Irreversibility and relaxation in the magnetization of Fe/Cr multilayer structures exhibiting a giant magnetoresistance (GMR) effect

Magnetization measurements on Fe/Cr multilayers, which have been prepared by ion beam deposition and which exhibit giant magnetoresistance (GMR) on the order of 20%, show an irreversible behavior. In sufficiently small external fields (H<500 Oe) and below an irreversibility temperature, T_irr(H), the zero-field-cooled (ZFC) and field-cooled (FC) curves do not coincide and hysteresis appears in the temperature-dependent magnetization. At fixed temperatures, magnetic relaxation with logarithmic time dependence is observed. The logarithmic relaxation rate shows a maximum at a temperature T_max that increases with decreasing magnetic field. At fixed fields and for T>T_max, the logarithmic rate decreases with increasing temperature but with a curvature that makes it difficult to accurately determine T_irr(H). These spin-glass-like effects will be correlated with GMR behavior and discussed with respect to the notion that the underlying behavior arises from frustrated interactions between ferromagnetic domains of adjacent Fe layers coupled antiferromagnetically across the Cr spacer layers.