Interfacial Ferromagnetic Coupling and Positive Spontaneous Exchange Bias in SrFeO3-x/La0.7Sr0.3MnO3 Bilayer.

Negative exchange bias (NEB) is usually discovered in ferromagnetic (FM)/antiferromagnetic(AFM) heterostructures after a field cooling (FC) process. Relatively, positive exchange bias (PEB) is a rarely observed phenomenon. So far, almost all of models for PEB whether undergo FC or zero-field cooling (ZFC) treatment have been explained by an interaction of strong AFM coupling at the interface. In this work, by selecting a special material of SrFeO3-x as AFM layer, couple with FM-La0.7Sr0.3MnO3 (LSMO), we obtain a novel PEB effect of the bilayer after ZFC measurement, of which the shift directions are unfixable and dependent on initial magnetization direction. According an transient magnetic field to control the remanence (Mr) direction of LSMO at room temperature (RT) and then cooling below the TN of SrFeO3-x without any magnetic field disturbance, the shift direction can be locked only toward the transient magnetic field. Combine with experimental results and first-principles calculations, we propose that the above phenomena are explained as field-induced AFM phase of SrFeO3-x transform into FM phase at a FM coupling bilayer interface. Thus, our finding may provide a new approach to realize and tune the zero-field cooling PEB with FM coupling heterostructures.