27 research outputs found
Very large magnetoresistance in FeTaS single crystals
Magnetic moments intercalated into layered transition metal dichalcogenides
are an excellent system for investigating the rich physics associated with
magnetic ordering in a strongly anisotropic, strong spin-orbit coupling
environment. We examine electronic transport and magnetization in
FeTaS, a highly anisotropic ferromagnet with a Curie temperature
K. We find anomalous Hall data confirming a
dominance of spin-orbit coupling in the magnetotransport properties of this
material, and a remarkably large field-perpendicular-to-plane MR exceeding 60%
at 2 K, much larger than the typical MR for bulk metals, and comparable to
state-of-the-art GMR in thin film heterostructures, and smaller only than CMR
in Mn perovskites or high mobility semiconductors. Even within the
FeTaS series, for the current = 0.28 single crystals the MR is
nearly higher than that found previously in the commensurate
compound FeTaS. After considering alternatives, we argue that
the large MR arises from spin disorder scattering in the strong spin-orbit
coupling environment, and suggest that this can be a design principle for
materials with large MR.Comment: 8 pages, 8 figures, accepted in PR
Direct evidence for the magnetic ordering of Nd ions in NdFeAsO by high resolution inelastic neutron scattering
We investigated the low energy excitations in the parent compound NdFeAsO of
the Fe-pnictide superconductor in the eV range by a back scattering
neutron spectrometer. The energy scans on a powder NdFeAsO sample revealed
inelastic peaks at E = 1.600 eV at T = 0.055 K on both energy
gain and energy loss sides. The inelastic peaks move gradually towards lower
energy with increasing temperature and finally merge with the elastic peak at
about 6 K. We interpret the inelastic peaks to be due to the transition between
hyperfine-split nuclear level of the Nd and Nd isotopes with
spin . The hyperfine field is produced by the ordering of the
electronic magnetic moment of Nd at low temperature and thus the present
investigation gives direct evidence of the ordering of the Nd magnetic
sublattice of NdFeAsO at low temperature
Superconductivity in NdFe1-xCoxAsO (0.05 < x < 0.20) and rare-earth magnetic ordering in NdCoAsO
The phase diagram of NdFe1-xCoxAsO for low cobalt substitution consists of a
superconducting dome (0.05 < x < 0.20) with a maximum critical temperature of
16.5(2) K for x = 0.12. The x = 1 end member, NdCoAsO, is an itinerant
ferromagnet (TC = 85 K) with an ordered moment of 0.30(1) BM at 15 K. Below TN
= 9 K, Nd spin-ordering results in the antiferromagnetic coupling of the
existing ferromagnetic planes. Rietveld analysis reveals that the
electronically important two-fold tetrahedral angle increases from 111.4 to
115.9 deg. in this series. Underdoped samples with x = 0.046(2) and x =
0.065(2) show distortions to the orthorhombic Cmma structure at 72(2) and 64(2)
K, respectively. The temperature dependences of the critical fields Hc2(T) near
Tc are linear with almost identical slopes of 2.3(1) T K-1 for x = 0.065(2), x
= 0.118(2) and x = 0.172(2). The estimated critical field Hc2(0) and
correlation length for optimally doped samples are 26(1) T and 36(1) Angstrom.
A comparison of the maximum reported critical temperatures of
well-characterized cobalt doped 122- and 1111-type superconductors is
presented.Comment: accepted to PR
Iron spin-reorientation transition in NdFeAsO
The low-temperature magnetic structure of NdFeAsO has been revisited using
neutron powder diffraction and symmetry analysis using the Sarah
representational analysis program. Four magnetic models with one magnetic
variable for each of the Nd and Fe sublattices were tested. The best fit was
obtained using a model with Fe moments pointing along the c-direction, and Nd
moments along the a-direction. This signals a significant interplay between
rare-earth and transition metal magnetism, which results in a
spin-reorientation of the Fe sublattice upon ordering of the Nd moments. All
models that fit the data well, including collinear models with more than one
magnetic variable per sublattice, were found to have an Fe moment of 0.5 BM and
a Nd moment of 0.9 BM, demonstrating that the low-temperature Fe moment is not
substantially enhanced compared to the spin-density wave (SDW) state.Comment: accepted to J. Phys.: Cond. Ma
Nd induced Mn spin-reorientation transition in NdMnAsO
A combination of synchrotron X-ray, neutron powder diffraction,
magnetization, heat capacity and electrical resistivity measurements reveals
that NdMnAsO is an antiferromagnetic semiconductor with large Neel temperature
(TN = 359(2) K). At room temperature the magnetic propagation vector k = 0 and
the Mn moments are directed along the crystallographic c-axis (mMn = 2.41(6)
BM). Upon cooling a spin reorientation (SR) transition of the Mn moments into
the ab-plane occurs (TSR = 23 K). This coincides with the long range ordering
of the Nd moments, which are restricted to the basal plane. The magnetic
propagation vector remains k = 0. At base temperature (1.6 K) the fitted
moments are mab,Mn = 3.72(1) BM and mab,Nd = 1.94(1) BM. The electrical
resistivity is characterized by a broad maximum at 250 K, below which it has a
metallic temperature dependence but semiconducting magnitude (rho250K = 50 Ohm
cm, residual resistivity ratio = 2), and a slight upturn at the SR transition
Topological metal behavior in GeBi2Te4 single crystals
The metallic character of the GeBi2Te4 single crystals is probed using a
combination of structural and physical properties measurements, together with
density functional theory (DFT) calculations. The structural study shows
distorted Ge coordination polyhedra, mainly of the Ge octahedra. This has a
major impact on the band structure, resulting in bulk metallic behavior of
GeBi2Te4, as indicated by DFT calculations. Such calculations place GeBi2Te4 in
a class of a few known non-trivial topological metals, and explains why an
observed Dirac point lies below the Fermi energy at about -0.12eV. A
topological picture of GeBi2Te4 is confirmed by the observation of surface
state modulations by scanning tunneling microscopy (STM).Comment: 10 pages, 8 figure
A Magnetic Transition Probed by the Ce Ion in Square-Lattice Antiferromagnet CeMnAsO
We examined the magnetic properties of the square-lattice antiferromagnets
CeMnAsO and LaMnAsO and their solid solutions La1-xCexMnAsO by resistivity,
magnetic susceptibility, and heat capacity measurements below room temperature.
A first-order phase transition is observed at 34.1 K, below which the
ground-state doublet of the Ce ion splits by 3.53 meV. It is likely that Mn
moments already ordered above room temperature are reoriented at the
transition, as reported for related compounds, such as NdMnAsO and PrMnSbO.
This transition generates a large internal magnetic field at the Ce site in
spite of the fact that simple Heisenberg interactions should be cancelled out
at the Ce site owing to geometrical frustration. The transition takes place at
nearly the same temperature with the substitution of La for Ce up to 90%. The
Ce moment does not undergo long-range order by itself, but is parasitically
induced at the transition, serving as a good probe for detecting the magnetism
of Mn spins in a square lattice.Comment: 11 pages, 5 figures, to be published in J. Phys. Soc. Jp
Colossal Magnetoresistance in the Mn2+ Oxypnictides NdMnAsO1-xFx
Colossal magnetoresistance (CMR) is a rare phenomenon in which the electronic
resistivity of a material can be decreased by orders of magnitude upon
application of a magnetic field. Such an effect could be the basis of the next
generation of magnetic memory devices. Here we report CMR in the
antiferromagnetic oxypnictide NdMnAsO1-xFx as a result of competition between
an antiferromagnetic insulating phase with strong electron correlations and a
paramagnetic semiconductor upon application of a magnetic field. The discovery
of CMR in antiferromagnetic Mn2+ oxypnictide materials could open up an array
of materials for further investigation and optimisation for technological
applications