Difference between blocking and Neel temperatures in the exchange biased Fe3O4/CoO system

The blocking temperature T-B has been determined as a function of the antiferromagnetic layer thickness in the Fe3O4/CoO exchange biased system. For CoO layers thinner than 50 Angstrom, T-B is reduced below the Neel temperature T-N of bulk CoO (291 K), independent of crystallographic orientation or film substrate (alpha-Al2O3, SrTiO3, and MgO). Neutron diffraction studies show that T-B does not track the CoO ordering temperature and, hence, that this reduction in T-B does riot arise from finite-size scaling. Instead, the ordering temperature of the CoO layers is enhanced above the bulk T-N for layer thicknesses less than or similar to 100 Angstrom due to the proximity of magnetic Fe3O4 layers.