Theory of static magnetization of magnetopolymer composites: The second virial approximation

Magnetostatic response and field-controlled haloing in binary superparamagnetic mixtures

Nowadays, magnetoresponsive soft materials, based not simply on magnetic nanoparticles but rather on multiple components with distinct sizes and magnetic properties in both liquid and polymeric carriers, are becoming more and more widespread 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 nonuniform internal field caused by spatial inhomogeneity in multicomponent systems. In this work, we present a combined analytical and simulation study of binary superparamagnetic systems containing nanoclusters and dispersed single-domain nanoparticles in both liquid and solid carrier matrices. We investigate the equilibrium magnetic response of these systems for wide ranges of concentrations and interaction energies. It turns out that, while the magnetization 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, an effect that is reminiscent of the so-called haloing effect previously observed experimentally on micro- and milliscales.

Effect of Polydispersity on the Structural and Magnetic Properties of a Magnetopolymer Composite

When using magnetopolymer composites in high-precision industrial and biomedical technologies, the problem of predicting their properties in an external magnetic field arises. In this work, we study theoretically the influence of the polydispersity of a magnetic filler on a composite's equilibrium magnetization and on the orientational texturing of magnetic particles formed during polymerization. The results are obtained using rigorous methods of statistical mechanics and Monte Carlo computer simulations in the framework the bidisperse approximation. It is shown that by adjusting the dispersione composition of the magnetic filler and the intensity of the magnetic field at which the sample's polymerization occurs, it is possible to control the composite's structure and magnetization. The derived analytical expressions determine these regularities. The developed theory takes into account dipole-dipole interparticle interactions and therefore can be applied to predict the properties of concentrated composites. The obtained results are a theoretical basis for the synthesis of magnetopolymer composites with a predetermined structure and magnetic properties.

Influence of field amplitude and dipolar interactions on the dynamic response of immobilized magnetic nanoparticles: Perpendicular mutual alignment of an alternating magnetic field and the easy axes

In this paper, the dynamic magnetic properties of an ensemble of interacting immobilized magnetic nanoparticles with aligned easy axes in an applied ac magnetic field directed perpendicular to the easy axes are considered. The system models soft, magnetically sensitive composites synthesized from liquid dispersions of the magnetic nanoparticles in a strong static magnetic field, followed by the carrier liquid's polymerization. After polymerization, the nanoparticles lose translational degrees of freedom; they react to an ac magnetic field via Néel rotation, when the particle's magnetic moment deviates from the easy axis inside the particle body. Based on a numerical solution of the Fokker-Planck equation for the probability density of the magnetic moment orientation, the dynamic magnetization, frequency-dependent susceptibility, and relaxation times of the particle's magnetic moments are determined. It is shown that the system's magnetic response is formed under the influence of competing interactions, such as dipole-dipole, field-dipole, and dipole-easy-axis interactions. The contribution of each interaction to the magnetic nanoparticle's dynamic response is analyzed. The obtained results provide a theoretical basis for predicting the properties of soft, magnetically sensitive composites, which are increasingly used in high-tech industrial and biomedical technologies.

Structure and magnetization of a magnetoactive ferrocomposite

This work is devoted to the theoretical study of the structural and magnetic properties of an ensemble of single-domain interacting magnetic nanoparticles immobilized in a non-magnetic medium. This model is typical for describing magnetically active soft materials, "smart" polymer ferrocomposites, which have been applied in science-intensive industrial and biomedical technologies. It is assumed that the ferrocomposite is obtained by solidification of the carrier medium in a ferrofluid under an external magnetic field, the intensity of which is determined by the Langevin parameter α_p; after the solidification of the carrier liquid, the nanoparticles retain the spatial distribution and orientation of their easy magnetization axes. The features of the orientational texture formed in the sample are analyzed depending on the intensity of the magnetic field α_p and interparticle dipole-dipole interactions. The magnetization of a textured ferrocomposite in the magnetic field α is also investigated. Our results show that in the case of a co-directional arrangement of the considered fields and if α < α_p, the ferrocomposites are magnetized much more efficiently than ferrofluids due to their texture. In the fields α > α_p, the ferrocomposite is magnetized less efficiently than the ferrofluid due to the internal magnetic anisotropy of the nanoparticles. The analytical expressions presented here make it possible to predict the magnetization of a ferrocomposite depending on its internal structure and synthesis conditions, which is the theoretical basis for the synthesis of ferrocomposites with a predetermined magnetic response in a given magnetic field.