207 research outputs found
YbPtPb: Magnetic frustration in the Shastry-Sutherland lattice
We have synthesized single crystals of YbPtPb, which crystallize
in the layered UPtSn-type structure, where planes of Yb ions lie on
a triangular network. We report here the first results of magnetization,
specific heat, and electrical resistivity experiments. The lattice constants
and high temperature magnetic susceptibility indicate that the Yb ions are
trivalent, while Schottky peaks in the specific heat show that the ground state
is a well isolated doublet. Significant magnetic anisotropy is observed, with
the ratio of susceptibilities perpendicular and parallel to the magnetic planes
differing by as much as a factor of 30 at the lowest temperatures.
Antiferromagnetic order occurs at a N\'eel temperature =2.07 K, a
transition temperature which is more than an order of magnitude smaller than
the mean field interactions reflected by the in-plane Weiss temperature.
Further evidence for short ranged magnetic fluctuations is found in the
magnetic susceptibility and electrical resistivity, which have broad peaks
above , and in the slow development of the magnetic entropy at
. Our experiments indicate that YbPtPb is a quasi-two
dimensional and localized moment system, where strong magnetic frustration may
arise from the geometry of the underlying Shastry-Sutherland lattice.Comment: 8 pages, 8 figure
Searching for hexagonal analogues of the half-metallic half-Heusler XYZ compounds
The XYZ half-Heusler crystal structure can conveniently be described as a
tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X
species. This description is well suited to understand the electronic structure
of semiconducting 8-electron compounds such as LiAlSi (formulated
Li[AlSi]) or semiconducting 18-electron compounds such as TiCoSb
(formulated Ti[CoSb]). The basis for this is that [AlSi]
(with the same electron count as Si) and [CoSb] (the same electron
count as GaSb), are both structurally and electronically, zinc-blende
semiconductors. The electronic structure of half-metallic ferromagnets in this
structure type can then be described as semiconductors with stuffing magnetic
ions which have a local moment: For example, 22 electron MnNiSb can be written
Mn[NiSb]. The tendency in the 18 electron compound for a
semiconducting gap -- believed to arise from strong covalency -- is carried
over in MnNiSb to a tendency for a gap in one spin direction. Here we similarly
propose the systematic examination of 18-electron hexagonal compounds for
semiconducting gaps; these would be the "stuffed wurtzite" analogues of the
"stuffed zinc blende" half-Heusler compounds. These semiconductors could then
serve as the basis for possibly new families of half-metallic compounds,
attained through appropriate replacement of non-magnetic ions by magnetic ones.
These semiconductors and semimetals with tunable charge carrier concentrations
could also be interesting in the context of magnetoresistive and thermoelectric
materials.Comment: 11 pages, 6 figures, of which 4 are colou
Laves phases: superstructures induced by coloring and distortions
The structural chemistry of Laves phases, especially with respect to their superstructures induced by coloring and distortions is discussed. Starting from the three classical Laves phases MgCu2, MgZn2 and MgNi2, the more complex Komura phases are derived. Different possibilities of their description are summarized. In the second part, the superstructures are discussed based on their respective prototypes. The crystal chemical relationships are illustrated based on group-subgroup descriptions using the Bärnighausen formalism
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
Electronic structure of AuMg and AgMg ( = Eu, Gd, Yb)
We have investigated the electronic structure of the equiatomic EuAuMg,
GdAuMg, YbAuMg and GdAgMg intermetallics using x-ray photoelectron
spectroscopy. The spectra revealed that the Yb and Eu are divalent while the Gd
is trivalent. The spectral weight in the vicinity of the Fermi level is
dominated by the mix of Mg , Au/Ag and bands, and not by the
. We also found that the Au and Ag bands are extraordinarily
narrow, as if the noble metal atoms were impurities submerged in a low density
metal host. The experimental results were compared with band structure
calculations, and we found good agreement provided that the spin-orbit
interaction in the Au an Ag bands is included and correlation effects in an
open shell are accounted for using the local density approximation +
Hubbard scheme. Nevertheless, limitations of such a mean-field scheme to
explain excitation spectra are also evident.Comment: 4 pages, 3 figures, Brief Repor
Magnetic behaviour of Eu_2CuSi_3: Large negative magnetoresistance above Curie temperature
We report here the results of magnetic susceptibility,
electrical-resistivity, magnetoresistance (MR), heat-capacity and ^{151}Eu
Mossbauer effect measurements on the compound, Eu_2CuSi_3, crystallizing in an
AlB_2-derived hexagonal structure. The results establish that Eu ions are
divalent, undergoing long-range ferromagnetic-ordering below (T_C=) 37 K. An
interesting observation is that the sign of MR is negative even at temperatures
close to 3T_C, with increasing magnitude with decreasing temperature exhibiting
a peak at T_C. This observation, being made for a Cu containing magnetic
rare-earth compound for the first time, is of relevance to the field of
collosal magnetoresistance.Comment: To appear in PRB, RevTex, 4 pages text + 6 psFigs. Related to our
earlier work on Gd systems (see cond-mat/9811382, cond-mat/9811387,
cond-mat/9812069, cond-mat/9812365
Ferromagnetic Ordering in the Thallide EuPdTl 2
The new thallide EuPdTl 2 , synthesized from the elements in a sealed tantalum tube in a highfrequency furnace, was investigated by X-ray diffraction on powders and single crystals: MgCuAl 2 type, Cmcm, Z = 4, a = 446.6(1), b = 1076.7(2), c = 812.0(2) pm, wR2 = 0.0632, 336 F 2 values, 16 variables. The structure can be considered as an orthorhombically distorted, palladium-filled variant of the binary Zintl phase EuTl 2 . The palladium and thallium atoms build up a three-dimensional [PdTl 2 ] polyanion with significant Pd-Tl (286 -287 pm) and Tl-Tl (323 -329 pm) interactions. The europium atoms fill distorted hexagonal channels of the [PdTl 2 ] polyanion. Susceptibility measurements show a magnetic moment of 7.46(5) µ B /Eu atom, indicative of divalent europium. EuPdTl 2 is a soft ferromagnet with a Curie temperature of T C = 12.5(5) K
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