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.

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Magnetic ferroelectrics or multiferroics, which are currently extensively explored, may provide a good arena to realize a novel magnetoelectric function. Here we demonstrate the genuine electric control of the spiral magnetic structure in one of such magnetic ferroelectrics, TbMnO3. A spinpolarized neutron scattering experiment clearly shows that the spin helicity, clockwise or counterclockwise, is controlled by the direction of spontaneous polarization and hence by the polarity of the small cooling electric field.Electric control of magnetic spins or their ordering structure has long been a big challenge in condensed matter physics. Furthermore, manipulating the magnetization by electric field may provide a low energy-consuming way in spin-electronics and a higher data density in information storages [1,2]. There are a number of magnetoelectric materials whose magnetization can be changed, though minutely, with an external electric field, yet only a very few are known whose magnetic structure itself can be controlled by an electric field [1,3,4,5]. The use of ferroelectricity is perhaps indispensable to enhance the electric field action on the magnetic spins. [2] One of the robust mechanisms to produce the ferroelectricicty of magnetic origin has been recently proposed by Katsura, Nagaosa, and Balatsky (KNB) [6]. The overlap of the electronic wave function between the two atomic sites (i and i + 1) with mutually canted spins (S i and S i+1 ) can generate electric polarization,, where e i,i+1 denotes the unit vector connecting the two sites and A is a constant determined by the spin exchange interaction and the spin-orbit interaction. (Note that the similar theoretical results have been obtained independently also in refs. [7,8]). In case the transverse-spiral (cycloidal) spin order is realized ( Fig. 1(b)), the uniform spontaneous polarization is expected to emerge as the sum of the local polarization p i in the direction perpendicular to the spiral propagation vector and the vector chirality,). This spin-driven ferroelectricity has recently been found in several transversespiral magnets such as TbMnO 3 (ref.[9]), Ni 3 V 2 O 8 (ref.[10]), MnWO 4 (ref.[11]), and also in a transverse conespiral magnet CoCr 2 O 4 (ref.[12]). We report here the quantitative elucidation of such magnetically induced ferroelectricity in terms of the spin ellipticity as the order parameter and show the successful electric control between the clockwise (CW) and counter-clockwise (CCW) spin helixes.A family of perovskite manganites, RMnO 3 with R being Tb, Dy, and their solid solution, have recently been demonstrated to undergo a ferroelectric transition at the Curie temperature T C of 20 − 30 K (see the example shown in Fig. 1(c)) [13,14]. Below T N ∼ 40 K, the compounds undergo a long-range spin ordering with the modulation vector Q s = (0, ±q, 1) with q = 1/2 − 1/4 (in P bnm orthorhombic setting) [9,15]. This has been ascribed to the spin frustration effect caused by the combination of GdFeO 3 -type distortion and staggered orbital or...

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.

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Neutral (N)-ionic (I) transitions in organic donor (D)/acceptor (A) charge-transfer complexes are intriguing because a 'reservoir of functions' is available. For systematically controlling N-I transitions, tuning the ionization potential of D and the electron affinity of A is extremely important. However, the effect of Coulomb interactions, which likely causes a number of charge-gap states at once in a system bringing about stepwise transitions, is a long-standing mystery. Here, we show definite evidence for stepwise N-I transitions caused by contributions from anisotropic interchain Coulomb interactions in a metal-complex-based covalently bonded DA chain compound, [Ru(2)(2,3,5,6-F(4)PhCO(2))(4)(DMDCNQI)]·2(p-xylene) (1; 2,3,5,6-F(4)PhCO(2)(-) = 2,3,5,6-tetrafluorobenzoate; DMDCNQI = 2,5-dimethyl-N,N'-dicyanoquinonediimine), where the [Ru(2)(II,II)(2,3,5,6-F(4)PhCO(2))(4)] moiety has a paddlewheel diruthenium(II,II) motif with a Ru-Ru bond. An intermediate-temperature phase involving self-organized N and I chains was observed in the temperature range between 210 K (= T(2)) and 270 K (= T(1)) with N phase at T > T(1) and I phase at T < T(2). Accompanying the charge transitions, the spin-ground states as well as the ferrimagnetic ordering in the I phase vary. The stepwise feature of the N-I transition with a highly sensitive magnetic response should bring about new dynamical functionalities associated with charge, spin, and lattice.