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Register a new userOn the Extraction of Exchange Parameters in Transition-Metal Perovskites
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The extraction of exchange parameters from measured spin-wave dispersion relations has severe limitations particularly for magnetic compounds such as the transition-metal perovskites, where the nearest-neighbor exchange parameter usually dominates the couplings between the further-distant-neighbor spins. Very precise exchange parameters beyond the nearest-neighbor spins can be obtained by neutron spectroscopic investigations of the magnetic excitation spectra of isolated multimers in magnetically diluted compounds. This is exemplified for manganese trimers in the mixed three- and two-dimensional perovskite compounds KMnxZn1-xF3 and K2MnxZn1-xF4, respectively. It is shown that the small exchange couplings between the second-nearest and the third-nearest neighboring spins can be determined unambiguously and with equal precision as the dominating nearest-neighbor exchange coupling.
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High-temperature thermopower is interpreted as entropy that a carrier carries. Owing to spin and orbital degrees of freedom, a transition metal perovskite exhibits large thermopower at high temperatures. In this paper, we revisit the high-temperature thermopower in the perovskites to shed light on the degrees of freedom. Thus, we theoretically derive an expression of thermopower in one-dimensional octahedral-MX6-clusters chain using linear-response theory and electronic structure calculation of the chain based on the tight-binding approximation. The derived expression of the thermopower is consistent with the extended Heikes formula and well reproduced experimental data of several perovskite oxides at high temperatures. In this expression, a degeneracy of many electron states in octahedral ligand field (which is characterized by multiplet term) appears instead of the spin and orbital degeneracies. Complementarity in between our expression and the extended Heikes formula is discussed.
We construct an effective Hamiltonian for the motion of electrons among the transition metal ions of ordered double perovskites like Sr2FeMoO6. in which strong intra-atomic Coulomb repulsion U is present in only one of the inequivalent transition metal sites. Using a slave-boson formalism, we construct a phase diagram which describes a charge transfer transition between insulating and metallic behavior as the parameters of the model are changed. The parameters for Sr2FeMoO6 are estimated from first-principles calculations and a transition to the insulating state with negative pressure is obtained.
Epitaxial strain offers an effective route to tune the physical parameters in transition metal oxides. So far, most studies have focused on the effects of strain on the bandwidths and crystal field splitting, but recent experimental and theoretical works have shown that also the effective Coulomb interaction changes upon structural modifications. This effect is expected to be of paramount importance in current material engineering studies based on epitaxy-based material synthesization. Here, we perform constrained random phase approximation calculations for prototypical oxides with a different occupation of the d shell, LaTiO3 (d1), LaVO3 (d2), and LaCrO3 (d3), and systematically study the evolution of the effective Coulomb interactions (Hubbard U and Hunds J) when applying epitaxial strain. Surprisingly, we find that the response upon strain is strongly dependent on the material. For LaTiO3, the interaction parameters are determined by the degree of localization of the orbitals, and grow with increasing tensile strain. In contrast, LaCrO3 shows the opposite trends: the interactions parameters shrink upon tensile strain. This is caused by the enhanced screening due to the larger electron filling. LaVO3 shows an intermediate behavior.
Magnetism of transition metal (TM) oxides is usually described in terms of the Heisenberg model, with orientation-independent interactions between the spins. However, the applicability of such a model is not fully justified for TM oxides because spin polarization of oxygen is usually ignored. In the conventional model based on the Anderson principle, oxygen effects are considered as a property of the TM ion and only TM interactions are relevant. Here, we perform a systematic comparison between two approaches for spin polarization on oxygen in typical TM oxides. To this end, we calculate the exchange interactions in NiO, MnO, and hematite (Fe2O3) for different magnetic configurations using the magnetic force theorem. We consider the full spin Hamiltonian including oxygen sites, and also derive an effective model where the spin polarization on oxygen renormalizes the exchange interactions between TM sites. Surprisingly, the exchange interactions in NiO depend on the magnetic state if spin polarization on oxygen is neglected, resulting in non-Heisenberg behavior. In contrast, the inclusion of spin polarization in NiO makes the Heisenberg model more applicable. Just the opposite, MnO behaves as a Heisenberg magnet when oxygen spin polarization is neglected, but shows strong non-Heisenberg effects when spin polarization on oxygen is included. In hematite, both models result in non-Heisenberg behavior. General applicability of the magnetic force theorem as well as the Heisenberg model to TM oxides is discussed.
The interplay of symmetry and quenched disorder leads to some of the most fundamentally interesting and technologically important properties of correlated materials. It also poses the most vexing of theoretical challenges. Nowhere is this more apparent than in the study of spin glasses. A spin glass is characterized by an ergodic landscape of states - an innumerable number of possibilities that are only weakly distinguished energetically, if at all. We show in the material Fe$_x$NbS$_2$, this landscape of states can be biased by coexisitng antiferromagnetic order. This process leads to a phenomenon of broad technological importance: giant, tunable exchange bias. We observe exchange biases that exceed those of conventional materials by more than two orders of magnitude. This work illustrates a novel route to giant exchange bias by leveraging the interplay of frustration and disorder in exotic materials.
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