Memory effects in superparamagnetic and nanocrystalline Fe50Ni50 alloy

Nanocrystalline Fe50Ni50 alloy is prepared using the sol-gel route in an amorphous silica host at different volume fractions (phi). The average particle size is 8.9 nm having log-normal distribution = 0.19, which is confirmed by transmission electron microscopy for phi = 1%. The blocking temperature (T-B) is 30 K, as observed in dc magnetization. The frequency-dependent peak-shift in ac susceptibility satisfies Neel-Arrhenius formalism with more reasonable physical parameters than Vogel-Fulcher and dynamical scaling formalisms. Analysis of the relaxation dynamics below T-B points toward weak interparticle interaction, signifying superparamagnetic behavior. Relaxation dynamics following stretched exponential function implies that it is involved with the activation against multiple anisotropy barriers, which is correlated to the distribution of particle size. Memory effects are observed in different experimental protocols below T-B, which has been discussed pertaining to superparamagnetic behavior. (C) 2012 American Institute of Physics. [doi :10.1063/1.3684624]