Size-Dependent Spin Structures in Iron Nanoparticles

Using photoemission electron microscopy, we study the magnetization orientation in single 5–25 nm iron particles coupled to a ferromagnetic cobalt support. We find a noncollinear alignment between the particle and substrate magnetization above a particle size of $\sim 6\mathrm{nm}$ and a parallel alignment for smaller sizes. Numerical calculations reveal a transition from an exchange-dominated to an anisotropy-dominated regime on increasing the particle height: the smaller particles are in a single-domain collinear state while larger particles exhibit a spin-spiral magnetic structure determined by the magnetic anisotropy energy.

Figure 1

(a) PEEM elemental contrast recorded at the Fe $L_3$ edge (708.1 eV) of Fe nanoparticles with sizes from 5 to 25 nm. (b) AFM image of the marked box in (a) with the dark gray (red) circles showing single nanoparticles and the light gray (yellow) circle indicating two close-lying particles. The inset shows an AFM image of the Fe nanoparticle in the white circle in (a) and (b) with a height of $13\pm 1\mathrm{nm}$. (c) Size distribution of the single Fe nanoparticles in (a).

Figure 2

XMCD magnetic contrast of (a) the Fe nanoparticles shown in Fig. 1a, and (b) the Co film measured in the same area. (c) Superposition of the Fe and Co XMCD images in (a) and (b). Different gray levels correspond to dissimilar magnetization orientations.

Figure 3

(a) XMCD images at the Fe $L_3$ edge (708.1 eV) of the same region as in Figs. 1a, 2 acquired with different azimuthal sample orientations ${\varphi }_s$ relative to the photon propagation vector $\mathbf{\sigma }$ (indicated by arrows). (b) Sketch of the sample geometry. (c) Quantitative determination of the magnetization direction for the single Fe particle in the inset of Fig. 1b [indicated by square frames in Fig. 3a]. The fit of the angular dependent XMCD asymmetry ratio gives ${\varphi }_m=-170\pm 30°$ and ${\theta }_m=140\pm 30°$.

Figure 4

(a) Size dependence of the absolute value of the in-plane canting angles between the film and the upper particle magnetization. The white dots are the experimental data with statistical error bars and the solid line and dashed line are the calculated curves for two values of $K_u$. The particle with a square symbol shows a representative error bar of the particle size. (b) Sketch of the spin structure of a Wulff-shaped 10-nm particle and $K_u=50\mu \mathrm{eV}/\mathrm{atom}$. (c) Size dependence of the experimental spread of $|\Delta \varphi |$ in (a), represented by the standard deviation (diamonds). The solid line is a guide to the eye.