Magnetization reversal mechanism of ramified and compact Co islands on Pt(111)

We report on the magnetization reversal mechanism of Co islands on Pt(111) as a function of their size and shape. We measure the zero-field susceptibility $\chi \left(T\right)$ and low-temperature magnetization curves $M\left(H\right)$ with in situ magneto-optical Kerr effect. Together with the island morphology deduced from scanning tunneling microscopy, this creates sufficient information to determine both the magnetization reversal mechanism and the distribution of anisotropy energies between perimeter and surface atoms. We find a transition from quasicoherent rotation to domain wall nucleation and propagation with a critical size of 350 atoms for ramified, and of 600 atoms for compact islands.

Figure 1

STM images of (a) compact and (b) ramified one ML high Co islands on Pt(111). (c) Perimeter lengths $p$ and (d) island cross sections $u$ as a function of island size $s$ for the compact (black) and ramified (red) islands for which the magnetism has been characterized in Figs. 2 and 3. [Growth parameters: (a) $\Theta =0.1$ ML at $T_{\mathrm{dep}}=150$ K, $T_{\mathrm{ann}}=270$ K for 120 s followed by $\Theta =0.2$ ML at $T_{\mathrm{dep}}=270$ K; (b) $\Theta =0.08$ ML at $T_{\mathrm{dep}}=150$ K followed by $\Theta =0.08$ ML at $T_{\mathrm{dep}}=250$ K; (c) compact islands as in (a) but first deposition at $T_{\mathrm{dep}}=160$ K; and (d) ramified islands $\Theta =0.25$ ML at $T_{\mathrm{dep}}=150$ K, $T_{\mathrm{ann}}=250$ K for 300 s.]

Figure 2

Out-of-plane zero-field susceptibility and magnetization curves of compact islands with morphology characterized by black symbols $(\times )$ in Figs. 1 and 1. (a) Real and (b) imaginary part of $\chi \left(T\right)$. The peak in ${\chi }^{\prime }$ has been normalized to 1 thereby defining the scale for ${\chi }^{\prime \prime }$. (c) $M\left(H\right)$ at $T=74$ K. Measured data: black dots. (d), (e), and (f) Experimental data as in (a), (b), and (c), respectively, but different fits. Simulation curves with respective reversal mechanisms and anisotropy values are given in Table 1. (g) Zero-field energy barriers $E_0$ for qCR (red) vs DW (blue) reversal. Minimizing $E_0$ leads to a crossover from qQR to DW at 600 atoms. (h) Zoom of the crossover region.

Figure 3

Out-of-plane zero-field susceptibility and magnetization curves of ramified islands with morphology characterized by red symbols (+) in Figs. 1 and 1. (a) Real and (b) imaginary part of $\chi$. Normalization as in Fig. 2. (c) $M\left(H\right)$ recorded at $T=66$ K. Experiments: black dots. Simulations: curves with respective reversal mechanisms and anisotropy values given in Table 1. (d) Zero-field energy barriers $E_0$ for qCR (red) vs DW (blue) reversal. Transition from qCR to DW at 350 atoms.

Figure 4

${\chi }^{\prime }\left(T\right)$ curves are modified when $T$ approaches $T_{\mathrm{C}}$, as shown for a single island with $E_0=0.25$ eV.

Figure 5

(a) DW $E_0(K_{\mathrm{s}},K_{\mathrm{p}})$ (eV) for a single island with $u=70$ atoms, and (b) corresponding $T_b(K_{\mathrm{s}},K_{\mathrm{p}})$ (K). The blue dots indicate the qCR-DW model parameters returning the green and gray curves in Fig. 2, $(K_{\mathrm{s}},K_{\mathrm{p}})=(-0.01,1.85),(0.00,1.50)$, and $(0.01,1.15)$ meV/atom.

Figure 6

Compact islands: (a) measured size distribution and relative Gaussian model, $N\left(s\right)\propto e^{-{\left(\frac{s-500}{3000}\right)}^2}$. (b), (c), and (d) Experimental data and green curve as in Figs. 2, 2, and 2, respectively, with additional magnetic simulations. Simulation curves with respective morphology $s,p,u$ input data and anisotropy values are given in Table 2. (f) and (e) $E_0$ and $E_{0.25\mathrm{T}}$ distributions, respectively.