470 research outputs found
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 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 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
Mode coupling theory for the critical dynamics of dipolar ferromagnets Hyperfine Interactions
Band structure of helimagnons in MnSi resolved by inelastic neutron scattering
A magnetic helix realizes a one-dimensional magnetic crystal with a period
given by the pitch length . Its spin-wave excitations -- the
helimagnons -- experience Bragg scattering off this periodicity leading to gaps
in the spectrum that inhibit their propagation along the pitch direction. Using
high-resolution inelastic neutron scattering the resulting band structure of
helimagnons was resolved by preparing a single crystal of MnSi in a single
magnetic-helix domain. At least five helimagnon bands could be identified that
cover the crossover from flat bands at low energies with helimagnons basically
localized along the pitch direction to dispersing bands at higher energies. In
the low-energy limit, we find the helimagnon spectrum to be determined by a
universal, parameter-free theory. Taking into account corrections to this
low-energy theory, quantitative agreement is obtained in the entire energy
range studied with the help of a single fitting parameter.Comment: 5 pages, 3 figures; (v2) slight modifications, published versio
Helimagnon Bands as Universal Spin Excitations of Chiral Magnets
MnSi is a cubic compound with small magnetic anisotropy, which stabilizes a
helimagnetic spin spiral that reduces to a ferromagnetic and antiferromagnetic
state in the long- and short-wavelength limit, respectively. We report a
comprehensive inelastic neutron scattering study of the collective magnetic
excitations in the helimagnetic state of MnSi. In our study we observe a rich
variety of seemingly anomalous excitation spectra, as measured in well over
twenty different locations in reciprocal space. Using a model based on only
three parameters, namely the measured pitch of the helix, the measured
ferromagnetic spin wave stiffness and the amplitude of the signal, as the only
free variable, we can simultaneously account for \textit{all} of the measured
spectra in excellent quantitative agreement with experiment. Our study
identifies the formation of intense, strongly coupled bands of helimagnons as a
universal characteristic of systems with weak chiral interactions.Comment: 8 pages, 4 figures, references updated, introduction updated,
reformatte
Soft phonons and structural phase transitions in LaBaCuO
Soft phonon behavior associated with a structural phase transition from the
low-temperature-orthorhombic (LTO) phase ( symmetry) to the
low-temperature-tetragonal (LTT) phase ( symmetry) was investigated
in LaBaCuO using neutron scattering. As temperature
decreases, the TO-mode at -point softens and approaches to zero energy
around K, where the LTO -- LTT transition occurs. Below , the phonon hardens quite rapidly and it's energy almost saturates below
50 K. At , the energy dispersion of the soft phonon along in-plane
direction significantly changes while the dispersion along out-of-plane
direction is almost temperature independent. Coexistence between the LTO phase
and the LTT phase, seen in both the soft phonon spectra and the peak profiles
of Bragg reflection, is discussed in context of the order of structural phase
transitions.Comment: 6 pages, 8 figure
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