Microwave resonances of magnetic skyrmions in thin film multilayers

Static and dynamic magnetic properties of circular and square cobalt nanodots in hexagonal cells

In this work we performed a detailed numerical analysis to investigate the static and dynamic magnetic properties of hexagonal cells of square and circular cobalt nanodots as a function of the distance between them and the external magnetic field to which they are subjected. By simulating hysteresis curves with the external magnetic field applied parallel and perpendicular to the plane of these nanostructures, we can conclude that the cobalt nanodots presented a significant perpendicular magnetic anisotropy. We also obtained that the coercivity increases with decreasing volume, which implies that the circular dots have a higher coercivity than the square dots. Furthermore, we studied the dynamic susceptibility of these systems and found that it is possible to control both the position and the number of resonance peaks by controlling the geometry and the distance between the magnetic nanodots. This work provides useful information on the behaviour of cobalt nanodot arrays, opening paths for the design and improvement of two-dimensional technological devices.

Tunable Spin Textures in a Kagome Antiferromagnetic Semimetal via Symmetry Design

Kagome antiferromagnetic semimetals such as Mn3Sn have attracted extensive attention for their potential application in antiferromagnetic spintronics. Realizing high manipulation of kagome antiferromagnetic spin states at room temperature can reveal rich emergent phenomena resulting from the quantum interactions between topology, spin, and correlation. Here, we achieved tunable spin textures of Mn3Sn through symmetry design by controlling alternate Mn3Sn and heavy-metal Pt thicknesses. The various topological spin textures were predicted with theoretical simulations, and the skyrmion-induced topological Hall effect, strong spin-dependent scattering, and vertical gradient of spin states were obtained by magnetotransport and magnetic circular dichroism (MCD) spectroscopy measurements in Mn3Sn/Pt heterostructures. Our work provides an effective strategy for the innovative design of topological antiferromagnetic spintronic devices.

Ferroelectrically tunable magnetic skyrmions in two-dimensional multiferroics

Magnetic skyrmions are topologically protected entities that are promising for information storage and processing. Currently, an essential challenge for future advances of skyrmionic devices lies in achieving effective control of skyrmion properties. Here, through first-principles and Monte-Carlo simulations, we report the identification of nontrivial topological magnetism in two-dimensional multiferroics of Co2NF2. Because of ferroelectricity, monolayer Co2NF2 exhibits a large Dzyaloshinskii-Moriya interaction. This together with exchange interaction can stabilize magnetic skyrmions with the size of sub-10 nm under a moderate magnetic field. Importantly, arising from the magnetoelectric coupling effect, the chirality of magnetic skyrmions is ferroelectrically tunable, producing the four-fold degenerate skyrmions. When interfacing with monolayer MoSe2, the creation and annihilation of magnetic skyrmions, as well as phase transition between skyrmion and skyrmion lattice, can be realized in a ferroelectrically controllable fashion. A dimensionless parameter κ' is further proposed as the criterion for stabilizing magnetic skyrmions in such multiferroic lattices. Our work greatly enriches the two-dimensional skyrmionics and multiferroics research.

Skyrmion-Excited Spin-Wave Fractal Networks

Magnetic skyrmions exhibit unique, technologically relevant pseudo-particle behaviors which arise from their topological protection, including well-defined, 3D dynamic modes that occur at microwave frequencies. During dynamic excitation, spin waves are ejected into the interstitial regions between skyrmions, creating the magnetic equivalent of a turbulent sea. However, since the spin waves in these systems have a well-defined length scale, and the skyrmions are on an ordered lattice, ordered structures from spin-wave interference can precipitate from the chaos. This work uses small-angle neutron scattering (SANS) to capture the dynamics in hybrid skyrmions and investigate the spin-wave structure. Performing simultaneous ferromagnetic resonance and SANS, the diffraction pattern shows a large increase in low-angle scattering intensity, which is present only in the resonance condition. This scattering pattern is best fit using a mass fractal model, which suggests the spin waves form a long-range fractal network. The fractal structure is constructed of fundamental units with a size that encodes the spin-wave emissions and are constrained by the skyrmion lattice. These results offer critical insights into the nanoscale dynamics of skyrmions, identify a new dynamic spin-wave fractal structure, and demonstrate SANS as a unique tool to probe high-speed dynamics.

Tailoring of the Interfacial Dzyaloshinskii-Moriya Interaction in Perpendicularly Magnetized Epitaxial Multilayers by Crystal Engineering

The interplay between the interfacial crystalline structure and Dzyaloshinskii-Moriya interaction (DMI) was investigated by Fe insertion in epitaxial Pt/Co/Ir perpendicular magnetized multilayers. The experimental results with the support of first-principles calculation indicate that the Fe/Ir interface exhibits a positive interfacial DMI (iDMI) originating from the fcc crystalline structure inserted by 2 monolayers (ML) Fe, while a negative one from the structure with a layer shifting of 1-ML Fe insertion. The total iDMI of the multilayers increases (decreases) due to the additive enhancement (competitive counteraction) between the iDMI of Fe/Ir and Pt/Co interfaces. Comparing the iDMI of single-crystalline and textured multilayers, the iDMI of multilayers is found to be particularly sensitive to the crystallinity nearby the heterointerfaces. This work is of vital importance to reveal a deeper insight into the physical mechanism of the iDMI and provides a viable strategy for tailoring the iDMI of the multilayers by crystal engineering.

Effect of Multilayered Structure on the Static and Dynamic Properties of Magnetic Nanospheres

The development of flexible and lightweight electromagnetic interference (EMI)-shielding materials and microwave absorbers requires precise control and optimization of core-shell constituents within composite materials. Here, a theoretical model is proposed to predict the static and dynamic properties of multilayered core-shell particles comprised of exchange-coupled layers, as in the case of a spherical iron core coupled to an oxide shell across a spacer layer. The theory of exchange resonance in homogeneous spheres is shown to be a limiting special case of this more general theory. Nucleation of magnetization reversal occurs through either quasi-uniform or curling magnetization processes in core-shell particles, where a purely homogeneous magnetization configuration is forbidden by the multilayered morphology. The energy is minimized through mixing of modes for specific interface conditions, leading to many inhomogeneous solutions, which grow as 2ⁿ with increasing layers, where n represents the number of magnetic layers. The analytical predictions are confirmed using numerical simulations.

Flexoresponses of Synthetic Antiferromagnetic Systems Hosting Skyrmions

While strain gradients break lattice centrosymmetry, ferromagnetism is a time-reversal symmetry breaking product. Flexomagnetic effect in ferromagnets is usually indirect and weak. In this Letter, we reveal a topologically enhanced flexomagnetic effect in synthetic antiferromagnetic systems based on Dzyaloshinskii-Moriya interaction and the large deformability of skyrmion. Moreover, the synthetic antiferromagnetic skyrmion exhibits an unexpected Hall effect under strain gradient. We propose that this flexo-Hall effect originates from a geometric Magnus force related to the asymmetric deformation of skyrmion. Our results shed new insights into the flexoresponses in systems hosting topological structures and may open up a new field-"flexoskyrmionics".