Ptychographic nanoscale imaging of the magnetoelectric coupling in freestanding BiFeO$_3$

TL;DR

This study uses soft X-ray ptychography to achieve nanoscale imaging of ferroelectric and antiferromagnetic domains in freestanding BiFeO$_3$, revealing strong magnetoelectric coupling and domain structures at room temperature.

Contribution

It demonstrates the application of soft X-ray ptychography for simultaneous nanoscale imaging of magnetic and ferroelectric domains in multiferroic materials, providing new insights into their coupling mechanisms.

Findings

Resolved the antiferromagnetic spin cycloid with 64 nm period.

Visualized the coupling between magnetic and ferroelectric domains.

Observed in-plane propagation preference and 71° polarization changes.

Abstract

Understanding the magnetic and ferroelectric ordering of magnetoelectric multiferroic materials at the nanoscale necessitates a versatile imaging method with high spatial resolution. Here, soft X-ray ptychography is employed to simultaneously image the ferroelectric and antiferromagnetic domains in an 80 nm thin freestanding film of the room-temperature multiferroic BiFeO$_3$ (BFO). The antiferromagnetic spin cycloid of period 64 nm is resolved by reconstructing the corresponding resonant elastic X-ray scattering in real space and visualized together with mosaic-like ferroelectric domains in a linear dichroic contrast image at the Fe L$_3$ edge. The measurements reveal a near perfect coupling between the antiferromagnetic and ferroelectric ordering by which the propagation direction of the spin cycloid is locked orthogonally to the ferroelectric polarization. In addition, the study evinces both a preference for in-plane propagation of the spin cycloid and changes of the ferroelectric polarization by 71{\deg} between multiferroic domains in the epitaxial strain-free, freestanding BFO film. The results provide a direct visualization of the strong magnetoelectric coupling in BFO and of its fine multiferroic domain structure, emphasizing the potential of ptychographic imaging for the study of multiferroics and non-collinear magnetic materials with soft X-rays.