PACS:Understanding the physical properties of magnetic skyrmions is important for fundamental research with the aim to develop new spintronic device paradigms where both logic and memory can be integrated at the same level. Here, we show a universal model based on the micromagnetic formalism that can be used to study skyrmion stability as a function of magnetic field and temperature. We consider ultrathin, circular ferromagnetic magnetic dots. Our results show that magnetic skyrmions with a small radius-compared to the dot radius-are always metastable, while large radius skyrmions form a stable ground state. The change of energy profile determines the weak (strong) size dependence of the metastable (stable) skyrmion as a function of temperature and/or field. These results can open a path toward the design of optimal materials for skyrmion based devices.2 Non-linear localized excitations have attracted the attention of physicists for a long time.Such excitations, including solitary waves or solitons, play an important role in optics, quantum field theory, condensed matter and other fields. It is sometimes possible to associate integer numbers (topological charges) to the solitons, which are preserved in their dynamics. Topologically non-trivial magnetization configurations in ferromagnets, such as domain walls, vortices, and skyrmions are currently the focus of a lot of research activity. These spin textures are also candidates for nanoscale device applications-computational paradigms, magnetic storage and programmable logic-due to their small size [1][2][3][4][5][6][7][8][9][10][11][12][13].Skyrmion solutions were obtained first by Skyrme in the non-linear field theory [14].Subsequently chiral skyrmions were predicted [15], and discovered experimentally in noncentrosymmetric cubic B20 compounds [16][17][18][19][20] which permit an antisymmetric anisotropic interaction, namely the Dzyaloshinskii-Moriya interaction (DMI). This arises from a relativistic correction and relies on spin-orbit interactions [21,22]. Recent efforts have focused on materials with interfacial DMI-especially ultra-thin transition metal/heavy metal multilayers with large spinorbit coupling such as Co/Pt and Co/Ir [8,23,24]. The DMI, which corresponds to Lifshitz invariants in the micromagnetic energy functional, is necessary to yield axisymmetric skyrmions in ultrathin magnetic elements and the chiral skyrmions can be further stabilized by external magnetic field [8,23,24]. Temperature is usually considered to be detrimental to skyrmion stability, leading to either the transformation of the skyrmion state into a more energetically favorable state [25] or to nucleation of multiple skyrmions and labyrinth domains [8,23,24]. In addition, recent roomtemperature experiments with an external out-of-plane magnetic field [8,23], showed a strong nonlinear dependence of the skyrmion radius on the external field strength pointing out the key role of the external field. Here, we develop a theoretical approach to skyrmion stability based on minimization ...

Abstract. Magnetic skyrmions are topologically protected states that are very promising for the design of the next generation of ultralow-power electronic devices. In this letter, we propose a magnetic tunnel junction based spin-transfer torque diode with a magnetic skyrmion as ground state and a perpendicular polarizer patterned as nanocontact for a local injection of the current. The key result is the possibility to achieve sensitivities (i.e. detection voltage over input microwave power) larger than 2000V/W for optimized contact diameters. Our results can be very useful for the identification of a new class of spintorque diodes with a non-uniform ground state.

Solitons are very promising for the design of the next generation of ultralow power devices for storage and computation. The key ingredient to achieve this goal is the fundamental understanding of their stabilization and manipulation. Here, we show how the interfacial Dzyaloshinskii-Moriya Interaction (i-DMI) is able to lift the energy degeneracy of a magnetic vortex state by stabilizing a topological soliton with radial chirality, hereafter called radial vortex. It has a non-integer skyrmion number S (0.5<|S|<1) due to both the vortex core polarity and the magnetization tilting induced by the i-DMI boundary conditions. Micromagnetic simulations predict that a magnetoresistive memory based on the radial vortex state in both free and polarizer layers can be efficiently switched by a threshold current density smaller than 10 6 A/cm 2 . The switching processes occur via the nucleation of topologically connected vortices and vortexantivortex pairs, followed by spin-wave emissions due to vortex-antivortex annihilations. 2Magnetic solitons, such as domain walls (DWs) [1,2,3,4], vortices [5,6,7,8,9,10,11] and skyrmions [12,13,14,15,16,17] From a fundamental point of view, the stabilization of a radial vortex gives rise to the possibility to create current densities with radial polarization for particle-trapping applications, such as skyrmion [22], analogously to what radially polarized beams can do in many optical systems [23].In order to show the main features and the possible applications of the radial vortex, extensive micromagnetic simulations have been performed. The first part of this letter focuses on the fundamental properties of the radial vortex, in terms of stability as a function of the i-DMI, 3 topology, nucleation as well as response to an in-plane field. The second part is dedicated to the analysis of the switching process of the radial vortex, underlining the differences with the circular vortex.We consider a CoFeB disk having a diameter d=250nm and a thickness t=1.0nm to have an in-plane easy axis at zero external field. To add the i-DMI, we consider the CoFeB coupled with a Pt (heavy metal) layer as already experimentally observed [24]. The study is carried out by means of a state-of-the-art micromagnetic solver which numerically integrates the Landau-Lifshitz-Gilbert (LLG) equation [25,26,27], that includes the i-DMI contribution Fig.4(a)). An H ext =5.0mT yields a uniform magnetic state. Fig.4(b) shows the results for the circular vortex (|D|=0.0mJ/m 2 ). The displacement occurs along the direction perpendicular to the external field (insets Fig.4(b)) [7,32,33,34,35]. Together with the qualitatively different vortex core shift, the field value that leads to the radial vortex core expulsion is twice the one of the circular vortex. This is ascribed to the i-DMI and, in particular, to its boundary conditions, in fact, the expulsion field increases as a function of |D|. The key reason for that is the need of a larger field to align the out-of-plane tilted spins at the sample edges 5 perpendicu...

Spin-Hall oscillators (SHO) are promising sources of spin-wave signals for magnonics applications, and can serve as building blocks for magnonic logic in ultralow power computation devices. Thin magnetic layers used as “free” layers in SHO are in contact with heavy metals having large spin-orbital interaction, and, therefore, could be subject to the spin-Hall effect (SHE) and the interfacial Dzyaloshinskii-Moriya interaction (i-DMI), which may lead to the nonreciprocity of the excited spin waves and other unusual effects. Here, we analytically and micromagnetically study magnetization dynamics excited in an SHO with oblique magnetization when the SHE and i-DMI act simultaneously. Our key results are: (i) excitation of nonreciprocal spin-waves propagating perpendicularly to the in-plane projection of the static magnetization; (ii) skyrmions generation by pure spin-current; (iii) excitation of a new spin-wave mode with a spiral spatial profile originating from a gyrotropic rotation of a dynamical skyrmion. These results demonstrate that SHOs can be used as generators of magnetic skyrmions and different types of propagating spin-waves for magnetic data storage and signal processing applications.

Nonlinear dynamics has been the key ingredient to improve the performance, in terms of sensitivity, of biased resonant spintronic diodes beyond their semiconductor counterparts. We experimentally demonstrate a nonlinear regime broadband detection for nanoscale spintronic diodes (NSD) where the rectification properties are independent of the input microwave frequency, and compare the device performance with the state of the art Schottky diode for low-power rectification. This regime is achieved in magnetic tunnel junctions with a canted magnetization of the free layer. We further show that the developed NSD provides sufficient dc voltage to supply a low-power nanodevice − a black phosphorus photosensor. Our results could pave the way for using spintronic detectors as building blocks for self-powered nanosystems, such as implantable biomedical devices, wireless sensors, and portable electronics.BIN FANG et al.PHYS. REV. APPLIED 0, XXXXXX (2019) spintronic diodes (NSD) are capable of powering a black phosphorus (BP) photosensor [15][16][17].