Structure-induced spin reorientation in magnetic nanostructures

We report on structuring-induced changes of the magnetic anisotropy of cylindrical nanostructures which are carved out of thin Pt/Co/Pt films. The magnetic properties of films and structures with a diameter of about 34 nm were investigated via magneto-optic Kerr effect. The magnetic anisotropy is determined for both films and nanostructures for varying Co thicknesses (0.5–7 nm). In general, the nanostructures exhibit larger perpendicular anisotropy than the films. On thickness increase of the Co layer two spin reorientation transitions at about 2.2 and 5 nm are found. At 2.2 nm the nanostructures exhibit the transition from perpendicular to in-plane orientation of magnetization while at 5 nm the reversed transition is found. The variation of the magnetic anisotropy of the Co nanostructures is not solely caused by the change of shape anisotropy. The net change, corrected for the shape, reveals a reduction of strain in the thinnest Co layers while the increase of the anisotropy of the nanostructures at higher Co thicknesses is caused by a transformation of the Co lattice from fcc to hcp.

Figure 1

SEM micrograph of magnetic nanostructures (left) and sketch of multilayer composition (right). The nanostructures displayed on the left have a mean diameter of about 34 nm and are carved out of films with the composition shown on the right.

Figure 2

Dependence of magnetic anisotropy on Co thickness for initial films and nanostructures. The predicted shape anisotropies for nanostructures of different shapes are plotted in the graphs. The results for cylinder, truncated cone, and double truncated oblate spheroid are very close and almost not distinguishable (red/black dashed line). (a) Anisotropy constant times thickness versus thickness. The green (blue) filled circles give the values found for nanostructures (initial films), respectively. The SRT found in the films is slightly shifted to higher Co thicknesses in the nanostructures (from 1.7 nm to about 3.2 nm) due to shape effects. Experimentally, a second SRT is observed for thicknesses above 5 nm. (b) Change of anisotropy constant $\mathrm{\Delta }K$ of nanostructures with respect to initial films (green symbols). The reddish line gives the calculated shape contribution. The change of the anisotropy in the nanostructures reveals a strong deviation from the supposed shape anisotropy.

Figure 3

Magnetic anisotropy change of the nanostructures corrected for the shape contribution of a double truncated oblate spheroid. The plot reveals the pure magnetic anisotropy changes that appear after structuring via ion milling. For thickness from 0 to 1.6 nm a nearly linear decrease of $(-108\pm 11)\mathrm{kJ}/\left({\mathrm{m}}^3\mathrm{nm}\right)$ is observed. For thicker films in the range from 1.6 to 5.5 nm a linear increase of about $(100\pm 5)\mathrm{kJ}/\left({\mathrm{m}}^3\mathrm{nm}\right)$ is determined. For thicker films above 5.5 nm the anisotropy change appears to be constant.