Unquenched e(g)(1) orbital moment in the Mott-insulating antiferromagnet KOsO4

Abstract

Applying the correlated electronic structure method based on density functional theory plus the Hubbard U interaction, we have investigated the tetragonal scheelite structure Mott insulator KOsO4, whose e(g)(1) configuration should be affected only slightly by spin-orbit coupling (SOC). The method reproduces the observed antiferromagnetic Mott-insulating state, populating the Os d(z)(2) majority orbital. The quarter-filled e(g) manifold is characterized by a symmetry breaking due to the tetragonal structure, and the Os ion shows a crystal field splitting Delta(cf) = 1.7 eV from the t(2g) complex, which is relatively small considering the high formal oxidation state Os7+. The small magnetocrystalline anisotropy before including correlation (i.e., in the metallic state) is increased by more than an order of magnitude in theMott-insulating state, a result of a strong interplay between large SOC and a strong correlation. In contrast to conventional wisdom that the eg complex will not support orbital magnetism, we find that for the easy axis [100] direction the substantial Os orbital moment M-L approximate to -0.2 mu B compensates half of the Os spin moment M-S = 0.4 mu B. The origin of the orbital moment is analyzed and understood in terms of additional spin-orbital lowering of symmetry, and beyond that due to structural distortion, for magnetization along [100]. Further interpretation is assisted by analysis of the spin density and theWannier function with SOC included.