Memory in a magnetic nanoparticle system: Polydispersity and interaction effects

We report here a theory of relaxation of single domain magnetic nanoparticles, appropriate for analyzing measurements of Mossbauer spectra, magnetization response, and hysteretic coercivity. Our special focus of attention in the theoretical formulation is the presence of dipolar interaction between the magnetic particles. We discuss in detail the effect of interaction as well as particle size distribution on the measured relaxation spectra and irreversible, nonequilibrium magnetization response in field-cooled and zero-field-cooled situations. Some of the memory effects, similar to those seen in spin glass systems, may be put to important device applications by tuning the interaction and the particle size.