Ferromagnetic resonance evidence for superparamagnetism in a partially crystallized metallic glass

Ferromagnetic resonance (FMR) measurements have been performed on samples of the metallic glass ${\mathrm{Fe}}_{40}$ ${\mathrm{Ni}}_{40}$ ${\mathrm{P}}_{14}$ ${\mathrm{B}}_6$ (Metglas 2826) annealed at 350 °C for several periods of time. Two absorption lines are seen, one due to the amorphous phase and another to a crystalline phase. The apparent anisotropy field of the crystalline phase, as calculated from FMR data, increases with annealing time up to an annealing time of 100 h. The results are consistent with the assumption that particles of the crystalline phase smaller than about 300 Å exhibit superparamagnetic behavior. Calculations based on this assumption yield a value for the Avrami exponent, n=1.52, which is close to the value obtained from linewidth-broadening measurements and consistent with parabolic growth at a constant or slightly decreasing nucleation rate.