Engineering Magnetic Anisotropy in Permalloy Films via Atomic Force Nanolithography

Abstract

Atomic force nanolithography provides a precise method for sculpting magnetic thin films, enabling controlled engineering of magnetic anisotropy in soft ferromagnets at the microscale. We demonstrate that nanoscale groove arrays patterned into permalloy (Ni80Fe20) films induce a robust in-plane uniaxial anisotropy, with the easy axis aligned along the groove direction. The anisotropy field is shown to increase with decreasing groove period and increasing engraving depth, offering continuous tunability of magnetic hardness within a single fabrication step. Artificially engraved microstructures further allow domain configurations and domain-wall trajectories to be directed along predefined pathways, exemplified by the creation of a chessboard-like magnetic landscape. Owing to its adaptability to diverse ferromagnetic materials and arbitrary corrugation geometries, this approach provides a versatile platform for tailoring in-plane magnetic anisotropy. Concrete applications are demonstrated in the design of magnonic elements and anisotropic magnetoresistance sensors.

Figures (6)

• Figure 1: (a) Illustration of the surface artificial grooves engraving (SAGE) process, highlighting the relevant geometrical parameters. The grooves define the magnetic easy axis and the associated uniaxial magnetic anisotropy $K_u$ . (b) AFM topography image of a $5 \times 5$$\mu$m$^2$ region with SAGE pattern. The cut-view profile along the 3 µm-long A-B section shows triangular grooves with homogeneous width and depth. (c) Normalized magnetic hysteresis loops showing the magnetization components parallel ($M_{\parallel}$, black dots) and perpendicular ($M_{\perp}$, red dots) to the magnetic field applied 5$^\circ$ off the SAGE hard axis (HA). The switching fields for the backward and forward branches are denoted as $H_s^-$ and $H_s^+$, respectively. The normalized hysteresis loop for the magnetization component along the SAGE easy axis ($M_{\mathrm{EA}}$, blue dots) is also shown for the field applied along the SAGE EA. The inset shows a scanning electron microscopy image of the surface structure of the investigated sample after SAGE.
• Figure 2: (a,b) Variation of the coercive field $H_c$ and (c,d) the anisotropy field $H_k$ as a function of the SAGE groove depth and period, respectively. Red dots represent experimental results, while black dots correspond to micromagnetic simulations incorporating a corrugated top surface. Dashed lines serve as guides to the eye. The upper horizontal axis in (a,b) shows the estimated energy density $K_u$, derived from micromagnetic simulations assuming an ideal homogeneous UMA. Blue solid lines represent the predictions of Schlömann’s model for ideal periodic rectangular grooves, while blue square symbols correspond to the same model using the roughness values measured by AFM. The green shaded area in (c) indicates the transition in the magnetization-reversal mechanism. (e) Magneto-Optic Kerr microscopy (MOKE) images of three devices with different scratching depths, acquired at two successive applied fields close to $H_a \simeq H_c$.
• Figure 3: Angular dependence of (a) $H_c$ and (b) $M_r$ for SAGE-patterned ($d=1.5$ nm, $\lambda=600$ nm) and pristine 50-µm-wide Py disks. Experimental results have been obtained by L-MOKE. Results are fitted using the Two-Phase Pinning model, assuming the presence of two independent uniaxial anisotropies. Solid dots denote measured data and empty dots show their mirrored counterparts for clarity.
• Figure 4: (a) Schematic illustration of the SAGE pattern designed to promote a chessboard-like domain arrangement. Each square has a side of 12.5 µm. (b) AFM topography at the junction between regions of alternating SAGE direction. (c–d) L-MOKE images showing (c) the domain configuration of a pristine square Py device, and (d) the emergence of a chessboard-like domain arrangement induced by the SAGE-patterned surface.
• Figure 5: (a) False-colored optical microscopy image of the pristine AMR sensor, illustrating the directions of current density ($J$, blue arrow) and applied magnetic field ($H_a$, red arrow). The inset presents a schematic of the corrugated Py stripe with an angle of 45$^\circ$ between the EA and the current direction. The applied in-plane magnetic field is perpendicular to the EA. The SAGE-patterned region spans approximately 90% of the stripe. (b) Comparison of the AMR response ($\mathrm{MR} = \Delta R/R_0$) as a function of the applied magnetic field for the corrugated Py stripe sensor (red) and the pristine Py sensor (black). For clarity, the pristine data have been vertically offset by 0.27% to enhance visual distinction. The blue line represents a fit of the signal near its working point. The inset shows a magnified view of the magnetic field range over which the device operates.