Speed and reliability of magnetic domain wall (DW) motion are key parameters that must be controlled to realize the full potential of DW-based magnetic devices for logic and memory applications. A major hindrance to this is extrinsic DW pinning at specific sites related to shape and material defects, which may be present even if the sample synthesis is well controlled. Understanding the origin of DW pinning and reducing it is especially desirable in electrochemically-deposited cylindrical magnetic nanowires (NWs), for which measurements of the fascinating physics predicted by theoretical computation have been inhibited by significant pinning. We experimentally investigate DW pinning in Co$_x$Ni$_{100-x}$ NWs, by applying quasistatic magnetic fields. Wire compositions were varied with $x=20,30,40$, while the microstructure was changed by annealing or varying the pH of the electrolyte for deposition. We conclude that pinning due to grain boundaries is the dominant mechanism, decreasing inversely with both the spontaneous magnetization and grain size. Second-order effects include inhomogeneities in lattice strain and the residual magnetocrystalline anisotropy. Surface roughness, dislocations and impurities are not expected to play a significant role in DW pinning in these wire samples.

We establish a method to produce cylindrical magnetic nanowires displaying several segments, with a large versatility in terms of segment diameter and length. It is based on electroplating in alumina templates, the latter being prepared by several steps of anodization, wet etching and atomic layer deposition to produce, widen or shrink pores, respectively. We propose an analytical model to analyze the in-plane and out-of-plane magnetization loops of dense assemblies of multisegmented wires. The model considers inter-wires dipolar fields, end-domain curling and predicts the switching field of individual wires with no adjustable parameter. Its ingredients are crucial to extract reliable parameters from the fitting of loops, such as magnetization or the porosity of the array.

We report the imaging of magnetic domains in ferromagnetic CoNiB nanotubes with very long aspect ratio, fabricated by electroless plating. While axial magnetization is expected for long tubes made of soft magnetic materials, we evidence series of azimuthal domains. We tentatively explain these by the interplay of anisotropic strain and/or grain size, with magneto-elasticity and/or anisotropic interfacial magnetic anisotropy. This material could be interesting for dense data storage, as well as curvature-induced magnetic phenomena such as the non-reciprocity of spin-wave propagation.

We report the fabrication and magnetic imaging of high-aspect ratio CoNiB nanotubes. With XMCD-PEEM we evidence multiple magnetic domains and domain walls in these nanotubes. Surprisingly, magnetization in the domains is orthoradial (azimuthal, vortex-like), a situation not anticipated by theory for long nanotubes. The material is therefore technologically appealing for a dense 3D magnetic device such as the racetrack memory (based on shifting magnetic walls), as flux-closure domains should efficiently prevent cross-talk related to internal dipolar fields. Further, we show tuning of a growth-induced anisotropy and thus of the magnetic state of the tube by annealing.

A preparative strategy is presented towards a three-dimensional magnetic data storage medium based on arrays of parallel, cylindrical metallic nanowires structured along their length. The preparation utilizes porous anodic alumina as an inert template that defines the geometry (cylinder length, diameter and pitch), combined with galvanic deposition of the functional materials inside the pores. We demonstrate that the composition of nickel-cobalt alloys can be tuned systematically at one constant deposition potential, whereby the magnetocrystalline anisotropy can be minimized. The wires then display maximal shape anisotropy. The definition of bits along the 'vertical' direction of each wire is performed either by the introduction of non-magnetic segments or by modulation of the pore diameter. The former strategy is made possible by pulsed electrodeposition from a ternary electrolyte. The latter structure is achieved by combining several anodization steps. We demonstrate the presence of magnetic domain boundaries at such designed pinning points using bulk magnetometry and single-wire imaging techniques. As future read/write elements we embed ultrathin oxide layers at mid-height of the wires by atomic layer deposition, and we demonstrate a measurable magnetoresistance effect of these layers. Figure 1