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 happens when there are oxygen vacancies. In both cases, the cohesive energy is of the same order of magnitude but only the AlO2/TiO2 interface presents an stable magnetic solution.
Graphene and boron nitride (GPBN) heterostructures provide a viable way to realize tunable bandgap, promising new opportunities in graphene-based nanoelectronic and optoelectronic devices. In the present study, we investigated the interplay between vacancies and graphene/h-BN interfaces in monolayer GPBN heterostructures. The energetics and kinetics of monovacancies and divacancies in monolayer GPBN heterostructures were examined using first-principle calculations. The interfaces were shown to be preferential locations for vacancy segregation. Meanwhile the kinetics of vacancies was found to b…
We present an ab initio DFT study of the magnetic moments that arise in graphite by creating single carbon vacancies in a 3-D graphite network, using a full potential, all electron, spin polarized electronic structure calculations. In previous reports the appearance of magnetic moments was explained in a 2-D graphene sheet just through the existence of the vacancies itself [1-5]. The dependence of the arising magnetic moment on the nature and geometry of the vacancies for different supercells is reported. We found that the highest value of magnetic moment is obtained for a 3x3x1 supercell and …
Using density-functional-theory (DFT) calculations with the HSE06 hybrid functional, we accurately evaluate the critical thickness of LaAlO3 film for the intrinsic doping in LaAlO3/SrTiO3 (LAO/STO) heterstructures. The calculated critical thickness of 6 unit-cell (uc) layers suggests to rule out the intrinsic doping mechanism. We also calculate the density of oxygen vacancies on the LAO surface at varying LAO thicknesses, preparation oxygen pressures and temperatures by using the condition of chemical equilibrium and DFT calculations. We find that once LAO thickness >=3 uc high-density (~ 10^1…
We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size / tolerance factor, epitaxial strain, ferroelectricity and magnetism in the perovskite manganite series, AMnO3 (A=Ca2+, Sr2+, Ba2+). We find that, as expected, increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. Increased volume also favors polar distortions, both because competing rotations of the o…
Density functional theory calculations indicate that the incorporation of V into Ti lattice positions of rutile TiO2 leads to magnetic V4+ species, but the extension and sign of the coupling between dopant moments confirm that ferromagnetic order cannot be reached via low-concentration doping in the non-defective oxide. Oxygen vacancies can introduce additional magnetic centres, and we show here that one of the effects of vanadium doping is to reduce the formation energies of these defects. In the presence of both V dopants and O vacancies all the spins tend to align with the same orientation.…
We performed first-principles calculations to investigate the possible magnetism induced by the different concentrations of non-magnetic impurities and vacancies in BN sheet. The atoms of Be, B, C, N, O, Al and Si are used to replace either B or N in the systems as impurities. We discussed the changes in density of states as well as the extent of the spatial distributions of the defect states, the possible formation of magnetic moments, the magnitude of the magnetization energies and finally the exchange energies due to the presence of these defects. It is shown that the magnetization energies…
We propose intrinsic magnetism in nanosheets of SnO$_{2}$, based on first-principles calculations. The electronic structure and spin density reveal that $p$ orbitals of the oxygen atoms, surrounding Sn vacancies, have a non itinerant nature which gives birth to localized magnetism. A giant decrease in defect formation energies of Sn vacancies in nanosheets is observed. We, therefore, believe that native defects can be stabilized without any chemical doping. Nanosheets of different thicknesses are also studied, and it is found that it is easier to create vacancies, which are magnetic, at the su…
\textit{Ab-initio} calculations based on density functional theory with local spin density approximation are used to study defects-driven magnetism in bulk $α$-Li$ _{3}$N. Our calculations show that bulk Li$ _{3} $N is a non-magnetic semiconductor. Two types of Li vacancies (Li-I and Li-II) are considered, and Li-vacancies (either Li-I or Li-II type) can induce magnetism in Li$ _{3}$N with a total magnetic moment of 1.0 $μ_{\rm B}$ which arises mainly due to partially occupied N-$p$-orbitals around the Li vacancies. The defect formation energies dictate that Li-II vacancy, which is in the Li$ …
Using the quantum-mechanical approach combined with the image charge method we calculated the lowest energy levels of the impurities and neutral vacancies with two electrons or holes located in the vicinity of flat surface of different solids. We obtained that the magnetic triplet state is the ground state of the impurities and neutral vacancies in the vicinity of surface, while the nonmagnetic singlet is the ground state in the bulk for e.g. He atom, Li+, Be++, etc. ions. The energy difference between the lowest triplet and singlet states strongly depends on the electron (hole) effective mass…
The effect of lithium vacancies in the hexagonal structure of $α-$Li$_3$N, is studied within the framework of density functional theory. Vacancies ($\square$) substituting for lithium in $α-$Li$_2$(Li$_{1-x}\square_x$)N are treated within the coherent potential approximation as alloy components. According to our results long range N($p$)-ferromagnetism ($\sim 1$ $μ_B$) sets in for vacancy substitution within the [Li$_2$N] layers ($x \ge 0.7$) with no significant change in unit cell dimensions. By total energies differences we established that in-plane exchange couplings are dominant. Vacancies…
Among two-dimensional atomic crystals, hexagonal boron nitride (hBN) is one of the most remarkable materials to fabricate heterostructures revealing unusual properties. We perform first-principles calculations to determine whether intercalated metal atoms and vacancies can mediate interfacial coupling and influence the structural and electronic properties of the graphene/hBN heterostructure. Metal impurity atoms (Li, K, Cr, Mn, Co, and Cu) as extrinsic defects between the graphene and hBN sheets produce $n$-doped graphene. We also consider intrinsic vacancy defects and find that a boron monova…
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…
In this paper we study the possible relation between the electronic and magnetic structure of the TiO2/LaAlO3 interface and the unexpected magnetism found in undoped TiO2 films grown on LaAlO$_3$. We concentrate on the role played by structural relaxation and interfacial oxygen vacancies. LaAlO3 has a layered structure along the (001) direction with alternating LaO and AlO2 planes, with nominal charges of +1 and -1, respectively. As a consequence of that, an oxygen deficient TiO2 film with anatase structure will grow preferently on the AlO2 surface layer. We have therefore performed ab-initio …
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…
We investigate the effect of oxygen vacancies and hydrogen dopants at the surface and inside slabs of LaAlO3, SrTiO3, and LaAlO3/SrTiO3 heterostructures on the electronic properties by means of electronic structure calculations as based on density functional theory. Depending on the concentration, the presence of these defects in LaAlO3 slab can suppress the surface conductivity. In contrast, in insulating SrTiO3 slabs already very small concentrations of oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the conduction band. Surface defects in insulating LaAlO3/SrTiO3 het…
The electronic structure, bonding and magnetism in graphene containing vacancies are studied using density-functional methods. The single-vacancy graphene ground state is spin polarized and structurally flat. The unpolarized state is non planar only for finite segments. Systems containing periodic arrays of vacancies displays magnetic transitions and metal-insulator transitions.
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…
Electronic transport near the insulator-metal transition is investigated in the molecular beam epitaxy-grown SrTiO3/Nd1-xTiO3/SrTiO3 heterostructures using temperature dependent magnetotransport measurements. It was found that Nd-vacancies introduce localized electronic states resulting in the variable range hopping transport at low temperatures. At a fixed Nd-vacancies concentration, a crossover from Mott to Efros-Shklovskii (ES) variable range hopping transport was observed with decreasing temperature. With increasing disorder, a sign reversal of magnetoresistance from positive to negative w…
Using ab-initio calculations based on density functional theory, we investigate the effects of vacancies on the electronic and magnetic properties of zigzag SiC nanoribbons (Z-SiCNR). Single (V_C and V_Si) and double (V_SiV_Si and V_SiV_C) vacancies are observed to induce magnetism in Z-SiCNRs. The presence of a single V_Si does not affect the half-metallic behavior of pristine Z-SiCNRs; however, a single V_C leads to a transition from half-metallic to metallic behavior in Z-SiCNRs due to the edge Si p orbitals and the atoms surrounding the vacancy. The interactions of vacancies with foreign i…
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…
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…
We report that in unannealed LaAlO3/SrTiO3 (LAO/STO) heterostructures the critical thickness for the appearance of the two-dimensional electron gas can be less than 4 unit cell (uc), the interface is conducting even for STO substrates with mixed terminations and the low-temperature resistance upturn in LAO/STO heterostructures with thick LAO layers strongly depends on laser fluence. Our experimental results provide fundamental insights into the different roles played by oxygen vacancies and polarization catastrophe in the two-dimensional electron gas in crystalline LAO/STO heterostructures.
Atomic vacancies have a strong impact in the mechanical, electronic and magnetic properties of graphene-like materials. By artificially generating isolated vacancies on a graphite surface and measuring their local density of states on the atomic scale, we have shown how single vacancies modify the electronic properties of this graphene-like system. Our scanning tunneling microscopy experiments, complemented by tight binding calculations, reveal the presence of a sharp electronic resonance at the Fermi energy around each single graphite vacancy, which can be associated with the formation of loc…
Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (VO) is a key issue for the future development of transition metal oxides. Progress in this field is currently addressed through VO variations and their impact on mainly one order parameter. Here we reveal a new mechanism for tuning both magnetism and ferroelectricity simultaneously by using VO. Combined experimental and density-functional theory studies of Eu0.5Ba0.5TiO3-δ, we demonstrate that oxygen vacancies create Ti3+ 3d1 defect states, mediating the ferromagnetic coupling between the localize…
We demonstrate that room-temperature ferromagnetism in epitaxial Co-doped TiO2 anatase is driven by electron-mediated exchange interaction, and not by metallic Co clusters. Co(II) substitutes for Ti(VI) in the lattice and produces oxygen vacancies that do not contribute carriers. Free electrons originate with oxygen vacancies resulting from an oxygen deficiency during growth.
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…
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…
Magnetisms in $p$-type monolayer GaX (X=S,Se) is investigated by performing density-functional calculations. Due to the large density of states near the valence band edge, these monolayer semiconductors are ferromagnetic within a small range of hole doping. The intrinsic Ga vacancies can promote local magnetic moment while Se vacancies cannot. Magnetic coupling between vacancy-induced local moments is ferromagnetic and surprisingly long-range. The results indicate that magnetization can be induced by hole doping and can be tuned by controlled defect generation.
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.
We study the effect of pressure on the localized magnetic moments induced by vacancies in bilayer graphene in the presence of topological defects breaking the bipartite nature of the lattice. By using a mean-field Hubbard model we address the two inequivalent types of vacancies that appear in the Bernal stacking bilayer graphene. We find that by applying pressure in the direction perpendicular to the layers the critical value of the Hubbard interaction needed to polarize the system decreases. The effect is particularly enhanced for one type of vacancies, and admits straightforward generalizati…
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…
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…
The origin of the two-dimensional electron gas (2DEG) in the interface between $γ$-Al$_2$O$_3$ (GAO) and SrTiO$_3$ (STO) (GAO/STO) as well as the reason for the high mobility of the 2DEG is still in debate. In this paper, the electronic structures of [001]-oriented GAO/STO heterostructures with and without oxygen vacancies are investigated by first-principle calculations based on the density functional theory. The calculation results show that the necessary condition for the formation of 2DEG is that the GAO/STO heterostructure has the interface composed of Al and TiO$_2$ layers. For the heter…
We present ab initio calculations for single-atom vacancies in graphene. In agreement with earlier work, we find that vacancies are responsible for the magnetism recently observed experimentally, creating a center with net spin. For small supercells, there is a strong symmetry breaking in the supercell with respect to the perfect graphene structure, and this symmetry breaking is much smaller for larger supercells. The influence of the adsorption of H, O and N atoms on the spin center of the bare single-atom vacancy and its influence on the magnetism induced in these samples is investigated. A …
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…
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…
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…
We have performed accurate \emph{ab--initio} pseudopotential calculations for the structural and electronic properties of ZnSe/GaAs(001) heterostructures with interface configurations accounting for charge neutrality prescriptions. Beside the simplest configurations with atomic interdiffusion we consider also some configurations characterized by As depletion and cation vacancies, motivated by the recent successfull growth of ZnSe/GaAs pseudomorphic structures with minimum stacking fault density characterized by the presence of a defected (Zn,Ga)Se alloy in the interface region. We find that--u…
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 …
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 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…
