448 research outputs found
Evolution of magnetic fluctuations through the Fe-induced paramagnetic to ferromagnetic transition in CrB
In itinerant ferromagnets, the quenched disorder is predicted to dramatically
affect the ferromagnetic to paramagnetic quantum phase transition driven by
external control parameters at zero temperature. Here we report a study on
Fe-doped CrB, which, starting from the paramagnetic parent, orders
ferromagnetically for Fe-doping concentrations larger than \%. In parent CrB, B nuclear magnetic resonance data reveal
the presence of both ferromagnetic and antiferromagnetic fluctuations. The
latter are suppressed with Fe-doping, before the ferromagnetic ones finally
prevail for . Indications for non-Fermi liquid behavior, usually
associated with the proximity of a quantum critical point, were found for all
samples, including undoped CrB. The sharpness of the ferromagnetic-like
transition changes on moving away from , indicating significant
changes in the nature of the magnetic transitions in the vicinity of the
quantum critical point. Our data provide constraints for understanding quantum
phase transitions in itinerant ferromagnets in the limit of weak quenched
disorder.Comment: 8 pages, 7 figure
Lone Pair Effect, Structural Distortions and Potential for Superconductivity in Tl Perovskites
Drawing the analogy to BaBiO3, we investigate via ab-initio electronic
structure calculations potential new superconductors of the type ATlX3 with A =
Rb, Cs and X = F, Cl, and Br, with a particular emphasis on RbTlCl3. Based on
chemical reasoning, supported by the calculations, we show that Tl-based
perovskites have structural and charge instabilities driven by the lone pair
effect, similar to the case of BaBiO3, effectively becoming A2Tl1+Tl3+X6. We
find that upon hole doping of RbTlCl3, structures without Tl1+, Tl3+ charge
disproportionation become more stable, although the ideal cubic perovskite,
often viewed as the best host for superconductivity, should not be the most
stable phase in the system. The known superconductor (Sr,K)BiO3 and hole doped
RbTlCl3, predicted to be most stable in the same tetragonal structure, display
highly analogous calculated electronic band structures.Comment: 5 pages, 5 figure
Strain and order-parameter coupling in Ni-Mn-Ga Heusler alloys from resonant ultrasound spectroscopy
Resonant ultrasound spectroscopy and magnetic susceptibility experiments have
been used to characterize strain coupling phenomena associated with structural
and magnetic properties of the shape-memory Heusler alloy series
NiMnGa (, 2.5, 5.0, and 7.5). All samples exhibit
a martensitic transformation at temperature and ferromagnetic ordering at
temperature , while the pure end member () also has a premartensitic
transition at , giving four different scenarios: ,
without premartensitic transition, , and .
Fundamental differences in elastic properties i.e., stiffening versus
softening, are explained in terms of coupling of shear strains with three
discrete order parameters relating to magnetic ordering, a soft mode and the
electronic instability responsible for the large strains typical of martensitic
transitions. Linear-quadratic or biquadratic coupling between these order
parameters, either directly or indirectly via the common strains, is then used
to explain the stabilities of the different structures. Acoustic losses are
attributed to critical slowing down at the premartensite transition, to the
mobility of interphases between coexisting phases at the martensitic transition
and to mobility of some aspect of the twin walls under applied stress down to
the lowest temperatures at which measurements were made.Comment: 9 pages, 5 figure
Direct measurements of the magnetocaloric effect in pulsed magnetic fields: The example of the Heusler alloy NiMnIn
We have studied the magnetocaloric effect (MCE) in the shape-memory Heusler
alloy NiMnIn by direct measurements in pulsed magnetic
fields up to 6 and 20 T. The results in 6 T are compared with data obtained
from heat-capacity experiments. We find a saturation of the inverse MCE,
related to the first-order martensitic transition, with a maximum adiabatic
temperature change of K at 250 K and a conventional
field-dependent MCE near the second-order ferromagnetic transition in the
austenitic phase. The pulsed magnetic field data allow for an analysis of the
temperature response of the sample to the magnetic field on a time scale of
to 100 ms which is on the order of typical operation frequencies (10
to 100 Hz) of magnetocaloric cooling devices. Our results disclose that in
shape-memory alloys the different contributions to the MCE and hysteresis
effects around the martensitic transition have to be carefully considered for
future cooling applications.Comment: 5 pages, 4 figure
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
Monitoring surface resonances on Co2MnSi(100) by spin-resolved photoelectron spectroscopy
The magnitude of the spin polarization at the Fermi level of ferromagnetic
materials at room temperature is a key property for spintronics. Investigating
the Heusler compound CoMnSi a value of 93 for the spin polarization has
been observed at room temperature, where the high spin polarization is related
to a stable surface resonance in the majority band extending deep into the
bulk. In particular, we identified in our spectroscopical analysis that this
surface resonance is embedded in the bulk continuum with a strong coupling to
the majority bulk states. The resonance behaves very bulk-like, as it extends
over the first six atomic layers of the corresponding (001)-surface. Our study
includes experimental investigations, where the bulk electronic structure as
well as surface-related features have been investigated using spin-resolved
photoelectron spectroscopy (SR-UPS) and for a larger probing depth
spin-integrated high energy x-ray photoemission spectroscopy (HAXPES). The
results are interpreted in comparison with first-principles band structure and
photoemission calculations which consider all relativistic, surface and
high-energy effects properly.Comment: 9 pages, 8 figures, Heusler alloy, electronic structure and
photoemissio
Thermopower and thermal conductivity in the Weyl semimetal NbP
The Weyl semimetal NbP exhibits an extremely large magnetoresistance (MR) and
an ultra-high mobility. The large MR originates from a combination of the
nearly perfect compensation between electron- and hole-type charge carriers and
the high mobility, which is relevant to the topological band structure. In this
work we report on temperature- and field-dependent thermopower and thermal
conductivity experiments on NbP. Additionally, we carried out complementary
heat capacity, magnetization, and electrical resistivity measurements. We found
a giant adiabatic magnetothermopower with a maximum of 800 V/K at 50 K in
a field of 9 T. Such large effects have been observed rarely in bulk materials.
We suggest that the origin of this effect might be related to the high
charge-carrier mobility. We further observe pronounced quantum oscillations in
both thermal conductivity and thermopower. The obtained frequencies compare
well with our heat capacity and magnetization data.Comment: 6 pages, 3 figure
Topological insulators in filled skutterudites
We propose new topological insulators in cerium filled skutterudite (FS)
compounds based on ab initio calculations. We find that two compounds CeOs4As12
and CeOs4Sb12 are zero gap materials with band inversion between Os-d and Ce-f
orbitals, which are thus parent compounds of two and three-dimensional
topological insulators just like bulk HgTe. At low temperature, both compounds
become topological Kondo insulators, which are Kondo insulators in the bulk,
but have robust Dirac surface states on the boundary. This new family of
topological insulators has two advantages compared to previous ones. First,
they can have good proximity effect with other superconducting FS compounds to
realize Majarona fermions. Second, the antiferromagnetism of CeOs4Sb12 at low
temperature provides a way to realize the massive Dirac fermion with novel
topological phenomena.Comment: 4 page, 3 figure
Epitaxial film growth and magnetic properties of Co_2FeSi
We have grown thin films of the Heusler compound Co_2FeSi by RF magnetron
sputtering. On (100)-oriented MgO substrates we find fully epitaxial
(100)-oriented and L2_1 ordered growth. On Al_2O_3 (11-20) substrates, the film
growth is (110)-oriented, and several in-plane epitaxial domains are observed.
The temperature dependence of the electrical resistivity shows a power law with
an exponent of 7/2 at low temperatures. Investigation of the bulk magnetic
properties reveals an extrapolated saturation magnetization of 5.0 mu_B/fu at 0
K. The films on Al_2O_3 show an in-plane uniaxial anisotropy, while the
epitaxial films are magnetically isotropic in the plane. Measurements of the
X-ray magnetic circular dichroism of the films allowed us to determine element
specific magnetic moments. Finally we have measured the spin polarization at
the surface region by spin-resolved near-threshold photoemission and found it
strongly reduced in contrast to the expected bulk value of 100%. Possible
reasons for the reduced magnetization are discussed.Comment: 9 pages, 12 figure
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