Theory of Magnetic-Field-Induced Polarization Flop in Spin-Spiral Multiferroics

TL;DR

This paper presents a theoretical study of how magnetic fields induce a polarization flop in spin-spiral multiferroics, revealing a deterministic mechanism driven by magnetic torques and uncovering complex spin structures.

Contribution

It introduces a microscopic model explaining the polarization flop mechanism and clarifies the deterministic nature of polarization switching in multiferroics.

Findings

Polarization flop direction is governed by magnetic torques, not probabilistic.

The P-flop enables controlled multiferroic domain switching.

The magnetic structure in P||a phase under H||b is a conical spin structure.

Abstract

The magnetic-field-induced 90-degree flop of ferroelectric polarization P in a spin-spiral multiferroic material TbMnO3 is theoretically studied based on a microscopic spin model. We find that the direction of the P-flop or the choice of +Pa or -Pa after the flop is governed by magnetic torques produced by the applied magnetic field H acting on the Mn spins and thus is selected in a deterministic way, in contradistinction to the naively anticipated probabilistic flop. This mechanism resolves a puzzle of the previously reported memory effect in the P direction depending on the history of the magnetic-field sweep, and enables controlled switching of multiferroic domains by externally applied magnetic fields. Our Monte-Carlo analysis also uncovers that the magnetic structure in the P||a phase under H||b is not a so-far anticipated simple ab-plane spin cycloid but a conical spin structure.