Models for the magnetic ac susceptibility of granular superferromagnetic CoFe/Al₂O₃

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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 Co₈₀Fe₂₀/Al₂O₃ 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.