Finger-Printing Exchange Bias

Kai Liu, H. G. Katzgraber, C. R. Pike, L. Zhao, R. T. Scalettar, K. L. Verosub, G. T. Zimanyi, and I. K. Schuller
University of California, Davis, US

Keywords: exchange bias, first order reversal curve, coercivity, exchange field

Abstract:
Details of the magnetization reversal processes have been ?finger-printed? by a first order reversal curve (FORC) technique in an exchange biased Fe (27nm) / FeF2 (20nm) thin film. After measuring a family of 100 reversal curves along the major hysteresis loop (Fig. 1a), we transform the second order mixed derivative of the magnetization relative to the reversal field and the applied field to map out the distribution of coercivity and exchange field in a FORC diagram (Fig. 1b). At 100K, above the FeF2 Néel temperature of 80K, the FORC diagram shows a narrow distribution of coercivity with zero bias, centered at the value obtained from the major loop. However, there is a small tail in the FORC diagram due to regions in the sample with higher coercivity. This corresponds to a tail-like feature in the major loop near saturation. After an exchange bias is established, at 50 K, the FORC diagram shows a distribution of coercivity and exchange field that are consistent with the major-loop values. Surprisingly, the tail of higher coercivity regions bends towards lower exchange fields, contrary to the belief that the exchange field measured in a major loop is the lower limit of the exchange field across the sample. Comparisons of the FORC diagrams generated from the decreasing- and increasing-field branches of the major loop will also be presented.

NSTI Nanotech 2003 Conference Technical Program Abstract