Dynamic magnetic-transformation-induced exchange bias in (Fe2O3)0.1-(FeTiO3)0.9

TL;DR

This study reveals a novel exchange bias mechanism in a special Fe2O3-FeTiO3 system where the pinned phase can dynamically transform into the pinning phase under an external magnetic field, leading to unique magnetic behaviors.

Contribution

It demonstrates the coexistence of two pinning phases and a dynamically produced pinned phase, with phase transformations causing anomalous exchange bias behaviors.

Findings

Coexistence of two types of short-range antiferromagnetic order.

The pinned phase can be dynamically generated from the pinning phase.

Exchange bias decreases with temperature and high cooling field, showing unique behaviors.

Abstract

Up to now, for the conventional exchange bias (EB) systems there has been one pinning phase and one pinned phase, and the pinning and pinned phases are inherent to the material and do not mutually transform into each other. Interestingly, we show here that EB is observed in a special system (Fe2O3)0.1(FeTiO3)0.9 (HI9) different from the conventional EB system. Neutron powder diffraction and magnetic measurement confirm that for HI9: i) two types of short range antiferromagnetic ordering coexist, ii) there are two pinning phases and one pinned phase, iii) the pinned phase is not intrinsic to the structure but can be dynamically produced from the pinning phase with the help of an external magnetic field. Consequently, two anomalous EB behaviors are observed: i) both the coercivity (HC) and the exchange bias field (HE) simultaneously decrease to zero at 30 K, ii) for a high cooling field (Hcool) HE decreases logarithmically with increasing Hcool. Using Arrott plots it is confirmed that the first order magnetic phase transformation (FOMPT) from the AFM Fe2+ to ferromagnetic (FM) Fe2+ and the second order magnetic phase transformation (SOMPT) for the process whereby the FM Fe2+ aligns with the external field direction coexist in HI-9. The Morin transition and FOMPT cause the anomalous EB behaviors. This work may provide fresh ideas for research into EB behavior.