Meronlike Spin Textures in In-Plane-Magnetized Thin Films

The interfacing of magnetic materials with nonmagnetic heavy metals with a large spin-orbit coupling, such as $\mathrm{Pt}$, results in an asymmetric exchange interaction at the interface due to the Dzyaloshinskii-Moriya interaction (DMI), which in turn leads to the formation of skyrmions and topological spin structures in perpendicularly magnetized multilayers. Here, we show that out-of-plane spin domains with lateral dimensions from 200 nm to 2 $\mu \mathrm{m}$ are stabilized in in-plane magnetized $\mathrm{Ta}$/$\mathrm{Co}$/$\mathrm{Pt}$ trilayers. We show that these spin textures are largely insensitive to the direction of the in-plane magnetization switched by either magnetic fields or electric fields applied across a ${\mathrm{Si}}_3{\mathrm{N}}_4$ gate dielectric. The results of micromagnetic simulations indicate that the DMI is required for the stabilization of such out-of-plane domains and that the presence of surface roughness helps to stabilize larger structures, in agreement with experimental results. We identify these spin structures as meronlike topological textures, characterized by a perpendicular spin texture in an uniformly in-plane magnetized system.

Figure 1

(a) A schematic of the fabricated sample structure, showing the silicon frame in blue, the membrane in yellow, and the two rectangular top electrodes in orange; the $\mathrm{Ta}$/$\mathrm{Co}$/$\mathrm{Pt}$ film is deposited on top of the membrane and overlapping the electrodes. The angle $\theta$ represents the azimuthal rotation of the sample in the microscope. Atomic force microscopy images of (b) ${\mathrm{Si}}_3{\mathrm{N}}_4$ and (c) ${\mathrm{Si}\mathrm{O}}_x$ substrates. (d),(e) Longitudinal and polar MOKE characterization of $\mathrm{Ta}$/$\mathrm{Co}$(6.5 Å)/$\mathrm{Pt}$ on ${\mathrm{Si}}_3{\mathrm{N}}_4$, respectively. (f) An XPEEM $\mathrm{Co}$ elemental-contrast image of $\mathrm{Ta}$/$\mathrm{Co}$(5.5 Å)/$\mathrm{Pt}$, showing a uniform $\mathrm{Co}$ distribution and the $\mathrm{Cr}$ marker structures (stripe and label “4” in the center). (g) The domain configuration of ${\mathrm{Si}}_3{\mathrm{N}}_4/\mathrm{Ta}/\mathrm{Co}/\mathrm{Pt}$ for different $\mathrm{Co}$ layer thicknesses, as denoted in the individual images. The length of the scale bars is 10 $\mu \mathrm{m}$; the direction of the magnetic contrast and the XMCD amplitude range (set to $\pm 0.03$ for 5–6.5 Å $\mathrm{Co}$ and to $\pm 0.06$ for 10 Å $\mathrm{Co}$) is indicated by the arrow.

Figure 2

XMCD images of $\mathrm{Ta}$/$\mathrm{Co}$(6.5 Å)/$\mathrm{Pt}$ on ${\mathrm{Si}}_3{\mathrm{N}}_4$, acquired at $0^{\circ }$, ${45}^{\circ }$, and ${90}^{\circ }$ with respect to the x-ray direction. The circles denote out-of-plane magnetized domains. The square at the bottom and the label “4” in the middle of the image are nonmagnetic $\mathrm{Cr}$ markers (indicated by black arrows). The direction of the magnetic contrast is indicated by the bidirectional arrow, with the XMCD amplitude range set to $\pm 0.03$.

Figure 3

(a) The roughness profile and anisotropy direction used in the simulation. (b) The initial domain configuration. (c),(d) The final domain configuration for $\mathrm{DMI}=1.6{\mathrm{mJ/m}}^2$ (c) without surface roughness and (d) with surface roughness. (e) The vector map of the out-of-plane magnetization component obtained in (d). (f),(g) Enlarged views of individual antimeron and meron structures, respectively. The contour map represents the direction of the in-plane $x$- and $y$-magnetization-vector components in (b)–(d), while the color scale bar represents the $z$ component of the magnetization vector in (e)–(g).

Figure 4

An overview of the change in the magnetic domain structure after an in-plane magnetic field of (a) 2 mT and (b) $-2$ mT is applied to the sample and measured at remanence. (c),(d) Enlarged images of the domain configuration in the white box shown in (a) and (b). The dashed white circles indicate regions with perpendicular spins. The direction of the magnetic contrast is indicated by the arrow (XMCD amplitude range set to $\pm 0.06$). (e) XMCD profiles across the dotted blue and red lines marked in (c) and (d), respectively. (f) Micromagnetic simulations showing the effect of the in-plane magnetic field on the perpendicular spin structure shown in Figs. 3 and 3, where the contour map represents the in-plane components of the magnetization vector.

Figure 5

(a) An overview of the change in the domain structure with an applied out-of-plane magnetic field of 0 mT, 2.5 mT, and 0 mT. (b)–(f) Enlarged images of out-of-plane spin structures marked with a white box in (a) as a function of the applied out-of-plane magnetic field. The direction of the magnetic contrast is indicated by the arrow (XMCD amplitude range set to $\pm 0.03$). (g) The effect of the out-of-plane magnetic field on the out-of-plane spin structures determined by micromagnetic simulations. The contour map represents the direction of the $x$- and $y$-magnetization-vector components for the top row of (g) and the color scale bar represents the direction of the $z$-magnetization component for the bottom row of (g).

Figure 6

(a)–(f) The effect of the electric field on the magnetic domain configuration for the ${\mathrm{Si}}_3{\mathrm{N}}_4/\mathrm{Ta}/\mathrm{Co}(6\AA )/\mathrm{Pt}$ sample. The area marked by a white square indicates an area with an out-of-plane magnetized region that remains unaffected by the reversal of the surrounding in-plane magnetization. The direction of the magnetic contrast is indicated by the arrow, with the XMCD amplitude range set to $\pm 0.07$. (g) The XMCD line profile of the out-of-plane component across the dashed line shown in the insets, corresponding to the structure indicated by a white box in (a) and (b).