47 research outputs found
Exotic magnetism in the alkali sesquoxides Rb4O6 and Cs4O6
Among the various alkali oxides the sesquioxides Rb4O6 and Cs4O6 are of
special interest. Electronic structure calculations using the local
spin-density approximation predicted that Rb4O6 should be a half-metallic
ferromagnet, which was later contradicted when an experimental investigation of
the temperature dependent magnetization of Rb4O6 showed a low-temperature
magnetic transition and differences between zero-field-cooled (ZFC) and
field-cooled (FC) measurements. Such behavior is known from spin glasses and
frustrated systems. Rb4O6 and Cs4O6 comprise two different types of dioxygen
anions, the hyperoxide and the peroxide anions. The nonmagnetic peroxide anions
do not contain unpaired electrons while the hyperoxide anions contain unpaired
electrons in antibonding pi*-orbitals. High electron localization (narrow
bands) suggests that electronic correlations are of major importance in these
open shell p-electron systems. Correlations and charge ordering due to the
mixed valency render p-electron-based anionogenic magnetic order possible in
the sesquioxides. In this work we present an experimental comparison of Rb4O6
and the related Cs4O6. The crystal structures are verified using powder x-ray
diffraction. The mixed valency of both compounds is confirmed using Raman
spectroscopy, and time-dependent magnetization experiments indicate that both
compounds show magnetic frustration, a feature only previously known from d-
and f-electron systems
Rapid Assessment of the Ce-Co-Fe-Cu System for Permanent Magnetic Applications
This work focuses on the rapid synthesis and characterization of quaternary Ce(CoFeCu)5 alloy libraries to assess their potential viability as permanent magnets. Arrays of bulk specimens with controlled compositions were synthesized via laser engineered net shaping (LENS) by feeding different ratios of alloy powders into a melt pool created by a laser. Based on the assessment of the magnetic properties of the LENS printed samples, arc-melted and cast ingots were prepared with varying Fe (5–20 at.%) and Co (60–45 at.%) compositions while maintaining constant Ce (16 at.%) and Cu (19 at.%) content. The evolution of the microstructure and phases with varying chemical compositions and their dependence on magnetic properties are analyzed in as-cast and heat-treated samples. In both the LENS printed and cast samples, we find the best magnetic properties correspond to a predominantly single-phase Ce(CoFeCu)5 microstructure in which high coercivity (Hc \u3e 10 kOe) can be achieved without any microstructural refinement
Upper critical field, lower critical field and critical current density of FeTe0.60Se0.40 single crystal
The transport and magnetic studies are performed on high quality
FeTe0.60Se0.40 single crystals to determine the upper critical fields (Hc2),
lower critical field (Hc1) and the Critical current density (Jc). The value of
upper critical field Hc2 are very large, whereas the activation energy as
determined from the slope of the Arrhenius plots are was found to be lower than
that in the FeAs122 superconductor. The lower critical field was determined in
ab direction and c direction of the crystal, and was found to have a anisotropy
of 'gamma'{=(Hc1//c) / (Hc1//b)} ~ 4. The magnetic isotherms measured up to 12
Tesla shows the presence of fishtail behavior. The critical current densities
at 1.8K of the single crystal was found to almost same in both ab and c
direction as 1X105 Amp/cm2 in low field regime.Comment: 9 pages, 6 figure
High temperature superconductivity (Tc onset at 34K) in the high pressure orthorhombic phase of FeSe
We have studied the structural and superconducting properties of tetragonal
FeSe under pressures up to 26GPa using synchrotron radiation and diamond anvil
cells. The bulk modulus of the tetragonal phase is 28.5(3)GPa, much smaller
than the rest of Fe based superconductors. At 12GPa we observe a phase
transition from the tetragonal to an orthorhombic symmetry. The high pressure
orthorhombic phase has a higher Tc reaching 34K at 22GPa.Comment: 15 pages, 4 figure
Superconductivity at 36 K in beta-Fe1.01Se with the compression of the interlayer separation under pressure
In this letter, we report that the superconductivity transition temperature
in beta-Fe1.01Se increases from 8.5 to 36.7 K under applied pressure of 8.9
GPa. It then decreases at higher pressure. A dramatic change in volume is
observed at the same time Tc rises, due to a collapse of the separation between
the Fe2Se2 layers. A clear transition to a linear resistivity normal state is
seen on cooling at all pressures. No static magnetic ordering is observed for
the whole p-T phase diagram. We also report that at higher pressure (starting
around 7 GPa and completed at 38 GPa), Fe1.01Se transforms to a hexagonal
NiAs-type structure and displays non-magnetic, insulating behavior. The
inclusion of electron correlation in band structure caculations is necessary to
describe this behavior, signifying that such correlations are important in this
chemical system. Our results strongly support unconventional superconductivity
in beta-Fe1.01Se.Comment: 17 pages, 4 figure
High pressure studies of the erbium-hydrogen system
High-pressure X-ray diffraction investigations up to 25 GPa using diamond anvil cell techniques (DAC) have been carried out on erbium and a series of erbium hydrides. The equations of state have been evaluated for ErH1.95, ErH2.091 (in the beta-phase) and for gamma-ErH3. For comparison, the compressibility of pure erbium metal has also been determined in the same pressure range. A rapid drop of lattice volume at a pressure of about 14.5 GPa has been observed for ErH2.091 accompanied by a color change of reflected light. This phenomenon was not observed in ErH1.95 where the molar volume varied smoothly up to the highest pressure. A pressure-induced transformation from hexagonal to cubic phase has been detected for erbium trihydride. For pure erbium metal, a transition from hexagonal to samarium structure has been revealed, confirming previously reported behavio
High pressure studies of GdMn 2 and its hydrides
Abstract High pressure energy dispersive X-ray diffraction (EDXD) studies have been performed on GdMn 2 and its GdMn 2 H x (x = 1, 2) hydrides up to 20 GPa at room temperature in a diamond anvil cell. The C15 structure of GdMn 2 is retained in the whole pressure range. Structural transformations from the C15 to a yet unidentified phase were observed for both hydrides in the 4-10 GPa pressure range. For higher hydrogen content, the transition pressure region shifts to higher values. Equations of state of pure GdMn 2 and its hydrides have been determined. For all these compounds, an unusually high compressibility at lower pressures has been revealed