Controlled antivortex propagation at bifurcations in reconfigurable NdCo/NiFe racetracks

Abstract

The controlled propagation of spin textures at bifurcations is a critical challenge for racetrack-based logic devices. Here, we investigate the effect of longitudinal and transverse magnetic fields on the propagation of magnetic antivortices at bifurcations within the stripe domain pattern of a reconfigurable NdCo/NiFe racetrack in order to control the preferred antivortex trajectory. Magnetic Transmission X-ray Microscopy experiments were employed to correlate the observed propagation path with the local magnetic configuration. We demonstrate that Zeeman coupling to the magnetization components at the bifurcation core enables switching of the preferred propagation branch using low-amplitude transverse magnetic fields, without modifying the global stripe domain configuration that defines the guiding racetrack landscape. In-plane magnetic anisotropy provides an additional mechanism to break the symmetry between the upper and lower bifurcation branches by tuning the relative orientation between the stripe domain pattern and the longitudinal magnetic fields.

Figures (4)

• Figure 1: (a) Sketch of a tail-to-tail DW within the stripe pattern of a NdCo/NiFe multilayer (arrows indicate the unit magnetization vector $\mathbf{m}=(m_x,m_y,m_z)$ at each domain): at NdCo, there is a BP on the boundary between up/down stripes; at NiFe, the combination of $m_x$ reversal with $m_y$ signs given by the closure domain structure creates an AV with $-m_z$ core and a V with $+m_z$ core in adjacent lanes. (b) Sketch of a stripe bifurcation with an AV propagating along the upper bifurcation branch.
• Figure 2: (a) MOTKE hysteresis loop of a 80 nm NdCo$_5$/40 nm Ni$_{80}$Fe$_{20}$ bilayer. Labels indicate $H_x$ sequence used to nucleate and propagate spin textures: $H_S\rightarrow H_R\rightarrow H_P\rightarrow H_R)$. (b-e) MTXM images at Fe L$_3$ edge with $\theta=30^\circ$, sensitive to $(m_x,m_z)$ within the Ni$_{80}$Fe$_{20}$ layer (contrast legend is indicated in the insets), and $H_y=0$: (b) initial remanence after $\mu_0H_S=+800$ mT, (c) partial reversal after $\mu_0 H_P= -13$ mT pulse. Dotted squares indicate different bifurcation types. (d-e) Zoom views of (c) showing B3-B2 bifurcations with partial in-plane magnetization reversal. Sketches show MTXM contrast depending on $(m_x,m_z)$ signs. Note that the circulation of magnetization around the core is directly related to the AV propagation path in each bifurcation.
• Figure 3: (a) Statistics of $m_y$ orientation at bifurcation cores as a function of transverse dc field with $\mu_0H_S=+800$ mT and $\mu_0 H_P=- 13$ mT; (b) Fraction of $+m_y$ bifurcation cores vs. $\mu_0 H_y$: red squares ($\mu_0 H_S = +800$ mT, $\mu_0 H_P=- 13$ mT), blue dots ($\mu_0H_S =-800$ mT, $\mu_0 H_P=+ 13$ mT). Note that bifurcations with AV propagation beyond the field of view have also been included to increase the number of events. Dashed lines indicates $\mu_0 H_y^{*}$, corresponding to 50% $\pm m_y$ probability; shaded areas indicate the field intervals $\Delta\mu_0H_y$ with mixed presence of $\pm m_y$ cores for each $H_S$ polarity. (c) $H_y$vs.$H_S$ diagram of preferred bifurcation branches for AV propagation.
• Figure 4: (a) Orientation of stripe domains at remanence ($\alpha$) vs.$H_S$ field orientation ($\beta$) with $\mu_0 H_S= 150$ mT. Inset is a sketch of the magnetic field and stripe domain geometry relative to the membrane frame and to the easy axis of $K_u($NdCo$)$. Solid line indicates $\alpha=\beta$, corresponding to stripe pattern orientation under pure rotatable anisotropy. Shaded area indicates the angular region dominated by $K_u($NdCo$)$. (b-c) Sketches of orientation of the stripe pattern at remanence relative to in-plane anisotropy axis at $\alpha_0=12^\circ$ and the fields applied during the AV propagation experiment ($H_S$ and $H_P$ at $\beta=0^\circ$ and $H_y$ at $\beta=90^\circ$).