To resolve the microscopic origin of magnetism in the Fe2O3/FeTiO3-system, we have performed density functional theory calculations taking into account on-site Coulomb repulsion. By varying systematically the concentration, distribution and charge state of Ti in a hematite host, we compile a phase diagram of the stability with respect to the end members and find a clear preference to form layered arrangements as opposed to solid solutions. The charge mismatch at the interface is accommodated through Ti4+ and a disproportionation in the Fe contact layer into Fe2+, Fe3+, leading to uncompensated moments in the contact layer and giving first theoretical evidence for the lamellar magnetism hypothesis. This interface magnetism is associated with impurity levels in the band gap showing halfmetallic behavior and making Fe2O3/FeTiO3 heterostructures prospective materials for spintronics applications.
Possible ferromagnetism induced in otherwise non-magnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetic order is stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated to Ti3+ ions at the interface itself for LaA…
Magnetism and transport are two key functional ingredients in modern electronic devices. In oxide heterostructures, ferroelectricity can provide a new route to control these two properties via electrical operations, which is scientifically interesting and technologically important. In this Brief Review, we will introduce recent progresses on this fast developing research field. Several subtopics will be covered. First, the ferroelectric polarization tuning of interfacial magnetism will be introduced, which includes the tuning of magnetization, easy axis, magnetic phases, as well as exchange bi…
We present a study of the electronic structure and magnetism of Co$_2$MnAl, CoMnVAl and their heterostructure. We employ a combination of density-functional theory and dynamical mean-field theory (DFT+DMFT). We find that Co$_2$MnAl is a half-metallic ferromagnet, whose electronic and magnetic properties are not drastically changed by strong electronic correlations, static or dynamic. Non-quasiparticle states are shown to appear in the minority spin gap without affecting the spin-polarization at the Fermi level predicted by standard DFT. We find that CoMnVAl is a semiconductor or a semi-metal, …
Magnetic complex-oxide heterostructures are of keen interest because a wealth of phenomena at the interface of dissimilar materials can give rise to fundamentally new physics and potentially valuable functionalities. Altered magnetization, novel magnetic coupling and emergent interfacial magnetism at the epitaxial layered-oxide interfaces are under intensive investigation, which shapes our understanding on how to utilize those materials, particularly for spintronics. Neutron and x-ray based techniques have played a decisive role in characterizing interfacial magnetic structures and clarifying …
In heterostructures of LaAlO3 (LAO) and SrTiO3 (STO), two nonmagnetic insulators, various forms of magnetism have been observed [1-7], which may [8, 9] or may not [10] arise from interface charge carriers that migrate from the LAO to the interface in an electronic reconstruction [11]. We image the magnetic landscape [5] in a series of n-type samples of varying LAO thickness. We find ferromagnetic patches that appear only above a critical thickness, similar to that for conductivity [12]. Consequently we conclude that an interface reconstruction is necessary for the formation of magnetism. We ob…
Using polarized neutron reflectometry (PNR) we measured the neutron spin dependent reflectivity from four LaAlO3/SrTiO3 superlattices. This experiment implies that the upper limit for the magnetization induced by an 11 T magnetic field at 1.7 K is 2 emu/cm3. SQUID magnetometry of the superlattices sporadically finds an enhanced moment, possibly due to experimental artifacts. These observations set important restrictions on theories which imply a strongly enhanced magnetism at the interface between LaAlO3 and SrTiO3.
Optical methods for magnetism manipulation have been considered as a promising strategy for ultralow-power and ultrahigh-speed spin switches, which becomes a hot spot in the field of spintronics. However, a widely applicable and efficient method to combine optical operation with magnetic modulation is still highly desired. Here, the strongly correlated electron material VO2 is introduced to realize phase-transition based optical control of the magnetism in NiFe. The NiFe/VO2 bilayer heterostructure features appreciable modulations in electrical conductivity (55%), coercivity (60%), and magneti…
Topological magnetism in low-dimensional systems is of fundamental and practical importance in condensed-matter physics and material science. Here, using first-principles and Monte-Carlo simulations, we present that multiple topological magnetism (i.e., skyrmion and bimeron) can survive in van der Waals Heterostructure of MnTe2ZrS2. Arising from interlayer coupling, MnTe2ZrS2 can harbor a large Dzyaloshinskii-Moriya interaction. This, combined with ferromagnetic exchange interaction, yields an intriguing skyrmion phase consisting of sub-10 nm magnetic skyrmions under a tiny magnetic field of 7…
In a recent article, A. Chen et al. claimed that they have switched (rotated by ~90 degrees) the magnetization of the soft layer of a magnetic tunnel junction (MTJ) in a non-volatile way with volatile strain by exploiting dipole interaction with the hard layer. I show that dipole interaction cannot cause this effect and the authors' explanation cannot be correct. An alternate explanation is offered.
Voltage modulation of yttrium iron garnet (YIG) with compactness, high speed response, energy efficiency and both practical/theoretical siginificances can be widely applied to various YIG based spintronics such as spin Hall, spin pumping, spin Seeback effects. Here we initial an ionic modulation of interfacial magnetism process on YIG/Pt bilayer heterostructures, where the Pt capping would influence the ferromagnetic (FMR) field position significantly, and realize a significant magnetism enhancement in bilayer system. A large voltage induced FMR field shifts of 690 Oe has been achieved in YIG …
We address the importance of the modern theory of orbital magnetization for spintronics. Based on an all-electron first-principles approach, we demonstrate that the predictive power of the routinely employed "atom-centered" approximation is limited to materials like elemental bulk ferromagnets, while the application of the modern theory of orbital magnetization is crucial in chemically or structurally inhomogeneous systems such as magnetic thin films, and materials exhibiting non-trivial topology in reciprocal and real space,~e.g.,~Chern insulators or non-collinear systems. We find that the mo…
In this work we study the electronic structure and magnetism of a TiO2 film grown on another non-magnetic oxide such as a LaAlO3 (001) substrate, concentrating on the role played by structural relaxation and oxygen vacancies. Using Density Functional Theory ab-initio methods, we study the free-standing anatase film as well as the interfaces with either the LaO or AlO2 planes of LaAlO3, focusing on the possibility of magnetic solutions. Our results show that the interface LaO/TiO2 is favored against the AlO2/TiO2 one if no oxygen vacancies are present in the interface whereas the contrary happe…
Proximity Magnetism in Mn(Bi,Sb)2Te4-(Bi,Sb)2Te3/MnTe Natural Heterostructures
DEVCOM Army Research Laboratory（DEVCOM陆军研究实验室）；Francis Bitter Magnet Laboratory, Plasma Science and Fusion Center, Massachusetts Institute of Technology（麻省理工学院等离子体科学与聚变中心弗朗西斯·比特磁实验室）；Department of Electrical Engineering, National Cheng Kung University（国立成功大学电气工程系）；Department of Physics, University of Ottawa（渥太华大学物理系）；Department of Materials Science and Engineering, North Carolina State University（北卡罗来纳州立大学材料科学与工程系）
Manipulating the interlayer magnetic coupling in van der Waals magnetic materials and heterostructures is the key to tailoring their magnetic and electronic properties for various electronic applications and fundamental studies in condensed matter physics. By utilizing the MnBi2Te4-family compounds and their heterostructures as a model system, we systematically studied the dependence of the sign and strength of interlayer magnetic coupling on constituent elements by using first-principles calculations. It was found that the coupling is a long-range superexchange interaction mediated by the cha…
In this work we show the presence of a magnetoelectric coupling in silicon-nitride gated Pt/Co/Pt heterostructures using X-ray photoemission electron microscopy (XPEEM). We observe a change in magnetic anisotropy in the form of domain wall nucleation and a change in the rate of domain wall fluctuation as a function of the applied electric field to the sample. We also observe the coexistence of in-plane and out of plane magnetization in Pt/Co/Pt heterostructures in a region around the spin reorientation transition whose formation is attributed to substrate surface roughness comparable to the fi…
Two-dimensional materials and their heterostructures have opened up new possibilities for magnetism at the nanoscale. In this study, we utilize first-principles simulations to investigate the structural, electronic, and magnetic properties of $\textrm{Fe}/\textrm{WSe}_2/\textrm{Pt}$ systems containing pristine, defective, or doped $\textrm{WSe}_2$ monolayers. The proximity effects of the ferromagnetic Fe layer are studied by considering defective and vanadium-doped $\textrm{WSe}_2$ monolayers. All heterostructures are found to be ferromagnetic, and the insertion of the transition-metal dichalc…
Magnetoelectric effect is a fundamental physics phenomenon that synergizes electric and magnetic degrees of freedom to generate distinct material responses like electrically tuned magnetism, which serves as a key foundation of the emerging field of spintronics. Here, we show by first-principles studies that ferroelectric (FE) polarization of an In2Se3 monolayer can modulate the magnetism of an adjacent transition-metal (TM) decorated graphene layer via an FE induced electronic transition. The TM nonbonding d-orbital shifts downward and hybridizes with carbon p states near the Fermi level, supp…
We present a phenomenological scheme to study the size-dependent electric voltage-controlled magnetic anisotropy in ferromagnetic (FM)/ferroelectric (FE) heterostructures. The FM layers are either metallic Fe(001), Ni(001), Co(0001), or half-metallic (La, Sr)MnO3 films. Two magnetoelectric mechanisms, i.e., interface-charge and strain-mediated couplings, are considered. We show that the interface-charge mediated coupling is the main mechanism for the magnetoelectic coupling when the FM film thickness is below a certain transition thickness dtr while the strain-mediated coupling dominates above…
We investigate the magnetic properties of the isostructural spinel-spinel interface of NiMn2O4(NMO)-Fe3O4. Although the magnetic transition temperature of the NMO film is preserved, both bulk and interface sensitive measurements demonstrate that the interface exhibits strong interfacial magnetic coupling up to room temperature. While NMO thin films have a ferrimagnetic transition temperature of 60K, both NiFe2O4 and MnFe2O4 are ferrimagnetic at room temperature. Our experimental results suggest that these magnetic properties arise from a thin interdiffused region of (Fe,Mn,Ni)3O4 at the interf…
The layered antiferromagnetic MnBi2Te4 films have been proposed to be an intrinsic quantum anomalous Hall (QAH) insulator with a large gap. To realize this proposal, it is crucial to open a magnetic gap of surface states. However, recent experiments have observed gapless surface states, indicating the absence of out-of-plane surface magnetism, and thus the quantized Hall resistance can only be achieved at the magnetic field above 6 T. In this work, we propose to induce out-of-plane surface magnetism of MnBi2Te4 films via the magnetic proximity with magnetic insulator CrI3. Our calculations hav…
Enhancement of magnetism of Fe that occurs by alloying with Cr or V is discussed on the basis of first-principles electronic structure calculation. The d states of Fe next to Cr(V) are pushed down to the lower energy side compared to those of pure Fe due to the hybridization with Cr(V) d states, which leads a Co-like electronic structure for the Fe atom. This enhances magnetic moments of other Fe atoms and also the exchange couplings among them. The same mechanism works as well for the magnetism of Fe/Cr heterostructures. The idea is extended to design of a new type of antiferromagnets with hi…
The electronic properties of the polar interface between insulating oxides is a subject of great current interest. An exciting new development is the observation of robust magnetism at the interface of two non-magnetic materials LaAlO_3 (LAO) and SrTiO_3 (STO). Here we present a microscopic theory for the formation and interaction of local moments, which depends on essential features of the LAO/STO interface. We show that correlation-induced moments arise due to interfacial splitting of orbital degeneracy. We find that gate-tunable Rashba spin-orbit coupling at the interface influences the exc…
Among the class of strongly-correlated oxides, La1-xSrxMnO3 $-$ a half metallic ferromagnet with a Curie temperature above room temperature $-$ has sparked a huge interest as a functional building block for memory storage and spintronic applications. In this respect, defect engineering has been in the focus of a long-standing quest for fabricating LSMO thin films with highest quality in terms of both structural and magnetic properties. Here, we discuss the correlation between structural defects, such as oxygen vacancies and impurity islands, and magnetism in La0.74Sr0.26MnO3/SrTiO3 (LSMO/STO) …
Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO3/SrTiO3 (001) heterostructures. Using a combination of element-specific X-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO3. Our results show that the …
In the bulk, LaCoO3 (LCO) is a paramagnet, yet in tensile strained thin films at low temperature ferromagnetism (FM) is observed, and its origin remains unresolved. Polarized neutron reflectometry (PNR) is a powerful tool to determine the depth profiles of the structure and magnetization simultaneously and thus the evolution of the interfacial FM with strain can be accurately revealed. Here we quantitatively measured the distribution of atomic density and magnetization in LCO films by PNR and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced at…
We demonstrate the electrical and optical control of ferromagnetism in semiconductor heterostructures at high temperatures of 100-117 K. The heterostructures consist of Mn delta-doped GaAs and p-type AlGaAs, where the overlap of the hole wavefunction with the Mn delta-doping profile leads to high ferromagnetic transition temperature TC of over 100 K (max. 172 K). We are able to isothermally change the paramagnetic state to the ferromagnetic state and vice versa, by applying a gate electric-field or by light irradiation. The large modulation of TC (DTC ~15 K) at high temperatures (> ~100 K) dem…
Neutron reflectometry (NR) has emerged as a unique technique for the investigation of structure and magnetism of thin films of both biologically relevant and magnetic materials. The advantage of NR with respect to many other surface-sensitive techniques is its sub-nanometer resolution that enables structural characterizations at the molecular level. While in the case of bio-relevant samples, NR can be used to probe thin films at buried interfaces, non-destructively, even adopting a complex sample environment. Whereas the polarized version of NR is best suited for revealing the interface magnet…
The emergence of magnetism in otherwise nonmagnetic compounds and its underlying mechanisms have become the subject of intense research. Here we demonstrate that the nonmagnetic oxygen vacancies are responsible for an unconventional magnetic state common for titanate interfaces and surfaces. Using an effective multiorbital modelling, we find that the presence of localized vacancies leads to an interplay of ferromagnetic order in the itinerant t2g band and complex magnetic oscillations in the orbitally-reconstructed eg-band, which can be tuned by gate fields at oxide interfaces. The magnetic ph…
Fabrication of custom-built heterostructures based on stacked 2D materials provides an effective method to controllably tune electronic and optical properties. To that end, optimizing fabrication techniques for building these heterostructures is imperative. A common challenge in layer-by-layer assembly of 2D materials is the formation of bubbles at the atomically thin interfaces. We propose a technique for addressing this issue by removing the bubbles formed at the heterostructure interface in a custom-defined area using the heat generated by a laser, equipped with raster scanning capabilities…
Controlling magnetism by using electric fields is a goal of research towards novel spintronic devices and future nano-electronics. For this reason, multiferroic heterostructures attract much interest. Here we provide experimental evidence, and supporting DFT analysis, of a transition in La0.65Sr0.35MnO3 (LSMO) thin film to a stable ferromagnetic phase, that is induced by the structural and strain properties of the ferroelectric BaTiO3 (BTO) substrate, which can be modified by applying external electric fields. X-ray Magnetic Circular Dichroism (XMCD) measurements on Mn L edges with a synchrotr…
While tremendous success has been achieved to date in creating both single phase and composite magnetoelectric materials, the quintessential electric-field control of magnetism remains elusive. In this work, we demonstrate a linear magnetoelectric effect which arises from a novel carrier-mediated mechanism, and is a universal feature of the interface between a dielectric and a spin-polarized metal. Using first-principles density functional calculations, we illustrate this effect at the SrRuO$_3$/SrTiO$_3$ interface and describe its origin. To formally quantify the magnetic response of such an …
The conductance confined at the interface of complex oxide heterostructures provides new opportunities to explore nanoelectronic as well as nanoionic devices. Herein we show that metallic interfaces can be realized in SrTiO3-based heterostructures with various insulating overlayers of amorphous LaAlO3, SrTiO3 and yttria-stabilized zirconia films. On the other hand, samples of amorphous La7/8Sr1/8MnO3 films on SrTiO3 substrates remain insulating. The interfacial conductivity results from the formation of oxygen vacancies near the interface, suggesting that the redox reactions on the surface of …
The discovery of conducting two-dimensional electron gas (2DEG) and magnetism at the interface between insulating nonmagnetic oxides, as exemplified by the polar LaAlO3 and nonpolar SrTiO3 has raised prospects for attaining interfacial functionalities absent in the component materials. Yet, the microscopic origin of such emergent phenomena remains unclear, posing obstacles to design of improved functionalities. Using first principles defect calculations, we reveal a unifying polarity-induced defect mechanism for both conductivity and magnetism at polar-nonpolar interfaces of nonmagnetic insula…
Controlling the strain level in nanowire heterostructures is critical for obtaining coherent interfaces of high crystalline quality and for the setting of functional properties such as photon emission, carrier mobility or piezoelectricity. In a nanowire axial heterostructure featuring a sharp interface, strain is set by the materials lattice mismatch and the nanowire radius. Here, we show that introducing a graded interface in nanowire heterostructures offers an additional parameter to control strain. For a given interface length and lattice mismatch, we first derive theoretically the maximum …
The problem for interface solutions in the quantum theory of heterostructures comprising narrow-gap semiconductors is reformulated in the language of commutative diagrams. By this way the theory of interface states in such heterostructures is naturally factorized in the two subproblems: (i) Criterion for the existence of interface states, (ii) Their localization in the common energy gap; the solution of both of them being presented by the requirement for commutativity of the relevant diagrams. It is shown that these two problems are dual in the sense of categorical duality, the passing from th…
We have studied the magnetic layer thickness dependence of the orbital magnetic moment in magnetic heterostructures to identify contributions from interfaces. Three different heterostructures, Ta/CoFeB/MgO, Pt/Co/AlO$_x$ and Pt/Co/Pt, which possess significant interface contribution to the perpendicular magnetic anisotropy, are studied as model systems. X-ray magnetic circular dichroism spectroscopy is used to evaluate the relative orbital moment, i.e. the ratio of the orbital to spin moments, of the magnetic elements constituting the heterostructures. We find that the relative orbital moment …
The structures of epitaxial ultrathin Co2FeAl/MgO(001) heterostructures relating to the interface-induced perpendicular magnetic anisotropy (PMA) were investigated using scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray magnetic circular dichroism. We found that Al atoms from the Co2FeAl layer significantly interdiffuse into MgO, forming an Al-deficient Co-Fe-Al/Mg-Al-O structure near the Co2FeAl/MgO interface. This atomic replacement may play an additional role for enhancing PMA, which is consistent with the observed large perpendicular orbital magneti…
The signature of magnetism without a ferromagnet in a non-magnetic heterostructure is novel as well as fascinating from fundamental research point of view. It has been shown by Al'Mari et al: that magnetism can be induced at the interface of Cu/C60 due to change in density of states. However, the quantification of such interfacial magnetic moment has not been performed yet. In order to quantify the induced magnetic moment in Cu, we have performed X-ray magnetic circular dichroism (XMCD) measurements on Cu/C$_{60}$ multilayers. We have observed room temperature ferromagnetism in Cu/C$_{60}$ sta…
We show that oxygen vacancies at titanate interfaces induce a complex multiorbital reconstruction which involves a lowering of the local symmetry and an inversion of t2g and eg orbitals resulting in the occupation of the eg orbitals of Ti atoms neighboring the O vacancy. The orbital reconstruction depends strongly on the clustering of O vacancies and can be accompanied by a magnetic splitting between the local eg orbitals with lobes directed towards the vacancy and interface dxy orbitals. The reconstruction generates a two-dimensional interface magnetic state not observed in bulk SrTiO3. Using…
Heterostructures made by stacking different materials on top of each other are expected to exhibit unusual properties and new phenomena. Interface of the heterostructures plays a vital role in determining their properties. Here, we report the observation of a two-dimensional (2D) spinodal interface in graphene-molybdenum carbide (α-Mo2C) heterostructures, which arises from spinodal decomposition occurring at the heterointerface, by using scanning tunneling microscopy. Our experiment demonstrates that the 2D spinodal interface modulates graphene into whispering gallery resonant networks filled …
We examine the possibility of intrinsic interface states bound to the plane of In-Sb chemical bonds at InAs/AlSb interfaces. Careful parameterization of the bulk materials in the frame of the extended basis spds^* tight-binding model and recent progress in predictions of band offsets severely limit the span of tight-binding parameters describing this system. We find that a heavy-hole like interface state bound to the plane of In-Sb bonds exists for a large range of values of the InSb/InAs band offset.
We apply the envelope function approach to abrupt heterostructures starting with the least action principle for the microscopic wave function. The interface is treated nonperturbatively, and our approach is applicable to mismatched heterostructure. We obtain the interface connection rules for the multiband envelope function and the short-range interface terms which consist of two physically distinct contributions. The first one depends only on the structure of the interface, and the second one is completely determined by the bulk parameters. We discover new structure inversion asymmetry terms …
In this paper, density functional theory calculations are used to explore the electronic and atomic reconstruction at interfaces between III-III/I-V oxides. In particular, at these interfaces, two dimensional electron gases (2DEGs) with twice the interfacial charge densities of the prototypical LaTiO3/SrTiO3 heterostructure are observed. Furthermore, a significant decrease in the band effective masses of the conduction electrons is shown, suggesting that possible enhancements in electron mobilities may be achievable. These findings represent a framework for chemically modulating 2DEGs, thereby…
Depositing disordered Al on top of SrTiO$_3$ is a cheap and easy way to create a two-dimensional electron system in the SrTiO$_3$ surface layers. To facilitate future device applications we passivate the heterostructure by a disordered LaAlO$_3$ capping layer to study the electronic properties by complementary x-ray photoemission spectroscopy and transport measurements on the very same samples. We also tune the electronic interface properties by adjusting the oxygen pressure during film growth.
