Dynamics of magnetic single domain particles embedded in a viscous liquid

TL;DR

This study develops kinetic equations for magnetic nanoparticles in viscous liquids, analyzing their orientation, energy absorption, and thermal effects under varying magnetic field amplitudes, revealing complex behavior and size-dependent power absorption features.

Contribution

The paper introduces a generalized kinetic model including thermal fluctuations for magnetic nanoparticles in liquids, providing new insights into their dynamic behavior and energy absorption mechanisms.

Findings

Particles orient perpendicular or parallel to the field depending on amplitude

Thermal fluctuations increase power absorption at low and intermediate fields

Size-dependent absorption peaks are observed in numerical simulations

Abstract

Kinetic equations for magnetic nano particles dispersed in a viscous liquid are developed and analyzed numerically. Depending on the amplitude of an applied oscillatory magnetic field the particles orient their time averaged anisotropy axis perpendicular to the applied field for low magnetic field amplitudes and nearly parallel to the direction of the field for high amplitudes. The transition between these regions takes place in a narrow field interval. In the low field region the magnetic moment is locked to some crystal axis and the energy absorption in an oscillatory driving field is dominated by viscous losses associated with particle rotation in the liquid. In the opposite limit the magnetic moment rotates within the particle while its easy axis being nearly parallel to the external field direction oscillates. The kinetic equations are generalized to include thermal fluctuations. This leads to a significant increase of the power absorption in the low and intermediate field region with a pronounced absorption peak as function of particle size. In the high field region, on the other hand, the inclusion of thermal fluctuations reduces the power absorption. The illustrative numerical calculations presented are performed for magnetic parameters typical for iron oxide.