Pinning, Rotation, and Metastability of BiFeO$_3$ Cycloidal Domains in a Magnetic Field

TL;DR

This paper investigates the behavior of cycloidal domains in BiFeO3 under magnetic fields, revealing how anisotropy and impurities influence domain pinning, rotation, and metastability, aligning theoretical models with recent experimental observations.

Contribution

It introduces the role of three-fold anisotropy K3 in domain wavevector restrictions and explains domain behavior and metastability under magnetic fields, improving understanding of BiFeO3's magnetic properties.

Findings

Domain wavevectors are restricted along the three-fold axis in zero field.

Metastability of certain domains persists below 7 T when aligned with the field.

Impurities cause domain wavevector rotation above 5.6 T, explaining experimental results.

Abstract

Earlier models for the room-temperature multiferroic BiFeO3 implicitly assumed that a very strong anisotropy restricts the domain wavevectors q to the three-fold symmetric axis normal to the static polarization P. However, recent measurements demonstrate that the domain wavevectors rotate so that q rotates within the hexagonal plane normal to P away from the field orientation m. We show that the previously neglected three-fold anisotropy K3 restricts the wavevectors to lie along the three-fold axis in zero field. For m along a three-fold axis, the domain with q parallel to m remains metastable below Bc1 = 7 T. Due to the pinning of domains by non-magnetic impurities, the wavevectors of the other two domains start to rotate away from m above 5.6 T, when the component of the torque t = M x B along P exceeds a threshold value tpin. Since t =0 when m is perpendicular to q, the wavevectors of those domains never become completely perpendicular to the magnetic field. Our results explain recent measurements of the critical field as a function of field orientation, small-angle neutron scattering measurements of the wavevectors, as well as spectroscopic measurements with m along a three-fold axis.