Skyrmion dynamics under uniaxial anisotropy

Abstract

Broadband microwave spectroscopy has been performed on single-crystalline GaV4S8, which exhibits a complex magnetic phase diagram including cycloidal, Néel-type skyrmion lattice, as well as field-polarized ferromagnetic phases below 13 K. At zero and small magnetic fields two collective modes are found at 5 and 15 GHz, which are characteristic of the cycloidal state in this easy-axis magnet. In finite fields, entering the skyrmion lattice phase, the spectrum transforms into a multimode pattern with absorption peaks near 4, 8, and 15 GHz. The spin excitation spectra in GaV4S8 and their field dependencies are found to be in close relation to those observed in materials with Bloch-type skyrmions. Distinct differences arise from the strong uniaxial magnetic anisotropy of GaV4S8 not present in so-far known skyrmion hosts. The occurence of nontrivial topology in the spin pattern of magnets has gained considerable interest in condensed matter physics. Recent research focuses on magnetic skyrmions which are thermodynamically stabilized in compounds with noncentrosymmetric crystal structures, in a limited region of the magnetic field versus temperature phase diagram [1][2][3]. Skyrmions are whirllike objects of spins which can crystallize in skyrmion lattices (SkLs) with typical lattice constants from ten to hundred nanometers and give rise to emergent electrodynamics, like the topological Hall effect [4][5][6] or magnetic monopoles [7]. Individual skyrmions have been proposed as building blocks for novel nanomagnetic storage devices [8,9]. The SkL has raised high interest for microwavetechnology applications after collective spin excitations predicted in the GHz range [10] were evidenced in the insulating chiral magnet Cu 2 OSeO 3 [11][12][13][14][15][16]. Later it was found that different metallic, semiconducting, and insulating chiral magnets support the same set of characteristic excitations, i.e., three SkL modes characterized as clockwise (CW), counterclockwise (CCW) and breathing (BR) modes, that all follow a universal behavior [17].Besides the Bloch-type skyrmions reported in the aforementioned works, a Néel-type SkL has recently been discovered in GaV 4 S 8 [18], where the spins rotate radially towards the vortex core. In this semiconductor characterized by V 4 S 4 clusters with spin S = 1 2 [19], a structural Jahn-Teller transition [20] at 44 K is followed by the onset of magnetic order at the Curie temperature T C = 13 K. At the structural transition the lattice is stretched along one of the four body diagonals, resulting in a strongly anisotropic easy-axis magnet. The magnetic multi-domain structure strongly depends on the orientation and strength of the applied magnetic field and gives rise to complex magnetic phase diagrams [see Figures 1(a), 1(c) and 2(a)] including cycloidal (Cyc), SkL, and ferromagnetic (FM) regions. Specifically, the skyrmions do not follow the external magnetic field but are confined to the magnetic easy axes. The phases have been interpreted in terms of a competition of symmetric and ...

Resonant properties of dipole skyrmions in amorphous Fe/Gd multilayers

“…First, the continuous decay of the mode branches associated with dipole skyrmions shows that these textures cannot be identified directly in f-H dispersions given there is no splitting of the mode branches at magnetic fields where the skyrmion phase forms out of the stripe phase. Similar continuous mode branches have been observed for Néel-type DMI skyrmions [25]. Experimental results also show that the arrangement of dipole skyrmions, whether in closed-packed lattices or isolated skyrmions, is not reflected in the resonant spectra.…”

Eigenmodes of Néel skyrmions in ultrathin magnetic films

“…5, are similar to those experimentally observed for Néel skyrmion lattices hosted in bulk single crystals of GaV 4 S 8 . 22 The simulations also confirm the experimental energy hierarchy of the fundamental modes. 36 Additionally, the figure reveals that the CCW mode is unaffected by the SkL ISk phase transition, as its frequency exhibits the same linear magnetic field dependence in both phases.…”

“…22 It is worth noting that simulations performed by Liu et al 23 have unveiled a rich variety of internal modes of the Bloch skyrmion excited by an ac electric field in the multiferroic Cu 2 OSeO 3 crystal. As the eigenmodes of skyrmions provide information on their stability and rigidity, an elucidation of the eigenmodes is important for understanding the behavior of these magnetic quasiparticles.…”

Magnetic phases of skyrmion-hostingGaV4S8−ySey(y=0,2Franke1, Huddart2, Hicken3 et al. 2018Phys. Rev. B322323View full textAdd to dashboardCite

“…An increase in the relaxation rate in LF μ + SR measurements thus most likely reflects a contribution of the SkL to dynamics perpendicular to the applied field directions. As the dynamics probed by μ + SR can reach the GHz regime, our observation is attributable to the emergent excitation modes of individual skyrmions that have been observed in the GHz regime, where clockwise, counterclockwise, and breathing modes occur in the skyrmion plane, but not perpendicular to it [24]. It is notable that dynamics coinciding with the SkL 054428-7 occur over a broad spectral range, with our AC susceptibility (on the kHz frequency scale) showing an increase in the imaginary component χ in the SkL phases, indicating an increase in dissipation [39].…”

“…Below the respective magnetic ordering temperatures (T c ≈ 12.5 K for GaV 4 S 8 [15,22] and T c ≈ 17.5 K for GaV 4 Se 8 [17,23]), applied magnetic fields drive successive transitions between cycloidal (C), SkL, and field-polarized (FP) phases. In contrast to the Bloch-type SkL plane commonly observed to align perpendicular to the applied magnetic field direction, the lattice vectors of the Néel-type SkL are constrained to a plane perpendicular to the rhombohedral lattice distortion [15,[24][25][26]. For GaV 4 S 8 , a uniaxial magnetocrystalline anisotropy is found along this distortion, thought to favor ferromagnetic (FM) alignment below T FM ≈ 5 K [15,16].…”