Microscopic origin of room-temperature ferromagnetism in the double perovskite Sr2FeReO6

Abstract

The puzzling observation of room temperature ferromagnetism in double perovskites $({A}_{2}{\phantom{\rule{4pt}{0ex}}BB}^{\ensuremath{'}}{O}_{6})$, despite having the magnetic lattice of B ions diluted by nonmagnetic ${B}^{\ensuremath{'}}$ ions, have been examined for ${\mathrm{Sr}}_{2}{\mathrm{FeReO}}_{6}$. Ab initio spin spiral electronic structure calculations along various high symmetry directions in reciprocal space are used to determine the exchange interactions entering an extended Heisenberg model, which is then solved classically using Monte Carlo simulations to determine the ferromagnetic transition temperature ${\phantom{\rule{4pt}{0ex}}T}_{c}$. We find that one must consider on-site Coulomb interactions at the nonmagnetic Re sites $(U)$ in order to obtain a ${T}_{c}$ close to the experimental value. Analysis of the ab initio electronic structure as well as an appropriate model Hamiltonian trace the origin of enhancement in ${T}_{c}$ with $U$ to the enhanced exchange splitting that is introduced at these sites. This in turn destabilizes the antiferromagnetic exchange channels, thereby enhancing the ${T}_{c}$. The role of occupancy at the nonmagnetic sites is examined by contrasting with the case of ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$.