Exchange interactions in transition metal oxides: the role of oxygen spin polarization

Abstract

Abstract. 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 (Fe 2 O 3 ) 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.

Electronic structure, magnetism, and exchange integrals in transition-metal oxides: Role of the spin polarization of the functional in DFT+ U calculations

“…In summary, the set of inter-site J ij 's obtained by means of MFT provides good estimates of ordering temperatures and magnon dispersions, if combined with atomistic spin dynamics simulations [23][24][25][26]. However, our results strongly suggest that if one extracts the J ij 's from the DFT+U total energies, the use of spin nonpolarized functional (e.g.…”

“…Moreover, the MFT-derived J ij 's depend on the magnetic ground state from which they are extracted. It has been argued [25] that correlated oxides with relatively large U values can be expected to be prototypical Heisenberg magnets characterized by configuration-independent exchange interaction although at large U charge transfer processes involving the ligand ions and controlled by the band gap become more important [32] so ratio of the bandwidth to the charge transfer gap may be a more relevant criterion. In several works, the configurationdependence of the MFT-derived J ij is argued to be a signature of higher-order magnetic interactions [33,34], in line with the work of Fedorova et al [35], highlighting the importance of biquadratic interactions in manganites.…”

Effective Heisenberg Model and Exchange Interaction for Strongly Correlated Systems

“…As it was already noted in [51], the difference between the lattice (40) and dual (45) polarization functions is that the first one is irreducible with respect to the (local and nonlocal parts of) EDMFT susceptibility X E , while the dual one is irreducible only with respect to the bare dual susceptibility, which is identically equal to the nonlocal part of X E . Expression for the spin susceptibility (43) can be rewritten in the more convenient way.…”

Identification of spin wave resonances and crystal field levels in simple chromites RCrO3 (R = Pr, Sm, Er) at low temperatures in the THz spectral region

“…It contrasts the standard approach that by handling only transition metal interactions the oxygen effects are considered a property of the transition metal. 23 Thus, the spin Hamiltonian for our compounds may next be written taking into account a single isotropic exchange constant coupling nearest neighbor transition metal spins, a single antisymmetric (canting) exchange constant, and two anisotropy constants. 24,25,26 .This reduces to…”

“…For fields above 3 T, and higher, the AFM then hardens while moving toward merging and disappearing into a growing background that itself, field induced, grows continuously from the start. The ferromagnetic FM mode at ~26 cm -1 and 5 K although remains well defined also undergoes broadening under the applied field implying 23 some kind of delocalization until merging into the continuum. We understand this last global feature as likely originated in out of equilibrium magnetic and electric dipoles (domain walls?)…”