Extraction of Exchange-Bias Field from Domain Wall Creep Dynamics in Ferromagnet/Antiferromagnet Thin Film Heterostructures

Domain wall creep in ultrathin ferromagnets is generally a well understood phenomena that has been observed in a wide range of systems. Although many technologically important magnetic systems utilize or host different types of interlayer coupling such as exchange bias (antiferromagnet/ferromagnet) or RKKY (ferromagnet/ferromagnet) - where an “effective” field which has its origin in the interlayer coupling is very important in dictating the overall behavior of the system - domain wall creep in such systems has not been studied as much. In this work, we use a simple modification to the typically used creep equation in order to take into account the presence of the "effective" field due to the antiferromagnet in an antiferromagnet/ferromagnet heterostructure. The utility of this method is demonstrated in Pt/Co/Pt/Co_1-xNi_xO heterostructures, by accurately extracting the exchange-bias field using MOKE microscopy at a local (10-100 microns) level. In systems where magnetization reversal occurs via domain nucleation and growth, we show that focused laser MOKE can sometimes provide erroneous exchange-bias values because of the stochastic nature of the domain nucleation process, and that a reliable measurement can be performed using the method discussed. Furthermore, we show that by combining focused laser MOKE and domain wall creep based experiments, the exchange-bias experienced by domain nucleation sites and the exchange-bias experienced by the propagating domain wall away from the nucleation sites can be separately measured and be significantly different from each other.