Magnetic irreversibility and relaxation in assembly of ferromagnetic nanoparticles

TL;DR

This study investigates magnetic irreversibility and relaxation behaviors in different ferromagnetic nanoparticle assemblies, developing a phenomenological model that accounts for time-dependent energy barriers influenced by particle interactions and anisotropies.

Contribution

The paper introduces a novel phenomenological model where the magnetic relaxation barrier depends on instantaneous magnetization, explaining experimental relaxation data.

Findings

Regular amorphous nanoparticles show the highest relaxation rate and lowest irreversibility temperature.

Crystalline nanoparticles exhibit the lowest relaxation rate and highest irreversibility temperature.

The model successfully explains the time-logarithmic decay of magnetization and the influence of particle interactions.

Abstract

Measurements of the magnetic irreversibility line and time-logarithmic decay of the magnetization are described for three $Fe_{2}O_{3}$ samples composed of regular amorphous, acicular amorphous and crystalline nanoparticles. The relaxation rate is the largest and the irreversibility temperature is the lowest for the regular amorphous nanoparticles. The crystalline material exhibits the lowest relaxation rate and the largest irreversibility temperature. We develop a phenomenological model to explain the details of the experimental results. The main new aspect of the model is the dependence of the barrier for magnetic relaxation on the instantaneous magnetization and therefore on time. The time dependent barrier yields a natural explanation to the time-logarithmic decay of the magnetization. Interactions between particles as well as shape and crystalline magnetic anisotropies define a new energy scale that controls the magnetic irreversibility. Introducing this energy scale yields a self-consistent explanation of the experimental data.