94 research outputs found
On the low-field Hall coefficient of graphite
We have measured the temperature and magnetic field dependence of the Hall
coefficient () in three, several micrometer long multigraphene
samples of thickness between to ~nm in the temperature range
0.1 to 200~K and up to 0.2~T field. The temperature dependence of the
longitudinal resistance of two of the samples indicates the contribution from
embedded interfaces running parallel to the graphene layers. At low enough
temperatures and fields is positive in all samples, showing a
crossover to negative values at high enough fields and/or temperatures in
samples with interfaces contribution. The overall results are compatible with
the reported superconducting behavior of embedded interfaces in the graphite
structure and indicate that the negative low magnetic field Hall coefficient is
not intrinsic of the ideal graphite structure.Comment: 10 pages with 7 figures, to be published in AIP Advances (2014
Effect of spin orbit scattering on the magnetic and superconducting properties of nearly ferromagnetic metals: application to granular Pt
We calculate the effect of scattering on the static, exchange enhanced, spin
susceptibility and show that in particular spin orbit scattering leads to a
reduction of the giant moments and spin glass freezing temperature due to
dilute magnetic impurities. The harmful spin fluctuation contribution to the
intra-grain pairing interaction is strongly reduced opening the way for BCS
superconductivity. We are thus able to explain the superconducting and magnetic
properties recently observed in granular Pt as due to scattering effects in
single small grains.Comment: 9 pages 3 figures, accepted for publication in Phys. Rev. Letter
Observation of long range magnetic ordering in pyrohafnate Nd2Hf2O7: A neutron diffraction study
We have investigated the physical properties of a pyrochlore hafnate Nd2Hf2O7
using ac magnetic susceptibility \chi_ac(T), dc magnetic susceptibility
\chi(T), isothermal magnetization M(H) and heat capacity C_p(T) measurements,
and determined the magnetic ground state by neutron powder diffraction study.
An upturn is observed below 6 K in C_p(T)/T, however both C_p(T) and \chi(T) do
not show any clear anomaly down to 2 K. The \chi_ac(T) shows a well pronounced
anomaly indicating an antiferromagnetic transition at T_N = 0.55 K. The long
range antiferromagnetic ordering is confirmed by neutron diffraction. The
refinement of neutron diffraction pattern reveals an all-in/all-out
antiferromagnetic structure, where for successive tetrahedra, the four Nd3+
magnetic moments point alternatively all-into or all-out-of the tetrahedron,
with an ordering wavevector k = (0, 0, 0) and an ordered state magnetic moment
of m = 0.62(1) \mu_B/Nd at 0.1 K. The ordered moment is strongly reduced
reflecting strong quantum fluctuations in ordered state.Comment: 10 pages, 9 figures and 2 tables; to appear in Phys. Rev.
Evolution of antiferromagnetic domains in the all-in-all-out ordered pyrochlore NdZrO
We report the observation of magnetic domains in the exotic,
antiferromagnetically ordered all-in-all-out state of NdZrO,
induced by spin canting. The all-in-all-out state can be realized by Ising-like
spins on a pyrochlore lattice and is established in NdZrO below
0.31 K for external magnetic fields up to 0.14 T. Two different spin
arrangements can fulfill this configuration which leads to the possibility of
magnetic domains. The all-in-all-out domain structure can be controlled by an
external magnetic field applied parallel to the [111] direction. This is a
result of different spin canting mechanism for the two all-in-all-out
configurations for such a direction of the magnetic field. The change of the
domain structure is observed through a hysteresis in the magnetic
susceptibility. No hysteresis occurs, however, in case the external magnetic
field is applied along [100].Comment: Accepted for publication in Phys. Rev. B, 6 pages, 6 figure
On the dc Magnetization, Spontaneous Vortex State and Specific Heat in the superconducting state of the weakly ferromagnetic superconductor RuSrGdCuO
Magnetic-field changes 0.2 Oe over the scan length in magnetometers that
necessitate sample movement are enough to create artifacts in the dc
magnetization measurements of the weakly ferromagnetic superconductor
RuSrGdCuO (Ru1212) below the superconducting transition
temperature 30 K. The observed features depend on the specific
magnetic-field profile in the sample chamber and this explains the variety of
reported behaviors for this compound below . An experimental procedure
that combines improvement of the magnetic-field homogeneity with very small
scan lengths and leads to artifact-free measurements similar to those on a
stationary sample has been developed. This procedure was used to measure the
mass magnetization of Ru1212 as a function of the applied magnetic field H (-20
Oe H 20 Oe) at and discuss, in conjunction with
resistance and ac susceptibility measurements, the possibility of a spontaneous
vortex state (SVS) for this compound. Although the existence of a SVS can not
be excluded, an alternative interpretation of the results based on the granular
nature of the investigated sample is also possible. Specific-heat measurements
of SrGdRuO (Sr2116), the precursor for the preparation of Ru1212
and thus a possible impurity phase, show that it is unlikely that Sr2116 is
responsible for the specific-heat features observed for Ru1212 at .Comment: 17 pages, 6 figure
Insights into intermetallic phases on pulse welded dissimilar metal joints
The Magnetic Pulse Welding (MPW) process has been developed to an industrially used joining method which is considered to be a fast, noncontact, clean and "cold" solid state welding process. Unlike fusion welding, the absence of direct heat during the welding cycle makes it possible to join dissimilar metals, for instance aluminium to copper or copper to steel, without noticeable detrimental metallurgical defects. This is very desirable, as today s industry lacks technologies to join often not fusion-weldable dissimilar materials effectively. However, current metallographic studies show that for many material combinations the formation of intermetallic seams in the joint region of magnetic pulse welds can not be completely avoided. Modern technical equipment for MPW is used to join aluminium with copper in order to study the microstructure and the intermetallic phases formed in the weld region in dependence of the processing parameters. The welds are analysed by means of metallographic and electron microscopic (SEM) methods. Relations between the parameters and the microstructures formed within the weld joints are shown. Based on the obtained results conclusions will be drawn with respect to the intermetallic phase formation process and the optimization of the weld microstructure and properties
Mode Conversion and Period Doubling at Plasma- Unity in an Alfv\'en-Wave Experiment with Liquid Rubidium
We report Alfv\'en-wave experiments with liquid rubidium at the Dresden High
Magnetic Field Laboratory (HLD). Reaching up to 63 T, the pulsed magnetic field
exceeds the critical value of 54 T at which the Alfv\'en speed becomes equal to
the sound speed (plasma- unity). At this threshold we observe a period
doubling of an applied 8 kHz CW excitation, a clear footprint for a parametric
resonance between magnetosonic waves and Alfv\'en waves.Comment: 5 pages, 4 figure
Influence of magnetic impurities on the heat capacity of nuclear spins
It is found that in a wide range of temperatures and magnetic fields even a
small concentration of magnetic impurities in a sample leads to a
temperature dependence of the nuclear heat capacity. This effect is related to
a nuclear-spin polarization by the magnetic impurities. The parameter that
controls the theory turns out not to be the impurity concentration
but instead the quantity , where and are
the magnetic moments of an electron and a nucleus, respectively. The ratio of
and is of order of
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