Low Temperature Magnetic Behaviour of γ-Fe<sub>2</sub>O<sub>3</sub> Nanoparticles

Abstract

γ-Fe 2 O 3 magnetic nanoparticles exhibit both strong exchange anisotropy and magnetic training effect together with high field irreversibility in M(H) curves and zero field cooling-field cooling (ZFC-FC) processes. A low temperature magnetic phase transition is evidenced by the existence of a freezing process with strong irreversibility between the ZFC and the FC branches even at H= 55 kOe. The cusp of the ZFC branch moves as a function of the magnetic field following the well known de Almeida-Thouless line δT F ∞ H 2/3 . Strong exchange anisotropy is observed after cooling the sample in high field through the freezing temperature T F ≃40 K. The temperature dependence of the exchange anisotropy field HE has been determined and analyzed observing approximately a linear falloff (1-T/T crit ) at a critical temperature T crit ≃25 K slightly below the spin glass transition temperature T F in good agreement with the predictions of the random-field model of exchange anisotropy. These experimental results, together with the high field magnetic relaxation processes, clearly point to the existence of a surface spin glass-like layer that undergoes a magnetic transition to a frozen state below T F . A close relation between H C and HE is observed, which would imply that the increase of H C observed in nanoparticles at low temperatures may be due to the pinning effect of the blocked spin glass-like surface layer upon the single-domained core.