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...
We describe two spin-echo instruments for neutron small angle scattering, which have been installed at the reactor institute in Delft. The first setup is using a monochromatic beam and magnetized foils as spin flippers, while the second uses resonant spin flippers in a pulsed neutron beam. The components that play an essential role for operation are described in some detail. Each setup has specific advantages in its range of spin-echo lengths that covers the range of correlation lengths that could be measured. This is demonstrated in a comparative measurement, the setup with magnetized foils measuring at spin-echo-lengths up to 20 μm and the setup with resonant flippers measuring in the range up to 0.5 μm.
X-ray diffraction using synchrotron radiation and polarized neutron small-angle diffraction have been used to evaluate the absolute crystallographic structure and the spin helix chirality of Mn 1−x Fe x Si. Contrary to previous observations we show that left-and right-handed crystals can be found for MnSi and its iron substituted analogs. The structural chirality rigorously determines the magnetic chirality of these compounds: left-͑right-͒handed crystalline chirality establishes left ͑right͒ handedness of the magnetic helix.
Monosilicides of 3d-metals frequently show a chiral magnetic ordering with the absolute configuration defined by the chirality of the crystal structure and the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and magnetic chiralities are probed here for Fe1−xCoxSi series and their mutual relationship is found to be dependent on the chemical composition. The chirality of crystal structure was previously shown to be governed by crystal growth, and the value of the DMI is nearly the same for all monosilicides of Fe, Co and Mn. Our findings indicate that the sign of the DMI in Fe1−xCoxSi is controlled by the Co composition x. We have been able to directly measure the change of the link between structure and magnetism in this helimagnetic B20 alloy.PACS numbers: 61.12.Ex, Scattering of polarized neutrons on chiral magnetic structures allows one to determine the absolute magnetic configuration, thus left-and right-handed helices can be easily distinguished [1]. On the other hand, knowing the magnetic configuration, one can analyse the polarization of a scattering beam [2]. Similar effects could also help to manipulate spin polarization of an electron current providing that the electrons interact with the known chiral magnetic structure.The ability to manipulate the electron spin is a necessary component for the spintronics [3], thus magnetic chiral organic molecules [4] or large scale magnetic structures have been proposed as such tools [5]. However, the question how to get the magnetic structure of a necessary chirality for spintronics applications is still open.Here we address the question for the case of Fe 1−x Co x Si solid solutions which, for certain compositions, show chiral (spiral) magnetic ordering [6][7][8].The structural chirality in monosilicides of 3d-metals is solely controlled by crystal growth [9]. A link between the structural and magnetic chiralities is provided by the Dzyaloshinskii-Moriya interaction (DMI) and has been experimentally proved for many monosilicides of 3dmetals [9][10][11][12].
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