470 research outputs found
Superconductivity in Weyl semimetal NbP: Bulk vs. surface
Transition metal monopnictides belong to the new class of semimetals where the bulk properties are determined by the presence of pairs of nodes with different chirality formed by linear dispersive states in the k-space. Beside the anomaly in the bulk magnetotransport superconductivity is frequently found in some Weyl semimetals. We found signatures of superconductivity in ac and dc magnetization measurements of highly pure and stoichiometric NbP powder. We determined the lower and upper critical field and the Ginzburg-Landau parameter. The relative small superconducting volume fraction is related to either effect of finite grain size and/or surface superconductivity. The last mentioned may originate from either off stoichiometric (Nb-rich) surface layers or a strained surface with different electronic properties. Furthermore the intrinsic normal state susceptibility is determined taking into account a paramagnetic contribution of a few ppm of magnetic impurities
Heusler 4.0: Tunable Materials
Heusler compounds are a large family of binary, ternary and quaternary
compounds that exhibit a wide range of properties of both fundamental and
potential technological interest. The extensive tunability of the Heusler
compounds through chemical substitutions and structural motifs makes the family
especially interesting. In this article we highlight recent major developments
in the field of Heusler compounds and put these in the historical context. The
evolution of the Heusler compounds can be described by four major periods of
research. In the latest period, Heusler 4.0 has led to the observation of a
variety of properties derived from topology that includes: topological metals
with Weyl and Dirac points; a variety of non-collinear spin textures including
the very recent observation of skyrmions at room temperature; and giant
anomalous Hall effects in antiferromagnetic Heuslers with triangular magnetic
structures. Here we give a comprehensive overview of these major achievements
and set research into Heusler materials within the context of recent emerging
trends in condensed matter physics
Large magnetocrystalline anisotropy in tetragonally distorted Heuslers: a systematic study
With a view to the design of hard magnets without rare earths we explore the
possibility of large magnetocrystalline anisotropy energies in Heusler
compounds that are unstable with respect to a tetragonal distortion. We
consider the Heusler compounds FeYZ with Y = (Ni, Co, Pt), and CoYZ
with Y = (Ni, Fe, Pt) where, in both cases, Z = (Al, Ga, Ge, In, Sn). We find
that for the CoNiZ, CoPtZ, and FePtZ families the cubic phase is
always, at , unstable with respect to a tetragonal distortion, while, in
contrast, for the FeNiZ and FeCoZ families this is the case for only 2
compounds -- FeCoGe and FeCoSn. For all compounds in which a tetragonal
distortion occurs we calculate the MAE finding remarkably large values for the
Pt containing Heuslers, but also large values for a number of the other
compounds (e.g. CoNiGa has an MAE of -2.11~MJ/m). The tendency to a
tetragonal distortion we find to be strongly correlated with a high density of
states at the Fermi level in the cubic phase. As a corollary to this fact we
observe that upon doping compounds for which the cubic structure is stable such
that the Fermi level enters a region of high DOS, a tetragonal distortion is
induced and a correspondingly large value of the MAE is then observed.Comment: 8 pages, 5 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
Huge quadratic magneto-optical Kerr effect and magnetization reversal in the CoFeSi Heusler compound
CoFeSi(100) films with L2 structure deposited onto MgO(100) were
studied exploiting both longitudinal (LMOKE) and quadratic (QMOKE)
magneto-optical Kerr effect. The films exhibit a huge QMOKE signal with a
maximum contribution of up to 30 mdeg, which is the largest QMOKE signal in
reflection that has been measured thus far. This large value is a fingerprint
of an exceptionally large spin-orbit coupling of second or higher order. The
CoFeSi(100) films exhibit a rather large coercivity of 350 and 70 Oe for
film thicknesses of 22 and 98 nm, respectively. Despite the fact that the films
are epitaxial, they do not provide an angular dependence of the anisotropy and
the remanence in excess of 1% and 2%, respectively
Properties of the quaternary half-metal-type Heusler alloy CoMnFeSi
This work reports on the bulk properties of the quaternary Heusler alloy
CoMnFeSi with the Fe concentration . All samples, which
were prepared by arc melting, exhibit long range order over the complete
range of Fe concentration. Structural and magnetic properties of
CoMnFeSi Heusler alloys were investigated by means of X-ray
diffraction, high and low temperature magnetometry, M{\"o\ss}bauer
spectroscopy, and differential scanning calorimetry. The electronic structure
was explored by means of high energy photo emission spectroscopy at about 8 keV
photon energy. This ensures true bulk sensitivity of the measurements. The
magnetization of the Fe doped Heusler alloys is in agreement with the values of
the magnetic moments expected for a Slater-Pauling like behavior of
half-metallic ferromagnets. The experimental findings are discussed on the hand
of self-consistent calculations of the electronic and magnetic structure. To
achieve good agreement with experiment, the calculations indicate that on-site
electron-electron correlation must be taken into account, even at low Fe
concentration. The present investigation focuses on searching for the
quaternary compound where the half-metallic behavior is stable against outside
influences. Overall, the results suggest that the best candidate may be found
at an iron concentration of about 50%.Comment: 26 pages, 9 figures Phys. Rev. B accepte
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
Completely compensated ferrimagnetism and sublattice spin crossing in the half-metallic Heusler compound Mn1.5FeV0.5Al
The Slater-Pauling rule states that L21 Heusler compounds with 24 valence
electrons do never exhibit a total spin magnetic moment. In case of strongly
localized magnetic moments at one of the atoms (here Mn) they will exhibit a
fully compensated half-metallic ferrimagnetic state instead, in particular,
when symmetry does not allow for antiferromagnetic order. With aid of magnetic
and anomalous Hall effect measurements it is experimentally demonstrated that
Mn1.5V0.5FeAl follows such a scenario. The ferrimagnetic state is tuned by the
composition. A small residual magnetization, that arises due to a slight
mismatch of the magnetic moments in the different sublattices results in a
pronounced change of the temperature dependence of the ferrimagnet. A
compensation point is confirmed by observation of magnetic reversal and sign
change of the anomalous Hall effect. Theoretical models are presented that
correlate the electronic structure and the compensation mechanisms of the
different half-metallic ferrimagnetic states in the Mn-V-Fe-Al Heusler system.Comment: Under revie
Hall effect in laser ablated Co_2(Mn,Fe)Si thin films
Pulsed laser deposition was employed to grow thin films of the Heusler
compounds Co_2MnSi and Co_2FeSi. Epitaxial growth was realized both directly on
MgO (100) and on a Cr or Fe buffer layer. Structural analysis by x-ray and
electron diffraction shows for both materials the ordered L2_1 structure. Bulk
magnetization was determined with a SQUID magnetometer. The values agree with
the Slater-Pauling rule for half-metallic Heusler compounds. On the films grown
directly on the substrate measurements of the Hall effect have been performed.
The normal Hall effect is nearly temperature independent and points towards a
compensated Fermi surface. The anomalous contribution is found to be dominated
by skew scattering. A remarkable sign change of both normal and anomalous Hall
coefficients is observed on changing the valence electron count from 29 (Mn) to
30 (Fe).Comment: 9 pages, 6 figures submitted to J Phys
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