Most spin-1/2 transition-metal ions do have single ion anisotropy

TL;DR

This study demonstrates that spin-1/2 transition-metal ions can exhibit single ion anisotropy due to spin-orbit coupling, challenging the common belief that such ions lack magnetic anisotropy.

Contribution

The paper provides a theoretical analysis showing that spin-orbit coupling induces anisotropy in spin-1/2 ions, supported by density functional theory and perturbation theory calculations.

Findings

Spin-1/2 Cu2+ ions exhibit easy-plane anisotropy due to spin-orbit coupling.

Magnetic dipole-dipole and exchange interactions do not cause the observed anisotropy.

Predicted in-plane anisotropy for Cu2+ ions in Bi2CuO4 and Li2CuO2.

Abstract

The cause for the preferred spin orientation in magnetic systems containing spin-1/2 transition-metal ions was explored by studying the origin of the easy-plane anisotropy of the spin-1/2 Cu2+ ions in CuCl2.2H2O, LiCuVO4, CuCl2 and CuBr2 on the basis of density functional theory and magnetic dipole-dipole energy calculations as well as a perturbation theory treatment of the spin-orbit coupling. We find that the spin orientation observed for these spin-1/2 ions is not caused by their anisotropic spin exchange interactions, nor by their magnetic dipole-dipole interactions, but by the spin-orbit coupling associated with their crystal-field split d-states. Our study also predicts in-plane anisotropy for the Cu2+ ions of Bi2CuO4 and Li2CuO2. The results of our investigations dispel the mistaken belief that magnetic systems with spin-1/2 ions have no magnetic anisotropy induced by spin-orbit coupling.