Difference between Blocking and Néel Temperatures in the Exchange BiasedFe3O4/CoOSystem

The blocking temperature $T_{\mathrm{B}}$ has been determined as a function of the antiferromagnetic layer thickness in the ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{CoO}$ exchange biased system. For CoO layers thinner than 50 Å, $T_{\mathrm{B}}$ is reduced below the Néel temperature $T_{\mathrm{N}}$ of bulk CoO (291 K), independent of crystallographic orientation or film substrate ( $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$, ${\mathrm{SrTiO}}_3$, and MgO). Neutron diffraction studies show that $T_{\mathrm{B}}$ does not track the CoO ordering temperature and, hence, that this reduction in $T_{\mathrm{B}}$ does not arise from finite-size scaling. Instead, the ordering temperature of the CoO layers is enhanced above the bulk $T_{\mathrm{N}}$ for layer thicknesses $\lesssim 100Å$ due to the proximity of magnetic ${\mathrm{Fe}}_3{\mathrm{O}}_4$ layers.