Electron paramagnetic resonance spectra near the spin-glass transition in iron oxide nanoparticles

Abstract

Electron paramagnetic resonance (EPR) in iron-oxide nanoparticles (\ensuremath{\sim}2.5 nm) embedded in a polyethylene matrix reveals the sharp line broadening and the resonance field shift on sample cooling below ${T}_{F}\ensuremath{\approx}40\mathrm{K}.$ At the same temperature a distinct anomaly in the field-cooled magnetization is detected. The temperature dependences of EPR parameters below ${T}_{F}$ are definitely different than those found for various nanoparticles in the superparamagnetic regime. In contrast to canonical bulk spin glasses, a linear fall-off of the EPR linewidth is observed. Such behavior can be explained in terms of the random-field model of exchange anisotropy.