[en] A general theoretical approach to polarized neutron chiral scattering is described. It is shown that using polarized neutrons one can study the projection of the spin chirality on the axial-vector interactions and investigate critical chiral fluctuations. Applications of this method to the triangular lattice antiferromagnets, helimagnets and spin glasses are discussed

[en] Magnetic properties of noncentrosymmetric cubic helimagnets (MnSi, etc.) at low T are studied theoretically using conventional exchange, Dzyaloshinskii-Moriya interaction, anisotropic exchange and magnetic dipolar interaction. Structure in magnetic field and spin-wave spectrum are considered. In low field gμ_BH₁=Δ√(2), where Δ is the spin-wave gap, the helix axis k turns along the field. A transition to ferromagnetic state occurs at gμ_BH_c=Ak², where A is the spin-wave stiffness at momenta q>>k. It is observed that the spin-wave spectrum is strongly anisotropic: excitations with q parallel k and q perpendicular k have linear and quadratic dispersion at q<< k respectively if one neglects the gap Δ. This is a result of umklapp interaction connecting the spin-waves with momenta q and q±k. This interaction leads to infinite set of equations for the Green functions. The simplest n=1 approximation is applicable at q=0 and q>>k. Further n=2 approximation gives correct result at q<< k. Results of both approximations are qualitative at q∼k. The weak field (H< H₁) perpendicular to k deforms the helix and the second harmonics of the spin rotation appears. The spin-wave gap is a result of the spin-wave interaction considered in the Hartree-Fock approximation and cubic anisotropy. Properties of the ESR and neutron scattering are considered. The theoretical results are in agreement with existing experimental data

[en] The magneto-elastic interaction in cubic helimagnets with B 20 symmetry is considered. It is shown that this interaction is responsible for a negative contribution to the square of the spin-wave gap Δ and it alone appears to disrupt the assumed helical structure. It is suggested that competition between the positive part of Δ_I², which stems from magnon-magnon interaction, and its negative magneto-elastic part leads to the quantum phase transition observed at high pressure in MnSi and FeGe. This transition has to occur when Δ²→0. For MnSi it was shown using rough estimations that at ambient pressure both parts Δ_I and |Δ_ME| are comparable with the experimentally observed gap. The magneto-elastic interaction is responsible for 2k modulation of the lattice where k is the helix wavevector and contributes to the magnetic anisotropy. Properties of the magnetic state above the quantum phase transition are also discussed. Experimental observation of the lattice modulation by x-ray and neutron scattering allows the determination of the strength of the anisotropic part of the magneto-elastic interaction responsible for the above phenomena and the lattice helicity.

[en] We suggest a new approach for describing the low-energy sector of spin-1/2 Heisenberg antiferromagnets (KAFs). The Kagome lattice is represented as a set of blocks (stars) arranged in a triangular lattice. Each of these stars has two degenerate singlet ground states. It is shown using symmetry considerations that the KAF lower singlet band is made by inter-star interaction from these degenerate states. The general form of the effective Hamiltonian describing this band is established. Low-T peculiarities in the specific heat of Kagome clusters are also discussed

[en] A special Microsymposium on polarized neutron methods and technologies on existing and future pulsed neutron sources was held at PNCMI-2002. We present the recommendations formulated by three working groups on 'Diffraction', 'Inelastic Scattering' and 'Reflectometry'. Needs for method development as well as opportunities for novel polarized neutron instrumentation are identified

[en] The chiral spin fluctuations and the spiral structure of the single crystal MnSi has been studied by small angle polarized neutron scattering near T_c=29 K under applied field. A weak magnetic field applied along one of the <111> axes produces a single domain sample with the helix wave vector along the field. The 90 degree sign reorientation of the spin spiral from the [111] axis to [110] axis is observed in the field range from 130 to 180 mT in the close vicinity to T_c. Further increase of the field above 180 mT restores the original orientation of the helix and leads to the induced ferromagnetic state at 350 mT. This observation clarifies the nature of the structural spin instability found in the H-T phase diagram of MnSi by other techniques. We explain this phenomenon as a result of competition of cubic anisotropy and the spin-wave Bose condensation provoked by the field perpendicular to the helix axis

[en] Magnon Bose condensation (BC) in a symmetry breaking magnetic field is the result of an unusual form of Zeeman energy that has terms linear in the spin-wave operators and terms mixing excitations, the momenta of which differ in the wavevector of the magnetic structure. The following examples are considered: simple easy-plane tetragonal antiferromagnets (AFs), the frustrated AF family R₂CuO₄, where R = Pr, Nd etc, and cubic magnets with a Dzyaloshinskii-Moriya interaction (MnSi etc). In all cases BC is important when the magnetic field is comparable with the spin-wave gap. The theory is illustrated by existing experimental results

[en] The compound Fe_1-xCo_xSi is a good representative for a cubic magnet with Dzyaloshinskii-Moriya interaction. On the basis of the neutron diffraction and superconducting quantum interference device measurements, we built the H-T phase diagram for the compound with different x from 0.1 to 0.7. The same set of parameters governs the magnetic system for different x. These parameters are well interpreted in the framework of the recently developed theory [S. V. Maleyev, Phys. Rev. B 73, 174402 (2006)]. As a result, the spin-wave stiffness, the Dzyaloshinskii constant, the anisotropic exchange constant, and the spin-wave gap caused by the Dzyaloshinskii interaction have been obtained and plotted as a function of x. The changes of the magnetic structure with x can be well interpreted on the basis of our findings

[en] The magnetic structure of Fe_1-xCo_xSi single crystals with x=0.10,0.15,0.20,0.50 has been studied by small angle polarized neutron diffraction and superconducting quantum interference device measurements. Experiments have shown that in zero field the compounds with x=0.1,0.15 have a well-defined tendency to order in the one-handed spiral along <100> axes due to the anisotropic exchange, that, however, decreases with increasing Co concentration x. The magnetic structure of Fe_1-xCo_xSi with x=0.2,0.5 consists of spiral domains with randomly oriented spiral wave vector k. The applied magnetic field produces a single domain helix oriented along the field. The process of the reorientation starts at the field H_C1. Further increase of the field leads to a magnetic phase transition from a conical to a ferromagnetic state near H_C2. In the critical range near T_C the integral intensity of the Bragg reflection shows a well-pronounced minimum at H_fl attributed to a k flop of the helix wave vector. On the basis of our experiments we built the H-T phase diagram for each compound. It is shown that the same set of the parameters governs the magnetic properties of these compounds k, H_C1, H_fl, and H_C2. Our experimental findings are well interpreted in the framework of a recently developed theory [Phys. Rev. B 73, 174402 (2006)] for cubic magnets with Dzyaloshinskii-Moriya (DM) interaction. In particular, the theory suggests an additional quantum term in the magnetic susceptibility caused by the DM interaction which is in good agreement with the experiment

[en] The magnetic structure of the noncentrosymmetrical cubic ferromagnets with Dzyaloshinskii-Moriya interaction Mn_1-yFe_ySi with y = 0, 0.06, 0.08, and 0.1 has been studied by means of the small angle neutron diffraction and magnetization measurements. These compounds are shown to be ordered into the helix structure below the critical temperature T_C that decreases linearly with the Fe doping and approaches zero at y ∼ 0.13. We build the H - T (magnetic field - temperature) phase diagrams for each compound and interpret them taking into account the Bak-Jensen hierarchical model of the principal interactions. Following the concentration evolution of these principal interactions (such as the spin wave stiffness A, the Dzyaloshinskii constant D, and the gap Δ in spin wave spectrum) we show that A resembles behavior of T_C approaching zero at y ∼ 0.15. Thus, it is the isotropic ferromagnetic exchange interaction that plays a dominating role in determination of the critical temperature T_C of the compounds under study. The Dzyaloshinskii constant D has been found to be the same either in the system under study and in the related compounds Fe_1-xCo_xSi.