Implications of a temperature-dependent magnetic anisotropy for superparamagnetic switching

TL;DR

This paper investigates how temperature-dependent magnetic anisotropy influences the superparamagnetic switching rate, showing that accounting for this dependence yields realistic attempt frequencies and modifies the classical Arrhenius law.

Contribution

It introduces a quantitative analysis of the effects of temperature-dependent anisotropy on superparamagnetic switching rates, extending the classical Neel-Brown-Arrhenius model.

Findings

Temperature dependence of anisotropy alters switching rate predictions.

Realistic attempt frequencies are obtained when anisotropy variation is included.

The classical Arrhenius law is modified by temperature-dependent anisotropy.

Abstract

The macroscopic magnetic moment of a superparamagnetic system has to overcome an energy barrier in order to switch its direction. This barrier is formed by magnetic anisotropies in the material and may be surmounted typically after 10^9 to 10^12 attempts per second by thermal fluctuations. In a first step, the associated switching rate may be described by a Neel-Brown-Arrhenius law, in which the energy barrier is assumed as constant or a given temperature. Yet, magnetic anisotropies in general depend on temperature themselves which is known to modify the Neel-Brown-Arrhenius law. We illustrate quantitatively the implications of a temperature-dependent anisotropy on the switching rate and in particular for the interpretation of the prefactor as an attempt frequency. In particular, we show that realistic numbers for the attempt frequency are obtained when the temperature dependence of the anisotropy is taken into account.