Theory of orbital magnetic quadrupole moment and magnetoelectric susceptibility

Abstract

We derive a quantum-mechanical formula of the orbital magnetic quadrupole moment (MQM) in periodic systems by using the gauge-covariant gradient expansion. This formula is valid for insulators and metals at zero and finite temperature. We also prove a direct relation between the MQM and magnetoelectric (ME) susceptibility for insulators at zero temperature. It indicates that the MQM is a microscopic origin of the ME effect. Using the formula, we quantitatively estimate these quantities for room-temperature antiferromagnetic semiconductors BaMn${}_2$As${}_2$ and CeMn${}_2$Ge${}_{2-x}$Si${}_x$. We find that the orbital contribution to the ME susceptibility is comparable with or even dominant over the spin contribution.

6 pages, 1 figure, Supplemental Material is available in the source