Magnetocaloric effect and scaling analysis in superspin glass Co/Au nanoparticles

Abstract

Bimetallic Co/Au (core/shell) nanoparticle system exhibiting transition form superparamagnetic (SPM) to superspin glass state (SSG) at low temperatures has been investigated with respect to its magnetocaloric properties. Peaks of magnetic entropy change vs. temperature dependences ($\Delta S_M(T)$) have been observed at a temperature in the vicinity of SPM to SSG transition. Despite of significant thermal hysteresis of magnetization at low applied fields a master curve has been constructed from the $\Delta S_M(T)$ dependences employing scaling analysis. That confirms the investigated transition is of the second order type. Different values of critical exponents calculated from field dependence of relevant magnetocaloric response parameters in the proximity of critical temperature have been found for low and high applied fields. Moreover, the exponents exhibited significant disagreement with theoretical values characteristic of mean field theory as well as Ising models indicating distinct behavior of the SSG system at critical temperature.