal symmetry broken by magnetization, a QAHE system carries
dissipationless charge current at edges, similar to the QHE where an external
magnetic field is necessary. The QAHE and corresponding Chern insulators are
also closely related to other topological electronic states, such as
topological insulators and topological semimetals, which have been extensively
studied recently and have been known to exist in various compounds.
First-principles electronic structure calculations play important roles not
only for the understanding of fundamental physics in this field, but also
towards the prediction and realization of realistic compounds. In this article,
a theoretical review on the Berry phase mechanism and related topological
electronic states in terms of various topological invariants will be given with
focus on the QAHE and Chern insulators. We will introduce the Wilson loop
method and the band inversion mechanism for the selection and design of
topological materials, and discuss the predictive power of first-principles
calculations. Finally, remaining issues, challenges and possible applications
for future investigations in the field will be addressed.
... 46 Among various transition metal (TM) doped Sb 2 Te 2 , V-doped Sb 2 Te 2 exhibits the most stable ferromagnetism, with a high Curie temperature. 38,47,48 Apart from transition metals, rare-earth metals such as Gd, 49 Dy, and Sm 50,51 have also been explored as effective dopants to induce long-range magnetic ordering in Bi 2 Se 3 51 and Bi 2 Te 3 . 52 Despite the intensive research on TM-doped Bi 2 Se 3 type TIs, the interpretations for the observed FM properties have often been controversial. ...
We have studied the structural, electronic, and magnetic properties of the Cr-doped topological insulator Bi 2 Se 3 within the density functional theory using the generalized gradient approximation in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The X-ray absorption spectra and X-ray magnetic circular dichroism at the Cr K and L 2,3 edges have been investigated theoretically from the first principles. The calculated results are in good agreement with the experimental data. The complex fine structure of the Cr L 2,3 X-ray absorption spectra in Cr-doped Bi 2 Se 3 has been found to be not compatible with a Cr ²⁺ valency state. Its interpretation demands a mixed valent state.
... This has resulted in observing ferromagnetism in the pure compounds when doped by V, Cr, or Mn. [231][232][233][234][235] Furthermore, the intrinsic ambipolar defect chemistry in the tellurides, discussed in Sec. II, gives a chemical route to control the Fermi level. ...
Topology appears across condensed matter physics to describe a wide array of phenomena which could alter, augment, or fundamentally change the functionality of many technologies. Linking the basic science of topological materials to applications requires producing high-quality thin films. This will enable combining dissimilar materials while utilizing dimensionality, symmetry, and strain to create or control the electronic phase, as well as platforms to fabricate novel devices. Yet, one of the longstanding challenges in the field remains understanding and controlling the basic material properties of epitaxial thin films. The aim of this Perspective article is to discuss how understanding the fundamental properties of topological materials grown by molecular beam epitaxy (MBE) is key to deepening the knowledge of the basic physics, while developing a new generation of topological devices. A focus will be on the MBE growth of intrinsic materials, creation, and control of superconducting and magnetic topological phases. Addressing these questions in the coming decade will undoubtedly uncover many surprises as new materials are discovered and their growth as high-quality thin films is refined.
... We think vanadium could be the key ingredient that enabled the apparition of a 4π periodic ABS in our system. Indeed, vanadium doping is known to induce a quantum anomalous Hall regime in BiSbTe 3 , Bi 2 Te 3 and Sb 2 Te 3 [32,35,52]. As we have etched the TI crystal prior to vanadium deposition, the atoms of vanadium may have slightly doped the crystal at its surface, thus inducing magnetic moment at the interface between Bi 2 Se 3 and the vanadium. ...
Three dimensional topological insulators (3D TI) are a new state of matter composedof an electrically insulating bulk covered by metallic surface states. Theoretically, a topo-logical Josephson junction composed of these surface states can host an Andreev Boundstate (ABS) that has twice the periodicity of the conventional 2π periodic ABSs. The4π periodic ABS is expected to be the building block of topological quantum computing.Therefore, we study the dynamic of this particular ABS by performing Shapiro measure-ment on Josephson junctions built with bismuth based 3D TI.To identify the effects of a 4π periodic ABS in a Shapiro measurement, we use a phe-nomenological model that simulates the voltage-current characteristics of a TJJ. We predicttwo signatures of the 4π periodic ABS and estimate their robustness against Joule heatingand thermally activated quasiparticle poisoning of the 4π periodic mode.We study the Josephson junctions dynamics by performing Shapiro measurements onjunctions built on Bi2Se3. We observe the two previously anticipated signatures, whichare the non-conventional appearance order of the Shapiro steps and the remaining of asupercurrent at the closing of the Shapiro step n = 0. They prove the presence of a 4πperiodic ABS.We also study the topological insulator BiSbTeSe2 that we have grown by using themelting growth method. By superconducting interferometric measurements, we show asuperconducting surface transport without bulk electronic conduction.
... It has been experimentally challenging to achieve longrange ferromagnetic ordering in the bulk of Bi and Sb chalcogenides. Earlier work focused on their use as dilute magnetic semiconductors, [7] and successful ferromagnetic (a) E-mail: Thorsten.Hesjedal@physics.ox.ac.uk (corresponding author) doping with, e.g., Mn [8][9][10] and Fe [11], has been reported in thin film and bulk samples. Cr doping has been predicted to lead to an insulating ferromagnetic ground state in Bi 2 Se 3 [1,12], as required for the QAH effect, whereas Mn and Fe doping leads to a metallic state [13]. ...
We report the structural and magnetic study of Cr-doped Bi2Se3 thin films using x-ray diffraction (XRD), magnetometry and polarized neutron reflectometry (PNR). Epitaxial layers were grown on c-plane sapphire by molecular beam epitaxy in a two-step process. High-resolution XRD shows the exceptionally high crystalline quality of the doped films with no parasitic phases up to a Cr concentration of 12% (in % of the Bi sites occupied by substitutional Cr). The magnetic moment, measured by SQUID magnetometry, was found to be per Cr ion. The magnetic hysteresis curve shows an open loop with a coercive field of . The ferromagnetic transition temperature was determined to be analyzing the magnetization-temperature gradient. PNR shows the film to be homogeneously ferromagnetic with no enhanced magnetism near the surface or interface.
... For instance, ferromagnetic Curie temperatures (T C ) ranging up to 190 K were reported in bulk crystals of Bi 2 Te 3 doped with V, Cr, Mn, [16][17][18] and Fe, 19 as well as in thin films grown by molecular beam epitaxy (MBE). 20 More recent studies of magnetically-doped chalcogenides, primarily driven by an interest in topological phenomena, have shown ferromagnetism in a variety of samples: bulk crystals of p-type, Mn-doped Bi 2 Te 3 21 and Fe-doped Bi 2 Se 3 , 22 as well as thin films of Cr-doped Bi 2 Te 3 , 23 Cr-doped Bi 2 Se 3 , 24 n-type, Mn-doped Bi 2 Se 3 , 25 and Cr-doped (Bi 1−x Sb x ) 2 (Te 1−y Se y ) 3 . 26,27 Other magnetic phases (paramagnetic, antiferromagnetic, spin glass) have also been observed. ...
We describe a detailed study of the structural, magnetic, and
magneto-transport properties of single-crystal, n-type, Mn-doped Bi2Te3 thin
films grown by molecular beam epitaxy. With increasing Mn concentration, the
crystal structure changes from the tetradymite structure of the Bi2Te3 parent
crystal at low Mn concentrations towards a BiTe phase in the (Bi2Te3)m(Bi2)n
homologous series. Magnetization measurements reveal the onset of
ferromagnetism with a Curie temperature in the range 13.8 K - 17 K in films
with 2 % - 10 % Mn concentration. Magnetization hysteresis loops reveal that
the magnetic easy axis is along the c-axis of the crystal (perpendicular to the
plane). Polarized neutron reflectivity measurements of a 68 nm-thick sample
show that the magnetization is uniform through the film. The presence of
ferromagnetism is also manifest in a strong anomalous Hall effect and a
hysteretic magnetoresistance arising from domain wall scattering. Ordinary Hall
effect measurements show that the carrier density is n-type, increases with Mn
doping, and is high enough (> 2.8 x 10^{13} cm^{-2}) to place the chemical
potential in the conduction band. Thus, the observed ferromagnetism is likely
associated with both bulk and surface states. Surprisingly, the Curie
temperature does not show any clear dependence on the carrier density but does
increase with Mn concentration. Our results suggest that the ferromagnetism
probed in these Mn-doped Bi2Te3 films is not mediated by carriers in the
conduction band or in an impurity band.
... In fact, p-type Bi 2 Te 3 /Sb 2 Te 3 superlattices were found to possess the highest thermoelectric figure of merit (ZT) of about 2.4 at 300 K [35]. Vanadium-doped Sb 2–x V x Te 3 films (x ≈ 0.35) display ferromagnetism up to temperatures of 177 K while the Curie temperature (TC) in Sb 2–x Cr x Te 3 (x ≈ 0.59) is increased to 190 K [36]. Here we report the results of our systematic study of thin films of Sb 2 Te 3 grown layer-by-layer by MBE on Si(111). ...
The growth and characterization of single-crystalline thin films of topological insulators (TIs) is an important step towards
their possible applications. Using in situ scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), we show that moderately thick
Sb2Te3 films grown layer-by-layer by molecular beam epitaxy (MBE) on Si(111) are atomically smooth, single-crystalline, and intrinsically
insulating. Furthermore, these films were found to exhibit a robust TI electronic structure with their Fermi energy lying
within the energy gap of the bulk that intersects only the Dirac cone of the surface states. Depositing Cs in situ moves the Fermi energy of the Sb2Te3 films without changing the electronic band structure, as predicted by theory. We found that the TI behavior is preserved
in Sb2Te3 films down to five quintuple layers (QLs).
KeywordsTopological insulator-electronic structure-scanning tunneling microscopy-angle-resolved photoemission spectroscopy-molecular beam epitaxy
We report the growth of Mn-doped Bi2Se3 thin films by molecular beam epitaxy (MBE), investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), SQUID magnetometry and x-ray magnetic circular dichroism (XMCD). Epitaxial films were deposited on c-plane sapphire substrates by co-evaporation. The films exhibit a spiral growth mechanism typical of this material class, as revealed by AFM. The XRD measurements demonstrate a good crystalline structure which is retained upon doping up to ∼7.5 atomic-% Mn, determined by Rutherford backscattering spectrometry (RBS), and show no evidence of the formation of parasitic phases. However an increasing interstitial incorporation of Mn is observed with increasing doping concentration. A magnetic moment of 5.1 μB/Mn is obtained from bulk-sensitive SQUID measurements, and a much lower moment of 1.6 μB/Mn from surface-sensitive XMCD. At ∼2.5 K, XMCD at the Mn L2,3 edge, reveals short-range magnetic order in the films and indicates ferromagnetic order below 1.5 K.
Since the notion of topological insulator (TI) was envisioned in late 2000s, topology has become a new paradigm in condensed matter physics. Realization of topology as a generic property of materials has led to numerous predictions of topological effects. Although most of the classical topological effects, directly resulting from the presence of the spin-momentum-locked topological surface states (TSS), were experimentally confirmed soon after the theoretical prediction of TIs, many topological quantum effects remained elusive for a long while. It turns out that native defects, particularly interfacial defects, have been the main culprit behind this impasse. Even after quantum regime is achieved for the bulk states, TSS still tends to remain in the classical regime due to high density of interfacial defects, which frequently donate mobile carriers due to the very nature of the topologically-protected surface states. However, with several defect engineering schemes that suppress these effects, a series of topological quantum effects have emerged including quantum anomalous Hall effect, quantum Hall effect, quantized Faraday/Kerr rotations, topological quantum phase transitions, axion insulating state, zeroth-Landau level state, etc. Here, we review how these defect engineering schemes have allowed topological surface states to pull out of the murky classical regime and reveal their elusive quantum signatures, over the past decade.
Since the notion of topological insulator (TI) was envisioned in late 2000s, topology has become a new paradigm in condensed matter physics. Realization of topology as a generic property of materials has led to numerous predictions of topological effects. Although most of the classical topological effects, directly resulting from the presence of the spin-momentum-locked topological surface states (TSS), were experimentally confirmed soon after the theoretical prediction of TIs, many topological quantum effects remained elusive for a long while. It turns out that native defects, particularly interfacial defects, have been the main culprit behind this impasse. Even after quantum regime is achieved for the bulk states, TSS still tends to remain in the classical regime due to high density of interfacial defects, which frequently donate mobile carriers due to the very nature of the topologically-protected surface states. However, with several defect engineering schemes that suppress these effects, a series of topological quantum effects have emerged including quantum anomalous Hall effect, quantum Hall effect, quantized Faraday/Kerr rotations, topological quantum phase transitions, axion insulating state, zeroth-Landau level state, etc. Here, we review how these defect engineering schemes have allowed topological surface states to pull out of the murky classical regime and reveal their elusive quantum signatures, over the past decade.
Topological insulators (TIs) have a bulk bandgap and gapless surface states protected by time‐reversal symmetry, and have recently attracted much attention in condensed matter physics and material sciences. Preparation of high‐quality TI films is a starting point to understand their physical properties and explore applications. In this chapter, the molecular beam epitaxy (MBE) growth and magnetic doping of Bi2Se3, Bi2Te3, and Sb2Te3 topological insulators are reviewed. By combined scanning tunneling microscopy and angle‐resolved photoemission spectroscopy study, we demonstrate that MBE is an ideal technique to realize stoichiometric compound material and band engineering, as well as atomic‐level control of doping, which are crucial for discovering those theoretically predicted exotic quantum effects such as quantum anomalous Hall effect.
The interplay between magnetism and topology, as exemplified in the magnetic skyrmion systems, has emerged as a rich playground for finding novel quantum phenomena and applications in future information technology. Magnetic topological insulators (TI) have attracted much recent attention, especially after the experimental realization of quantum anomalous Hall effect. Future applications of magnetic TI hinge on the accurate manipulation of magnetism and topology by external perturbations, preferably with a gate electric field. In this work, we investigate the magneto transport properties of Cr doped Bi2(SexTe1-x)3 TI across the topological quantum critical point (QCP). We find that the external gate voltage has negligible effect on the magnetic order for samples far away from the topological QCP. But for the sample near the QCP, we observe a ferromagnetic (FM) to paramagnetic (PM) phase transition driven by the gate electric field. Theoretical calculations show that a perpendicular electric field causes a shift of electronic energy levels due to the Stark effect, which induces a topological quantum phase transition and consequently a magnetic phase transition. The in situ electrical control of the topological and magnetic properties of TI shed important new lights on future topological electronic or spintronic device applications.
This chapter gives a comprehensive overview of the epitaxial growth of three dimensional (3D) topological insulators (TI) thin films and heterostructures, and focuses principally on the molecular beam epitaxy (MBE) growth of the chalcogenide-based 3D TIs and their transport properties. TI films can be grown by a variety of methods. While MBE has been used predominantly, pulsed laser deposition (PLD), metal organic vapor deposition (MOCVD), and vapor liquid solid (VLS) methods, among others, have been used. The chapter discusses some of the most common materials used in the epitaxial growth of TI thin films. While HgTe is a semimetal, strain can modify the band structure, opening a gap in the bulk bands and leading to it becoming a 3D TI. Heterostructures provide an alternative route to interface TIs with the broken TR symmetry created by proximal ferromagnetism.
The discovery of the integer quantum Hall (QH) effect in 1980 led to the
realization of a topological electronic state with dissipationless currents
circulating in one direction along the edge of a two dimensional electron layer
under a strong magnetic field.1,2 The quantum anomalous Hall (QAH) effect
shares a similar physical phenomenon as the QH effect, whereas its physical
origin relies on the intrinsic spin-orbit coupling and ferromagnetism.3-13
Since the QAH effect does not require an external field and the associated
Landau levels, it is believed that this effect has unique potential for
applications in electronic devices with low-power consumption.3-16 Recently,
the QAH effect has been experimentally observed in thin films of the
ferromagnetic topological insulators (FMTI), Cr-doped (Bi,Sb)2Te3.14 However,
in this system, a large residual longitudinal resistance 0.098h/e2(~2.53kohm)
remains, and thus the Hall conductance is only around 0.987e2/h at zero
magnetic field, hampering the precise realization of this dissipationless
topological state. Here we report the experimental observation of the QAH state
in V-doped (Bi,Sb)2Te3 films with the zero-field longitudinal resistance down
to 0.00013+-0.00007h/e2 (~3.35+-1.76ohm), Hall conductance reaching
0.9998+-0.0006e2/h and the Hall angle becoming as high as 89.993+-0.004 degree
at T=25mK, thus realizing the anomalous Hall transport with negligible
dissipation in the absence of any initial magnetic field. The advantage of this
system comes from the fact that it is a hard ferromagnet with a large coercive
field (Hc>1.0T) and a relative high Curie temperature. This realization of
robust QAH state in hard FMTIs is a major step towards dissipationless
electronic applications without external fields.
The anomalous Hall effect is a fundamental transport process in solids arising from the spin-orbit coupling. In a quantum
anomalous Hall insulator, spontaneous magnetic moments and spin-orbit coupling combine to give rise to a topologically nontrivial
electronic structure, leading to the quantized Hall effect without an external magnetic field. Based on first-principles calculations,
we predict that the tetradymite semiconductors Bi2Te3, Bi2Se3, and Sb2Te3 form magnetically ordered insulators when doped with transition metal elements (Cr or Fe), in contrast to conventional dilute
magnetic semiconductors where free carriers are necessary to mediate the magnetic coupling. In two-dimensional thin films,
this magnetic order gives rise to a topological electronic structure characterized by a finite Chern number, with the Hall
conductance quantized in units of e2/h (where e is the charge of an electron and h is Planck’s constant).
Magnetotransport properties of p-type ferromagnetic (Ga,Mn)As, a diluted magnetic semiconductor based on III-V semiconductors, are measured and the p-d exchange between holes and Mn 3d spins is determined. The ferromagnetic transition temperatures calculated based on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction using the exchange reproduce remarkably well the observed ferromagnetic transition temperatures, demonstrating that ferromagnetism of (Ga,Mn)As has its origin in the RKKY interaction mediated by holes.
Reflection high-energy electron diffraction is one of the most powerful tools used in surface structural analysis to monitor epitaxial growth. This book serves as an introduction to RHEED for beginners and details experimental and theoretical treatments for experts. First, the principles of electron diffraction are explained, with many examples of RHEED patterns described for beginners. The second part contains in-depth descriptions of RHEED theory. Finally, applications of RHEED are explained with many examples.
www.cambridge.org/jp/academic/subjects/engineering/materials-science/reflection-high-energy-electron-diffraction?format=PB#4iMLgxpAvffJgEPX.97
We report on magnetic and electrical transport properties of Sb2-xCrxTe3 single crystals with 0 <= x <= 0.095 over temperatures from 2 K to 300 K. A ferromagnetic state develops in these crystals at low temperatures with Curie temperatures that are proportional to x (for x>0.014), attaining a maximum value of 20 K for x=0.095. Hysteresis below T-C for the applied field parallel to the c axis is observed in both magnetization and Hall-effect measurements. Magnetic as well as transport data indicate that Cr takes the 3+(3d(3)) valence state, substituting for antimony in the host lattice structure, and does not significantly affect the background hole concentration. Analysis of the anomalous Hall effect reveals that skew scattering is responsible for its presence. These results broaden the scope of ferromagnetism in the V-2-VI3 diluted magnetic semiconductors (DMS) and in ferromagnetic DMS structures, generally.
We studied the effect of free carriers in the nonmagnetic spacer layer on the magnetic properties of GaMnAs/GaAs superlattice (SL) systems. One of the SLs contained GaAs layers doped with Be (which acts as a p-type dopant), while the GaAs layers of the other SL were undoped. While the Be-doped SL exhibited relatively robust temperature dependence of remanent magnetization and large coercivity, the undoped SL showed a fast decrease of remanent magnetization with the temperature and rather small coercivity. We propose that the observed hardness of magnetization in the SL with Be-doped GaAs layers is related to interlayer coupling introduced by doping of the nonmagnetic layers. © 2004 American Institute of Physics.
A search for spin-dependent electron transport at the ferromagnet/semiconductor interface has been made by measuring the bias dependence of a photon excited current through the interface. A circularly polarized laser beam was used to excite electrons with a spin polarization perpendicular to the film plane. In samples of the form 3 nm Au/5 nm Ni <sub> 80 </sub> Fe <sub> 20 </sub> /GaAs (110), a significant transport current was detected with a magnitude dependent on the relative orientation of the spin polarization and the magnetization vector. At perpendicular saturation, the bias dependence of the photocurrent is observed to change in the range 0.7–0.8 eV when the helicity is reversed. © 1999 American Institute of Physics.
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.
The magnetic and magneto-optical properties of a Cr-doped II-VI semiconductor ZnTe were investigated. Magnetic circular dichroism measurements showed a strong interaction between the sp carriers and localized d spins, indicating that Zn(1-x)Cr(x)Te is a diluted magnetic semiconductor. The Curie temperature of the film with x=0.20 was estimated to be 300+/-10 K, which is the highest value ever reported for a diluted magnetic semiconductor in which sp-d interactions were confirmed. In spite of its high Curie temperature, Zn(1-x)Cr(x)Te film shows semiconducting electrical transport properties.
The search for ferromagnetism above room temperature in dilute magnetic semiconductors has been intense in recent years. We report the first observations of ferromagnetism above room temperature for dilute (<4 at.%) Mn-doped ZnO. The Mn is found to carry an average magnetic moment of 0.16 mu(B) per ion. Our ab initio calculations find a valance state of Mn(2+) and that the magnetic moments are ordered ferromagnetically, consistent with the experimental findings. We have obtained room-temperature ferromagnetic ordering in bulk pellets, in transparent films 2-3 microm thick, and in the powder form of the same material. The unique feature of our sample preparation was the low-temperature processing. When standard high-temperature (T > 700 degrees C) methods were used, samples were found to exhibit clustering and were not ferromagnetic at room temperature. This capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new spintronic devices as well as magneto-optic components.
p-(In,Mn)As/n-InAs/p-(In,Mn)As trilayer structures have been prepared by molecular beam epitaxy on GaSb/GaAs/GaAs(001) substrates. Ferromagnetic coupling has been found between the two magnetic p-(In,Mn)As layers with a rather thick n-InAs layer (dsimilar to30 nm). The magnitude of the ferromagnetic coupling is estimated to be J=7.6x10(-3) mJ/m(2), being one order of magnitude larger than that extracted for the (Ga,Mn)As-based trilayer structures [D. Chiba , Appl. Phys. Lett. 77, 1873 (2000)]. (C) 2002 American Institute of Physics.
Thin films of Sb2−xVxTe3 with x=0, 0.15, 0.32 and 0.35 have been grown on sapphire (0001) substrates using molecular-beam epitaxy. With the increasing concentration of vanadium, the in-plane lattice constant of Sb2−xVxTe3 films decreases while the c-axis lattice constant shows a very small increase. The electrical resistivity of Sb2−xVxTe3 films decreases by an order of magnitude when V is substituted on the Sb sublattice. The Hall effect measurements show an increase in the concentration of holes as the content of vanadium increases.
Reflection high energy electron diffraction patterns from quantum dots formed by depositing InAs on to GaAs(001) show pairs of streaks known as chevrons. A clear understanding of the mechanism by which chevrons are produced does not seem to exist in the literature, and both pure diffraction mechanisms and effects due to the refraction of electrons have been put forward. Scanning tunnelling microscopy has shown that these particular quantum dots have bounding surfaces which are curved, not planar. In this paper, a model is presented for the explanation of the production of chevrons from such quantum dots in terms of pure refraction effects. A method is given for determining the grazing angles of incidence of the various diffracted beams on the curved bounding surfaces of the quantum dots, and hence the refractive deviations have been calculated as a function of the direction of the incident electron beam. It is shown that the model provides a semi-quantitative explanation of the occurrence of the chevrons.
A criterion is proposed for determining the onset of ferromagnetism in a material as its temperature is lowered from a region in which the linearity of its magnetic moment versus field isotherm gives an indication of paramagnetism. Within the limits of validity of a molecular field treatment, the Curie temperature is shown to be in general indicated by the third power of the magnetization being proportional to the internal magnetic field. The method has been employed to redetermine the Curie point of nickel from the data of Weiss and Forrer, of Fe3O4 from the data of Smith and of some alloys from the data of Kaufmann and his collaborators and the author.
Various anomalous optical, magnetic, and transport phenomena have been found in Eu-chalcogenide alloys. Anomalous optical properties are explained by the magnetic exciton in which a hole in the 4f conduction band and an electron (for optically active magnetic exciton) in the 5d conduction band make a bound state. Strong d-f exchange interaction between the magnetic-exciton electron of 5d character and the 4f electrons in the surrounding Eu2+ ions is the origin of the anomaly. Anomalous magnetic properties in dilute R3+ alloys are explained by the properties of the giant spin molecule which comes from the strong s-f exchange interaction between the impurity electron and the 4f electrons at the central R3+ and the surrounding Eu2+. The anomalous transition from the hopping type to the metallic-impurity band conduction in dilute R3+ alloys is also explained by the temperature dependence of the activation energy due to the s-f exchange interaction in the magnetic-impurity state.
Carrier-induced ferromagnetism in the doped diluted magnetic semiconductors \(In,Mn↑,Mn↓,A\)As, where A is As or Sn and Mn↑ ( Mn↓) denotes Mn atoms whose local magnetic moment is parallel (antiparallel) to the magnetization, are investigated using the Korringa-Kohn-Rostoker coherent-potential approximation and local density approximation first-principles calculation. The result shows that (i) the ferromagnetic state is stable due to the double exchange at low concentrations of A, and (ii) a spin-glass-like local-moment disordered state, which stems from the superexchange, is more favorable when the Mn d holes are nearly compensated.
Resistivity and Hall-effect measurements in Eu0.95Gd0.05S give evidence for the onset of a well defined conduction band with critical scattering coexisting with magnetic impurity states.
DOI:https://doi.org/10.1103/PhysRevLett.19.786
Ruderman-Kittel-Kasuya-Yosida interaction between localized spins is considered for various dimensionality structures of doped diluted magnetic semiconductors. The influence of this interaction on the temperature and magnetic-field dependencies of magnetization and spin splitting of the bands are evaluated in the mean-field approximation. The results show that the hole densities that can presently be achieved are sufficiently high to drive a paramagnetic-ferromagnetic phase transition in bulk and modulation-doped structures of II-VI diluted magnetic semiconductors.
We grew single crystals of the new dilute magnetic semiconductor p-Sb2−xCrxTe3 (x=0,0.0115,0.0215) and investigated their transport, magneto-transport and magnetic properties in the temperature interval 1.7–300K. In high fields we clearly observed Shubnikov–de Haas oscillations, that show a decrease in hole concentration for Cr-doped samples. A negative magnetoresistance and anomalous Hall effect are observed in Cr-doped samples. Ferromagnetic order is found below the Curie temperature Tc≈5.8K (x=0.0215) and Tc≈2.0K (x=0.0115). In the ferromagnetic phase we observe large magnetic anisotropy with the easy-axis parallel to the C3 crystallographic axis.
It is assumed (1) that the interaction between the incomplete d shells of the transition elements is insufficient to disrupt the coupling between the d electrons in the same shells, and (2) that the exchange interaction between adjacent d shells always has the same sign irrespective of distance of separation. The direct interaction between adjacent d shells then invariably leads to a tendency for an antiferromagnetic alignment of d spins. The body-centered cubic structure of the transition metals V, Cr, Cb, Mo, Ta, and W is thereby interpreted, as well as more complex lattices of certain alloys. It is demonstrated that the spin coupling between the incomplete d shells and the conduction electrons leads to a tendency for a ferromagnetic alignment of d spins. The occurrence of ferromagnetism is thereby interpreted in a much more straightforward manner than through the ad hoc assumption of a reversal in sign of the exchange integral. The occurrence of antiferromagnetism and of ferromagnetism in various systems is readily understood, and certain simple rules are deduced for deciding which type of magnetism will occur in particular alloys.
A significant role of refraction effect on reflection high-energy electron diffraction (RHEED) from nanostructures is demonstrated. It was found that the chevron-shape spots in RHEED patterns from self-assembled InAs/GaAs(001) and InAs/InAlAs/InP(001) quantum dots at [11¯0] azimuth are well reproduced by kinematical calculations taking into account the refraction of electron beam at the curved surfaces of the dots. The dots must have (11¯0) cross sections steeper than (110) cross sections and consequently extend along [11¯0] since the refraction effects, considerable only at glancing incidence and departure, are invisible at [110] azimuth.
3d-transition-metal-doped ZnO films (n-type Zn1−xMxO (x = 0.05–0.25): M=Co, Mn, Cr, Ni) are formed on sapphire substrates using a pulsed-laser deposition technique, and their magnetic and electric properties are examined. The Co-doped ZnO films showed the maximum solubility limit. Some of the Co-doped ZnO films exhibit ferromagnetic behaviors with the Curie temperature higher than room temperature. The magnetic properties of Co-doped ZnO films depend on the concentration of Co ions and carriers. © 2001 American Institute of Physics.
We report on the growth, structural as well as magnetic characterization of (Ga,Mn)N epitaxial layers grown directly on 4H–SiC(0001) by reactive molecular-beam epitaxy. We focus on two layers grown under identical conditions except for the Mn/Ga flux ratio. Structural characterization reveals that the sample with the lower Mn content is a uniform alloy, while in the layer with the higher Mn content, Mn-rich clusters are found to be embedded in the (Ga,Mn)N alloy matrix. Although the magnetic behavior of both the samples is similar at low temperatures, showing antiferromagnetic characteristics with a spin-glass transition, the sample with higher Mn content additionally exhibits ferromagnetic properties at and above room temperature. This ferromagnetism most likely originates from the Mn-rich clusters in this sample. © 2003 American Institute of Physics.
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.
We review the physical properties of diluted magnetic semiconductors (DMS) of the type A<sup>II</sup> 1-x Mn x B<sup>VI</sup> (e.g., Cd 1-x Mn x Se, Hg 1-x Mn x Te). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic A<sup>II</sup>B<sup>VI</sup> ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d<sup>5</sup> shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloys. In particular, we discuss the mechanism of the Mn<sup>+</sup><sup>+</sup>‐Mn<sup>+</sup><sup>+</sup> exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susceptibility of DMS in the paramagnetic range; we describe a somewhat empirical picture of the spin‐glasslike freezing in the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloys, and its relationship to the short range antiferromagnetic order revealed by neutron scattering; and we point out some not yet fully understood questions concerning spin dynamics in DMS revealed by electron paramagnetic resonance. We then discuss the sp‐d exchange interaction between the sp band electrons of the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloy and the 3d<sup>5</sup> electrons associated with the Mn atoms. Here we present a general formulati-
on of the exchange problem, followed by the most representative examples of its physical consequences, such as the giant Faraday rotation, the magnetic‐field‐induced metal‐to‐insulator transition in DMS, and the properties of the bound magnetic polaron. Next, we give considerable attention to the extremely exciting physics of quantum wells and superlattices involving DMS. We begin by describing the properties of the two‐dimensional gas existing at a DMS interface. We then briefly describe the current status of the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> layers and superlattices (systems already successfully grown; methods of preparation; and basic nonmagnetic properties of the layered structures). We then describe new features observed in the magnetic behavior of the quasi‐two‐dimensional ultrathin DMS layers; and we discuss the exciting possibilities which the sp‐d exchange interaction offers in the quantum‐well situation. Finally, we list a number of topics which involve DMS but which have not been explicitly covered in this review such as elastic properties of DMS, DMS‐based devices, and the emerging work on diluted magnetic semiconductors other than the A<sup>II</sup> 1-x Mn x B<sup>VI</sup> alloys—and we provide relevant literature references to these omitted topics.
We have investigated the magnetic and magnetotransport properties of (Ga, Mn)As/(Al, Ga)As/(Ga, Mn)As semiconductor-based magnetic trilayer structures. We observe a weak ferromagnetic interlayer coupling between the two ferromagnetic (Ga, Mn)As layers as well as magnetoresistance effects due to spin-dependent scattering and to spin-dependent tunneling. Both the coupling strength and the magnetoresistance ratio decrease with the increase of temperature and/or the increase of Al composition of the nonmagnetic (Al, Ga)As layer. © 2000 American Institute of Physics.
We perform ab initio calculations to obtain the
formation energy of thin InAs films grown on the GaAs(001) substrate.
For the island surface morphology,
a hybrid method combining
ab initio calculations and elasticity theory is employed.
Our results show that two-dimensional growth is favored
for the first monolayer in a wide range of chemical potentials.
Additional deposited material may transform the surface morphology
into the three-dimensional island growth mode. The driving force behind
this surface morphology change is relieving the elastic energy in the films.
We present theoretical studies of basic magnetic response characteristics of superlattice structures formed from alternating materials, each described through use of a localized spin model. We explore two geometries here, both of which have body centered cubic crystal structure. The first geometry is one in which the antiferromagnetic consists of sheets of spins parallel to each interface within which the spins are aligned ferromagnetically. This geometry is realized when the interface between ferromagnetic and antiferromagnetic constituents is a (100) plane. In the second geometry explored here, the interface between the ferromagnetic and antiferromagnetic constituents is a (110) plane. In these systems, the antiferromagnet consists of sheets of spins parallel to each interface, within which the spins are aligned antiferromagnetically. We have investigated the influence of an external magnetic field on the classical ground state of these superlattice structures, and found a series of magnetic field induced spin reorientation transitions occur. For the second geometry described above, we find the zero field ground state spin configuration has spins canted to minimize total exchange energy at the interface. In addition, we find a different sequence of phase transitions occur within the phase diagram when we change the value of the interface exchange constant. We have also calculated the spin wave spectrum and the infrared absorption spectrum for the various phases of each superlattice structure. We find a subset of the spin waves are collective excitations of the structure as a whole, as opposed to modes localized in one or the other constituent.
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.
We report the discovery of antiferromagnetic interlayer exchange coupling and enhanced saturation magnetoresistance in two new metallic superlattice systems, Co/Cr and Co/Ru. In these systems and in Fe/Cr superlattices both the magnitude of the interlayer magnetic exchange coupling and the saturation magnetoresistance are found to oscillate with the Cr or Ru spacer layer thickness with a period ranging from 12 \AA{} in Co/Ru to \ensuremath{\simeq}18\char21{}21 \AA{} in the Fe/Cr and Co/Cr systems.
Magnetotransport properties of p-type (In,Mn)As, a new diluted magnetic semiconductor based on a III-V semiconductor, are studied. The interaction between the holes and the Mn 3d spins is manifested in the anomalous Hall effect, which dominates the Hall resistivity from low temperature (0.4 K) to nearly room temperature, and in the formation of partial ferromagnetic order below 7.5 K, which is a cooperative phenomenon related to carrier localization. The coexistence of remanent magnetization and unsaturated spins as well as the large negative magnetoresistance at low temperatures is explained by the formation of large bound magnetic polarons.
An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin,
and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped
d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for
understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance.
The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder
transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given
here on this “orbital physics,” which will be a key concept for the science and technology of correlated electrons.
Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From
a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their
magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with the
concentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating
the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic
moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at
2 kelvin.
Interlayer exchange coupling that oscillates between antiferromagnetic and ferromagnetic as a function of NiO thickness has been observed in [Pt(5 A)/Co(4 A)](3)/NiO(t(NiO) A)/[Co(4 A)/Pt(5 A)](3) multilayers with out-of-plane anisotropy. The period of oscillation corresponds to approximately 2 monolayers of NiO. This oscillatory behavior is possibly attributed to the antiferromagnetic ordering in NiO. The antiferromagnetic interlayer exchange coupling for the 11 A NiO layer shows an increase in coupling strength with increasing temperature, in agreement with the quantum interference model of Bruno for insulating spacer layers. A coexistence of exchange biasing and antiferromagnetic interlayer exchange coupling has been observed below T=250 K.
The interlayer coupling between (Ga,Mn)As ferromagnetic layers in all-semiconductor superlattices is studied theoretically within a tight-binding model, which takes into account the crystal, band and magnetic structure of the constituent superlattice components. It is shown that the mechanism originally introduced to describe the spin correlations in antiferromagnetic EuTe/PbTe superlattices, explains the experimental results observed in ferromagnetic semiconductor structures, i.e., both the antiferromagnetic coupling between ferromagnetic layers in IV-VI (EuS/PbS and EuS/YbSe) superlattices as well as the ferromagnetic interlayer coupling in III-V ((Ga,Mn)As/GaAs) multilayer structures. The model allows also to predict (Ga,Mn)As-based structures, in which an antiferromagnetic interlayer coupling could be expected. Comment: 4 pages, 3 figures