We have demonstrated a low temperature surface spin-glass layer in the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ high-pressed nanocompacts. The related core-shell magnetic structure arises during the compaction, which is evident by the field dependence of the freezing temperature following the de Almeida–Thouless relationship and obvious exchange bias effect. Moreover, we have also found a critical cooling field, above which both the surface spin-glass behavior and the exchange bias effect abruptly disappeared.

• Received 4 February 2004

DOI:https://doi.org/10.1103/PhysRevB.70.092409