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Register a new user3D versus 2D domain wall interaction in ideal and rough nanowires
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The interaction between transverse magnetic domain walls (TDWs) in planar (2D) and cylindrical (3D) nanowires is examined using micromagnetic simulations. We show that in perfect and surface deformed wires the free TDWs behave differently, as the 3D TDWs combine into metastable states with average lifetimes of 300ns depending on roughness, while the 2D TDWs do not due to 2D shape anisotropy. When the 2D and 3D TDWs are pinned at artificial constrictions, they behave similarly as they interact mainly through the dipolar field. This magnetostatic interaction is well described by the point charge model with multipole expansion. In surface deformed wires with artificial constrictions, the interaction becomes more complex as the depinning field decreases and dynamical pinning can lead to local resonances. This can strongly influence the control of TDWs in DW-based devices.
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Interactions between pairs of magnetic domain walls (DW) and pinning by radial constrictions were studied in cylindrical nanowires with surface roughness. It was found that a radial constriction creates a symmetric pinning potential well, with a change of slope when the DW is situated outside the notch. Surface deformation induces an asymmetry in the pinning potential as well as dynamical pinning. The depinning fields of the domain walls were found generally to decrease with increasing surface roughness. A DW pinned at a radial constriction creates a pinning potential well for a free DW in a parallel wire. We determined that trapped bound DW states appear above the depinning threshold and that the surface roughness facilitates the trapped bound DW states in parallel wires.
We study thermally assisted domain wall generation in perpendicular magnetic anisotropy CoFeB trilayer nanowires by the effect of Joule heating. The anomalous Hall effect is utilized to detect magnetization reversal in order to study the domain wall generation. We observe a statistical distribution in the switching process which is consistent with the thermal activation process. Our results show that the proposed method provides an efficient way for generating domain walls in perpendicular magnetic nanowires at predefined locations.
Spin-polarized electric current exerts torque on local magnetic spins, resulting in magnetic domain-wall (DW) motion in ferromagnetic nanowires. Such current-driven DW motion opens great opportunities toward next-generation magnetic devices controlled by current instead of magnetic field. However, the nature of the current-driven DW motion--considered qualitatively different from magnetic-field-driven DW motion--remains yet unclear mainly due to the painfully high operation current densities J_OP, which introduce uncontrollable experimental artefacts with serious Joule heating. It is also crucial to reduce J_OP for practical device operation. By use of metallic Pt/Co/Pt nanowires with perpendicular magnetic anisotropy, here we demonstrate DW motion at current densities down to the range of 10^9 A/m^2--two orders smaller than existing reports. Surprisingly the current-driven motion exhibits a scaling behaviour identical to the field-driven motion and thus, belongs to the same universality class despite their qualitative differences. Moreover all DW motions driven by either current or field (or by both) collapse onto a single curve, signalling the unification of the two driving mechanisms. The unified law manifests non-vanishing current efficiency at low current densities down to the practical level, applicable to emerging magnetic nanodevices.
Topological defects such as magnetic solitons, vortices, Bloch lines, and skyrmions have started to play an important role in modern magnetism because of their extraordinary stability, which can be exploited in the production of memory devices. Recently, a novel type of antisymmetric exchange interaction, namely the Dzyaloshinskii-Moriya interaction (DMI), has been uncovered and found to influence the formation of topological defects. Exploring how the DMI affects the dynamics of topological defects is therefore an important task. Here we investigate the dynamic domain wall (DW) under a strong DMI and find that the DMI induces an annihilation of topological vertical Bloch lines (VBLs) by lifting the four-fold degeneracy of the VBL. As a result, velocity reduction originating from the Walker breakdown is completely suppressed, leading to a soliton-like constant velocity of the DW. Furthermore, the strength of the DMI, which is the key factor for soliton-like DW motion, can be quantified without any side effects possibly arising from current-induced torques or extrinsic pinnings in magnetic films. Our results therefore shed light on the physics of dynamic topological defects, which paves the way for future work in topology-based memory applications.
Using Lorentz transmission electron microscopy we investigate the behavior of domain walls pinned at non-topographic defects in Cr(3 nm)/Permalloy(10 nm)/Cr(5 nm) nanowires of width 500 nm. The pinning sites consist of linear defects where magnetic properties are modified by a Ga ion probe with diameter ~ 10 nm using a focused ion beam microscope. We study the detailed change of the modified region (which is on the scale of the focused ion spot) using electron energy loss spectroscopy and differential phase contrast imaging on an aberration (Cs) corrected scanning transmission electron microscope. The signal variation observed indicates that the region modified by the irradiation corresponds to ~ 40-50 nm despite the ion probe size of only 10 nm. Employing the Fresnel mode of Lorentz transmission electron microscopy, we show that it is possible to control the domain wall structure and its depinning strength not only via the irradiation dose but also the line orientation.
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