Temperature-dependent impedance anomaly, microwave GMR and exchange-coupling in thin Ni/Cu multilayered films

We have observed a temperature-dependent impedance anomaly in Ni-Cu multilayer (ML) ultra-thin films grown epitaxially on Si(111) substrate. Here we report temperature (T) dependent surface resistance (Z(real)) anomaly as a function of de magnetic field (H); around T = T* (where T* is a characteristic temperature, H similar to 1.35 kOe for sample S1) Z(real) saturates at a constant value, for T > T* it exhibits a negative slope, but for T < T* this slope is positive. We report the Fu st measurements of microwave giant magneto resistance (mu W-GMR) and exchange coupling (J) on these MLs from surface impedance data of swept-frequency ferromagnetic resonance (FMR) experiments. The interlayer diffusion length was observed to increase with the annealing temperature of the sample, which suggests a spin-glass-like behaviour in Ni/Cu ML films.