Magnetization Reversal of Individual Co Nanoislands

We investigate the magnetization reversal of individual Co islands on Cu(111) in the size range of $N=700$ to 18 000 atoms by spin-polarized scanning tunneling microscopy at 8 K. The switching field $H_{\mathrm{sw}}$ changes with island size in a nonmonotonic manner: it increases with island size and reaches a maximum value of 2.4 T at $N=5500$ atoms, and it decreases for larger islands. We extract the energy barrier for magnetization reversal as a function of island size. The maximum $H_{\mathrm{sw}}$ corresponds to an energy barrier of 1 eV. Our results elucidate a crossover of the magnetization reversal from an exchange-spring behavior to domain wall formation with increasing size at around 7500 atoms.

Figure 1

Constant-current mode measurement of double layer high (0.4 nm) Co islands on Cu(111) ($I=1\mathrm{nA}$, $V=-0.27\mathrm{V}$, $T=8\mathrm{K}$). The labeled islands have been selected for the spin-STS measurements, and these labels are used in Figs. 1, 2, 3, 4.

Figure 2

Hysteresis curves of the differential conductance $dI/dV$ measured at the center of islands A to D. The magnetic field is applied perpendicularly to the substrate surface. Note, that due to the reorientation of the magnetization of the neighboring islands and of the tip, magnetic stray fields vary during a field scan. We estimate these field variations smaller than 0.1 T. ($V=-0.5\mathrm{V}$ and $T=8\mathrm{K}$).

Figure 3

Switching fields of 54 individual Co islands of different size. Data points of islands $A$ to $D$ from Fig. 1 are indicated. The red and blue lines are calculated from two different reversal mechanism, as described in the text. They do not represent fits through the data points.

Figure 4

Island size dependence of the energy barrier $\Delta E$. (a) The blue curve is a linear fit $\Delta E_{\mathrm{lin}}(N)=K(N-N_0)$, as described in the text. The red curve shows the calculated energy barrier for domain wall formation $\Delta E_{\mathrm{dw}}(N)=4\sigma \sqrt{AK}$. The labels identify the data for the islands of Fig. 1. (b) The blue line is the same as in (a). The function $K(N-N_{\mathrm{rim}})$ is plotted for different rim widths of 1 and 4 atomic rows, respectively, with dashed and dotted green lines.