First-principles study of KCoF$_3$: Jahn-Teller effect, dynamical magnetic charges, magnetoelectric multipoles and antimagnetoelectricity

Abstract

We study from \textit{ab~initio} density functional theory calculations the structural and magnetic properties of the crystal KCoF$_3$. We found that the experimentally reported cubic to tetragonal phase transition is due to an electronic first-order Jahn-Teller effect from the R zone boundary point. We also obtain that the magnetic ground state is the G-type antiferromagnetic order, in agreement with the R-point Jahn-Teller distortion and that the magnetic moment of the Co atoms contains a strong orbital contribution ($m_L=0.95$ $\mu_B$ in the cubic phase and 0.55 $\mu_B$ in the tetragonal phase). Furthermore, we compute the dynamical magnetic effective charges and show that it is zero by symmetry for the Co and they can reach a value as large as 200 $10^{-2}\mu_{\text{B}}/\text{\r{A}}$ for the apical F anion. This large magnetic effective charge comes from the spin-orbit coupling (50\% of the response is from the orbital moment) contrary to the rare-earth manganites and ferrites with similar order of magnitude but originating from the exchange striction mechanism. The fact that the dynamical magnetic effective charges are non-zero also proves that the tetragonal phase of KCoF$_3$ is antimagnetoelectric with a large magnetic sublattice magnetoelectric response of 210 ps/m per spin-channel. We also discuss the generality of these magnetic effective charges.