Controlled magnetization by magnetic fields in multisegmented cylindrical nanowires

Abstract

Cylindrical magnetic nanowires (NWs) are promising candidates for building blocks of 3D nanoarchitectures and 3D nanotechnologies [1,2]. They present great potential for 3D applications, offering responses to many external (electrical, magnetic, mechanical, thermal, etc.) stimuli [3].The emerging interest toward cylindrical magnetic NWs is related to the fact that their geometry promotes topologically non-trivial 3D magnetic structures such as Bloch-point domain walls, complex configurations that connect vortex domains with opposite chirality, vortices or skyrmion tubes [4,5].Here, we present a study on multisegmented NWs formed by ferromagnetic (FM) segments separated either by a different FM material or a non-magnetic (NM) one. On one hand we have alternating CoNi/Ni segments (Fig. 1 (a)), where the domain structure can be tailored by alternating magnetocrystalline anisotropy and the length of each segment. On other hand, FeCo ferromagnetic layers separated by thin Cu NM layers (Fig. 1 (c)). The multisegmented NWs were prepared by electroplating filling the pores of anodic aluminum oxide membranes.The HRTEM analysis confirms the multisegmented morphology with a uniform diameter, and periodic and sharp interfaces. Quantitative analysis reveals that the diameter of CoNi/Ni NWs is about 140 nm with lengths between 0.6 μm and 1.4 μm for CoNi and 1 μm for Ni segments while FeCo/Cu wires with 165 nm in diameter are formed by FeCo segments with 250 nm in length separated by 50 nm Cu layers.XMCD-PEEM imaging of multisegmented CoNi/Ni NWs reveals that the magnetic structure in Ni segment is in a dominantly axial magnetic state while in CoNi segments it depends on their length. In NWs with the shortest CoNi segments (0.6 μm), we observed single vortex domains, in longer segments of 1.2 μm we observed one to three vortex domain states (Fig. 1 (a)), while in the NWs with the longest CoNi segments either multi-vortex or multi-transverse domains were observed.The micromagnetic simulations show that the remanent magnetic structure of CoNi segments depends on the direction of the previously applied field. If the direction is perpendicular to both the NW and the magnetization easy axis, the vortex domains simply expand inside the segment following the structures formed at the interfaces and resulting in one vortex domain for short segments and two or more vortex structures for longer segments. In turn, when the field is applied perpendicular to the NW but along the magnetization easy axis, transverse domains are formed. They are separated by vortex domain structures with core at the surface pointing perpendicular to it.Furthermore, although Ni segments are in an almost single-domain state, they are not completely saturated, and present a small magnetization curling on the NW surface. Their chirality is formed prior to formation of magnetic structures in CoNi and imprints the chirality along the next CoNi segments (Fig. 1 (b)) [5].The second investigated multisegmented system, FeCo/Cu (Fig. 1 (c-d)), presents a tilt at the interface between FM/NM layer. From the XMCD-PEEM images (Fig. 1 (c)) we determined that each FeCo segment presents a single vortex state which switches its magnetization under the influence of perpendicular magnetic fields. This switching is sequential and each segment has a slightly different switching field. The simulations (Fig. 1 (d)) unveil the main differences between magnetization states when the field is applied along different directions with respect to the NW axis. The tilt between adjacent segments breaks the rotational symmetry providing different magnetic potentials for vortices moving in a certain direction under the influence of applied magnetic field.The proper design of segments anisotropy and geometry (e.g., surface tilting) in cylindrical multisegmented nanowires opens multiple opportunities for advanced nanotechnologies in 3D magnetic nanostructures. **