Nanoparticles of Fe in SiO2: A superparamagnet with a collective state?

We present a detailed experimental characterization of the irreversible response of a thin film of nanodimensional Fe particles embedded in SiO2, which includes measurements of the field and temperature dependence of the field-cooled moment, the zero-field-cooled moment, the thermoremanent moment, and the isothermal remanent moment, as well as measurements of the relaxation of the moment with time following various field application and removal protocols. The data are analyzed within the framework of a Preisach ensemble of temperature-dependent, thermally activated, asymmetric double-well subsystems. The analysis allows us to estimate the thermal fluctuation field, and to reconstruct the Preisach spectrum of double-well dissipation fields and bias fields, and its evolution with temperature. Relaxation isotherms exhibit an age dependence which suggests that the free-energy landscape of the thermally blocked state may possess an organization similar to that which characterizes the collectively frozen state ...