Spin dynamics and criteria for onset of exchange bias in superspin glass Fe/γ-Fe2O3core-shell nanoparticles

Abstract

A detailed study is presented on Fe/\ensuremath{\gamma}-Fe${}_{2}$O${}_{3}$ core-shell structured nanoparticles (mean size \ensuremath{\sim}10 nm) to understand the spin dynamics of the core and shell independently and their role in triggering exchange bias (EB) phenomena. The particle dynamics critically slow down at ${T}_{g}$ \ensuremath{\sim} 68 K, below which they exhibit memory effect in field-cooled and zero-field-cooled protocols associated with a superspin glass state. The field dependence of mean blocking temperature fits the de Almeida-Thouless line and shows two different linear responses in the low and high field regimes corresponding to the core and shell, respectively. We show that the energy barrier distribution estimated from the temperature decay of isothermal remanent magnetization shows two maxima that mark the freezing temperatures of the core (${T}_{\mathrm{f}\text{\ensuremath{-}}\mathrm{cr}}$ \ensuremath{\sim} 48 K) and shell (${T}_{\mathrm{f}\text{\ensuremath{-}}\mathrm{sh}}$ \ensuremath{\sim} 21 K). Last, hysteresis measurements after field cooling reveal strong EB indicated by a loop shift associated with unidirectional anisotropy. The onset of EB is at 35 K when the ferromagnetic core is frozen and the moments in the ferrimagnetic shell begin to block, resulting in enhanced exchange coupling.