Strain-assisted nanoscale patterning of Co thin films

The effect of a nanometer-scale oscillating surface strain field, due to a buried misfit dislocation array, on the growth of Co on Pt(001) is investigated by examining the structure and magnetic properties of the deposited Co film. $1\mathrm{nm}$ thick Co films are sputter-deposited on epitaxial Pt underlayers in which the strain field is generated by a square array of interfacial misfit dislocations due to the lattice mismatch of the Pt(001) film with the underlying MgO(001) substrate. The strength of the surface strain field is controlled by varying the thickness of the Pt underlayer from $2\text{to}10\mathrm{nm}$. Remarkable changes in the magnetic remanence behavior, measured using the $\Delta m$ technique, are observed as a function of the thickness of the Pt underlayer. These changes can be attributed to changes in the Co nucleation density and growth morphology associated with the different strength of the surface strain field for different Pt underlayer thicknesses. This indirect approach suggests that nanopatterned, magnetically isolated Co films are produced over a very narrow range of Pt underlayer thickness.

Figure 1

(a) Strain field exerting the free surface in terms of the film thickness by a dislocation; $10.0\mathrm{nm}$ (●), 7.0 (엯), 5.0 (▴), 4.0 (◻), and 3.0 (∎) (b) high-resolution transmission electron microscopy (HRTEM) cross-sectional image of misfit dislocations (indicated by arrows) with the electron beam parallel to [010].

Figure 2

X-ray diffraction phi-scan from (a) {111} planes of (001) MgO substrate and (b) {111} planes of $10\mathrm{nm}$ thick epitaxial (001) Pt film.

Figure 3

TEM diffraction patterns showing the epitaxial relationship of (a) $2\mathrm{nm}$ thick Pt film on (001) MgO substrate and (b) Pt film only after removing the MgO by chemical etching using dilute ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2\mathrm{S}{\mathrm{O}}_4$ solution, taken near the [001] zone axis.

Figure 4

Surface reflectivity measurement using x-ray low angle scattering: Measured patterns from (a) $10.0\mathrm{nm}$, (b) $5.0\mathrm{nm}$, and (c) $2.0\mathrm{nm}$ underlayer samples (left column); simulated patterns of the corresponding thickness (right column).

Figure 5

Hysteresis loops of $1.0\mathrm{nm}$ thick Co films with various Pt underlayer thickness. Magnetic moments are normalized by saturation magnetization (Ms).

Figure 6

The $\Delta m$ plots of the samples extracted from IRM and DCD measurements. Magnetic moments and fields were normalized by the sample volume and remanent coercivity $\left(H_{\mathrm{cr}}\right)$, respectively.