Onset of Magnetic Order in Strongly-Correlated Systems from First Principles: Application to Transition Metal Oxides

Understanding the interplay between electron charge and spin degrees of freedom is crucial in materials of possible technological interest. This requires a full quantum description of the electron spin. For low temperatures, a magnetic material has an electronic structure which has a fixed spin polarization, e.g. a uniform spin polarization for a ferromagnet or fixed sublattice spin polarizations for an antiferromagnet. With increasing temperature, spin fluctuations are induced, eventually destroying the longrange magnetic order and hence the overall spin polarization. These collective electron modes interact as the temperature increases, depending upon and affecting the underlying electronic structure. The implications can be explored by invoking a timescale separation between the fast electronic motions and the much slower spin fluctuations. For intermediate times, τ, the spin orientations of electrons leaving an atomic site are sufficiently correlated with those arriving that the magnetization, averaged over τ, is nonzero. `Localmoments' are thus established. Although set up by the collective motion of the interacting electrons, these can be described as classical spin-like variables, provided the temperature is not so low that the dynamics of the spin fluctuations become important. For most finite temperatures, the magnetic properties of a system are determined by ensemble averages over the static orientational configurations of the local moments. In the so-called disordered local moment (DLM) approach, these averages are carried out using a mean-field technique. A first-principles implementation of DLM, using density functional theory (DFT) in the local spin density (LSD) approximation and a multiple scattering effective medium theory, to handle the local moment disorder, has proved extremely successful at describing the magnetic properties of many transition metal (TM) systems, with fully itinerant electrons. However, for the TM oxides (TMOs), containing localized 3d electron states, LSD fails and one has to employ the self-interaction corrected local spin density (SIC-LSD) approximation to account for strong correlations and reproduce the ground state properties.