Onset of magnetic order in strongly-correlated systems fromab initioelectronic structure calculations: application to transition metal oxides

Abstract

We describe an ab initio theory of finite temperature magnetism in strongly-correlated electron systems. The formalism is based on spin density functional theory, with a self-interaction corrected local spin density approximation (SIC-LSDA). The self-interaction correction is implemented locally, within the KKR multiplescattering method. Thermally induced magnetic fluctuations are treated using a meanfield 'disordered local moment' (DLM) approach and at no stage is there a fitting to an effective Heisenberg model. We apply the theory to the 3d transition metal oxides, where our calculations reproduce the experimental ordering tendencies, as well as the qualitative trend in ordering temperatures. We find a large insulating gap in the paramagnetic state which hardly changes with the onset of magnetic order.

Verification of Anderson Superexchange in MnO via Magnetic Pair Distribution Function Analysis andab initioTheory

“…We find the hS 0 · S n i's using an Onsager cavity field calculation [28] by solving the coupled integral equations, CðqÞ −1 ¼ ½1 − β(S ð2Þ ðqÞ − Λ) and Λ ¼ R S ð2Þ ðqÞCðqÞdq for the LFT of the magnetic correlation function, CðqÞThis pair of equations ensures that the sum rule hS 2 0 i ¼ 1 is met Furthermore, the real space S ð2Þ 0n quantities describe the magnetic exchange interactions, the J n 's, between the Mn spins on different shells. When analyzed in concert with the underlying electronic structure [17], an ab initio prediction of a "textbook" picture of Anderson superexchange between O 2p and Mn 3d orbitals emerges.…”

“…In this Letter, we utilize magnetic pair distribution function (mPDF) analysis [13], a recently developed method for investigating local magnetic structure, to measure directly the short-range magnetic correlations in the paramagnetic state of MnO from temperature-dependent neutron total scattering experiments. We use these results to evaluate competing theories of magnetic exchange in MnO, finding that the Anderson superexchange [1] obtained from recent DFT calculations with the self-interaction-corrected (SIC) local spin density approximation [14,15] in the "disordered local moment" (DLM) approach [16,17] describes the data exceptionally well with no need for an additional direct exchange contribution present in other models. In addition to resolving this longstanding question about MnO, this work highlights the mPDF technique and the DLM-DFT(+SIC) scheme as valuable tools for studying the magnetic properties of strongly correlated electron systems.…”

Role of nonlocal exchange in the electronic structure of correlated oxides

“…For these compounds it was recently shown that a calculation, based on the self-interaction-corrected (SIC) functional and taking explicitly into account the disordered local moments of the paramagnetic phase, was able to correctly describe the insulating phases. 87 Thus, a similar calculation would be suitable in the (disordered) paramagnetic phase also for the present perovskite compounds (for which a modified SIC implementation has been recently used for the magnetically ordered phases).…”

Néel Temperature of Antiferromagnets for Phase Transitions Driven by Spin-wave Interactions

“…It is noteworthy that through these fluctuations at the transitions, local magnetic moments remain non-negligible above the transition temperature. Of course, the remanence of local moments is guaranteed using either the previous [12][13][14] or Hartree-Fock 11 results or the mappings onto Heisenberg models for the temperature description. Furthermore, good experimental and theoretical agreement is found for the Néel temperatures of MnO, FeO, and CoO, except for NiO, for which better correlation schemes or dynamical correlations including temperature dependence seem to be needed.…”

“…Well known works 18 have traditionally dealt with spin fluctuations, and they have divided these into transverse and longitudinal susceptibilities. The transverse modes are easily included in oxides, 14 but this is not the case for the longitudinal ones. We feel that the λ constant can be ascribed to how the longitudinal spin fluctuations depart from average values at T = 0; in fact, they could end in spin canted solutions, unless locally and close to the transition.…”