# Temperature induced phase transition from cycloidal to collinear antiferromagnetism in multiferroic Bi$_{0.9}$Sm$_{0.1}$FeO$_3$ driven by $f$-$d$ induced magnetic anisotropy

## TL;DR

This study reveals that substituting samarium into BiFeO$_3$ induces a temperature-driven transition from cycloidal to collinear antiferromagnetic order, driven by $f$-$d$ exchange-induced magnetic anisotropy, impacting multiferroic device stability.

## Contribution

It demonstrates how $f$-$d$ exchange coupling from samarium substitution causes a phase transition in BiFeO$_3$, revealing the role of magnetic anisotropy in non-collinear magnetic structures.

## Key findings

- Cycloidal order increases in periodicity with Sm substitution.
- Below ~15 K, magnetic structure transitions to G-type antiferromagnet.
- Transition driven by $f$-$d$ exchange coupling inducing local anisotropy.

## Abstract

In multiferroic BiFeO$_3$ a cycloidal antiferromagnetic structure is coupled to a large electric polarization at room temperature, giving rise to magnetoelectric functionality that may be exploited in novel multiferroic-based devices. In this paper, we demonstrate that by substituting samarium for 10% of the bismuth ions the periodicity of the room temperature cycloid is increased, and by cooling below $\sim15$ K the magnetic structure tends towards a simple G-type antiferromagnet, which is fully established at 1.5 K. We show that this transition results from $f-d$ exchange coupling, which induces a local anisotropy on the iron magnetic moments that destroys the cycloidal order - a result of general significance regarding the stability of non-collinear magnetic structures in the presence of multiple magnetic sublattices.