Enhanced controllability of domain-wall pinning by asymmetric control of domain-wall injection

We investigate a control scheme for enhancing the controllability of domain-wall (DW) pinning on ferromagnetic devices using an interaction between magnetic charges distributed on a nanobar and at a notch, respectively. The scheme is realized at an artificial notch with a nanobar vertical to it on Permalloy nanowires with an asymmetrical pad. Injection fields for injecting the DWs from the asymmetrical pad to the nanowire show an asymmetrical dependence on the saturation angle for initializing the magnetization of the nanowire, and the injected DWs are pinned by the notch with the nanobar vertical to it. We have found that the landscape of the pinning potential energy experienced by the DWs depends on the magnetized direction of the nanobar and that its level is shifted by the injection field, leading to an increase or decrease in the depinning field with respect to the saturation angle. This is consistent with our estimation based on micromagnetic simulation. (Some figures may appear in colour only in the online journal) Studies concerning the magnetic charge distribution in a magnetic domain wall (DW) pinned at a pinning geometry are important for a comprehensive characterization of the pinning potential landscape for reproducible positioning of the DW on DW-based spintronic devices, because the positioning of the DW is closely connected to the pinning behaviors of the DW at a constriction and the DW pinning is sensitive to the initial DW states, the DW structure, and the chirality [1–3]. Moreover, the pinning behavior of the DW in a ferromagnetic nanowire provides models crucial for understanding the fundamental physical properties of DW motion [4–6]. However, the stochastic nature of DW propagation leads to unstable positioning of the DW on the spintronic devices because the lithographically patterned constrictions and edges in the nanowire mainly give rise to stochastic behaviors of the DW. Recently, using magnetostatic interactions via the magnetic charge distributions, a few solutions for manipulating the DW pinning without additional alterations to the nanowire have been reported [7, 8]. Even though the DW pinning via the magnetostatic interaction is locally controlled, the pinning strength is insufficient for reliably manipulating the DW propagation in the real DW-mediated device. In this study, we suggest a new control scheme of enhancing the controllability by employing an asymmetrical injection pad for the nanowire with the pinning geometry composed of a notch and a nanobar vertical to it. When the DW injected from the asymmetrical pad propagates under the magnetostatic interaction between the magnetic charge distribution in the DW at the notch and the stray field generated by the magnetized nanobar, the pinning behavior of the DW at the pinning geometry is revealed by presenting the local DW energy landscapes depending on the direction of the magnetized nanobar. For this study, a Permalloy (Ni81Fe19) nanowire, 280 nm wide and 10 nm thick, and a nanobar perpendicular to each other were fabricated by electron beam lithography, dc-magnetron sputtering, and a lift-off process; the nanowire 1 0957-4484/13/105304+05$33.00 c © 2013 IOP Publishing Ltd Printed in the UK & the USA Nanotechnology 24 (2013) 105304 S-M Ahn and K-W Moon Figure 1. Scanning electron microscope image of an Ni81Fe19 nanowire, 280 nm wide and 10 nm thick, and a nanobar perpendicular to each other with an edge-to-edge separation of approximately 30 nm and an elliptical pad. Regions enclosed by dashed lines indicate an asymmetrical junction between the pad and the nanowire with an expected position A in DW injection, and the notch with expected position B in DW pinning, respectively. I and II on the wire indicate the positions of the probing laser spot for the magneto-optic Kerr effect (MOKE) measurement. The inset shows the high-resolution image of the notch. The arrows indicate the magnetic field directions of HS and HA with the saturation angle θS. and nanobar had an edge-to-edge separation of approximately 30 nm and an asymmetrically positioned elliptical pad (0.71μm×1.29 μm) for DW nucleation, as shown in figure 1. The nanowire structure consists of Ta (5 nm)/Ni81Fe19 (10 nm)/Ta (5 nm) films on Si/SiO2 (100 nm) substrates. A notch is placed in the middle of the nanowire, as depicted in the figure. The notch depth is approximately 107 nm, which is roughly 40% of the nanowire width. The DW propagation along the nanowire is measured by using a longitudinal magneto-optic Kerr effect (MOKE) measurement system [9] with a laser (wavelength: 405 nm) and an objective lens (numerical aperture: 0.9). In order to quantize the DW propagation, we denote the two critical fields as the injection field HINJ for injecting the DW from the pad to the nanowire and the depinning field HDEP for escape of the DW from the notch with the nanobar perpendicular to it. The laser spot is placed at a position I on the nanowire in the vicinity of the junction for measuring HINJ and then at a position II on the nanowire near the notch for HDEP, as shown in figure 1. The measurement procedure is similar to the previous one, as follows [7]: to fully saturate the magnetization of the pad structure, an external magnetic field pulse HS (∼400 Oe and 1 s) is first applied to the structure with respect to an angle θS (140–220), and a DW is nucleated at the junction between the pad and the wire after turning off the magnetic field [7, 10]. A magnetic field is then applied rightward up to small HA in the horizontal direction in order to inject the DW with a controlled magnetic structure into the nanowire, depending on the angle of the saturation field. Finally, the magnetic field is applied rightward up to large HA in the horizontal direction in order to depin the pinned DW from the notch with the nanobar to the right. Considering our nanowire dimensions, we used a vortex DW (VDW) with a characteristic round shape as the stable DW structure [11]. To previously check whether it is possible to asymmetrically control injection of the DW into the nanowire with respect to the angle of HS—the saturation angle— micromagnetic simulations have been performed by use of the OOMMF software package [12]. In the simulation, we use the same geometry as in the experiments and the magnetic properties of permalloy for the saturation magnetization MS (8.6 × 105 A m−1) and the exchange stiffness AX (1.3 × 10−11 J m−1). Using the same procedure as previously [10], the simulations are carried out with several saturation angles. Each equilibrium state is attained for each magnetic field and a maximum internal torque less than 2×10−4M2 S is employed as the criterion of equilibrium. At the saturation angles smaller than 180 corresponding to figure 2(i), the magnetization around the junction has abrupt changes with narrower structure than the magnetization at the saturation angles larger than 180 corresponding to figure 2(ii), as shown in figures 2(a) and (b), respectively. Figures 2(c) and (d) summarize the critical fields such as HINJ and HDEP depending on the symmetrically and asymmetrically positioned pad geometry—the symmetric and asymmetric pads with respect to the saturation angle, respectively. As shown in figure 2(c), unlike HINJ from the symmetric pad, HINJ from the asymmetric pad is 57 Oe for the upward magnetized range and 36 Oe for the downward magnetized range of the saturation angles, which implies that injection of the VDW from the pad to the nanowire is controlled by the pad geometry [10, 13]. Interestingly, HDEP follows the same trend as HINJ with respect to the saturation angle, as shown in figure 2(d). In the range with the saturation angle of less than 180, HDEP is observed to be 72 Oe, and in the range with the saturation angle of more than 180, 49 Oe, which reflects that behaviors of depinning from the notch with the nanobar depend on the pad geometry as well as the magnetized direction of the nanobar. In particular, we believe that the two levels of HINJ and HDEP with respect to the saturation angle are ascribable to vortex magnetization states, chiralities that are controlled by HS with specific directions in the asymmetric pad [13]. Considering the differences between HINJ and HDEP, the difference is about 18 Oe for the upward magnetization of the nanobar and about 12 Oe for the downward magnetization of the nanobar. The difference is mainly caused by the magnetostatic interaction at the notch with the nanobar but is not enough to properly control the DW position in the middle of the DW propagation in the DW-mediated device. Thus, a sufficient enhancement of the depinning field is required to compensate for uncontrollability caused by these unexpected depinning behaviors of the DWs. To understand our observation using the OOMMF package [12], micromagnetic calculations are performed. The modeled nanowire 300 nm wide and 10 nm thick is similar to the nanowire in the experiment and the nanobar is perpendicular to it with an edge-to-edge separation of 30 nm, as depicted in the insets of figure 3. The standard magnetic parameters for Permalloy, a cell size of 5 nm, and a damping constant α of 0.5, are used in the simulation. When we define E INT as the magnetostatic energy of interaction between the surface charge distribution of the