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Recent studies in three dimensional spintronics propose that the ��rsted field plays a significant role in cylindrical nanowires. However, there is no direct report of its impact on magnetic textures. Here, we use time-resolved scanning transmission X-ray microscopy to image the dynamic response of magnetization in cylindrical Co$_{30}$Ni$_{70}$ nanowires subjected to nanosecond ��rsted field pulses. We observe the tilting of longitudinally magnetized domains towards the azimuthal ��rsted field direction and create a robust model to reproduce the differential magnetic contrasts and extract the angle of tilt. Further, we report the compression and expansion, or breathing, of a Bloch-point domain wall that occurs when weak pulses with opposite sign are applied. We expect that this work lays the foundation for and provides an incentive to further studying complex and fascinating magnetization dynamics in nanowires, especially the predicted ultra-fast domain wall motion and associated spin wave emissions., 6 pages, 3 figures in main text and supplementary materials

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. Sch\"obitz \etal, 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.

The concept of perpendicular shape anisotropy spin-transfer torque magnetic random-access memory (PSA-STT-MRAM) consists in increasing the storage layer thickness to values comparable to the cell diameter, to induce a perpendicular shape anisotropy in the magnetic storage layer. Making use of that contribution, the downsize scalability of the STT-MRAM may be extended towards sub-20 nm technological nodes, thanks to a reinforcement of the thermal stability factor $\Delta$. Although the larger storage layer thickness improves $\Delta$, it is expected to negatively impact the writing current and switching time. Hence, optimization of the cell dimensions (diameter, thickness) is of utmost importance for attaining a sufficiently high $\Delta$ while keeping a moderate writing current. Micromagnetic simulations were carried out for different pillar thicknesses of fixed lateral size 20 nm. The switching time and the reversal mechanism were analysed as a function of the applied voltage and aspect-ratio (AR) of the storage layer. For AR $$ 1 a non-coherent reversal is observed, characterized by the nucleation of a transverse domain wall at the ferromagnet/insulator interface which then propagates along the vertical axis of the pillar. It was further observed that the inverse of the switching time is linearly dependent on the applied voltage. This study was extended to sub-20 nm width with a value of $\Delta$ around 80. It was observed that the voltage necessary to reverse the magnetic layer increases as the lateral size is reduced, accompanied with a transition from macrospin-reversal to a buckling-like reversal at high aspect-ratios., Comment: 7 pages with a total of 8 figures

Poster presented at the International Magnetics Conference (INTERMAG Europe 2017), held in Dublin (Ireland) on April 24-28th, 2017., Magnetic nanowires (NWs) play an important role in several future technologies such as magnetic recording, magnetic sensors or bio applications [1]. Control over the magnetization is essential in technologies where reversal process takes place via nucleation/propagation and pinning/depinning of domain wall (DW). This concerns concepts such as race track memory or logic devices [2]. So far, for ease of fabrication and detection, research has been mostly focused on lithographically prepared nanostrips, where DWs can be artificially nucleated and subsequently pinned [3]. The use of cylindrical nanowires is the route towards 3D memory devices[4]. A renewed attention towards cylindrical nanowires is due to their specific characteristics which theoretically allow a larger variety of magnetic topologies for DWs than in nanostrips, particularly Bloch-point walls, or the absence of the Walker limit for the DW motion [5,6]. Recent studies have reported the domain structure and magnetization reversal in individual nanowires by experimental (e.g., MFM, MOKE, PEEMS/XRCD) methods and micromagnetic simulations [7,8]. The modeling shows that demagnetization process in these cases takes place by domain wall motion mechanisms. Cylindrical nanowires are typically grown by electrochemical route with designed diameter in the range 15 to 200 nm. Recently, NWs with tailored longitudinal modulations in diameter or compositionally multisegmented (multilayer) NWs have been fabricated and characterized. A particular objective of such modulations is to achieve control for motion and pinning of DWs to be shifted [9]. The understanding of the corresponding magnetization configuration and the DW pinning at the modulations are essential for the development of such novel technologies. Several types of magnetic domains can appear in NWs, namely, with longitudinal, transverse or circumferential magnetization. On the other hand, transverse or azimuthal vortex, (that is Bloch-point like), domain walls can be found. In the present study nanowires with different diameters and composition have been considered where transverse or vortex domain walls can be nucleated. We investigate particularly the influence of diameter modulations. Two kinds of cylindrical nanowires have been simulated by micromagnetics: i) FeCo NWs with fcc polycrystalline structure and diameter modulations between 40 and 130 nm, and ii) FeNi NWs with diameter modulations between 20 and 50 nm. Our results show that FeCo NWs are characterized by vortex magnetization configurations. Different vortex chiralities were obtained depending on the polycrystalline structure distribution. The switching field depends on the difference of segments diameter and the chirality of the nucleated DWs along the nanowire constrictions. Pinning of domain walls at the diameter transitions is analysed for given chirality patterns and diameter values. Figure 1 shows the hysteresis loops and the corresponding DW chiralities in the simulated NW with particular distribution of grain structure. In the case of FeNi nanowires transverse domain walls are formed. Here we provide a simple analytic model for the DW pinning at the diameter transition under a uniform applied field. A flat transverse DW is assumed creating magnetic charges uniformly confined within a sphere of the same radius as that of the NW and centered at the DW position. This model leads to a scaling law for the depinning field and pinning potential, which fairly agrees with the results obtained in the micromagnetic simulations. Figure 2 shows the final position of the DW for different applied fields, H, obtained from micromagnetic simulations and analytical model for a nanowire with diameters 20 and 10 nm. The dependence of the DW width with the modulation size (height and width) was also numerically studied.