Models for the magnetic ac susceptibility of granular superferromagnetic CoFe/Al$_2$O$_3$

Abstract

The magnetization and magnetic ac susceptibility, $χ= χ' - i χ''$, of superferromagnetic systems are studied by numerical simulations. The Cole-Cole plot, $χ''$ vs. $χ'$, is used as a tool for classifying magnetic systems by their dynamical behavior. The simulations of the magnetization hysteresis and the ac susceptibility are performed with two approaches for a driven domain wall in random media. The studies are motivated by recent experimental results on the interacting nanoparticle system Co$_{80}$Fe$_{20}$/Al$_{2}$O$_{3}$ showing superferromagnetic behavior. Its Cole-Cole plot indicates domain wall motion dynamics similarly to a disordered ferromagnet, including pinning and sliding motion. With our models we can successfully reproduce the features found in the experimental Cole-Cole plots.