Magnetic Radial Vortex Stabilization and Efficient Manipulation Driven by the Dzyaloshinskii-Moriya Interaction and Spin-Transfer Torque

Abstract

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...

Experimental Demonstration of Spintronic Broadband Microwave Detectors and Their Capability for Powering Nanodevices

“…The experimentally observed behavior is well reproduced by micromagnetic computations [33][34][35][36][37][38] (see Supplementary Note 1 [27]) as shown in Fig. 5.…”

Collective antiskyrmion-mediated phase transition and defect-induced melting in chiral magnetic films

“…In ultra-thin systems with interface-induced anisotropic DMIs, such as Ir/Co/Pt multilayers 23 , 24 , skyrmions form even at room temperature and in zero external magnetic field due to strong PMA. Additionally, it has been shown numerically 25 and experimentally 26 that a new type of magnetic solitons, namely radial vortices, also occur in the presence of weak in-plane anisotropy. Contrarily, antiskyrmion lattices have only been observed in bulk systems such as Mn–Pt–Sn 27 , and some of the current research is focused on stabilizing antiskyrmions in a wider range of materials, including thin films 28 .…”

Impact of Dzyaloshinskii-Moriya interactions on the thermal stability factor of heavy metal/magnetic metal/oxide based nano-pillars

“…Particularly, the interfacial Dzyaloshinskii-Moriya interaction (DMI) 14,15 promotes states of noncollinear magnetization with an intrinsic tilt of magnetization at pillar edges 16,17 , stabilizes cycloidal states like chiral bubbles and skyrmions 10,18 , and assists fast magnetic domain wall (DW) motion 19,20 . Particularly, the creation and the manipulation of skyrmions (chiral bubbles) are mediated by the interfacial DMI, such spin structures being promising in view of conceiving various applications 21,22 . Furthermore, several studies have already pointed out that interfacial DMI significantly reduces the current density required for magnetization switching in Pt/CoFeB/MgO trilayers 23,24 but it also affects negatively the stability properties.…”