Exchange bias effect in magnetic materials

Thickness, roughness and density values of bilayers are analyzed by using X-ray reflectivity technique. The thickness and the roughness values are found as expected. However, from the theoretical fit it is calculated that the density of CoO is much less than the bulk values. This is explained by the superstoichiometric structure of CoO. For selected varying Fe thicknesses (4nm, 10nm, 20nm) the ferromagnetic resonance (FMR) spectra of bilayers have been studied as a function of temperature at X-band frequency (9.5GHz). M versus H loops were recorded at selected temperatures by using VSM magnetometry. The resonance lines of FMR spectra have a relatively small linewidth indicating a high crystallinity of the Fe films. The room temperature FMR data also shows that the easy axis of Fe is in the film plane and parallel to the [110] crystallographic direction of MgO substrate. Although all exchange-bias properties are in good agreement with the previous results on similar systems, the blocking temperatures of the samples are found to be too low comparing to the Neel temperature of CoO. The reason for this situation is explained by both effects of the superstoichiometric structure and the small thickness of CoO.