162 research outputs found
Magnetic properties of iron pnictides from spin-spiral calculations
The wave-vector (q) and doping dependences of the magnetic energy, iron
moment, and effective exchange interactions in LaFeAsO, BaFe2As2, and SrFe2As2\
are studied by self-consistent LSDA calculations for co-planar spin spirals.
For the undoped compounds, the calculated total energy, E(q), reaches its
minimum at q corresponding to stripe anti-ferromagnetic order. In LaFeAsO, this
minimum becomes flat already at low levels of electron-doping and shifts to an
incommensurate q at delta=0.2, where delta is the number of additional
electrons (delta>0) or holes (delta<0) per Fe. In BaFe2As2 and SrFe2As2, stripe
order remains stable for hole doping down to delta=-0.3. Under electron doping,
on the other hand, the E(q) minimum shifts to incommensurate q already at
delta=0.1.Comment: 4 pages, 2 figures, International Conference on Magnetism, Karlsruhe,
July 26 - 31, 200
Spin density wave anomaly at 140 K in the ternary iron arsenide BaFe2As2
The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type
structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to
LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is
a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase
transition at 140 K, accompanied by strong anomalies in the specific heat,
electrical resistance and magnetic susceptibility. In the course of this
transition, the space group symmetry changes from tetragonal (I4/mmm) to
orthorhombic (Fmmm). 57Fe Moessbauer spectroscopy experiments show a single
signal at room temperature and full hyperfine field splitting below the phase
transition temperature (5.2 T at 77 K). Our results suggest that BaFe2As2 can
serve as a new parent compound for oxygen-free iron arsenide superconductors.Comment: 4 pages, 6 figures, submitted to PR
Incommensurate spin density wave in Co-doped BaFe2As2
57Fe Mossbauer spectroscopy measurements are presented in the underdoped
Ba(Fe{1-x}Cox)2As2 series for x=0.014 (T_c < 1.4K) and x=0.03 and 0.045 (T_c ~
2 and 12K respectively). The spectral shapes in the so-called spin-density wave
(SDW) phase are interpreted in terms of incommensurate modulation of the
magnetic structure, and allow the shape of the modulation to be determined. In
undoped BaFe2As2, the magnetic structure is commensurate, and we find that
incommensurability is present at the lowest doping level (x=0.014). As Co
doping increases, the low temperature modulation progressively loses its
"squaredness" and tends to a sine-wave. The same trend occurs for a given
doping level, as temperature increases. We find that a magnetic hyperfine
component persists far above the SDW transition, its intensity being
progressively tranferred to a paramagnetic component on heating.Comment: 7 pages, 8 figures, published in EP
Comparison of crystal structures and effects of Co substitution in a new member of Fe-1111 superconductor family AeFeAsF(Ae = Ca and Sr): a possible candidate for higher Tc superconductor
We refined crystal structures of newly found members of the Fe-1111
superconductor family, CaFe\_{1-x}Co\_{x}AsF and SrFe\_{1-x}Co\_{x}AsF (x = 0,
0.06, 0.12) by powder synchrotron X-ray diffraction analysis. The tetragonal to
orthorhombic phase transitions were observed at ~120 K for unsubstituted
CaFeAsF and at ~180 K for unsubstituted SrFeAsF, the transition temperatures
agreeing with kinks observed in temperature-dependent resistivity curves.
Although the transition temperature decreases, the structural phase transitions
were observed below 100 K in both samples of x = 0.06, and finally they were
suppressed in the doping level of x = 0.12. The refined structures reveal that
distortions of the FeAs4 tetrahedron from the regular tetrahedron likely
originate from mismatches in atomic radii among the constituent elements. In
this system, the enlarged FeAs4 tetrahedron resulting from larger radius of Sr
than that of Ca is flattened along a-b plane, whereas the smaller radius of Ca
makes the tetrahedron closer to regular one, and their characteristic shapes
are further enhanced by Co substitution. These results suggest that the CaFeAsF
compound is a promising candidate for higher-Tc superconductor.Comment: 17 pages, 8 figures, 2 tables, Supplementary information is included
at the end of the documen
Phonon spectra in CaFe2As2 and Ca0.6Na0.4Fe2As2: Measurement of the pressure and temperature dependence and comparison with ab-initio and shell model calculations
We report the pressure and temperature dependence of the phonon
density-of-states in superconducting Ca0.6Na0.4Fe2As2 (Tc=21 K) and the parent
compound CaFe2As2, using inelastic neutron scattering. We observe no
significant change in the phonon spectrum for Ca0.6Na0.4Fe2As2 at 295 K up to
pressures of 5 kbar. The phonon spectrum for CaFe2As2 shows softening of the
low-energy modes by about 1 meV when decreasing the temperature from 300 K to
180 K. There is no appreciable change in the phonon density of states across
the structural and anti-ferromagnetic phase transition at 172 K. These results,
combined with our earlier temperature dependent phonon density of states
measurements for Ca0.6Na0.4Fe2As2, indicate that the softening of low-energy
phonon modes in these compounds may be due to the interaction of phonons with
electron or short-range spin fluctuations in the normal state of the
superconducting compound as well as in the parent compound. The phonon spectra
are analyzed with ab-initio and empirical model calculations giving partial
densities of states and dispersion relations.Comment: 14 pages, 6 figure
Static magnetic order of SrAOFeAs (A = Sc and V) revealed by local probes
Static magnetic order of quasi two-dimensional FeAs compounds
Sr4A2O6-xFe2As2, with A = Sc and V, has been detected by 57Fe Moessbauer and
muon spin relaxation ({\mu}SR) spectroscopies. The non-superconducting
stoichiometric (x = 0) A = Sc system exhibits a static internal/hyperfine
magnetic field both at the 57Fe and {\mu}+ sites, indicating antiferromagnetic
order of Fe moments below TN = 35 K with ~ 0.1 Bohr magneton per Fe at T = 2 K.
The superconducting and oxygen deficient (x = 0.4) A = V system exhibits a
static internal field only at the {\mu}+ site below TN ~ 40 K, indicating
static magnetic order of V moments co-existing with superconductivity without
freezing of Fe moments. These results suggest that the 42622 FeAs systems
belong to the same paradigm with the 1111 and 122 FeAs systems with respect to
magnetic behavior of Fe moments.Comment: 4 pages 4 figures: for information, contact
[email protected]
Paramagnetic Phase of a Heavy-Fermion Compound, CeFePO, Probed by 57Fe M\"{o}ssbauer Spectroscopy
57Fe M\"{o}ssbauer spectroscopy was applied to an iron-based layered compound
CeFePO. At temperatures from 9.4 to 293 K, no magnetic splitting was observed
in the M\"ossbauer spectra of CeFePO indicating a paramagnetic phase of the Fe
magnetic sublattice. All the spectra were fitted with a small quadrupole
splitting, and the Debye temperature of CeFePO was found to be \sim448 K. The
isomer shift at room temperature, 0.32 mm/s, was almost equal to those of
LnFeAsO (Ln = La, Ce, Sm). Comparing s-electron density using the isomer shifts
and unit cell volumes, it was found that the Fe of CeFePO has a similar valence
state to other layered iron-based quaternary oxypnictides except LaFePO
Tetragonal to Orthorhombic Transition of GdFeAsO Studied by Single-Crystal Synchrotron X-Ray Diffraction
A study of the tetragonal to orthorhombic phase transition of GdFeAsO is
presented. Planes of the reciprocal space were reconstructed form
single-crystal synchrotron X-ray diffraction data. By cooling below the
structural transition temperature splitting of the Bragg reflections was
observed corresponding to four different twin domain orientations. A model was
developed to quantify the distortion of the lattice from the position of the
splitted reflections relative to each other. Constrained 2D-Cauchy fits of
several splitted reflections provided positions of the reflections. The
influence of the structural distortion was detectable already above the
structural transition temperature hinting at fluctuations in the tetragonal
phase.Comment: 6 pages, 6 figure
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