Unquenched $e_g^1$ orbital moment in the Mott insulating antiferromagnet KOsO4

TL;DR

This study uses advanced density functional theory with Hubbard U to reveal a significant orbital magnetic moment in the Mott insulator KOsO4, challenging conventional expectations about $e_g$ orbital magnetism.

Contribution

It demonstrates the emergence of a substantial Os orbital moment in a $e_g^1$ Mott insulator due to strong spin-orbit coupling and correlations, supported by detailed electronic structure analysis.

Findings

The Os orbital moment is approximately -0.2 μ_B, compensating half of the spin moment.

Strong interplay between SOC and correlation enhances magnetocrystalline anisotropy.

The $e_g$ orbital supports orbital magnetism contrary to conventional wisdom.

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 KOsO$_4$, whose $e_g^1$ configuration should be affected only slightly by spin-orbit couping (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 Os$^{7+}$. The small magnetocrystalline anisotropy before including correlation (i.e., in the metallic state) is increased by more than an order of magnitude in the Mott-insulating state, a result of a strong interplay between large SOC and a strong correlation. In contrast to conventional wisdom that the $e_g$ complex will not support orbital magnetism, we find that for the easy axis [100] direction the substantial Os orbital moment $M_L\approx-0.2 \mu_B$ 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 the Wannier function with SOC included.