Evidence of Skyrmion-Tube Mediated Magnetization Reversal in Modulated Nanowires

Magnetic nanowires, conceived as individual building blocks for spintronic devices, constitute a well-suited model to design and study magnetization reversal processes, or to tackle fundamental questions, such as the presence of topologically protected magnetization textures under particular conditions. Recently, a skyrmion-tube mediated magnetization reversal process was theoretically reported in diameter modulated cylindrical nanowires. In these nanowires, a vortex nucleates at the end of the segments with larger diameter and propagates, resulting in a first switching of the nanowire core magnetization at small fields. In this work, we show experimental evidence of the so-called Bloch skyrmion-tubes, using advanced Magnetic Force Microscopy modes to image the magnetization reversal process of FeCoCu diameter modulated nanowires. By monitoring the magnetic state of the nanowire during applied field sweeping, a detected drop of magnetic signal at a given critical field unveils the presence of a skyrmion-tube, due to mutually compensating stray field components. That evidences the presence of a skyrmion-tube as an intermediate stage during the magnetization reversal, whose presence is related to the geometrical dimensions of the cylindrical segments

Domain Wall Propagation and Pinning Induced by Current Pulses in Cylindrical Modulated Nanowires

The future developments of three-dimensional magnetic nanotechnology require the control of domain wall dynamics by means of current pulses. While this has been extensively studied in planar magnetic strips (planar nanowires), few reports exist in cylindrical geometry, where Bloch point domain walls are expected to have intriguing properties. Here we report this investigation in cylindrical magnetic Ni nanowires with geometrical notches. Experimental work based on synchrotron X-ray magnetic circular dichroism (XMCD) combined with photoemission electron microscopy (PEEM) indicates that large current densities induce domain wall nucleation while smaller currents move domain walls preferably against the current direction. In the region where no pinning centers are present we found domain wall velocity of about 1 km/s. The domain wall motion along current was also detected in the vicinity of the notch region. Pinning of domain walls has been observed not only at geometrical constrictions but also outside of them. Thermal modelling indicates that large current densities temporarily raise the temperature in the nanowire above the Curie temperature leading to nucleation of domain walls during the system cooling. Micromagnetic modelling with spin-torque effect shows that for intermediate current densities Bloch point domain walls with chirality parallel to the Oersted field propagate antiparallel to the current direction. In other cases, domain walls can be bounced from the notches and/or get pinned outside their positions. We thus find that current is not only responsible for the domain wall propagation but is also a source of pinning due to the Oersted field action

Domain wall propagation and pinning induced by current pulses in cylindrical modulated nanowires

The future developments in 3D magnetic nanotechnology require the control of domain wall dynamics by means of current pulses. While this has been extensively studied in 2D magnetic strips (planar nanowires), few reports on this exist in cylindrical geometry, where Bloch point domain walls are expected to have intriguing properties. Here, we report an investigation on cylindrical magnetic Ni nanowires with geometrical notches. An experimental work based on synchrotron X-ray magnetic circular dichroism (XMCD) combined with photoemission electron microscopy (PEEM) indicates that large current densities induce domain wall nucleation, while smaller currents move domain walls preferably antiparallel to the current direction. In the region where no pinning centers are present, we found a domain wall velocity of about 1 km s. Thermal modelling indicates that large current densities temporarily raise the temperature in the nanowire above the Curie temperature, leading to nucleation of domain walls during the system cooling. Micromagnetic modelling with a spin-torque effect shows that for intermediate current densities, Bloch point domain walls with chirality parallel to the Oersted field propagate antiparallel to the current direction. In other cases, domain walls can be bounced from the notches and/or get pinned outside their positions. We thus found that current is not only responsible for domain wall propagation, but also is a source of pinning due to the Oersted field action.Grants PID2019-108075RB-C31 funded by the Ministry of Science and Innovation MCIN/AEI/10.13039/501100011033 and S2018/NMT-4321 NANOMAGCOST-CM funded by the Government of Madrid Region, Spain. We acknowledge the service from the MiNa Laboratory at IMN and the funding from CM (project SpaceTec, S2013/ICE2822), MINECO (project CSIC13-4E-1794), and EU (FEDER, FSE).Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr00455