Controlling the kinetic order of spin-reorientation transitions in $\mathrm{Ni}∕\mathrm{Cu}\left(100\right)$ films by tuning the substrate step structure

To study whether spin-reorientation transitions in 8–10 ML thick $\mathrm{Ni}∕\mathrm{Cu}\left(100\right)$ films take place by continuous or discontinuous rotation of the magnetization, we used spin-polarized low-energy electron microscopy to image the magnetization vector of magnetic domains during Ni growth. After substrate preparations that either promote or suppress bunching of atomic steps, we find strong evidence for either first- or second-order transition kinetics. The results are explained in terms of a magnetic phase diagram, taking the effect of topography on magnetic anisotropy contributions into account.

Figure 1

SRT of first order in $\mathrm{Ni}∕\mathrm{Cu}\left(100\right)$ film grown on substrate showing pronounced step bunches. $2\times 2\mu {\mathrm{m}}^2$ LEEM (a) and SPLEEM [(b)–(j)] images show topography and dependence of magnetic contrast on $\mathrm{Ni}∕\mathrm{Cu}\left(100\right)$ film thickness and polarization direction of illuminating electron beam (background gray level corresponds to null contrast). Dark contrast in (b), combined with null contrast in (d), indicates homogeneous in-plane magnetization. Strong bright and dark contrast in (g) and (j), combined with vanishing contrast in flat areas of (e), (f), (h), and (i) reveals abruptly formed, stripe-shaped, out-of-plane magnetized domains (Ref. 17). (k) Normalized $xyz$ components of $\vec{M}$ demonstrating the abruptness of the SRT. Rectangular insets show averaged gray and background levels of areas highlighted in (a)–(j).

Figure 2

SRT of second order in $\mathrm{Ni}∕\mathrm{Cu}\left(100\right)$ film grown on substrate showing no step bunches. $2\times 2\mu {\mathrm{m}}^2$ LEEM (a) and SPLEEM [(b)–(j)] images show topography and dependence of magnetic contrast on $\mathrm{Ni}∕\mathrm{Cu}\left(100\right)$ film thickness and polarization direction of illuminating electron beam (background gray level corresponds to null contrast). As a result of continuous rotation of the magnetization from in plane to out of plane with increasing Ni thickness, magnetic contrast gradually increases for spin polarization along the surface normal (bottom row), while magnetic contrast fades for in-plane spin polarization (upper and middle rows). (k) Normalized out-of-plane component of $\vec{M}$ recorded during Ni deposition [separate experiment, black squares represent corresponding results for panels (d), (g), and (j)].

Figure 3

Phase diagram for $\mathrm{Ni}∕\mathrm{Cu}\left(100\right)$ films. Square symbols: thickness-dependent anisotropies using experimental values given in Ref. 21. Triangular symbols: same data, shifted upward by assuming topography-induced change of fourth-order surface anisotropy constant $K_{4\Vert }^S$ by $15\mu \mathrm{eV}∕\mathrm{atom}$. See text for details.