$\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ 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 $M\left(H\right)$ curves and zero field cooled–field cooled (ZFC-FC) processes has also been detected. A low temperature spin-glass-like transition is evidenced at $T_F\approx 42\mathrm{K}$ with strong irreversibility even at $H=55\mathrm{}\mathrm{kOe}$. $T_F\left(H\right)$ evolves following the well known de Almeida–Thouless line $\delta T_F\propto H^{2/3}$. 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.

• Received 5 August 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.181