Domain Walls in Antiferromagnetically Coupled Multilayer Films

We report experimentally observed magnetic domain-wall structures in antiferromagnetically coupled multilayer films with perpendicular anisotropy. Our studies reveal a first-order phase transition from domain walls with no net moment to domain walls with ferromagnetic cores. The transition originates from the competition between dipolar and exchange energies, which we tune by means of layer thickness. Although observed in a synthetic antiferromagnetic system, such domain-wall structures may be expected to occur in $A$-type antiferromagnets with anisotropic exchange coupling.

Figure 1

Magnetic hysteresis loop for a $\mathbf{[}[\mathrm{C}\mathrm{o}(4\mathrm{\AA })/\mathrm{P}\mathrm{t}(7\mathrm{\AA }){]}_6/\mathrm{C}\mathrm{o}(4\mathrm{\AA })/\mathrm{R}\mathrm{u}(9\mathrm{\AA }){\mathbf{]}}_4$ multilayer sample with the field applied perpendicular to the film. Inset 1 is an MFM image in remanence after out-of-plane saturation that shows a laterally uniform ground state, while inset 2, measured after in-plane saturation, exhibits domains of the two possible ground states (up-down up-down vs down-up down-up). Both images are $(5\mu \mathrm{m}{)}^2$ in size.

Figure 2

Details of domain wall structures observed in $\mathbf{[}[\mathrm{C}\mathrm{o}(4\mathrm{\AA })/\mathrm{P}\mathrm{t}(7\mathrm{\AA }){]}_{X-1}/\mathrm{C}\mathrm{o}(4\mathrm{\AA })/\mathrm{R}\mathrm{u}(9\mathrm{\AA }){\mathbf{]}}_N$ multilayers. (a)–(d) MFM images where (a) and (c) are $(5\mu \mathrm{m}{)}^2$ and (b) and (d) are $(3\mu \mathrm{m}{)}^2$ in size. The individual images correspond to the following samples: (a) $(N=4,X=5)$; (b) $(N=10,X=5)$; (c) $(N=12,X=7)$; (d) $(N=18,X=8)$. (e),(f) MFM line scans across the domain walls for (e) $(N=4,X=5)$, (f) $(N=4,X=7)$, and (g) $(N=12,X=7)$. In order to achieve better statistics, the domain-wall profiles were averaged over a 200–300 nm wide domain-wall region. For the periodic domain-wall patterns only the dark part of the wall was used to determine the profile.

Figure 3

(a) Schematic illustration of the underlying magnetic domain-wall structure in the AF-coupled multilayers with finite shift $d$. (b) Contour plot of the calculated AF domain-wall energy versus layer thickness and domain-wall shift $d$ for $N=4$. The energy is plotted relative to the energy for $d=0$. The horizontal cuts through the contour plots [marked by the dashed lined and labeled $(X=5)$ and $(X=7)$] correspond to the first two energy profiles shown in (c). The solid line corresponds to the $d$ value that gives the lowest energy. (c) Energy versus domain wall shift $d$ for three different samples. (d) Calculated magnetic stray field profiles above the film across the equilibrium domain wall structure determined from the minimum in (c).