Cole-cole analysis of the superspin glass system Co80Fe20/Al2O3

Ac susceptibility measurements were performed on discontinuous magnetic multilayers [Co80Fe20(t)/Al2O3(3 nm)](10), t = 0.9and 1.0 nm, by Superconducting Quantum Interference Device (SQUID) magnetometry. The CoFe forms nearly spherical ferromagnetic single-domain nanoparticles in the diamagnetic Al2O3 matrix. Due to dipolar interactions and random distribution of anisotropy axes the system exhibits a spin-glass phase. We measured the ac susceptibility as a function of temperature 20 less than or equal to T less than or equal to 100 K at different dc fields and as a function of frequency 0.01 less than or equal to f less than or equal to 1000 Hz. The spectral data were successfully analyzed by use of the phenomenological Cole-Cole model, giving a power-law temperature dependence of the characteristic relaxation time T, and a high value for the polydispersivity exponent, alpha approximate to 0.8, typical of spin glass systems.