Damping of spin waves and singularity of the longitudinal modes in the dipolar critical regime of the Heisenberg-ferromagnet EuS

By inelastic scattering of polarized neutrons near the (200)-Bragg reflection, the susceptibilities and linewidths of the spin waves and the longitudinal spin fluctuations were determined separately. By aligning the momentum transfers q perpendicular to both \delta S_sw and the spontaneous magnetization M_s, we explored the statics and dynamics of these modes with transverse polarizations with respect to q. In the dipolar critical regime, where the inverse correlation length kappa_z(T) and q are smaller than the dipolar wavenumber q_d, we observe:(i) the static susceptibility of \delta S_sw^T(q) displays the Goldstone divergence while for \delta S_z^T(q) the Ornstein-Zernicke shape fits the data with a possible indication of a thermal(mass-)renormalization at the smallest q-values, i.e. we find indications for the predicted 1/q divergence of the longitudinal susceptibility; (ii) the spin wave dispersion as predicted by the Holstein-Primakoff theory revealing q_d=0.23(1)\AA^{-1}in good agreement with previous work in the paramagnetic and ferromagnetic regime of EuS; (iii) within experimental error, the (Lorentzian) linewidths of both modes turn out to be identical with respect to the q^2-variation, the temperature independence and the absolute magnitude. Due to the linear dispersion of the spin waves they remain underdamped for q<q_d. These central results differ significantly from the well known exchange dominated critical dynamics, but are quantitatively explained in terms of dynamical scaling and existing data for T>=T_C. The available mode-mode coupling theory, which takes the dipolar interactions fully into account, describes the gross features of the linewidths but not all details of the T- and q-dependencies. PACS: 68.35.Rh, 75.40.GbComment: 10 pages, 7 figure

Critical spin-flip scattering at the helimagnetic transition of MnSi

We report spherical neutron polarimetry (SNP) and discuss the spin-flip scattering cross sections as well as the chiral fraction $\eta$ close to the helimagnetic transition in MnSi. For our study, we have developed a miniaturised SNP device that allows fast data collection when used in small angle scattering geometry with an area detector. Critical spin-flip scattering is found to be governed by chiral paramagnons that soften on a sphere in momentum space. Carefully accounting for the incoherent spin-flip background, we find that the resulting chiral fraction $\eta$ decreases gradually above the helimagnetic transition reflecting a strongly renormalised chiral correlation length with a temperature dependence in excellent quantitative agreement with the Brazovskii theory for a fluctuation-induced first order transition.Comment: 5 pages, 3 figure

Universal behavior of the IMS domain formation in superconducting niobium

In the intermediate mixed state (IMS) of type-II/1 superconductors, vortex lattice (VL) and Meissner state domains coexist due to a partially attractive vortex interaction. Using a neutron-based multiscale approach combined with magnetization measurements, we study the continuous decomposition of a homogeneous VL into increasingly dense domains in the IMS in bulk niobium samples of varying purity. We find a universal temperature dependence of the vortex spacing, closely related to the London penetration depth and independent of the external magnetic field. The rearrangement of vortices occurs even in the presence of a flux freezing transition, i.e. pronounced pinning, indicating a breakdown of pinning at the onset of the vortex attraction

New Magnetic Excitations in the Spin-Density-Wave of Chromium

Low-energy magnetic excitations of chromium have been reinvestigated with a single-Q crystal using neutron scattering technique. In the transverse spin-density-wave phase a new type of well-defined magnetic excitation is found around (0,0,1) with a weak dispersion perpendicular to the wavevector of the incommensurate structure. The magnetic excitation has an energy gap of E ~ 4 meV and at (0,0,1) exactly corresponds to the Fincher mode previously studied only along the incommensurate wavevector.Comment: 4 pages, 4 figure