Magnetic structure of hexagonal YMnO3 and LuMnO3 from a microscopic point of view

TL;DR

This study investigates the microscopic magnetic structures of hexagonal YMnO3 and LuMnO3, revealing how lattice distortions and spin-orbit interactions influence their magnetic ground states and ferroelectric properties.

Contribution

The paper develops a low-energy electronic model from first-principles calculations that explains the different magnetic structures of YMnO3 and LuMnO3, highlighting the role of single-ion anisotropy and interlayer interactions.

Findings

Relativistic spin-orbit interactions lift magnetic degeneracy.

Different magnetic structures are due to lattice distortion effects.

Ferroelectric polarization depends on magnetic state and lattice distortions.

Abstract

The aim of this work is to unravel a basic microscopic picture behind complex magnetic properties of hexagonal manganites. For these purposes, we consider two characteristic compounds: YMnO3 and LuMnO3, which form different magnetic structures in the ground state. First, we establish an electronic low-energy model, which describes the behavior of the Mn 3d bands of YMnO3 and LuMnO3, and derive parameters of this model from the first-principles calculations. From the solution of this model, we conclude that, despite strong frustration effects in the hexagonal lattice, the relativistic spin-orbit interactions lift the degeneracy of the magnetic ground state so that the experimentally observed magnetic structures are successfully reproduced by the low-energy model. Then, we analyze this result in terms of interatomic magnetic interactions, which were computed using different approximations (starting from the model Hamiltonian as well as directly from the first-principles electronic structure calculations in the local-spin-density approximation). We argue that the main reason why YMnO3 and LuMnO3 tend to form different magnetic structures is related to the behavior of the single-ion anisotropy, which reflects the directional dependence of the lattice distortion: namely, the expansion and contraction of the Mn-trimers, which take place in YMnO3 and LuMnO3, respectively. On the other hand, the magnetic coupling between the haxagonal planes is controlled by the next-nearest-neighbor interactions, which are less sensitive to the direction of the trimerization. Finally, using the Berry-phase formalism, we evaluate the magnetic-state dependence of the ferroelectric polarization, and discuss potential applications of the latter in magnetoelectric switching phenomena.