Magnetism and Néel skyrmion dynamics in GaV$_{4}$S$_{8-y}$Se$_{y}$
T. J. Hicken, S. J. R. Holt, K. J. A. Franke, Z. Hawkhead, A. Štefančič, M. N. Wilson, M. Gomilšek, B. M. Huddart, S. J. Clark, M. R. Lees, F. L. Pratt, S. J. B… 展开作者
T. J. Hicken, S. J. R. Holt, K. J. A. Franke, Z. Hawkhead, A. Štefančič, M. N. Wilson, M. Gomilšek, B. M. Huddart, S. J. Clark, M. R. Lees, F. L. Pratt, S. J. Blundell, G. Balakrishnan, T. Lancaster
We present an investigation of the influence of low-levels of chemical substitution on the magnetic ground state and N{\' e}el skyrmion lattice (SkL) state in GaV$_4$S$_{8-y}$Se$_y$, where $y =0, 0.1, 7.9$, and $8$. Muon-spin spectroscopy ($μ$SR) measurements on $y=0$ and 0.1 materials reveal the magnetic ground state consists of microscopically coexisting incommensurate cycloidal and ferromagnetic environments, while chemical substitution leads to the growth of localized regions of increased spin density. $μ$SR measurements of emergent low-frequency skyrmion dynamics show that the SkL exists under low-levels of substitution at both ends of the series. Skyrmionic excitations persist to temperatures below the equilibrium SkL in substituted samples, suggesting the presence of skyrmion precursors over a wide range of temperatures.
Broadband microwave spectroscopy has been performed on single-crystalline GaV$_4$S$_8$, 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 GaV$_4$S$_8$ and t…
We report on the slow magnetization dynamics observed upon the magnetic phase transitions of GaV4S8, a multiferroic compound featuring a long-ranged cycloidal magnetic order and a Néel-type skyrmion lattice in a relatively broad temperature range below its Curie temperature. The fundamental difference between GaV4S8 and the chiral helimagnets, wherein the skyrmion phase was first discovered, lies within the polar symmetry of GaV4S8, promoting a cycloidal instead of a helical magnetic order and rendering the magnetic phase diagram essentially different from that in the cubic helimagnets. Our ac…
We report small-angle neutron scattering studies of the lacunar spinel GaV$_4$S$_8$, 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 GaV$_4$S$_8$, including the skyrmion state, gain their stability from thermal fluctuations, while at lower temperature the…
We present magnetization and muon-spin spectroscopy measurements of Néel skyrmion-host GaV$_4$S$_8$ under the application of hydrostatic pressures up to $P=2.29$ GPa. Our results suggest that the magnetic phase diagram is altered with pressure via a reduction in the crossover temperature from the cycloidal (C) to ferromagnetic-like state with increasing $P$, such that, by 2.29 GPa, the C state appears to persist down to the lowest measured temperatures. With the aid of micromagnetic simulations, we propose that the driving mechanism behind this change is a reduction in the magnetic anisotropy …
Skyrmions are whirl like topological spin objects with high potential for future magnetic data storage. It is a fundamental question, relevant for both basic research and application, if a ferroelectric (FE) polarization can be associated with their magnetic texture and if these objects can be manipulated by electric fields. Here, we study the interplay between magnetism and electric polarization in the lacunar spinel GaV4S8, which undergoes a structural transition associated with orbital ordering at 44 K and reveals a complex magnetic phase diagram below 13 K, including a ferromagnetic (FM), …
The lacunar spinel GaV$_{4}$S$_{8}$ was recently suggested to be a prototype multiferroic material hosting skyrmion lattice states with a sizeable polarization $\boldsymbol{P}$ coupled to magnetic order. We explain this phenomenon on the microscopic level. On the basis of density functional theory, we construct an effective model describing the behavior of magnetically active electrons in a weakly coupled lattice formed by molecular orbitals of the (V$_{4}$S$_{4}$)$^{5+}$ clusters. By applying superexchange theory combined with the Berry-phase theory for $\boldsymbol{P}$, we derive a compass m…
The orientation of Néel-type skyrmions in the lacunar spinels GaV$_4$S$_8$ and GaV$_4$Se$_8$ is tied to the polar axes of their underlying crystal structure through the Dzyaloshinskii-Moriya interaction. In these crystals, the skyrmion lattice phase exists for externally applied magnetic fields parallel to these axes and withstands oblique magnetic fields up to some critical angle. Here, we map out the stability of the skyrmion lattice phase in both crystals as a function of field angle and magnitude using dynamic cantilever magnetometry. The measured phase diagrams reproduce the major feature…
We report the feasibility of using magnetoentropic mapping for the rapid identification of magnetic cycloid and skyrmion phases in uniaxial systems, based on the GaV4S8 and GaV4Se8 model skyrmion hosts with easy-axis and easy-plane anisotropies respectively. We show that these measurements can be interpreted with the help of a simple numerical model for the spin Hamiltonian to yield unambiguous assignments for both single phase regions and phase boundaries. In the two lacunar spinel chemistries, we obtain excellent agreement between the measured magnetoentropic features and a minimal spin Hami…
GaV$_4$S$_8$ is a multiferroic semiconductor hosting magnetic cycloid (Cyc) and Néel-type skyrmion lattice (SkL) phases with a broad region of thermal and magnetic stability. Here, we use time-resolved magneto-optical Kerr spectroscopy and micro-magnetic simulations to demonstrate the coherent generation of collective spin excitations in the Cyc and SkL phases driven by an optically-induced modulation of uniaxial anisotropy. Our results shed light on spin-dynamics in anisotropic materials hosting skyrmions and pave a new pathway for the optical control of their magnetic order.
In the lacunar spinel GaV4S8, the interplay of spin, charge, and orbital degrees of freedom produces a rich phase diagram that includes an unusual Neel-type skyrmion phase composed of molecular spins. To provide insight into the interactions underlying this complex phase diagram, we study the spin excitations in GaV4S8 through inelastic neutron scattering measurements on polycrystalline and single-crystal samples. Using linear spin-wave theory, we describe the spin-wave excitations using a model where V4 clusters decorate an FCC lattice. The effective cluster model includes a ferromagnetic int…
We show how low-energy electronic models derived from the first-principles electronic structure calculations can help to rationalize the magnetic properties of two lacunar spinel compounds GaM4S8 with light (M=V) and heavy (M=Mo) transition-metal elements, which are responsible for different spin-orbit interaction strength. In the model, each magnetic lattice point was associated with the M4S4 molecule, and the model itself was formulated in the basis of molecular Wannier functions constructed for three magnetic t2 bands. The effects of rhombohedral distortion, spin-orbit interaction, band fil…
We construct the general free energy governing long-wavelength magnetism in two-dimensional oxide heterostructures, which applies irrespective of the microscopic mechanism for magnetism. This leads, in the relevant regime of weak but non-negligible spin-orbit coupling, to a rich phase diagram containing in-plane ferromagnetic, spiral, cone, and skyrmion lattice phases, as well as a nematic state stabilized by thermal fluctuations. The general conclusions are vetted by a microscopic derivation for a simple model with Rashba spin-orbit coupling.
We present the results of a muon-spin spectroscopy investigation of GaV$_4$S$_{8-y}$Se$_{y}$ with $y=0, 2, 4$ and 8. Zero-field measurements suggest that GaV$_{4}$Se$_{8}$ and GaV$_{4}$S$_{8}$ have distinct magnetic ground states, with the latter material showing an anomalous temperature-dependence of the local magnetic field. It is not possible to evolve the magnetic state continuously between these two systems, with the intermediate $y=2$ and $4$ materials showing glassy magnetic behaviour at low temperature. The skyrmion lattice (SkL) phase is evident in the $y=0$ and 8 materials through an…
Polycrystalline members of the GaV$_4$S$_{8-y}$Se$_y$ family of materials with small levels of substitution between $0 \leq y \leq 0.5$ and $7.5 \leq y\leq 8$ have been synthesized in order to investigate their magnetic and structural properties. Substitutions to the skyrmion hosting parent compounds GaV$_4$S$_8$ and GaV$_4$Se$_8$, are found to suppress the temperature of the cubic to rhombohedral structural phase transition that occurs in both end compounds and to create a temperature region around the transition where there is a coexistence of these two phases. Similarly, the magnitude of th…
We report a magnetic state in GaV$_4$Se$_8$ which emerges exclusively in samples with mesoscale polar domains and not in polar mono-domain crystals. Its onset is accompanied with a sharp anomaly in the magnetic susceptibility and the magnetic torque, distinct from other anomalies observed also in polar mono-domain samples upon transitions between the cycloidal, the Néel-type skyrmion lattice and the ferromagnetic states. We ascribe this additional transition to the formation of magnetic textures localized at structural domain walls, where the magnetic interactions change stepwise and spin text…
The lacunar spinel GaV4S8 exhibits orbital ordering at 44 K and shows a complex magnetic phase diagram below 12.7 K, which includes ferromagnetic and cycloidal spin order. At low but finite external magnetic fields, Néel-type skyrmions are formed in this material. Skyrmions are whirl-like spin vortices that have received great theoretical interest because of their non-trivial spin topology and that are also considered as basic entities for new data-storage technologies. Interestingly, we found that the orbitally ordered phase shows sizable ferroelectric polarization and that excess spin-driven…
The GaV$_4$S$_{8-y}$Se$_y$ $(y = 0$ to $8)$ family of materials have been synthesized in both polycrystalline and single crystal form, and their structural and magnetic properties thoroughly investigated. Each of these materials crystallizes in the $F\bar{4}3m$ space group at ambient temperature. However, in contrast to the end members GaV$_4$S$_8$ and GaV$_4$Se$_8$, that undergo a structural transition to the $R3m$ space group at 42 and 41 K respectively, the solid solutions $(y = 1$ to $7)$ retain cubic symmetry down to 1.5 K. In zero applied field the end members of the family order ferroma…
Magnetic skyrmions are vortex-like topological spin textures often observed to form a triangular-lattice skyrmion crystal in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Recently $β$-Mn structure-type Co-Zn-Mn alloys were identified as a new class of chiral magnet to host such skyrmion crystal phases, while $β$-Mn itself is known as hosting an elemental geometrically frustrated spin liquid. Here we report the intermediate composition system Co$_7$Zn$_7$Mn$_6$ to be a unique host of two disconnected, thermal-equilibrium topological skyrmion phases; one is a conventional s…
Following the early prediction of the skyrmion lattice (SkL) - a periodic array of spin vortices - it has been observed recently in various magnetic crystals mostly with chiral structure. Although non-chiral but polar crystals with C$_{nv}$ symmetry were identifed as ideal SkL hosts in pioneering theoretical studies this archetype of SkL has remained experimentally unexplored. Here, we report the discovery of a SkL in the polar magnetic semiconductor GaV$_4$S$_8$ with rhombohedral (C$_{3v}$) symmetry and easy axis anisotropy. The SkL exists over an unusually broad temperature range compared wi…
We report on the anomalous magnetization dynamics of the cycloidally-modulated spin textures under the influence of uniaxial anisotropy in multiferroic $\mathrm{GaV_4S_8}$. The temperature and field dependence of the linear ac susceptibility [$χ_{1ω}^{\prime}(T,H)$], ac magnetic loss [$χ_{1ω}^{\prime\prime}(T,H)$], and nonlinear ac magnetic response [$M_{3ω}(T,H)$] are examined across the magnetic phase diagram in the frequency range $f = 10-10000$ Hz. According to recent theory, skyrmion vortices under axial crystal symmetry are confined along specific orientations, resulting in enhanced robu…
A superposition of spin helices can yield topological spin textures, such as skyrmion and hedgehog lattices. Based on the analogy with the moiré in optics, we study the magnetic and topological properties of such superpositions in a comprehensive way by modulating the interference pattern continuously. We find that the control of the angles between the superposed helices and the net magnetization yields successive topological transitions associated with pair annihilation of hedgehogs and antihedgehogs. Accordingly, emergent electromagnetic fields, magnetic monopoles and antimonopoles, and Dira…
Polar lacunar spinels, such as GaV$_4$S$_8$ and GaV$_4$Se$_8$, were proposed to host skyrmion phases under magnetic field. In this work, we put forward, as a candidate for Néel-type skyrmion lattice, the isostructural GaMo$_4$S$_8$, here systematically studied via both first-principles calculations and Monte Carlo simulations of model Hamiltonian. Electric polarization, driven by Jahn-Teller distortion, is predicted to arise in GaMo$_4$S$_8$, showing a comparable size but an opposite sign with respect to that evaluated in V-based counterparts and explained in terms of different electron counti…
We report scanning transmission X-ray microscopy of mixed helical and skyrmion magnetic states in thin FeGe lamellae. This imaging of the out-of-plane magnetism allows clear identification of the different magnetic states, and reveals details about the coexistence of helical and skyrmion states. In particular, our data show that finite length helices are continuously deformable down to the size of individual skyrmions and are hence topologically equivalent to skyrmions. Furthermore, we observe transition states between helical and skyrmion states across the thickness of the lamella that are ev…
The intermetallic Gd2PdSi3 has recently generated a lot of excitement after reports that it hosts chiral magnetic nano-skyrmions despite its centrosymmetric crystal structure. Using magnetic-field-dependent polarized resonant elastic x-ray scattering (REXS), we find than an unexpected incommensurate charge density wave (CDW) appears below the ordering transition with a wavevector equal to that of the magnetic textures. Furthermore, we show these incommensurate magnetic textures in Gd2PdSi3 are highly anisotropic, with the vast majority of the spin modulation lying within the hexagonal ab-plane…
Topological transports of fermions are governed by the Chern numbers of the energy bands lying below the Fermi energy. For bosons, e.g. phonons and magnons in a crystal, topological transport is dominated by the Chern number of the lowest energy band when the band gap is comparable to the thermal energy. Here, we demonstrate the presence of topological transport by bosonic magnons in a lattice of magnetic skyrmions - topological defects formed by a vortex-like texture of spins. We find a distinct thermal Hall signal in the magnetic skyrmion phase of an insulating polar magnet GaV4Se8, identifi…
We show that continuous and spin-lattice models of chiral ferro- and antiferromagnets provide the existence of an infinite number of stable soliton solutions of any integer topological charge. A detailed description of the morphology of new skyrmions and the corresponding energy dependencies are provided. The considered model is general, and is expected to predict a plethora of particle-like states which may occur in various chiral magnets including atomic layers, e.g., PdFe/Ir(111), rhombohedral GaV$_4$S$_8$ semiconductor, B20-type alloys as Mn$_{1-x}$Fe$_x$Ge, Mn$_{1-x}$Fe$_x$Si, Fe$_{1-x}$C…
In polar magnets, such as GaV$_4$S$_8$, GaV$_4$Se$_8$ and VOSe$_2$O$_5$, modulated magnetic phases namely the cycloidal and the Néel-type skyrmion lattice states were identified over extended temperature ranges, even down to zero Kelvin. Our combined small-angle neutron scattering and magnetization study shows the robustness of the Néel-type magnetic modulations also against magnetic fields up to 2 T in the polar GaMo$_4$S$_8$. In addition to the large upper critical field, enhanced spin-orbit coupling produces a variety of modulated phases with sub-10 nm periodicity and a peculiar distributio…
Magnetic skyrmions are vortex-like topological spin textures often observed in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Among them, Co-Zn-Mn alloys with a $β$-Mn-type chiral structure host skyrmions above room temperature. In this system, it has recently been found that skyrmions persist over a wide temperature and magnetic field region as a long-lived metastable state, and that the skyrmion lattice transforms from a triangular lattice to a square one. To obtain perspective on chiral magnetism in Co-Zn-Mn alloys and clarify how various properties related to the skyrm…
The skyrmions generated by frustration in centrosymmetric structures host extra internal degrees of freedom: vorticity and helicity, resulting in distinctive properties and potential functionality, which are not shared by the skyrmions stemming from the Dzyaloshinskii-Moriya interaction in noncentrosymmetric structures. The present work indicates that the magnetism-driven electric polarization carried by skyrmions provides a direct handle for tuning helicity. Especially for the in-plane magnetized skyrmions, the helicity can be continuously rotated and exactly picked by applying an external el…
We report experimental coupling of chiral magnetism and superconductivity in [IrFeCoPt]/Nb heterostructures. The stray field of skyrmions with radius ~50nm is sufficient to nucleate antivortices in a 25nm Nb film, with unique signatures in the magnetization, critical current and flux dynamics, corroborated via simulations. We also detect a thermally-tunable Rashba-Edelstein exchange coupling in the isolated skyrmion phase. This realization of a strongly interacting skyrmion-(anti)vortex system opens a path towards controllable topological hybrid materials, unattainable to date.
Local magnetic properties of the rare-earth intermetallics $R$Mn$_2$Ge$_2$ ($R$=Ce, Pr, Nd)
Durham University（杜伦大学）；PSI Center for Neutron and Muon Sciences（瑞士保罗谢勒研究所中子与μ子科学中心）；ISIS Neutron and Muon Source, STFC Rutherford Appleton Laboratory（英国科学与技术设施委员会卢瑟福阿普尔顿实验室ISIS中子与μ子源）；University of Oxford（牛津大学）；University of Warwick（华威大学）
The magnetic skyrmions generated in a centrosymmetric crystal were recently first discovered in Gd2PdSi3. In light of this, we observe the electronic structure by angle-resolved photoemission spectroscopy (ARPES) and unveil its direct relationship with the magnetism in this compound. The Fermi surface and band dispersions are demonstrated to have a good agreement with the density functional theory (DFT) calculations carried out with careful consideration of the crystal superstructure. Most importantly, we find that the three-dimensional Fermi surface has extended nesting which matches well the…
The helicity-orbital coupling is an intriguing feature of magnetic skyrmions in frustrated magnets. Here, we explore the skyrmion dynamics in a frustrated magnet based on the $J_{1}$-$J_{2}$-$J_{3}$ classical Heisenberg model explicitly by including the dipole-dipole interaction. The skyrmion energy acquires a helicity dependence due to the dipole-dipole interaction, resulting in the current-induced translational motion with a fixed helicity. The lowest energy states are the degenerate Bloch-type states, which can be used for building the binary memory. By increasing the driving current, the h…
The magnetic ground state of polycrystalline Néel skyrmion hosting material GaV$_4$S$_8$ has been investigated using ac susceptibility and powder neutron diffraction. In the absence of an applied magnetic field GaV$_4$S$_8$ undergoes a transition from a paramagnetic to a cycloidal state below 13~K and then to a ferromagnetic-like state below 6~K. With evidence from ac susceptibility and powder neutron diffraction, we have identified the commensurate magnetic structure at 1.5 K, with ordered magnetic moments of $0.23(2)~μ_{\mathrm{B}}$ on the V1 sites and $0.22(1)~μ_{\mathrm{B}}$ on the V2 site…
The area of magnetic skyrmions, with potential revolution in certain applications, in condensed matter physics is considered about a decade old. In this article, we draw the attention of the community to the recent work of Kurumaji et al [Science 365, 914 (2019)] establishing magnetic skyrmion lattice behavior in Gd2PdSi3. We consider it important to bring out the two decades old history, in particular Hall transport anomalies characterizing magnetic skyrmions. The key experimental data in support of the skyrmion lattice that Kurumaji et al present is the observation of topological Hall resist…
Switching magnetic texture via in-plane magnetic field in noncentrosymmetric dipolar magnets: From skyrmions to antiskyrmions and nontopological magnetic bubbles
We have carried out neutron diffraction and small angle neutron scattering measurements on a high quality single crystal of the cubic lacunar spinel multiferroic, GaV$_4$S$_8$, as a function of magnetic field and temperature to determine the magnetic properties for the single electron that is located on the tetrahedrally coordinated V$_4$ molecular unit. Our results are in good agreement with the structural transition at 44 K from cubic to rhombohedral symmetry where the system becomes a robust ferroelectric, while long range magnetic order develops below 13 K in the form of an incommensurate …
The tri-layer La$_{3-3x}$Sr$_{1+3x}$Mn$_{3}$O$_{10}$ manganites of Ruddlesden-Popper (RP) series are naturally arranged layered structure with alternate stacking of m-MnO$_2$ (m = 3) planes and rock-salt type block layers (La, Sr)$_2$O$_2$ along c-axis. The dimensionality of the RP series manganites depends on the number of perovskite layers and significantly affects the magnetic and transport properties of the system. The tri-layer La$_{2.1}$Sr$_{1.9}$Mn$_{3}$O$_{10}$ shows second-order magnetic phase transition. The critical behavior of phase transition has been studied around the transition…
Motivated by the discovery of the quantum anomalous Hall effect in Cr-doped \ce{(Bi,Sb)2Te3} thin films, we study the generic states for magnetic topological insulators and explore the physical properties for both magnetism and itinerant electrons. First-principles calculations are exploited to investigate the magnetic interactions between magnetic Co atoms adsorbed on the \ce{Bi2Se3} (111) surface. Due to the absence of inversion symmetry on the surface, there are Dzyaloshinskii-Moriya-like twisted spin interactions between the local moments of Co ions. These nonferromagnetic interactions twi…
Anisotropic mesoscale magnetic scattering across the spin-reorientation transition in TbMn6Sn6
Spallation Neutron Source Science Center（散裂中子源科学中心）；Institute of High Energy Physics, Chinese Academy of Sciences (CAS)（中国科学院高能物理研究所）；Guangdong Provincial Key Laboratory of Extreme Conditions（广东省极端条件重点实验室）
