The effects of anisotropy on solvent-suspended ‘superparamagnetic’ nanoparticles:Magnetization step on melting

The magnetic anisotropy of single-domain magnetic nanoparticles can influence their behaviour significantly even at temperatures above the blocking temperature as conventionally defined. We compare the magnetic properties of such nanoparticles that are free to rotate, and the same nanoparticles with random but fixed orientations. When free to rotate, the particles show Langevin behaviour as expected, but when the orientations are fixed, their magnetic anisotropy causes deviations from this behaviour. These deviations may be observed directly in the M-H curves. They also cause a step in the M-T curve measured for a zero-field cooled sample of nanoparticles suspended in a solvent at the solvent's melting point. The step occurs because magnetic anisotropy causes M for particles with random but fixed orientation to be lower than for the same particles that are free to rotate when the solvent melts. The size of the step reaches a maximum at a finite applied field. This phenomenon is important because it can be used to determine the fraction of magnetic nanoparticles that are immobilized, for example by adsorption to ice in a freeze-concentrated solution.