Geometrically frustrated magnets provide abundant opportunities for discovering complex spin textures, which sometimes yield unconventional electromagnetic responses in correlated electron systems. It is theoretically predicted that magnetic frustration may also promote a topologically nontrivial spin state, i.e., magnetic skyrmions, which are nanometric spin vortices. Empirically, however, skyrmions are essentially concomitant with noncentrosymmetric lattice structures or interfacialsymmetry-breaking heterostructures. Here, we report the emergence of a Bloch-type skyrmion state in the frustrated centrosymmetric triangular-lattice magnet Gd2PdSi3. We identified the field-induced skyrmion phase via a giant topological Hall response, which is further corroborated by the observation of in-plane spin modulation probed by resonant x-ray scattering. Our results exemplify a new gold mine of magnetic frustration for producing topological spin textures endowed with emergent electrodynamics in centrosymmetric magnets.

A series of cyanide bridged Fe-Co molecular squares, [Co(2)Fe(2)(CN)(6)(tp*)(2)(dtbbpy)(4)](PF(6))(2)·2MeOH (1), [Co(2)Fe(2)(CN)(6)(tp*)(2)(bpy)(4)](PF(6))(2)·2MeOH (2), and [Co(2)Fe(2)(CN)(6)(tp)(2)(dtbbpy)(4)](PF(6))(2)·4H(2)O (3) (tp = hydrotris(pyrazol-1-yl)borate, tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bpy =2,2'-bipyridine, dtbbpy =4,4'-di-tert-butyl-2,2'-bipyridine), were prepared by the reactions of [Fe(CN)(3)(L)](-) (L = tp or tp*) with Co(2+) and bidentate ligands (bpy or dtbbpy) in MeOH. In the molecular squares, Fe and Co ions are alternately bridged by cyanide ions, forming macrocyclic tetranuclear cores. Variable temperature X-ray structural analyses and magnetic susceptibility measurements confirmed that 1 exhibits two-step charge-transfer induced spin transitions (CTIST) centered at T(1/2) = 275 and 310 K in the solid state. The Fe and Co ions in 1 are the low-spin (LS) Fe(III) and high-spin (HS) Co(II) ions, described here in the high-temperature (HT) phase ([Fe(III)(LS2)Co(II)(HS2)]) at 330 K, while a low-temperature (LT) phase ([Fe(II)(LS2)Co(III)(LS2)]) with LS Fe(II) and Co(III) ions was dominant below 260 K. X-ray structural analysis revealed that in the intermediate (IM) phase at 298 K 1 exhibits positional ordering of [Fe(III)(LS2)Co(II)(HS2)] and [Fe(II)(LS2)Co(III)(LS2)] species with the 2:2 ratio. In photomagnetic experiments on 1, light-induced CTIST from the LT to the HT phase was observed by excitation of Fe(II) → Co(III) intervalence charge transfer (IVCT) band at 5 K and the trapped HT phase thermally relaxed to the LT phase in a two-step fashion. On the other hand, 2 and 3 are in the HT and LT phases, respectively, throughout the entire temperature range measured, and no CTIST was observed. UV-vis-NIR absorption spectral measurements and cyclic voltammetry in solution revealed that the different electronic states in 1-3 are ascribable to the destabilization of iron and cobalt ion d-orbitals by the introduction of methyl and tert-butyl groups to the ligands tp and bpy, respectively. Temperature dependence of UV-vis-NIR spectra confirmed that 1 exhibited a one-step CTIST in butyronitrile, of which T(1/2) varied from 227 to 280 K upon the addition of trifluoroacetic acid.

Quantum transport of quasi–two-dimensional Dirac fermions is largely controlled by magnetic order in a layered magnet.

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Magnetic skyrmion textures are realized mainly in non-centrosymmetric, e.g. chiral or polar, magnets. Extending the field to centrosymmetric bulk materials is a rewarding challenge, where the released helicity/vorticity degree of freedom and higher skyrmion density result in intriguing new properties and enhanced functionality. We report here on the experimental observation of a skyrmion lattice (SkL) phase with large topological Hall effect and an incommensurate helical pitch as small as 2.8 nm in metallic Gd3Ru4Al12, which materializes a breathing kagomé lattice of Gadolinium moments. The magnetic structure of several ordered phases, including the SkL, is determined by resonant x-ray diffraction as well as small angle neutron scattering. The SkL and helical phases are also observed directly using Lorentz-transmission electron microscopy. Among several competing phases, the SkL is promoted over a low-temperature transverse conical state by thermal fluctuations in an intermediate range of magnetic fields.