Magnetostatic response and field-controlled haloing in binary superparamagnetic mixtures

Abstract

Nowadays, magnetoresponsive soft materials, based not simply on magnetic nanoparticles, but rather on multiple components with distinct sizes and magnetic properties, both in liquid and polymeric carriers, are becoming more and more wide-spread due to their unique and versatile macroscopic response to an applied magnetic field. The variability of the latter is related to a complex interplay of the magnetic interactions in a highly non-uniform internal fields caused by spatial inhomogeneity in multicomponent systems. In this work, we present a combine analytical and simulation study of binary superparamagnetic systems, containing nanoclusters and dispersed single-domain nanoparticles, both in liquid and solid carrier matrices. We investigate the equilibrium magnetic response of these systems in wide ranges of concentrations and interaction energies. It turns out that, while the magnetisation of a binary solid can be both above and below that of an ideal superparamagnetic gas, depending on the concentration of the dispersed phase and the interparticle interactions, the system in a liquid carrier is highly magnetically responsive. In liquid, a spatial redistribution of the initially homogeneously dispersed phase in the vicinity of the nanocluster is observed -- the effect that is reminiscent of the so-called ``haloing'' effect previously observed experimentally on micro- and milli-scales.