The ability to create and manipulate domain walls and vortices in ferromagnetic nanostructures is of a high importance to proposed applications in a range of novel magnetic sensors, in magnetic storage and computation. Precise understanding the magnetic domain wall configuration and its development in dependence on the field value and its orientation is required to optimise the performance of such novel sensors. Simple angular-dependent anisotropic magnetoresistance (AMR) measurements can be used to probe the domain wall configuration, providing that the electrical read-out is sufficiently reliable to unambiguously determine the domain wall state of sensors (Figs 1-2).

Figure 1: (Left) Chip with domain wall devices prepared for transport measurements
(Right) MFM image of an L-shaped domain wall device. Design of the structure is adapted from the work of M Donolato et al

Figure 2a: Mapping of the resistance in dependence on the magnitude and angle of the magnetic field

Figure 2b: Switching of the domain states and the relevant change of the device resistance

We have performed a detailed parametric mapping of the AMR signal in L-shaped permalloy nanowires in  dependence on the magnitude and orientation of the magnetic field and geometry of nanostructures, i.e. width (60-400 nm), shape and vertex of a corner structure. Figure 2 shows the angular dependence of domain-wall induced AMR in 120-nm thick L-shaped permaloy nanowires with a straight corner. Red colour corresponds to the state without a domain wall (high resistance), whereas blue colour - to the state with a domain wall (low resistance). By performing a small step rotation of the magnetic field in relation to the L-structure, we demonstrate that certain combinations of experimental parameters and device geometry can provide a stable domain wall presence in a relatively large field range (Fig 2), which is essential for the optimal device performance.

Figure 3: (Left) AFM image of the device
(Right) Comparison of MOKE (top and middle) and AMR (bottom) results for 150-nm wide device in the same field orientation as shown in the left image

Transport measurements are complemented by MOKE (Fig 3) and MFM (Fig 1) studies allowing for examination of the precise magnetic configuration and its effect on the nucleation and depinning fields. The MOKE experiment is performed in collaboration with the Thin Film Magnetism group at Cambridge University. While MOKE measures the Kerr signal from each individual arm (green line - for horizontal arm; blue line - for vertical arm), AMR signal is measured through the corner of the device and reflects switching of magnetization in both arms - horizontal arm in lower fields and the vertical arm in higher field. As the MOKE and AMR signals probe different aspects of the sample magnetisation, they give complimentary information about device performance, which is essential for optimisation of the design of the domain wall pinning site.