Modelling the influence of interphase and magnetostatic interaction on the magnetic characteristics of core/shell nanoparticles

Abstract

In this work, we present the results of a theoretical study of the dependence of the hysteresis characteristics of core/shell nanoparticles on their size, interphase exchange and interparticle magnetic interactions. By modelling the influence of the size effect on the saturation magnetisation Ms, the remanent magnetisation Mrs and the coercive field Hc of nanoparticles (Fe/Fe3O4andCo/Au), in which there is a difference in the magnetostatic interaction between the core and shell of the systems, we show that an increase in the size of the nanoparticles leads to an increase in Ms and Mrs,and non-monotonic (Co/Au) and monotonic (Fe/Fe3O4) changes in Hc. The increased hysteresis characteristics of the nanoparticles are due to both a decrease in the proportion in the superparamagnetic state and the peculiarities of the reversal of magnetisation. The interphase exchange interaction, regardless of its sign, leads to an increase in the coercive force and remanent saturation magnetisation of Fe/Fe3O4 nanoparticles, and has only very weak effects on the hysteresis characteristics of Co/Au nanoparticles. It was found that an increase in the interparticle magnetic interaction (volume concentration) of core/shell nanoparticles, and consequently their chaotisation, leads to a decrease in the coercive force and remanent saturation magnetisation. Moreover, an increase in the size of the nanoparticles leads to a higher rate of decrease in the hysteresis characteristics. A calculation of the blocking temperature TB for the core/shell nanoparticles shows that it is determined not only by their volume but also by the size dependence of the critical fields of the phase magnetisation reversal. With an increase in the interfacial exchange interaction constant Ain, the blocking temperature of the Fe/Fe3O4 nanoparticles quickly reaches saturation, while for Co/Au nanoparticles, TB is practically independent of Ain. The magnetic interaction between nanoparticles leads to chaos in the distribution of their magnetic moments, and hence to an increase in the blocking temperature. The results obtained here are in good agreement with experimental data.