The origin of the non-monotonic field dependence of the blocking temperature in magnetic nanoparticles

TL;DR

This study investigates the non-monotonic dependence of the blocking temperature on magnetic field in nanoparticles, attributing it to anisotropic energy barriers and magnetization behavior, supported by experiments and simulations.

Contribution

It reveals the origin of the non-monotonic field dependence of blocking temperature in magnetic nanoparticles through combined experimental and theoretical analysis.

Findings

Peak temperature increases then decreases with field

Non-Curies law magnetization causes non-monotonic behavior

Simulation aligns with experimental data

Abstract

The field dependence of the peak temperature of the zero-field-cooled (ZFC) magnetization curve of a magnetic nanoparticle system was studied using a diluted magnetic fluid composed of FePt nanoparticles. It is found that the peak temperature increases with increasing applied field below 3 kOe; it then decreases with further increasing the applied field. The non-monotonic field dependence of the peak temperature in magnetic particle systems is attributed to the anisotropic energy barrier distribution of the particles, and to the slowly decreasing magnetization (or to the non-Curies law dependence of magnetization) above the blocking temperature. The non-Curies law dependence of the magnetization is caused by large magnetic anisotropy and Zeeman energy of particles in high magnetic fields. Numerical simulation results, based on basic thermodynamics, and pure thermal relaxation and energy barrier distribution extracted from the low-field, experimental ZFC data show a good agreement with experimental results.