Exchange bias measurement methodologies and the role of hysteresis loop asymmetry

The phenomenon of exchange bias refers to the observation of a hysteresis loop field shift in ferromagnetic-antiferromagnetic (AFM) compound structures [1] and in all-ferromagnetic bilayer systems [2]. The exchange bias effect is typically quantified by determining the sum of the coercive fields from a hysteresis loop. Such a two-point (TP) measurement is, however, unambiguous only for time reversal symmetric hysteresis loops [3]. To account for the loop asymmetry, frequently observed in experiments, we recently proposed an alternative characterization scheme, called the center of mass method (CM) [3]. In the present study, we correlate the differences between TP and CM methods and the hysteresis loop asymmetry, using measurement data obtained from the all-ferromagnetic bilayer system, which are supported by model calculation results. We find the loop asymmetry to be a reliable indicator for the ambiguity of the conventional TP method. We will also discuss the applicability of the CM method to conventional AFM structures. [1] A. Berkowitz, K. Takano, J. Magn. Magn. Mater. 200, 552 (1999). [2] A. Berger et. al., Appl. Phys. Lett. 85, 1571 (2004). [3] O. Hovorka et. al., Appl. Phys. Lett. 89, 142513 (2006).