Three-dimensional skyrmionic cocoons in magnetic multilayers

Abstract

Three-dimensional spin textures emerge as promising quasi-particles for encoding information in future spintronic devices. The third dimension provides more malleability regarding their properties and more flexibility for potential applications. However, the stabilization and characterization of such quasi-particles in easily implementable systems remain a work in progress. Here we observe a three-dimensional magnetic texture that sits in the interior of magnetic thin films aperiodic multilayers and possesses a characteristic ellipsoidal shape. Interestingly, these objects that we call skyrmionic cocoons can coexist with more standard tubular skyrmions going through all the multilayer as evidenced by the existence of two very different contrasts in room temperature magnetic force microscopy. The presence of these novel skyrmionic textures as well as the understanding of their layer resolved chiral and topological properties have been investigated by micromagnetic simulations. Finally, we show that the skyrmionic cocoons can be electrically detected using magneto-transport measurements.

Fig. 1. Properties of Single Gradient (SG) multilayers with parameters S = 0.1 nm, X₁ = 1.7 nm, N = 13 layers.

a Evolution of the Co thickness for a typical SG multilayer, defined by the number of layers N, the thickness step S, and the starting thickness X₁. b Field evolution of the magnetic textures with micromagnetic simulations after an out-of-plane saturation displayed with isosurfaces (in red, m_z = −0.8, in white, m_z = 0 and in dark blue, m_z = 1). c Corresponding phase map images obtained with MFM accompanied by simulated ones (1 × 1 μm²) at the bottom left corner. The experimental magnetic fields correspond to the simulated ones ± 10 mT and point out-of-plane. The scales shown in the bottom image are common to all MFM maps. d) Magnetization cuts of a selected object, indicated in (b) by the green dotted ellipses, at two different magnetic fields to evidence the vertical evolution and the chirality (CW: clockwise, CCW: counterclockwise).

Fig. 2. Remanent magnetic configuration in DG multilayer with parameters X₁ = 2.0 nm, S = 0.1 nm, Y = 1.0 nm, N = 13 layers, M = 15 repetitions.

a Schematic structure with additional parameters of the thin Co thickness Y and the associated number of repetitions M. The lateral plot shows the Co thickness evolution. b Simulated and experimental phase maps for different magnetic histories. For the simulations (resp. the measurements), the sample was either saturated (resp. demagnetized) out-of-plane (OOP), in-plane (IP) or with a tilted magnetic field, 30^∘ away from the normal (resp. 60^∘). All images share the same length scale, displayed in the top left image.

Fig. 3. DG multilayer field dependency.

a Experimental MFM phase maps measured after a 30^∘ demagnetization (away from the normal). b Relaxed states of micromagnetic simulations, displayed with isosurfaces (in red, m_z = − 0.8, in white, m_z = 0 and in dark blue, m_z = 1), at various magnetic fields starting from a 60^∘ initialization. The magnetic field is applied perpendicularly to the sample.

Fig. 4. Electronic transport measurements on 20 × 100 μm² Hall bars for a DG, associated with the corresponding micromagnetic simulations.

a Measurements of R_xx(H) for different inclinations of the magnetic field. The inset shows the geometry of the experiment. b R_xy(H) at θ = 0^∘ (OOP field) and its derivative. The background colors corresponds to the magnetic phases (U: uniform, C: cocoons, W: worms) as predicted by the simulations. The displayed simulations are associated with the points indicated by a roman number and represent a 1 × 1 μm² area. The experimental R_xy has been normalized by $R_{AHE}^{\theta =0^{\circ }}$ to correspond to the associated m_z value. All the displayed curves, measured or simulated, are acquired from positive field saturation, sweeping the field towards negative values in the yz plane.