95 research outputs found
Vibrational and electronic entropy of β-cerium and γ-cerium measured by inelastic neutron scattering
Time-of-flight (TOF) inelastic neutron-scattering spectra were measured on β-cerium (double hcp) and γ-cerium (fcc) near the phase-transition temperature. Phonon densities of states (DOS) and crystal-field levels were extracted from the TOF spectra. A softening of the phonon DOS occurs in the transition from β- to γ-cerium, accounting for an increase in vibrational entropy of ΔSvibγ-β=(0.09±0.05)kB/atom. The entropy calculated from the crystal-field levels and a fit to calorimetry data from the literature were significantly larger in β-cerium than in γ-cerium below room temperature, but the difference was found to be negligible at the experimental phase-transition temperature. A contribution to the specific heat from Kondo spin fluctuations was consistent with the quasielastic magnetic scattering, but the difference between phases was negligible. To be consistent with the latent heat of the β-γ transition, the increase in vibrational entropy at the phase transition may be accompanied by a decrease in electronic entropy not associated with the crystal-field splitting or spin fluctuations. At least three sources of entropy need to be considered for the β-γ transition in cerium
Crystal growth and magnetic structure of MnBi2Te4
Millimeter-sized MnBiTe single crystals are grown out of Bi-Te flux
and characterized by measuring magnetic and transport properties, scanning
tunneling microscope (STM) and spectroscopy (STS). The magnetic structure of
MnBiTe below T is determined by powder and single crystal neutron
diffraction measurements. Below T=24\,K, Mn moments order
ferromagnetically in the \textit{ab} plane but antiferromagnetically along the
crystallographic \textit{c} axis. The ordered moment is 4.04(13) /Mn
at 10\,K and aligned along the crystallographic \textit{c}-axis. The electrical
resistivity drops upon cooling across T or when going across the
metamagnetic transition in increasing fields below T. A critical scattering
effect was observed in the vicinity of T in the temperature dependence of
thermal conductivity. However, A linear temperature dependence was observed for
thermopower in the temperature range 2K-300K without any anomaly around T.
These indicate that the magnetic order in Mn-Te layer has negligible effect on
the electronic band structure, which makes possible the realization of proposed
topological properties in MnBiTe after fine tuning of the electronic
band structure
Character of the structural and magnetic phase transitions in the parent and electron doped BaFe2As2 compounds
We present a combined high-resolution x-ray diffraction and x-ray resonant
magnetic scattering (XRMS) study of as-grown BaFe2As2. The structural/magnetic
transitions must be described as a two-step process. At T_S = 134.5 K we
observe the onset of a second-order structural transition from the
high-temperature paramagnetic tetragonal structure to a paramagnetic
orthorhombic phase, followed by a discontinuous step in the structural order
parameter that is coincident with a first-order antiferromagnetic (AFM)
transition at T_N = 133.75 K. These data, together with detailed
high-resolution x-ray studies of the structural transition in lightly doped
Ba(Fe{1-x}Co{x})2As2 and Ba(Fe{1-x}Rh{x})2As2 compounds, show that the
structural and AFM transitions do, in fact, occur at slightly different
temperatures in the parent BaFe2As2 compound, and evolve towards split
secondorder transitions as the doping concentration is increased. We estimate
the composition for the tricritical point for Co-doping and employ a mean-field
approach to show that our measurements can be explained by the inclusion of an
anharmonic term in the elastic free energy and magneto-elastic coupling in the
form of an emergent Ising-nematic degree of freedom.Comment: 10 pages, 11 figures; accepted for publication in Phys. Rev.
Effect of carbon addition on the single crystalline magnetostriction of Fe-X (X = Al and Ga) alloys
The effect of carbon addition on the magnetostriction of Fe–Ga and Fe–Al alloys was investigated and is summarized in this study. It was found that the addition of carbon generally increased the magnetostriction over binary alloys of Fe–Ga and Fe–Al systems. The formation of carbide in the Fe–Ga–C alloys with a composition near D03 phase region decreased the magnetostriction drastically. Fe–Al–C and Fe–Ga–C alloys responded differently to thermal treatments; the magnetostriction in the quenched Fe–Al–C alloys is equal to or slightly lower than that of the slow cooled as is observed in binary Fe–Al alloy; in contrast, the magnetostriction is generally higher in quenched Fe–Ga–C alloys than slow cooled condition, consistent with the behavior of binary alloys of Fe–Ga. A significant increase in magnetostriction between 25% and 165% depending on the phase region in Fe–Ga–C alloys by quenching was observed in the A2+D03 two-phase region and D03 single phase region
Low temperature heat capacity of Fe1−xGax alloys with large magnetostriction
The low temperature heat capacity Cp of Fe1−xGax alloys with large magnetostriction has been investigated. The data were analyzed in the standard way using electron (γT) and phonon (βT3) contributions. The Debye temperature ΘD decreases approximately linearly with increasing Ga concentration, consistent with previous resonant ultrasound measurements and measured phonon dispersion curves. Calculations of ΘD from lattice dynamical models and from measured elastic constants C11,C12, and C44 are in agreement with the measured data. The linear coefficient of electronic specific heat γ remains relatively constant as the Ga concentration increases, despite the fact that the magnetoelastic coupling increases. Band structure calculations show that this is due to the compensation of majority and minority spin states at the Fermi level
Magnetic excitations in underdoped Ba(Fe1-xCox)2As2 with x=0.047
The magnetic excitations in the paramagnetic-tetragonal phase of underdoped
Ba(Fe0.953Co0.047)2As2, as measured by inelastic neutron scattering, can be
well described by a phenomenological model with purely diffusive spin dynamics.
At low energies, the spectrum around the magnetic ordering vector Q_AFM
consists of a single peak with elliptical shape in momentum space. At high
energies, this inelastic peak is split into two peaks across the direction
perpendicular to Q_AFM. We use our fittings to argue that such a splitting is
not due to incommensurability or propagating spin-wave excitations, but is
rather a consequence of the anisotropies in the Landau damping and in the
magnetic correlation length, both of which are allowed by the tetragonal
symmetry of the system. We also measure the magnetic spectrum deep inside the
magnetically-ordered phase, and find that it is remarkably similar to the
spectrum of the paramagnetic phase, revealing the strongly overdamped character
of the magnetic excitations.Comment: 12 pages, 7 figure
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