Nature of magnetism in Ca$_3$Co$_2$O$_6$

TL;DR

This study uses LSDA+U calculations to reveal that Ca$_3$Co$_2$O$_6$ is a Mott insulator with highly anisotropic Ising-like magnetism, giant orbital moments, and significant oxygen hole contributions.

Contribution

It provides a detailed electronic and magnetic characterization of Ca$_3$Co$_2$O$_6$, highlighting the role of spin-orbit coupling and oxygen holes in its magnetic properties.

Findings

Ca$_3$Co$_2$O$_6$ is a Mott insulator, not a ferromagnetic half-metal.

The trigonal Co ions have a large orbital moment due to spin-orbit coupling.

The material exhibits large magnetocrystalline anisotropy (~70 meV).

Abstract

We find using LSDA+U band structure calculations that the novel one-dimensional cobaltate Ca$_3$Co$_2$O$_6$ is not a ferromagnetic half-metal but a Mott insulator. Both the octahedral and the trigonal Co ions are formally trivalent, with the octahedral being in the low-spin and the trigonal in the high-spin state. The inclusion of the spin-orbit coupling leads to the occupation of the minority-spin $d_{2}$ orbital for the unusually coordinated trigonal Co, producing a giant orbital moment (1.57 $\mu_{B}$). It also results in an anomalously large magnetocrystalline anisotropy (of order 70 meV), elucidating why the magnetism is highly Ising-like. The role of the oxygen holes, carrying an induced magnetic moment of 0.13 $\mu_{B}$ per oxygen, for the exchange interactions is discussed.