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 report on the structural, electric and magnetic properties of (NixCo1−x)B ferromagnetic nanotubes, displaying azimuthal magnetization. The tubes are fabricated using electroless plating in polycarbonate porous templates, with lengths of several tens of micrometers, diameters from 100 nm to 500 nm and wall thicknesses from 10 nm to 80 nm. The resistivity is ∼1.5×10−6Ω m, and the anisotropic magnetoresistance (AMR) of 0.2 to 0.3%, one order of magnitude larger (resp. smaller) than in the bulk material, which we attribute to the resistance at grain boundaries. We determined the azimuthal anisotropy field from M(H) AMR loops of single tubes contacted electrically. Its magnitude is around 10 mT, and tends to increase with the tube wall thickness, as well as the Co content. However, surprisingly it does not dependent much on the diameter nor on the curvature.

We analyze the micromagnetics of short longitudinal modulations of a high-magnetization material in cylindrical nanowires made of a soft-magnetic material of lower magnetization such as permalloy, combining magnetic microscopy, analytical modeling, and micromagnetic simulations. The mismatch of magnetization induces curling of magnetization around the axis in the modulations, in an attempt to screen the interfacial magnetic charges. The curling angle increases with modulation length, until a plateau is reached with nearly full charge screening for a specific length scale~$\Delta_\mathrm{mod}$, larger than the dipolar exchange length of any of the two materials. The curling circulation can be switched by the Oersted field arising from a charge current with typical magnitude $10^{12} A/m^{2}$ for a diameter of $\sim$100 nm, and reaching a maximum for $\Delta_\mathrm{mod}$.

Two types of domain walls exist in magnetically soft cylindrical nanowires: the transverse-vortex wall (TVW) and the Bloch-point wall (BPW). The latter is expected to prevent the usual Walker breakdown, and thus enable high domain wall speed. We showed recently [M. Schöbitz , Phys. Rev. Lett. 123, 217201 (2019)] that the previously overlooked OErsted field associated with an electric current is a key in experiments to stabilize the BPW and reach speed above 600 m/s with spin-transfer. Here, we investigate in detail this situation with micromagnetic simulations and modeling. The switching of the azimuthal circulation of the BPW to match that of the OErsted field occurs above a threshold current scaling with 1/R^3 (R is the wire radius), through mechanisms that may involve the nucleation and/or annihilation of Bloch points. The domain wall dynamics then remains of a below-Walker type, with speed largely determined by spin-transfer torque alone.

High-entropy alloys (HEAs), i.e., multicomponent alloys where (typically five or more) elements are combined in equal, or roughly equal, quantities, are of great current interest, due to their formation of single, simple structured phases, and the unusual properties they can potentially exhibit. Phase presence may be predicted using semi-empirical methods, but deviations from predictions may be seen during the course of alloy synthesis, with the formation of unexpected phases. The generation of such phases may be controlled with knowledge of the effective solidification temperature; in this full article, Chvorinov's rule for solidification time is used to estimate this temperature as part of the design of a new multiphase alloy system, TiVCrZr-Si-x. Further heat treatment of the TiVCrZr-Si system confirms the applicability of this approach. The new compositions demonstrate mechanical properties that suggest potential for optimization for high-temperature applications.

Proccessible FePt 3 alloy nanoparticles with sizes smaller than 50 nm open the avenue to novel magnetic sensor, catalytic and biomedical applications. Our research objective was to establish a highly scalable synthesis technique for production of single-crystalline FePt 3 alloy nanoparticles. We have elaborated a one-pot thermal decomposition technique for the synthesis of superparamagnetic FePt 3 nanoparticles (FePt 3 NPs) with mean sizes of 10 nm. Subsequent tiron coating provided water solubility of the FePt 3 NPs and further processibility as bidental ligands enable binding to catalyst surfaces, smart substrates or biosensors. The chemical composition, structure, morphology, magnetic, optical and crystallographic properties of the FePt 3 NPs were examined using high resolution transmission electron microscopy, high-angle annular dark field-scanning transmission electron microscopy, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy mapping, Fourier transform infrared-attenuated total reflection, X-ray powder diffraction, X-ray photoelectron spectroscopy, vibrating sample magnetometry and UV–Vis absorption spectroscopy.

This corrects the article DOI: 10.1103/PhysRevLett.123.217201.

While the usual approach to tailor the behavior of condensed matter and nanosized systems is the choice of material or finite-size or interfacial effects, topology alone may be the key. In the context of the motion of magnetic domain walls (DWs), known to suffer from dynamic instabilities with low mobilities, we report unprecedented velocities >600  m/s for DWs driven by spin-transfer torques in cylindrical nanowires made of a standard ferromagnetic material. The reason is the robust stabilization of a DW type with a specific topology by the Œrsted field associated with the current. This opens the route to the realization of predicted new physics, such as the strong coupling of DWs with spin waves above >600  m/s.

Dynamique induite par le courant des parois de domaines magnétiques dites à point de Bloch dans les nanofils cylindriques Les nanofils magnétiques cylindriques constituent une plateforme parfaite pour étudier la dynamique de parois de domaine magnétiques, en raison de la mobilité et de la stabilité attendues de la topologiquement unique des parois dites à point de Bloch (BPW). Cette thèse explore expérimentalement le comportement des BPWs dans des nanofils cylindriques individuels soumis à des impulsions de courant électrique d'une durée de quelques nanosecondes. J’ai synthétisé des nanofils de matériaux magnétiquement doux (alliages CoNi et CoFe) d'un diamètre de 60-200 nm, par dépôt électrochimique dans des gabarits nanoporeux. Pour étudier la dynamique de l'aimantation sans entrave, j’ai d'abord optimisé les matériaux des nanofils en fonction du potentiel de piégeage de leur paroi de domaine. J’ai ainsi montré que le champ de propagation est réduit dans les matériaux avec de petits grains cristallins ou avec des valeurs élevées d'aimantation spontanée, ce qui est cohérent avec le piégeage des parois de domaine aux frontières des grains. Le contactage électrique de nanofils individuels dispersés sur des substrats a permis d'envoyer des impulsions de courant électrique de quelques nanosecondes. En utilisant des techniques d'imagerie magnétique, nous avons découvert que, bien que négligé jusqu'à présent, le champ azimutal d'Œrsted induit par le courant électrique a un effet crucial sur l'aimantation : il sélectionne et stabilise les parois exclusivement de type point de Bloch, avec une circulation gauche ou droite déterministe. De plus, j’ai utilisé une imagerie magnétique résolue en temps pour visualiser les effets du champ d'Œrsted en temps réel, et j’ai constaté que le champ induit un enroulement hélicoidal de l'aimantation dans des domaines longitudinaux au repos, et dans un régime de faible densité de courant, il provoque la compression ou l'expansion d'un BPW. Enfin, les mesures du déplacement des BPW induits par le courant montrent qu'en dépit de vitesses pouvant atteindre 2400 m/s, la principale force motrice n'est pas le couple de transfert de spin et pourrait plutôt être liée à l’augmentation de température due au chauffage par effet Joule.