Modification of the magnetic properties of Co films grown on MgO (100) by treatment with NaOH solution

We investigated the surface morphology and magnetic properties of Co films grown on MgO (100) substrates before and after treatment with a NaOH solution. The surface morphology evolves from a continuous film into isolated hexagonal nanodisks resulting from the formation of cobalt hydroxide. In the as-grown Co films the magnetic anisotropy behaves as a superposition of fourfold magnetocrystalline anisotropy and uniaxial magnetic anisotropy (UMA), and the magnetization reversal proceeds by two-step domain-wall motion. After the NaOH treatment the UMA is reduced and the magnetization reversal appears to be dominated by magnetization rotation. In addition, the exchange bias effect, which in the as-deposited films results from the presence of a cobalt oxide layer, disappears after the NaOH treatment, and can be reinduced by annealing the sample. The observed significant changes in the magnetic properties can be related to the disappearance and reappearance of a cobalt oxide top layer.

Figure 1

AFM topography images at different magnifications of (a) and (c) an as-grown Co film on MgO (100) and (b) and (d) this Co film after treatment with NaOH solution. (e) presents the line profile of the Co island marked by the green line in (c). (f) presents the line profile of the hexagonal Co(OH)${}_2$ nanodisk indicated by the green line in (d).

Figure 2

XRD diffraction patterns of the as-grown Co film on MgO (100), this Co film after treatment with NaOH solution, and this same Co film after annealing.

Figure 3

Magnetic hysteresis loops measured by MOKE along (a) the [100] direction and (b) the [110] direction of the MgO (100) substrate for the as-grown Co film and for this Co film after treatment with NaOH solution. The insets of (a) and (b) present the magnetic domain structure measured by MFM at the applied magnetic fields labeled in the respective hysteresis loops for the as-grown Co film. (c) The azimuthal angular dependence of the switching field in the hysteresis loops for the as-grown Co film. The straight lines correspond to the fitted curves based on the model for two-jump domain-wall motion. (d) The azimuthal angular dependence of the coercive field in the hysteresis loops for the Co film after treatment with NaOH solution.

Figure 4

The first two hysteresis loops measured by SQUID magnetometry along the [100] direction of the MgO (100) substrate after field cooling for (a) the as-grown Co film, (b) this Co film after treatment with NaOH solution, and (c) this same Co film after annealing.

Figure 5

XPS high-resolution O $1s$ spectrum for the as-grown Co film on MgO (100), for this Co film after treatment with NaOH solution, and for this same Co film after annealing.