Classifying magnetic interactions in nanoparticle assemblies using Cole-Cole plots

Measuring the magnetic ac susceptibility, $\chi = \chi' - i \chi''$, of nanoparticle assemblies leads to important conclusions about the role of inter-particle interactions. We demonstrate that the Cole-Cole plot, $\chi''$ vs. $\chi'$, can be used as a tool for classifying the magnetic behavior of interacting or non-interacting nanoparticles. Distinguishing non-interacting superparamagnetic from superspin glass (SSG) or superferromagnetic (SFM) behavior is easily possible. We performed measurements of the magnetization hysteresis and ac susceptibility on both SSG and SFM granular multilayers [Co$_{80}$Fe$_{20}$/Al$_{2}$O$_{3}$] $_n$. The SFM results are successfully compared to simulations of a driven domain wall in a random medium. \noindent Work supported by the DFG, Alexander von Humboldt Foundation and DAAD.