Low temperature surface spin-glass transition in γ-Fe2O3 nanoparticles

gamma-Fe2O3 magnetic nanoparticles, with a very high surface to volume ratio, exhibit both strong exchange anisotropy and magnetic training effect. At the same time high field irreversibility in M(H) curves and zero field cooled-field cooled (ZFC-FC) processes has also been detected. A low temperature spin-glass-like transition is evidenced at T-F approximate to 42 K with strong irreversibility even at H = 55 kOe. TF(H) evolves following the well known de Almeida-Thouless line delta T-F proportional to H-2/3. The thermal dependence of the exchange anisotropy field H-E is described by the random-field model of exchange anisotropy. In the framework of this theory, a surface spin-glass layer about 0.6 nm thick is determined.