Antiferromagnetic Transition-metal Oxides Research Articles

Antiferromagnetic insulatronics: Spintronics in insulating 3d metal oxides with antiferromagnetic coupling

Antiferromagnetic transition metal oxides are an established and widely studied materials system in the context of spin-based electronics, commonly used as passive elements in exchange bias-based memory devices. Currently, major interest has resurged due to the recent observation of long-distance spin transport, current-induced switching, and THz emission. As a result, insulating transition metal oxides are now considered to be attractive candidates for active elements in future spintronic devices. Here, we discuss some of the most promising materials systems and highlight recent advances in reading and writing antiferromagnetic ordering. This article aims to provide an overview of the current research and potential future directions in the field of antiferromagnetic insulatronics.

Natural orbitals and two-particle correlators as tools for the analysis of effective exchange couplings in solids.

Using generalizations of spin-averaged natural orbitals and two-particle charge correlators for solids, we investigate the electronic structure of antiferromagnetic transition-metal oxides with a fully self-consistent, imaginary-time GW method. Our findings disagree with the Goodenough-Kanamori (GK) rules that are commonly used for the qualitative interpretation of such solids. First, we found a strong dependence of the natural orbital occupancies on momenta, contradicting GK assumptions. Second, along the momentum path, the character of natural orbitals changes. In particular, the contributions of oxygen 2s orbitals are important, which has not been considered in the GK rules. To analyze the influence of the electronic correlation on the values of effective exchange coupling constants, we use both natural orbitals and two-particle correlators and show that electronic screening modulates the degree of superexchange by stabilizing the charge-transfer contributions, which greatly affects these coupling constants. Finally, we give a set of predictions and recommendations regarding the use of density functional, Green's function, and wave-function methods for evaluating effective magnetic couplings in molecules and solids.

Correct and Accurate Polymorphic Energy Ordering of Transition-Metal Monoxides Obtained from Semilocal and Onsite-Hybrid Exchange-Correlation Approximations

The relative energetic stability of the structural phases of common antiferromagnetic transition-metal oxides (MnO, FeO, CoO, and NiO) within the semilocal and hybrid density functionals are fraught with difficulties. In particular, MnO is known to be the most difficult case for almost all common semilocal and hybrid density approximations. Here, we show that the meta-generalized gradient approximation (meta-GGA) constructed from the cuspless hydrogen model and Pauli kinetic energy density (MGGAC) can lead to the correct phase as the ground-state of MnO. The relative energy differences of zinc blende (zb) and rock salt (rs) structures as computed using MGGAC are found to be in nice agreement with those obtained from high-level correlation methods like the random phase approximation or quantum Monte Carlo techniques. Besides, we have also applied the onsite hybrid functionals (closely related to DFT+U) based on GGA and meta-GGA functionals, and it is shown that a relatively high amount of Hartree–Fock exchange is necessary to obtain rs as the ground-state phase. Our present investigation suggests the semilocal MGGAC and onsite hybrids, both being computationally cheap, as methods of choice for the calculation of the relative stability of antiferromagnetic transition-metal oxides having potential applications in solid-state physics and structural chemistry.

Lattice dynamical properties of antiferromagnetic oxides calculated using self-consistent extended Hubbard functional method

We study the lattice dynamics of antiferromagnetic transition-metal oxides by using self-consistent Hubbard functionals. We calculate the ground states of the oxides with the on-site and intersite Hubbard interactions determined self-consistently within the framework of density functional theory. The on-site and intersite Hubbard terms fix the errors associated with the electron self-interaction in the local and semilocal functionals. Inclusion of the intersite Hubbard terms in addition to the on-site Hubbard terms produces accurate phonon dispersion of the transition-metal oxides. Calculated Born effective charges and high-frequency dielectric constants are in good agreement with experiment. Our study provides a computationally inexpensive and accurate set of first-principles calculations for strongly-correlated materials and related phenomena.

High-pressure synthesis of the BiVO3 perovskite

We report the high-pressure, high-temperature synthesis of $\mathrm{BiV}{\mathrm{O}}_{3}$, a cubic perovskite that thus far has remained inaccessible under ambient pressure conditions. We created this material at $\ensuremath{\sim}25$ GPa and 1500 K in a laser-heated diamond-anvil cell and recovered it to ambient pressure and temperature. Our synthetic approach circumvents the oxidative chemistry that, at ambient pressures, previously rendered the cubic $\mathrm{BiV}{\mathrm{O}}_{3}$ perovskite inaccessible. We find through density-functional theory calculations that this material is a unique metallic and antiferromagnetic transition-metal oxide.

Realizing Magnetoelectric Coupling with Hydrogen Intercalation.

Materials with a coexistence of magnetic and ferroelectric order (i.e., multiferroics) provide an efficient route for the control of magnetism by electric fields. Unfortunately, a long-sought room temperature multiferroic with strongly coupled ferroelectric and ferromagnetic (or ferrimagnetic) orderings is still lacking. Here, we propose that hydrogen intercalation in antiferromagnetic transition-metal oxides is a promising way to realize multiferroics with strong magnetoelectric coupling. Taking brownmillerite SrCoO_{2.5} as an example, we show that hydrogen intercalated SrCoO_{2.5} displays strong ferrimagnetism and large electric polarization in which the hydroxide acts as a new knob to simultaneously control the magnetization and polarization at room temperature. We expect that ion intercalation will become a general way to design magnetoelectric and spintronic functional materials.

Physical-chemical properties of M@Fe3O4 core@shell nanowires (M = Cu, Co, CoO).

Interest in low dimensional magnetic systems has been growing due to the novel and dramatically differentiated effects of their physical properties, which give them special behaviors and uses in biomedical, environmental and technological fields. In this study we report extensive first-principles calculations on the geometric optimization as well as electronic, magnetic, mechanical and thermal properties of several quasi one-dimensional core/shell nanowires: Cu/Fe3O4, Co/Fe3O4, and CoO/Fe3O4. The main focus lies on the quantum confinement effects as well as on the effect of the interaction between the ferrimagnetic semiconductor shell material (magnetite nanotube) and core compounds with differentiated magnetic behavior such as (i) a ferromagnetic material (Co), (ii) an antiferromagnetic transition metal oxide (CoO) and (iii) a non-magnetic simple metal (Cu). The mechanical properties of the related nanosystems are studied through the effects of axial deformations, and their thermal behavior is evaluated by considering the electronic contribution of each sample to the heat capacity, and some potential technological applications are suggested.

Exploring the energy landscape of resistive switching in antiferromagnetic Sr3Ir2O7

We study the resistive switching triggered by an applied electrical bias in antiferromagnetic Mott insulator Sr(3)Ir(2)O(7). The switching was previously associated with an electric-field driven structural transition. Here we use time-resolved measurements of the switching to probe the energy barrier associated with the transition. We quantify the changes in the energy barrier height with respect to the applied bias and find a linear decrease of the barrier with increasing bias. Our observations support the potential of antiferromagnetic transition metal oxides for spintronic applications.

Identification of ground-state spin ordering in antiferromagnetic transition metal oxides using the Ising model and a genetic algorithm

We identify ground-state collinear spin ordering in various antiferromagnetic transition metal oxides by constructing the Ising model from first-principles results and applying a genetic algorithm to find its minimum energy state. The present method can correctly reproduce the ground state of well-known antiferromagnetic oxides such as NiO, Fe2O3, Cr2O3 and MnO2. Furthermore, we identify the ground-state spin ordering in more complicated materials such as Mn3O4 and CoCr2O4.

Approximations to the exact exchange potential: KLI versus semilocal

In the search for an accurate and computationally efficient approximation to the exact exchange potential of Kohn-Sham density functional theory, we recently compared various semilocal exchange potentials to the exact one [F. Tran et al., Phys. Rev. B 91, 165121 (2015)]. It was concluded that the Becke-Johnson (BJ) potential is a very good starting point, but requires the use of empirical parameters to obtain good agreement with the exact exchange potential. In this work, we extend the comparison by considering the Krieger-Li-Iafrate (KLI) approximation, which is a beyond-semilocal approximation. It is shown that overall the KLI- and BJ-based potentials are the most reliable approximations to the exact exchange potential, however, sizable differences, especially for the antiferromagnetic transition-metal oxides, can be obtained.

Observation of weak ferromagnetism and the sizable magnetocaloric effect in Co2V2O7

The magnetic behavior of cobalt pyrovanadate compound Co2V2O7 with dichromate structure is reported. The compound undergoes long range magnetic ordering below TC=8K and our study identifies the ground state to be a canted antiferromagnetic type with a weak ferromagnetic component. The transition at TC is found to be first-order in nature as evident from the presence of distinct thermal hysteresis in the temperature dependent magnetization data. Below TC, a significantly large value of magnetic relaxation is observed which is possibly due to the metastability associated with the first order phase transition. Interestingly, the sample exhibits a sizable magneto-caloric effect around TC (∼4.1Jkg−1. K−1 for 50kOe of field change) which is reasonably high among antiferromagnetic transition metal oxides with weak ferromagnetism.

Exchange bias and training effect in NiCr2O4/Cr2O3 composite

Abstract

Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides.

Polar oxide interfaces are an important focus of research due to their novel functionality which is not available in the bulk constituents. So far, research has focused mainly on heterointerfaces derived from the perovskite structure. It is important to extend our understanding of electronic reconstruction phenomena to a broader class of materials and structure types. Here we report from high-resolution transmission electron microscopy and quantitative magnetometry a robust – above room temperature (Curie temperature TC ≫ 300 K) – environmentally stable- ferromagnetically coupled interface layer between the antiferromagnetic rocksalt CoO core and a 2–4 nm thick antiferromagnetic spinel Co3O4 surface layer in octahedron-shaped nanocrystals. Density functional theory calculations with an on-site Coulomb repulsion parameter identify the origin of the experimentally observed ferromagnetic phase as a charge transfer process (partial reduction) of Co3+ to Co2+ at the CoO/Co3O4 interface, with Co2+ being in the low spin state, unlike the high spin state of its counterpart in CoO. This finding may serve as a guideline for designing new functional nanomagnets based on oxidation resistant antiferromagnetic transition metal oxides.

Single Dirac-cone state and quantum Hall effects in a honeycomb structure

A honeycomb lattice system has four types of Dirac electrons corresponding to the spin and valley degrees of freedom. We consider a state that contains only one type of massless electrons and three types of massive ones, which we call the single Dirac-cone state. We analyze quantum Hall (QH) effects in this state. We make a detailed investigation of the Chern and spin-Chern numbers. We make clear the origin of unconventional QH effects discovered in graphene. We also show that the single Dirac-cone state may have arbitrary large spin-Chern numbers in magnetic field. Such a state will be generated in antiferromagnetic transition-metal oxides under electric field or in silicene with antiferromagnetic order under electric field.

Optical and energy-loss spectra of the antiferromagnetic transition metal oxides MnO, FeO, CoO, and NiO including quasiparticle and excitonic effects

We calculate the frequency-dependent dielectric function for the series of antiferromagnetic transition metal oxides (TMOs) from MnO to NiO using many-body perturbation theory. Quasiparticle, excitonic, and local-field effects are taken into account by solving the Bethe-Salpeter equation in the framework of collinear spin polarization. The optical spectra are based on electronic structures which have been obtained using density-functional theory with a hybrid functional containing screened exchange (HSE03) and a subsequent quasiparticle calculation in the $GW$ approximation to describe exchange and correlation effects adequately. These sophisticated quasiparticle band structures are mapped to electronic structures resulting from the computationally less expensive $\mathrm{GGA}+U+\ensuremath{\Delta}$ scheme that includes an on-site interaction $U$ and a scissors shift $\ensuremath{\Delta}$ and allows us to calculate the large number of electronic states that is necessary to construct the Bethe-Salpeter Hamiltonian. For an accurate description of the optical spectra, an appropriate treatment of the strong electron-hole attraction is mandatory to obtain agreement with the experimentally observed absorption-peak positions. The itinerant $s$ and $p$ states as well as the localized transition metal $3d$ states have to be considered on an equal footing. We find that a purely atomic picture is not suitable to understand the optical absorption spectra of the TMOs. Reflectivity spectra, absorption coefficients, and loss functions at vanishing momentum transfer are computed in a wide spectral range and discussed in light of the available experimental data.

Quasiparticle band structures of the antiferromagnetic transition-metal oxides MnO, FeO, CoO, and NiO

Quasiparticle (QP) band structures for antiferromagnetic MnO, FeO, CoO, and NiO are calculated within the GW approximation using wave functions and energy eigenvalues obtained from a generalized Kohn-Sham scheme with the nonlocal exchange-correlation functional HSE03 which accounts for screened exchange. This improved starting point for the exchange-correlation self-energy leads to an efficient solution of the QP equation and remedies the failure of the GW approach on top of (semi)local Kohn-Sham schemes for these materials. The resulting band gaps and densities of states (DOS) show good agreement with measurements for all four oxides. The fit of the results from $\text{GGA}+U$ calculations with an additional scissors shift $\ensuremath{\Delta}$ to these benchmark QP DOS allows to reproduce the QP DOS widely.

CaCrO3: An Anomalous Antiferromagnetic Metallic Oxide

Combining infrared reflectivity, transport, susceptibility, and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transition-metal oxide with a three-dimensional electronic structure. Local spin density approximation calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizable pd hybridization remarkably strong next-nearest-neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magnetoelastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.

DFT‐based solution to the gap problem of antiferromagnetic transition metal oxides and parent compounds of high‐Tc superconductors

AbstractCurrently used approximations to the exchange‐correlation potential in density functional theory (DFT) are known to fail in describing the properties of certain compounds, of which we discuss here only two examples: CoO and stoichiometric La2CuO4. Both materials are insulating and antiferromagnetic. A DFT calculation on CoO yields antiferromagnetic order, but Co‐associated magnetic moments that are by ∼1 μB smaller than the experimental value, and one obtains the electronic structure of a metal. The latter applies also to La2CuO4, and, in contrast to the experiment, the calculation does not even yield nonzero moments associated with the Cu atoms. We exploit the fact that approximate exchange‐correlation potentials lead necessarily to spin‐dependent densities that differ from the exact ones. We therefore derive modified Kohn–Sham (KS) equations in which the effective potentials depend on the exact spin densities rather than on the standard KS densities. If the latter are modified by adding small (spin‐up, spin‐down) portions that individually integrate to zero within the lattice unit cell and do not change the total charge density, the inconsistencies with the experiment can be removed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Three-body scattering theory for electronic excitations in transition metal oxides

Developing a local three-body scattering theory on a multi-orbital tight-binding model, we study the excitation spectra of the antiferromagnetic transition metal oxides, MnO, FeO, CoO and NiO. The multiplet structures of three-particle states are fully taken into account. It is shown that the self-energy correction drastically improves the excitation spectra, leading to the correct value of the band gap and the satellite structure in agreement with photoemission experiments. The quasiparticle dispersions are obtained in good agreement with the angle-resolved photoemission data of CoO and NiO.

Local approach to electronic excitations in MnO, FeO, CoO, and NiO.

The excitation spectra of the antiferromagnetic transition-metal oxides MnO, FeO, CoO, and NiO are studied by developing a local three-body scattering theory within a multiorbital tight-binding model. The self-energy corrections to the Hartree-Fock solutions are carried out by taking account of local three-body correlations. Thereby the multiplet structures of three-particle states are fully taken into account. It is shown that the self-energy correction improves drastically the excitation spectra for FeO, CoO, and NiO, leading to the correct value of the band gap and the satellite structure, in agreement with photoemission experiments. Quasiparticle dispersions are obtained in good agreement with the angle-resolved photoemission data of CoO and NiO. The self-energy correction for MnO is found to be quite small. \textcopyright{} 1996 The American Physical Society.