No abstract

Solitonic magnetic excitations such as domain walls and, specifically, skyrmionics enable the possibility of compact, high density, ultrafast, all-electronic, low-energy devices, which is the basis for the emerging area of skyrmionics. The topological winding of skyrmion spins affects their overall lifetime, energetics, and dynamical behavior. In this Perspective, we discuss skyrmionics in the context of the present-day solid-state memory landscape and show how their size, stability, and mobility can be controlled by material engineering, as well as how they can be nucleated and detected. Ferrimagnets near their compensation points are promising candidates for this application, leading to a detailed exploration of amorphous CoGd as well as the study of emergent materials such as Mn4N and inverse Heusler alloys. Along with material properties, geometrical parameters such as film thickness, defect density, and notches can be used to tune skyrmion properties, such as their size and stability. Topology, however, can be a double-edged sword, especially for isolated metastable skyrmions, as it brings stability at the cost of additional damping and deflective Magnus forces compared to domain walls. Skyrmion deformation in response to forces also makes them intrinsically slower than domain walls. We explore potential analog applications of skyrmions, including temporal memory at low density—one skyrmion per racetrack—that capitalizes on their near ballistic current–velocity relation to map temporal data to spatial data and decorrelators for stochastic computing at a higher density that capitalizes on their interactions. We summarize the main challenges of achieving a skyrmionics technology, including maintaining positional stability with very high accuracy and electrical readout, especially for small ferrimagnetic skyrmions, deterministic nucleation, and annihilation and overall integration with digital circuits with the associated circuit overhead.

Skyrmions can be stabilized in magnetic systems with broken inversion symmetry and chiral interactions, such as Dzyaloshinskii-Moriya interactions (DMi). further, compensation of magnetic moments in ferrimagnetic materials can significantly reduce magnetic dipolar interactions, which tend to favor large skyrmions. tuning DMi is essential to control skyrmion properties, with symmetry breaking at interfaces offering the greatest flexibility. However, in contrast to the ferromagnet case, few studies have investigated interfacial DMi in ferrimagnets. Here we present a systematic study of DMI in ferrimagnetic CoGd films by Brillouin light scattering. We demonstrate the ability to control DMI by the coGd cap layer composition, the stack symmetry and the ferrimagnetic layer thickness. the DMi thickness dependence confirms its interfacial nature. In addition, magnetic force microscopy reveals the ability to tune DMI in a range that stabilizes sub-100 nm skyrmions at room temperature in zero field. our work opens new paths for controlling interfacial DMi in ferrimagnets to nucleate and manipulate skyrmions. Magnetic skyrmions due to their non-trivial topology have interesting properties 1-3 that make them attractive for spintronic applications, such as racetrack memory and logic devices 4-6. A magnetic skyrmion designates a chiral spin texture with a whirling spin configuration 7. Skyrmions can be stabilized by broken inversion symmetry and chiral interactions, such as the Dzyaloshinskii-Moriya interactions (DMI) 8,9 , which is an antisymmetric exchange interaction that favors non-collinear neighboring spins. Ultrathin magnetic materials with interfaces to heavy non-magnetic metals with large spin-orbit coupling exhibit interfacial DMI that stabilizes skyrmions and chiral domain walls 10-13. The interfacial DMI and the nucleation of skyrmions have been extensively investigated in ferromagnetic materials 10,14-18. Very recently, magnetic skyrmions and chiral domains were reported in ferrimagnetic systems 19-21. Nearly compensated thin ferrimagnetic films with interfacial DMI are interesting materials due to their low stray fields, reduced sensitivity to external magnetic fields, and fast spin dynamics, which are predicted to lead to ultrasmall and ultrafast skyrmions 19,22. Unlike in ferromagnets where fast current-induced motion of chiral textures is impeded by the Walker breakdown and domain wall pinning 13,23-25 , high domain wall velocities-reaching 1000 m s-1-have been observed in ferrimagnetic CoGd films near the angular momentum compensation temperature 19. In addition, the large dipolar fields in ferromagnets are obstacles to the formation of ultrasmall skyrmion 22. Hence, ferrimagnetic thin films are promising candidates for ultrafast skyrmion-based spintronics. Recently, bulk DMI was reported in an amorphous ferrimagnetic GdFeCo alloy 26. However, the significant advantages of interfacial DMI are that it can be controlled by the nature of the interfaces and widely tuned to stabilize skyrmions. Yet, interfacial DM...

A hybrid semiconductor-superconductor nanowire on the top of a magnetic film in the stripe phase experiences a magnetic texture from the underlying fringing fields. The Zeeman interaction with the highly inhomogeneous magnetic textures generates a large synthetic spin-orbit coupling. We show that this platform can support the formation of Majorana bound states (MBS) localized at the ends of the nanowire. The transition to the topological superconducting phase not only depends on the nanowire parameters and stripe size but also on the relative orientation of the stripes with respect to the nanowire axis. Topological phase transitions with the corresponding emergence or destruction of MBS can be induced by reorienting the stripes or shifting their position, which can be achieved by passing a charge current through the magnetic film or by applying electrically-controlled strain to it. The proposed platform removes the need for external magnetic fields and offers a non-invasive electrical tuning of MBS with the perturbation (current or strain) acting only on thee magnetic film.