59 research outputs found
Large zero-field cooled exchange-bias in bulk Mn2PtGa
We report a large exchange-bias (EB) effect after zero-field cooling the new
tetragonal Heusler compound Mn2PtGa from the paramagnetic state. The
first-principle calculation and the magnetic measurements reveal that Mn2PtGa
orders ferrimagnetically with some ferromagnetic (FM) inclusions. We show that
ferrimagnetic (FI) ordering is essential to isothermally induce the exchange
anisotropy needed for the zero-field cooled (ZFC) EB during the virgin
magnetization process. The complex magnetic behavior at low temperatures is
characterized by the coexistence of a field induced irreversible magnetic
behavior and a spin-glass-like phase. The field induced irreversibility
originates from an unusual first-order FI to antiferromagnetic transition,
whereas, the spin-glass like state forms due to the existence of anti-site
disorder intrinsic to the material.Comment: 5 pages, 4 figures, supplementary material included in a separate
file; accepted for publication in PR
New iron-based Heusler compounds Fe2YZ: Comparison with theoretical predictions of the crystal structure and magnetic properties
The present work reports on the new soft ferromagnetic Heusler phases
Fe2NiGe, Fe2CuGa, and Fe2CuAl, which in previous theoretical studies have been
predicted to exist in a tetragonal regular Heusler structure. Together with the
known phases Fe2CoGe and Fe2NiGa these materials have been synthesized and
characterized by powder XRD, 57 Fe M\"ossbauer spectroscopy, SQUID and EDX
measurements. In particular M\"ossbauer spectroscopy was used to monitor the
degree of local atomic order/disorder and to estimate magnetic moments at the
Fe sites from the hyperfine fields. It is shown that in contrast to the
previous predictions all the materials except Fe2NiGa basically adopt the
inverse cubic Heusler (X-) structure with differing degrees of disorder. The
disorder is more enhanced in case of Fe2NiGa, which was predicted as an inverse
Heusler phase. The experimental data are compared with results from ab-inito
electronic structure calculations on LDA level incorporating the effects of
atomic disorder by using the coherent potential approximation (CPA). A good
agreement between calculated and experimental magnetic moments is found for the
cubic inverse Heusler phases. Model calculations on various atomic
configurations demonstrate that antisite disorder tends to enhance the
stability of the X-structure. Given the fundamental scientific and
technological importance of tetragonal Heusler phases the present results call
for further investigations to unravel the factors stabilizing tetragonal
Heusler materials
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
Reactive and organic halogen species in three different European coastal environments
International audienceWe present results of three field campaigns using active longpath DOAS (Differential Optical Absorption Spectroscopy) for the study of reactive halogen species (RHS) BrO, IO, OIO and I2. Two recent field campaigns took place in Spring 2002 in Dagebüll at the German North Sea Coast and in Spring 2003 in Lilia at the French Atlantic Coast of Brittany. In addition, data from a campaign in Mace Head, Ireland in 1998 was re-evaluated. During these field campaigns volatile halogenated organic compounds (VHOCs) were determined by GC/ECD-ICPMS in air and water. Due to the spatial distribution of macroalgae at the German North Sea Coast we found a clear connection between elevated levels of VHOCs and the appearance of macroalgae. Extraordinarily high concentrations of several VHOCs, especially CH3I and CH3Br of up to 1830 pptv and 875 pptv, respectively, were observed at the coast of Brittany, demonstrating the outstanding level of bioactivity there. We found CH2I2 at levels of up to 20 pptv, and a clear anti-correlation with the appearance of IO. The IO mixing ratio reached up to 7.7±0.5 ppt(pmol/mol) during the day, in reasonable agreement with model studies designed to represent the meteorological and chemical conditions in Brittany. For the two campaigns the DOAS spectra were evaluated for BrO, OIO and I2, but none of these species could be clearly identified (detection limits around 2 ppt, 3 ppt, 20 ppt, resp.). Only in the Mace Head spectra evidence was found for the presence of OIO. Since macroalgae under oxidative stress are suggested to be a further source for I2 in the marine boundary layer, we re-analyzed spectra in the 500?600 nm range taken during the 1998 PARFORCE campaign in Mace Head, Ireland, which had not previously been analyzed for I2. We identified molecular iodine above the detection limit (~20 ppt), with peak concentrations of 61±12 ppt. Since I2 was undetectable during the Brittany campaign, we suggest that iodine may not be released into the atmosphere by macroalgae in general, but only by a special type of the laminaria species under oxidative stress. Only during periods of extraordinarily low water (spring-tide), is the plant exposed to ambient air and may release gaseous iodine in some way to the atmosphere. The result of our re-analysis of spectra from the PARFORCE campaign in 1998 support this theory. Hence, we feel that we can provide an explanation for the different I2 levels in Brittany and Mace Head
Superconductivity in the Heusler Family of Intermetallics
Several physical properties of the superconducting Heusler compounds,
focusing on two systems (Y, Lu, Sc)Pd2Sn and APd2M, where A=Hf, Zr and M=Al,
In, are summarized and compared. The analysis of the data shows the importance
of the electron-phonon coupling for superconductivity in this family. We report
the superconducting parameters of YPd2Sn, which has the highest Tc among all
known Heusler superconductors.Comment: Accepted for publication in Phys. Rev.
Electronic and structural properties of palladium-based Heusler superconductors
This work reports on superconductivity in the Heusler compounds Pd2ZrAl and Pd2HfAl. Magnetisation and resistance measurements were carried out to verify their superconducting states. The compounds exhibit transition temperatures of 3.2 K (Zr) and 3.4 K (Hf). From their behaviour in external magnetic fields, it was determined that both compounds are type 11 superconductors. Similar to the half-metallic ferromagnets, the superconducting Heusler compounds follow an electron counting scheme based on theoretical considerations. As found from ab initio calculations, the superconductivity can be explained by a valence instability at the L-point, that has been used as design criterion. (C) 2007 Elsevier Ltd. All rights reserved
Mn<sub>3</sub>Ga, a compensated ferrimagnet with high Curie temperature and low magnetic moment for spin torque transfer applications
This work reports about the electronic, magnetic, and structural properties of the binary compound Mn3Ga. The tetragonal DO22 phase of Mn3Ga was successfully synthesized and investigated. It has been found that the material is hard magnetic with an energy product of H(c)xB(r)=52.5 kJ m(-3) and an average saturation magnetization of about 0.25 mu(B)/at. at 5 K. The saturation magnetization indicates a ferrimagnetic order with partially compensating moments at the Mn atoms on crystallographically different sites. The Curie temperature is above 730 K where the onset of decomposition is observed. The electronic structure calculations indicate a nearly half-metallic ferrimagnetic order with 88% spin polarization at the Fermi energy. (c) 2007 American Institute of Physics
Tuning the magnetism of the Heusler alloys Mn<sub>3-x</sub>Co<sub>x</sub>Ga from soft and half-metallic to hard-magnetic for spin-transfer torque applications
The magnetic properties of Mn3-xCoxGa Heusler alloys exhibit a very interesting feature. While the Mn-rich alloys (x = 0.1-0.4) are similar to Mn3Ga as they crystallize in a tetragonally distorted variation of the Heusler structure and exhibit comparable hard-ferrimagnetic properties highly useful for spin-transfer torque applications, the Co-rich samples (x = 0.6-1) exhibit cubic Heusler structures, have soft-magnetic hysteresis loops, and follow the generalized Slater-Pauling curve indicating half-metallic ferrimagnetism. The Curie temperatures of all alloys are above 700 K allowing for high operating temperatures. (C) 2011 American Institute of Physics. [doi:10.1063/1.3665260
Superconductivity in palladium-based Heusler compounds
This work reports on four more Heusler superconductors: Pd(2)ZrAl, Pd(2)HfAl, Pd(2)ZrIn, and Pd(2)HfIn. These compounds exhibit superconducting transition temperatures ranging from 2.4-3.8 K as determined by resistivity measurements. According to their behavior in an external magnetic field, all compounds are type II bulk superconductors. The occurrence of superconductivity was predicted for these compounds using electronic structure calculations. The electronic structures exhibit van Hove singularities (saddle points) at the L point. These lead to a maximum in the corresponding density of states and superconductivity according to the van Hove scenario. The superconducting properties of electron-doped and hole-doped substituted compounds Pd(2)B(1-x)B(x)(')Al, whereby B=Zr and Hf, and B(')=Y, Nb, and Mo, were investigated to obtain information about the dependence of the transition temperature on the density of states at the Fermi energy following the van Hove scenario. The calculated electronic structure reveals that the substituted compounds do not follow a rigid-band model. In addition, the random distribution of the substituted atoms strongly increases impurity-type electron scattering. The substituent concentrations used in this work lead to strongly enhanced impurity-type scattering and eventually to suppression of the superconducting state
Quaternary half-metallic Heusler ferromagnets for spintronics applications
This work reports on three quaternary Heusler compounds NiFeMnGa, NiCoMnGa, and CuCoMnGa. In contrast to their ternary relatives, quaternary Heusler compounds are still rarely investigated. A very large pool of interesting materials lies thus idle waiting for exploration. The difficulty consists in choosing prospective compositions, and trial and error is elaborate and expensive. We have identified several candidates employing ab initioelectronic-structure calculations. The compounds were synthesized, and the structural and magnetic properties were investigated experimentally. CuCoMnGa is a quaternary Heusler compound; NiFeMnGa and NiCoMnGa are unreported half-metallic ferromagnetic materials with potential for spintronics applications
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