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 relaxation close to the glass temperature of CuMn and AuFe spin glasses is shown, by neutron spin echo, to follow a generalized exponential function which explicitly introduces hierarchically constrained dynamics and macroscopic interactions. The interaction parameter is directly related to the normalized Tsallis non-extensive entropy parameter, q, and exhibits universal scaling with reduced temperature. At the glass temperature q = 5/3 corresponding, within Tsallis' q-statistics, to a mathematically defined critical value for the onset of strong disorder and non-linear dynamics. However the stretched exponential form does not adequately describe the relaxation of either structural or magnetic glasses close to T g and below, where selfsimilarity in time (i.e. fractal behaviour in time) occurs. This is demonstrated in Monte Carlo calculations by Ogielski [8], based upon a 3d ±J Ising spin glass model, which show that close to the spin glass temperature the time dependent spin autocorrelation function should take a phenomenological modified Kohlrausch form which incorporates a power law dependence:in which β increases from 1/3 at T g to 1 at ∼ 4T g , x increases from 0 below T g to 0.5 at high temperatures and the relaxation time, τ , diverges at T g . This prediction is also largely supported by neutron spin echo (NSE) measurements [11]. The origins and implications of such non-exponential relaxation remain the subject of some debate, not least because it can be demonstrated that Kohlrausch-like relaxation may arise from either a statistical distribution of independent (parallel) relaxation channels, or from more complex hierarchically constrained dynamics [12]. For a detailed physical insight it is necessary to explore global models of glassy relaxation which attempt intrinsically to embody both distributed dynamics and the interactions that may lead to hierarchical relaxation.Such a model was introduced by Weron [13] in an attempt to explain the apparently universal power law for dielectric relaxation. Weron's rigorous probabilistic approach, based on the cluster model of Dissado and Hill [14], considers a hierarchical progression of relaxation which results in a continuously changing energy landscape. The hierarchy is introduced through the formation of finite clusters which arise from interactions between relaxing dipoles, with each cluster being represented by an effective dipole related to its internal structure. The time taken for polarization fluctuations to reach equilibrium is a random variable that for each relaxing dipole depends upon two other random variables, namely the waiting time and the dissipation rate. Through these variables Weron accounts for the effects of both intercluster and intra-cluster interactions with the characteristic timescale of any relaxing entity being restricted by the structural reorganization of the surrounding clusters and derives a generalized relaxation functionwhere β is associated with the fractal geometry of the system and k(> 0) is an effective interaction par...

No abstract

Magnetic skyrmions are topologically protected nanoscale spin textures with particle-like properties. In bulk cubic helimagnets, they appear under applied magnetic fields and condense spontaneously into a lattice in a narrow region of the phase diagram just below the magnetic ordering temperature, the so-called A-phase. Theory, however, predicts skyrmions to be locally stable in a wide range of magnetic fields and temperatures. Our neutron diffraction measurements reveal the formation of skyrmion states in large areas of the magnetic phase diagram, from the lowest temperatures up to the A-phase. We show that nascent and disappearing spiral states near critical lines catalyze topological charge changing processes, leading to the formation and destruction of skyrmionic states at low temperatures, which are thermodynamically stable or metastable depending on the orientation and strength of the magnetic field. Skyrmions are surprisingly resilient to high magnetic fields: the memory of skyrmion lattice states persists in the field polarized state, even when the skyrmion lattice signal has disappeared. These findings highlight the paramount role of magnetic anisotropies in stabilizing skyrmionic states and open up new routes for manipulating these quasi-particles towards energy-efficient spintronics applications.