On the multiferroic skyrmion-host GaV₄S₈

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 multi-mode 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. PACS numbers: 76.50.+g, 12.39.Dc, 75.30.Gw 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 [1-3]. Skyrmions are whirl-like objects of spins which can crystallize in skyrmion lattices (SkLs) with typical lattice constants from ten to hundred nanometers and give rise to emergent electrody-namics, like the topological Hall effect [4-6] 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 microwave-technology applications after collective spin excitations predicted in the GHz range [10] were evidenced in the insulating chiral magnet Cu 2 OSeO 3 [11-16]. 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 comp...

Magnetic phases of skyrmion-hostingGaV4S8−ySey(y=0,2Franke1, Huddart2, Hicken3 et al. 2018Phys. Rev. B302323View full textAdd to dashboardCite

“…This effect is not observed in the Se-containing analog suggesting that the ground state magnetism in these systems is significantly different. Recent observations of the S-containing material suggest that, on cooling, the periodicity of the magnetic cycloid spin structure increases, eventually transforming into a soliton lattice with periodically arranged domain walls in the FM phase [16,47]. This continuous change in magnetic structure might provide a mechanism for the unusual T dependence of the local B field.…”

Relaxation dynamics of modulated magnetic phases in the skyrmion host GaV4S8 : An ac magnetic susceptibility study

“…2 [20,25,29]. Red and blue stars display the location of the maxima in the real and imaginary parts of the ac susceptibility using a modulation frequency of 1 kHz.…”

“…* butykai@mail.bme.hu Long-range magnetic ordering arises and thereby the compound becomes a type-I multiferroic at T C = 13 K [25,26,28,29]. The interplay of the symmetric exchange interaction and the antisymmetric Dzyaloshinskii-Moriya (DM) exchange interaction gives rise to a long-wavelength spin ordering.…”

“…The skyrmion lattice in GaV 4 S 8 was demonstrated to carry magnetoelectric polarization [25,29], offering possibilities of skyrmion manipulation with external electric fields.…”

Lattice modes and the Jahn-Teller ferroelectric transition ofGaV4S8

“…On the other hand, the complete ordering of the dynamical dipoles of V 4 clusters, estimated from the paraelectric Curie constant, would contribute only little (P s = 0.7μ C/cm 2 ). Since the transition is of a first-order nature [22,23], this difference is nevertheless well understandable.…”