94 research outputs found
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
Anomalous transport properties of the halfmetallic ferromagnets Co2TiSi, Co2TiGe, and Co2TiSn
In this work the theoretical and experimental investigations of Co2TiZ (Z =
Si, Ge, or Sn) compounds are reported. Half-metallic ferromagnetism is
predicted for all three compounds with only two bands crossing the Fermi energy
in the majority channel. The magnetic moments fulfill the Slater-Pauling rule
and the Curie temperatures are well above room temperature. All compounds show
a metallic like resistivity for low temperatures up to their Curie temperature,
above the resistivity changes to semiconducting like behavior. A large negative
magnetoresistance of 55% is observed for Co2TiSn at room temperature in an
applied magnetic field of 4T which is comparable to the large negative
magnetoresistances of the manganites. The Seebeck coefficients are negative for
all three compounds and reach their maximum values at their respective Curie
temperatures and stay almost constant up to 950 K. The highest value achieved
is -52muV/K m for Co2TiSn which is large for a metal. The combination of
half-metallicity and the constant large Seebeck coefficient over a wide
temperature range makes these compounds interesting materials for
thermoelectric applications and further spincaloric investigations.Comment: 4 pages 4 figure
Tuning of crystal structure and magnetic properties by exceptionally large epitaxial strains
Huge deformations of the crystal lattice can be achieved in materials with
inherent structural instability by epitaxial straining. By coherent growth on
seven different substrates the in-plane lattice constants of 50 nm thick
Fe70Pd30 films are continuously varied. The maximum epitaxial strain reaches
8,3 % relative to the fcc lattice. The in-plane lattice strain results in a
remarkable tetragonal distortion ranging from c/abct = 1.09 to 1.39, covering
most of the Bain transformation path from fcc to bcc crystal structure. This
has dramatic consequences for the magnetic key properties. Magnetometry and
X-ray circular dichroism (XMCD) measurements show that Curie temperature,
orbital magnetic moment, and magnetocrystalline anisotropy are tuned over broad
ranges.Comment: manuscript, 3 figures, auxiliary materia
Disentangling the Mn moments on different sublattices in the half-metallic ferrimagnet Mn3?xCoxGa
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Disentangling the Mn moments on different sublattices in the half-metallic ferrimagnet Mn3?xCoxGa
Electro deposited In2S3 buffer layers for CuInS2 solar cells
We report for the first time the electro deposition of In2S3 buffer layers for CuInS2 solar cells. All materials and deposition conditions were selected taking into account environmental, economic and technological aspects of a potential transfer to large volume industrial production. Different bath compositions and electro deposition parameters were studied. The obtained films exhibited complete substrate coverage, confirmed by SEM and XPS. In S ratio close to 2 3 was obtained. XPS measurements detected the presence of indium hydroxide, transforming into oxide upon anneal at 200 C. Maximum photoelectric conversion efficiency of 7.1 was obtained, limited mainly by a low fill factor 51 . Further process optimization is expected to lead to efficiencies comparable to CdS buffers. So far, open circuit voltages as high as 660mV were demonstrate
Temperature dependence of x-ray absorption spectra in the ferromagnetic Heusler alloys Mn2VAl and Co2FeAl
Electronic structure of the austenitic and martensitic state of magnetocaloric Ni Mn In Heusler alloy films
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