Ising magnetism and ferroelectricity in Ca$_3$CoMnO$_6$

TL;DR

This study combines ab initio calculations and spectroscopy to elucidate the origin of Ising magnetism and ferroelectricity in Ca$_3$CoMnO$_6$, revealing the roles of high-spin states, orbital moments, and exchange striction.

Contribution

It provides a detailed atomic-level understanding of how specific electronic states and structural instabilities lead to magnetism and ferroelectricity in Ca$_3$CoMnO$_6$, a multiferroic material.

Findings

Ca$_3$CoMnO$_6$ has high-spin Co$^{2+}$ and Mn$^{4+}$ ions in specific sites.

Co$^{2+}$ exhibits a large orbital moment of 1.7 μ_B, driving Ising magnetism.

Exchange striction causes structural inequivalence, leading to ferroelectricity.

Abstract

The origin of both the Ising chain magnetism and ferroelectricity in Ca$_3$CoMnO$_6$ is studied by $ab$ $initio$ electronic structure calculations and x-ray absorption spectroscopy. We find that Ca$_3$CoMnO$_6$ has the alternate trigonal prismatic Co$^{2+}$ and octahedral Mn$^{4+}$ sites in the spin chain. Both the Co$^{2+}$ and Mn$^{4+}$ are in the high spin state. In addition, the Co$^{2+}$ has a huge orbital moment of 1.7 $\mu_B$ which is responsible for the significant Ising magnetism. The centrosymmetric crystal structure known so far is calculated to be unstable with respect to exchange striction in the experimentally observed $\uparrow\uparrow\downarrow\downarrow$ antiferromagnetic structure for the Ising chain. The calculated inequivalence of the Co-Mn distances accounts for the ferroelectricity.