Coercivity of magnetic nanoparticles: a stochastic model

Abstract

The variation in magnetic properties with particle size for nanomagnetic particles at 300 Kand 10 K has been explained with the help of nonequilibrium statistical mechanics. At roomtemperature a maximum in the coercivity curve is observed at a critical diameter,dc, so that two different regimes can be distinguished. This clearly indicates two differentmechanisms of magnetization reversal as a function of particle size. Using Kramer’streatment, the increase in coercivity with an increase in particle size at room temperaturein the single-domain region has been clarified. Beyond a certain critical particle size, amulti-domain region is formed. Now we invoke supersymmetric quantum mechanics (SUSYQM) for these multi-domain region to explain the decrease in coercivity with an increase inparticle size. The decrease in coercivity with an increase in particle size at very lowtemperature (10 K) is also explained with the help of our two-state model by invoking theconcept of effective anisotropy. The variation in the saturation magnetizationMs and the remanence-to-saturation magnetization ratio, , with particle size are discussed in detail. The above results underscore the fact that atroom temperature thermal effects dominate, whereas at low temperature (10 K) surfaceeffects govern the magnetization reversal process. In this paper the effect of themagneto-crystalline anisotropic potential on the magnetization of non-interacting uniaxaialnanomagnetic particles is discussed in detail.