Dynamic Magnetic-Transformation-Induced Exchange Bias in [α-Fe2O3]0.1[FeTiO3]0.9

To date, for the known 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 [alpha-Fe2O3](0.1)[FeTiO3](0.9) (HI-9). Neutron powder diffraction and magnetic measurement confirm the following for HI-9: (i) two types of short-range antiferromagnetic (AFM) orderings coexist and serve as two pinning phases; (ii) 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 (H-C) and the exchange-bias field (H-E) simultaneously decrease to zero at 30 K; (ii) for a high cooling field (H-cool), H-E decreases logarithmically with increasing H-cool. 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 for the process whereby the FM Fe2+ align with the external-field direction coexist in HI-9. The Morin transition and FOMPT cause anomalous EB behaviors. This work may provide fresh ideas in the application of the EB effect.