Loop Bifurcation and Magnetization Rotation in Exchange Biased Ni/FeF2

TL;DR

This study investigates exchange biased Ni/FeF2 films, revealing loop bifurcation in epitaxial films and magnetization rotation behaviors linked to the crystal structure, using vector magnetometry.

Contribution

It demonstrates how epitaxial FeF2 causes loop bifurcation and magnetization rotation, highlighting the effects of crystal structure on exchange bias phenomena.

Findings

Loop bifurcation occurs with increasing cooling field in epitaxial FeF2.

Over 85% of Ni reverses magnetization via rotation.

Distinct behaviors between epitaxial and twinned FeF2 films.

Abstract

Exchange biased Ni/ FeF2 films have been investigated using vector coil vibrating sample magnetometry as a function of the cooling field strength H_FC. In films with epitaxial FeF2, a loop bifurcation develops with increasing H_FC as it divides into two sub-loops shifted oppositely from zero field by the same amount. The positively biased sub-loop grows in size with H_FC until only a single positively shifted loop is found. Throughout this process, the negative/positive (sub)loop shift has maintained the same discrete value. This is in sharp contrast to films with twinned FeF2 where the exchange field gradually changes with increasing H_FC. The transverse magnetization shows clear correlations with the longitudinal sub-loops. Interestingly, over 85% of the Ni reverses its magnetization by rotation, either in one step or through two successive rotations. These results are due to the single crystal nature of the antiferromagnetic FeF2, which breaks down into two opposite regions of large domains.