Field dependence of the temperature at the peak of the ZFC magnetization

TL;DR

This study investigates how an applied magnetic field influences the temperature at the peak of ZFC magnetization, emphasizing the importance of nonlinear effects and anisotropy over simple relaxation time models.

Contribution

It demonstrates that nonlinear magnetization and anisotropy effects are essential to understanding the peak temperature in ZFC magnetization, beyond traditional relaxation time predictions.

Findings

Precise Néel-Brown model calculations are insufficient to explain the field effect.

Nonlinear magnetization dependence is crucial for the maximum.

Magnetocrystalline anisotropy significantly influences the peak temperature.

Abstract

The effect of an applied magnetic field on the temperature at the maximum of the ZFC magnetization, $M_{ZFC}$, is studied using the recently obtained analytic results of Coffey et al. (Phys. Rev. Lett. {\bf 80}(1998) 5655) for the prefactor of the N\'{e}el relaxation time which allow one to precisely calculate the prefactor in the N\'{e}el-Brown model and thus the blocking temperature as a function of the coefficients of the Taylor series expansion of the magnetocrystalline anisotropy. The present calculations indicate that even a precise determination of the prefactor in the N\'{e}el-Brown theory, which always predicts a monotonic decrease of the relaxation time with increasing field, is insufficient to explain the effect of an applied magnetic field on the temperature at the maximum of the ZFC magnetization. On the other hand, we find that the non linear field-dependence of the magnetization along with the magnetocrystalline anisotropy appears to be of crucial importance to the existence of this maximum.