Direct evidence for cycloidal modulations in the thermal-fluctuation-stabilized spin spiral and skyrmion states of GaV4S8

Abstract

We report small-angle neutron scattering studies of the lacunar spinel GaV4S8, which reveal the long-wavelength magnetic states to be cycloidally modulated. This provides direct support for the formation of Néel-type skyrmions recently claimed to exist in this compound. In striking contrast with all other bulk skyrmion host materials, upon cooling the modulated magnetic states transform into a ferromagnetic state. These results indicate all of the modulated states in GaV4S8, including the skyrmion state, gain their stability from thermal fluctuations, while at lower temperature the ferromagnetic state emerges in accord with the strong easy-axis magnetic anisotropy. In the vicinity of the transition between the ferromagnetic and modulated states, both a phase coexistence and a soliton-like state are also evidenced by our study. PACS numbers:Particle-like magnetic skyrmions with topologically non-trivial spin textures receive continued attention since their creation, stability and annihilation, either individually or in skyrmion lattices (SkLs), raise fundamental questions related to topology in physics [1][2][3]. Moreover, the observations of metastable skyrmions [4,5] and their current-driven dynamics [6,7] inspires new ideas for using skyrmions in data storage and logic devices [8][9][10].The spin texture of a skyrmion can be described by a vector field S=S(cos(Φ(ϕ))sin(Θ(ρ)), sin(Φ(ϕ))sin(Θ(ρ)), cos(Θ(ρ)), where ϕ and ρ are the azimuth and radial spatial coordinates, respectively [8,11]. The non-trivial skyrmion topology is evidenced when its spin pattern is mapped on to a sphere, and the mapping wraps the surface of the sphere entirely. Two parameters classify the skyrmion type: the vorticity m which is a non-zero integer, and the helicity, γ where Φ(ϕ) = mϕ + γ.The bulk cubic chiral helimagnets with B20 structure, like MnSi and FeGe, host whirlpool-like Bloch-type skyrmions described by m=+1 and γ=±π/2 [4,11,12]. In these compounds the competition between a dominant ferromagnetic (FM) exchange and weaker DzyaloshinskiiMoriya interactions (DMIs) stabilize a long-wavelength (∼10-100 nm) helical ground state in zero magnetic field (B). The SkL phase exists under a finite B, where it competes with either a spin-flop conical phase or a field polarized state. Since the SkL is stable typically only over the narrow temperature (T ) range 0.9 ≤ T ≤ T C , where T C is the ordering T , thermal fluctuations are proposed to be crucial for the phase stability [11][12][13].Recently a new bulk SkL phase was reported in the lacunar spinel GaV 4 S 8 (GVS) [14]. At high T , GVS has a rock-salt like structure composed of alternating GaS 4 and V 4 S 4 clusters. The highest-energy unpaired electron of the V 4 S 4 cluster occupies a triply degenerate molecular orbital which drives a cubic (F 43m) to rhombohedral (R3m) Jahn-Teller transition at T JT =42 K [15][16][17][18][19]. Since this phase transition breaks the 4 symmetry [15], four structural domains develop below T JT . Each domain carries an electric polarization along its r...

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

“…According to earlier SANS measurements, the modulation wavelength indeed shows significant increase in the vicinity of the FP state [20]. Recent SANS measurements reveal strong fluctuations in the cycloidal wavelength along the temperature-driven phase transition from the Cyc to the zero-field ferromagnetic state below 6 K, supporting this scenario [30].…”

“…As demonstrated by recent SANS measurements [30], the q vectors are distributed evenly over rings within the 111 -type planes, indicating a negligible (or weak) in-plane anisotropy. The external field applied in the [111] direction does not break the isotropy of the (111) plane, therefore the modulation directions in the unique domain remain unaffected by the field, which is in accord with the absence of dissipation within the pure Cyc phase.…”

“…However, in contrast to the B20 compounds featuring a helical spin order in zero field and longitudinal conical spin structure in finite magnetic fields, in GaV 4 S 8 the different pattern of DM vectors, dictated by its polar C 3v symmetry, leads to a cycloidal spin order (Cyc) and precludes the emergence of the longitudinal conical structure in finite fields [20]. The cycloidal nature of the magnetic modulations has been confirmed experimentally by polarized small-angle neutron scattering (SANS) measurements [30].…”

“…(ii) The modulation vectors of the Cyc and SkL states are confined to the plane perpendicular to the rhombohedral axis in each structural variant irrespective of the direction of the applied magnetic field [20,30]. Therefore, the structural domain boundaries are necessarily boundaries to the consistent magnetic structures as well [27].…”

“…Due to the lack of the longitudinal conical state, being the main competitor of the SkL phase in the B20 compounds, the Néel-type SkL is stable over a broad temperature region [20]. On the other hand, the easy-axis anisotropy in the rhombohedral domains [31] promotes the ferromagnetic ordering (FM) of the spins in the ground state, suppressing the modulated phases below T F ∼ 5 K even in zero field [20,30]. In the temperature range between T F = 5 K and T C = 13 K moderate magnetic fields drive the modulated structures, Cyc and SkL, to a collinear, field-polarized (FP) state.…”