Shape-Dependent Thermal Switching Behavior of Superparamagnetic Nanoislands

The thermal switching behavior of individual perpendicularly magnetized nanoscale Fe islands consisting of 200–600 atoms only is studied by low-temperature spin-polarized scanning tunneling microscopy. Our results reveal that the switching rate is strongly affected by the particle shape; i.e., elongated islands switch much more rapidly than compact islands of the same volume. This observation is explained by different processes of magnetization reversal. Our results suggest that compact magnetic particles are an ideal choice for future perpendicular magnetic recording media because they are robust against thermal magnetization reversal.

Figure 1

(a) Topographic STM image and (b) the simultaneously measured out-of-plane sensitive magnetic $dI/dU$ signal of two Mo(110) terraces decorated with Fe islands (overall coverage 0.25 ML). The line section (lower panel) reveals that the substrate’s step edge and the islands are of monatomic height ($\approx 2\AA$). During image recording one island switches from dark to bright (inset). Measurement parameters are $T=13\mathrm{K}$, $U=90\mathrm{m}\mathrm{V}$, and $I=1\mathrm{n}\mathrm{A}$.

Figure 2

Magnetic switching behavior of the Fe islands a–d (inset) measured at $T=13\pm 1\mathrm{K}$ (left panel) and $T=19\pm 1\mathrm{K}$ (right panel). The switching rate increases with increasing temperature thereby proving the thermal nature of the observed effect. At $T=19\pm 1\mathrm{K}$ the switching rate of island b exceeds the line repetition rate resulting in an intermediate and blurred signal.

Figure 3

(a) Topography and (b) magnetic $dI/dU$ signal of numbered Fe islands on Mo(110). (c) Plot of the switching rate versus the area of individual islands. The scatter of the switching rate points to a shape-dependent crossover from coherent rotation of compact Fe islands towards nucleation and expansion of reversed domains in elongated islands. Insets: topography of selected Fe islands (scale bar: 5 nm).

Figure 4

(a) Overview showing the topography (left panel) and magnetic $dI/dU$ signal (middle panel) of Fe nanowires on Mo(110). Four domain walls can be recognized. Right panel: two domain walls at higher magnetification. (b) Zoom into the domain wall region (inset, rotated by $90°$). The domain wall width is $w=2.9\pm 0.1\mathrm{n}\mathrm{m}$ (orange line).