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

Abstract

The magnetization and magnetic ac susceptibility, $\chi ={\chi }^{\prime }-i{\chi }^{\prime \prime }$, of superferromagnetic systems are studied by numerical simulations. The Cole-Cole plot, ${\chi }^{\prime \prime }$ vs. ${\chi }^{\prime }$, 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.

8 pages, 6 figures