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...

This paper is devoted to the determination of the spin distribution in the spin triplet ground state of [Cu2(t-Bupy)4(N3)2](ClO4)2, with t-Bupy = p-tert-butylpyridine. The crystal structure, previously solved at room temperature from X-ray diffraction, has been redetermined at 18 K from unpolarized neutron diffraction. The structure consists of binuclear cations in which Cu2+ ions are doubly bridged by azido groups in the 1,1-fashion, and noncoordinated perchlorate anions. The experimental spin distribution has been determined from polarized neutron diffraction (PND) at 1.6 K under 50 kOe. The spin populations have been found to be strongly positive on the Cu2+ ions, weakly positive on the terminal and bridging nitrogen atoms of the azido groups as well as on the nitrogen atoms of the t-Bupy ligands, and weakly negative on the central nitrogen atoms of the N3 - bridges. The PND results have been discussed. The spin distribution in [Cu2(t-Bupy)4(N3)2](ClO4)2 has been analyzed as resulting from a spin delocalization from the Cu2+ ions toward the azido bridges, to which a spin polarization effect within the azido π orbitals is superimposed. The experimental data have been compared to the results of DFT calculations. The spin density map is qualitatively reproduced; however, the DFT calculations overestimate the spin delocalization from the Cu2+ ions toward the peripheral and bridging ligands.