We present a comprehensive study of chiral fluctuations in the reference helimagnet MnSi by polarized neutron scattering and Neutron Spin Echo spectroscopy, which reveals the existence of a completely left-handed and dynamically disordered phase. This phase may be identified as a spontaneous skyrmion phase: it appears in a limited temperature range just above the helical transition TC and coexists with the helical phase at TC .Chirality is ubiquitous in nature and of fundamental importance both on the microscopic level and in our everyday life. The break of symmetry between right and left manifests itself in parity violation, governs biological structures such as DNA and can also be experienced in the organisation of our own body. In magnetism, chirality is evident in solitons [1], systems with geometric frustration [2] and metallic systems with noncentro-symmetric lattice structures, where the resulting anti-symmetric Dzyaloshinski-Moriya (DM) interactions [3,4] introduce a parity breaking term in the Hamiltonian [5]. The DM term has the form M × ( ∇ × M ) and is more than a perturbation giving rise to the peculiar canted magnetic arrangements found in high temperature superconductors [6] or the cycloid spin structures in multiferroics [7,8]. In the non-centrosymmetric weak itinerant-electron ferromagnet MnSi, DM induced chirality comes in close interplay with Fermi liquid behavior and quantum fluctuations [9]. The Hamiltonian of MnSi comprises three hierarchically ordered magnetic interaction terms with well separated energy scales [10], which allow to distinguish between different contributions. The strongest ferromagnetic exchange interaction aligns the spins, the weaker chiral Dzyaloshinski-Moriya (DM) term twists them into a helix and the weakest Anisotropic Exchange (AE) or crystal field term pins the helix propagation vector τ along the 111 crystallographic directions.The helical order appears below T C ≈ 29 K. It is a lefthanded helix with a period of ℓ ∼ 175Å (τ ≈ 0.036Å −1 ) and all magnetic moments perpendicular to the helix vector [11].In this letter we concentrate on the chiral correlated paramagnetic or spin liquid phase of MnSi just above T C , where intense diffuse neutron scattering spreads homogeneously over the surface of a sphere with radius τ . This unusual feature emerges as a ring on the two-dimensional small angle neutron scattering patterns and the rings reduce to half-moons if the beam is polarized. This is illustrated by figure 1, which reproduces spectra from [12]. Numerous theoretical studies were devoted to explain this phase invoking possibilities such as unpinned helical order [12,13] or condensation of chiral order parameters [14]. Recent local mean-field calculations assuming the hierarchical hamiltonian of MnSi show that the helical phase is preceded by a disordered phase with skyrmionlike short range order similar to the partial order in liquid crystals [15], which sets in at T C ′ ≈ T C + 1K (see supplementary information of [15]). Skyrmions are solutions of the non-linear f...

Spin ordering in TbBaCo 2 O 5.5 and its temperature transformation reproducible for differently synthesized samples are studied. First of all, the polymorphism due to the oxygen ordering with the average content close to 5.5 is investigated. One of ceramic samples (I), in addition to the main phase a p × 2a p × 2a p , Pmmm (Z = 2), contained about 25% of the phase a p × a p × 2a p , Pmmm, (Z = 1) with statistical distribution of oxygen over the apical sites, where a p is parameter of perovskite cell. The other sample (II) contained a single phase a p × 2a p × 2a p , Pmmm (Z = 2) with well defined octahedral and pyramidal sublattices. Treatment of neutron diffraction patterns of the sample I itself gives a sophisticated spin structure. Knowing the structure of sample II, one can chose only proper magnetic lines, which give exactly the same results as for sample II. Above the Néel temperature T N ≈ 290 K, there is a structural transition to the phase 2a p × 2a p × 2a p , Pmma. At T N , the spins order with the wave vector k 19 = 0 (phase 1). At T 1 ≈ 255 K, a magnetic transition takes place to the phase 2 with k 22 = b 3 /2. At T 2 ≈ 170 K, the crystal structure changes to 2a p × 2a p × 4a p , Pcca (Z = 4). The wave vector of the spin structure becomes again k 19 = 0 (phase 3). The basis functions of irreducible representations of the group G k have been found. Using results of this analysis, the magnetic structure in all phases is determined. The spins are always parallel to the x axis, and the difference is in the values and mutual orientation of the moments in the ordered non-equivalent pyramidal or octahedral positions. Spontaneous moment M 0 = 0.30(3) µ B /Co at T = 260 K is due to ferrimagnetic ordering of the moments M Py1 = 0.46(9) µ B and M Py2 = −1.65(9) µ B in pyramidal sites (Dzyaloshinskii-Moriya canting is forbidden by symmetry). The moments in the non-equivalent octahedral sites are: M Oc1 = −0.36(9) µ B , M Oc2 = 0.39(9) µ B . At T = 230 K, M Py1 = 0.28(8) µ B , M Py2 = 1.22(8) µ B , M Oc1 = 1.39(8) µ B , M Oc2 = −1.52(8) µ B . At T = 100 K, M Py1 = 1.76(6) µ B , M Py2 = −1.76 µ B , M Oc1 = 3.41(8) µ B , M Oc2 = −1.47(8) µ B . The moment values together with the ligand displacements are used to analyze the picture of spin-state/orbital ordering in each phase.

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We show, with the help of polarized neutrons, that the cubic magnets Fe1-xCoxSi with Dzyaloshinskii-Moriya interaction can be switched between left (for x=0.1, 0.15) and right (for x=0.2, 0.25, 0.3, 0.5) chiral states of the spin helix. The absolute structure was evaluated using x-ray diffraction. The crystals are shown to be enantiopure and the structural chirality changes from right handed for x<0.2 to left handed for x>0.2. These compounds are compared with the etalon sample of MnSi which is identified as having the left-handed chirality both in the magnetic and crystallographic sense.