Coercivity ratio and anisotropy distribution in chemically synthesized L1₀FePt nanoparticle systems

The ratio of the remanent coercivity to hysteresis coercivity, H_CR/H_C (the “coercivity ratio”), has been calculated as a function of the width of the anisotropy distribution for uniaxial, single-domain particles with randomly oriented easy axis distributions. Both a Stoner-Wohlfarth mean-field model and a Monte Carlo model that explicitly accounted for interparticle interactions and thermal effects were used. At low temperature and in the absence of interactions, the two models are in agreement and give a linear increase in the coercivity ratio with the anisotropy distribution width. In the Stoner-Wohlfarth model, which did not include thermal effects, a positive mean-field interaction decreases the ratio and a negative interaction increases the ratio. The coercivity ratio was shown to increase with decreasing saturating field used in measuring the remanent curve and the hysteresis loop. In the Monte Carlo model, exchange interactions reduce the coercivity ratio and magnetostatic interactions increase the coercivity ratio if the mean anisotropy constant is sufficiently small. The coercivity ratio increases with temperature and the temperature dependence is larger for smaller mean anisotropy constant. Measurements are presented for partially ordered L1₀ nanoparticle arrays that are in qualitative agreement with the calculations.