Interpretation of thermal dependence of magnetic aftereffect for magnetic nanocomposite with slow decay rates

Abstract

A new experimental characterization is presented of time-, field-, and temperature-dependent dynamic effects in magnetization of a nanocomposite which displays slow decay. Field and temperature variations of irreversible susceptibility, χ _irr , decay coefficient, S, fluctuation field, h_f , and activation volume, V, have been calculated for the nanocomposite sample (Co₈₀Ni₂₀) using a recently developed modified Preisach–Arrhenius (MPA) model. The sample is composed of non-interacting nanoparticles having negligible reversible magnetization. Non-Arrhenius behavior is observed in both the maximum decay coefficient, S _max, and the fluctuation field, h_f , as a function of temperature T. The peak of both temperature curves are identical and occur at a critical temperature T_k of ∼50 K, which agrees with our experimental results. Based on the effect of a temperature-dependent chemical potential on energy barrier, h_f is studied for T < T_k and T ≥ T_k , respectively. A more complete MPA model that can predict the magnetization as function of time, field and temperature for a magnetic material with slow decay rates is proposed. This model uses a multi-variable analytical formula, , which incorporates the characteristic parameters.

Keywords:

• magnetic aftereffect
• fluctuation field
• numerical modeling
• Co₈₀Ni₂₀ nanocomposite

Acknowledgements

The authors thank Dr. Vázquez of CISC, Madrid, Spain for providing us with the nanocomposite sample. The authors are grateful to Prof. M. Wagner and Dr. C. Yan of The George Washington University for collaboration and to Drs. C. Dennis and V. Provenzano of the National Institute of Standards and Technology for fruitful discussions. The work is partially supported by National Science Foundation under Contract no. 0733526 and no. 1031619.