173 research outputs found
Iron substitution in NdCoAsO: crystal structure and magnetic phase diagram
The effects of replacing small amounts of Co with Fe in NdCoAsO are reported.
Polycrystalline materials with compositions NdCo1-xFexAsO (x = 0.05, 0.10,
0.15, and 0.20) are studied and the results compared to previous reports for
NdCoAsO. Rietveld analysis of powder x-ray diffraction data shows that as Fe
replaces Co on the transition metal (T) site, the T-As distance increases, and
the As tetrahedra surrounding the T-site become more regular. Electrical
resistivity and magnetization measurements indicate that the three magnetic
phase transitions in NdCoAsO are suppressed as Co is replaced by Fe, and these
transitions are not observed above 1.8 K for x = 0.20. Based on these results,
the magnetic phase diagram for the Co-rich side of the NdCoAsO-NdFeAsO system
is constructed.Comment: Accepted for publication in Physical Review B, revised text and
figures, 5 pages, 5 figure
Anomalous Hall Effect in three ferromagnets: EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30
The Hall resistivity (Rho_xy), resistivity (Rho_xx), and magnetization of
three metallic ferromagnets are investigated as a function of magnetic field
and temperature. The three ferromagnets, EuFe4Sb12 (Tc = 84 K), Yb14MnSb11 (Tc
= 53 K), and Eu8Ga16Ge30 (Tc = 36 K) are Zintl compounds with carrier
concentrations between 1 x 10^21 cm^-3 and 3.5 x 10^21 cm^-3. The relative
decrease in Rho_xx below Tc [Rho_xx(Tc)/Rho_xx(2 K)] is 28, 6.5, and 1.3 for
EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30 respectively. The low carrier
concentrations coupled with low magnetic anisotropies allow a relatively clean
separation between the anomalous (Rho_'xy), and normal contributions to the
measured Hall resistivity. For each compound the anomalous contribution in the
zero field limit is fit to alpha Rho_xx + sigma_xy rho_xx^2 for temperatures T
< Tc. The anomalous Hall conductivity, sigma_xy, is -220 +- 5 (Ohm^-1 cm^-1),
-14.7 +- 1 (Ohm^-1 cm^-1), and 28 +- 3 (Ohm^-1 cm^-1) for EuFe4Sb12,
Yb14MnSb11, and Eu8Ga16Ge30 respectively and is independent of temperature for
T < Tc if the change in spontaneous magnetization (order parameter) with
temperature is taken into account. These data are consistent with recent
theories of the anomalous Hall effect that suggest that even for stochiometric
ferromagnetic crystals, such as those studied in this article, the intrinsic
Hall conductivity is finite at T = 0, and is a ground state property that can
be calculated from the electronic structure.Comment: 22 pages, 13 figures Submitted to PR
Unusual Phase Transitions and Magnetoelastic Coupling in TlFe1.6Se2 Single Crystals
Structural, magnetic, electrical transport, and heat capacity data are
reported for single crystals of TlFe1.6Se2. This compound crystallizes in a
tetragonal structure similar to the ThCr2Si2 structure, but with vacancies in
the Fe layer. The vacancies can be ordered or disordered depending on
temperature and thermal history. If the vacancies are ordered, the basal plane
lattice constant increases from a to \sqrt{5}a. Antiferromagnetic order with
the Fe spins along the c-axis occurs below T_N ~ 430K as shown by single
crystal neutron diffraction and the magnetic structure is reported. In
addition, for the vacancy ordered crystal, two other phase transitions are
found at T_1 ~ 140K, and T_2 ~ 100K. The phase transitions at T_1 and T_2 are
evident in heat capacity, magnetic susceptibility, resistivity data, a and c
lattice parameters, and in the unusual temperature dependence of the magnetic
order parameter determined from neutron scattering. The phase transitions at
T_1 and T_2 result in significant changes in the magnetic moment per iron, with
1.72(6)\mu_B observed at 300K, 2.07(9)\mu_B at 140\,K, 1.90(9)\,\mu_B at
115\,K, and 1.31(8)\mu_B for 5\,K if the same "block checkerboard" magnetic
structure is used at all temperatures. The phase transitions appear to be
driven by small changes in the c lattice constant, large magnetoelastic
coupling, and the localization of carriers with decreasing temperature.Comment: Accepted for publication in Physical Review
Thermoelectric properties of Co, Ir, and Os-Doped FeSi Alloys: Evidence for Strong Electron-Phonon Coupling
The effects of various transition metal dopants on the electrical and thermal
transport properties of Fe1-xMxSi alloys (M= Co, Ir, Os) are reported. The
maximum thermoelectric figure of merit ZTmax is improved from 0.007 at 60 K for
pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal
conductivity data among Os, Ir and Co doped alloys indicates strong
electron-phonon coupling in this compound. Because of this interaction, the
common approximation of dividing the total thermal conductivity into
independent electronic and lattice components ({\kappa}Total =
{\kappa}electronic + {\kappa}lattice) fails for these alloys. The effects of
grain size on thermoelectric properties of Fe0.96Ir0.04Si alloys are also
reported. The thermal conductivity can be lowered by about 50% with little or
no effect on the electrical resistivity or Seebeck coefficient. This results in
ZTmax = 0.125 at 100 K, still about a factor of five too low for solid-state
refrigeration applications
Magnetic Phase Transitions in NdCoAsO
Magnetization measurements reveal that NdCoAsO undergoes three magnetic phase
transitions below room temperature. The crystal and magnetic structures of
NdCoAsO have been determined by powder neutron diffraction, and the effects of
the phase transitions on physical properties are reported. Near 69 K a
ferromagnetic state emerges with a small saturation moment of about 0.2 Bohr
magnetons, likely on Co atoms. At 14 K the material enters an antiferromagnetic
state with propagation vector (0 0 1/2) and small ordered moments (~0.4 Bohr
magnetons) on Co and Nd. Near 3.5 K a third transition is observed, and
corresponds to the antiferromagnetic ordering, with the same propagation
vector, of larger moments on Nd reaching 1.30(2) Bohr magnetons at 1.4 K. In
addition, transport properties and heat capacity results are presented, and
show anomalies at all three phase transitions.Comment: Some minor changes made, and lower temperature neutron diffraction
results are included. Accepted for publication in Physical Review
Superconductivity at 22 K in Co-doped BaFe2As2 Crystals
Here we report bulk superconductivity in BaFe1.8Co0.2As2 single crystals
below Tc = 22 K, as demonstrated by resistivity, magnetic susceptibility, and
specific heat data. Hall data indicate that the dominant carriers are
electrons, as expected from simple chemical reasoning. This is the first
example of superconductivity induced by electron doping in this family of
materials. In contrast to the cuprates, the BaFe2As2 system appears to tolerate
considerable disorder in the FeAs planes. First principles calculations for
BaFe1.8Co0.2As2 indicate the inter-band scattering due to Co is weak.Comment: 9 pages, 3 figure
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