Interfacial Ferromagnetic Coupling and Positive Spontaneous Exchange Bias in SrFeO_3-/La_0.7Sr_0.3MnO₃ Bilayers

Negative exchange bias 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 the models for PEB whether undergoing 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 SrFeO_3- as the AFM layer, coupled with FM-La_0.7Sr_0.3MnO₃ (LSMO), we obtain a novel PEB effect of the bilayer after ZFC measurement, of which the shift directions are unfixable and dependent on the initial magnetization direction. Based on a transient magnetic field to control the remanence (M_r) direction of LSMO at room temperature and then cooling below the T_N of SrFeO_3- without any magnetic field disturbance, the shift direction can be locked only toward the transient magnetic field. Combined with experimental results and first-principles calculations, we propose that the above phenomena are explained as the field-induced AFM phase of SrFeO_3- transforming into the FM phase at an 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.