186 research outputs found
Effects of stoichiometry, purity, etching and distilling on resistance of MgB2 pellets and wire segments
We present a study of the effects of non-stoichiometry, boron purity, wire
diameter and post-synthesis treatment (etching and Mg distilling) on the
temperature dependent resistance and resistivity of sintered MgB2 pellets and
wire segments. Whereas the residual resistivity ratio (RRR) varies between RRR
\~ 4 to RRR > 20 for different boron purity, it is only moderately affected by
non-stoichiometry (from 20% Mg deficiency to 20% Mg excess) and is apparently
independent of wire diameter and presence of Mg metal traces on the wire
surface. The obtained set of data indicates that RRR values in excess of 20 and
residual resistivities as low as rho{0} ~ 0.4 mu Ohm cm are intrinsic material
properties of high purity MgB2
Angular dependent planar metamagnetism in the hexagonal compounds TbPtIn and TmAgGe
Detailed magnetization measurements, M(T,H,theta), were performed on single
crystals of TbPtIn and TmAgGe (both members of the hexagonal Fe_2P/ZrNiAl
structure type), for the magnetic field H applied perpendicular to the
crystallographic c axis. These data allowed us to identify, for each compound,
the easy-axes for the magnetization, which coincided with high symmetry
directions ([120] for TbPtIn and [110] for TmAgGe). For fixed orientations of
the field along each of the two six-fold symmetry axes, a number of
magnetically ordered phases is being revealed by M(H,T) measurements below T_N.
Moreover, T ~ 2 K, M(H)|_theta measurements for both compounds (with H applied
parallel to the basal plane), as well as T = 20 K data for TbPtIn, reveal five
metamagnetic transitions with simple angular dependencies: H_{ci,j} ~
1/cos(theta +/- phi), where phi = 0^0 or 60^0. The high field magnetization
state varies with theta like 2/3*mu_{sat}(R^{3+})*cos(theta), and corresponds
to a crystal field limited saturated paramagnetic, CL-SPM, state. Analysis of
these data allowed us to model the angular dependence of the locally saturated
magnetizations M_{sat} and critical fields H_c with a three coplanar Ising-like
model, in which the magnetic moments are assumed to be parallel to three
adjacent easy axes. Furthermore, net distributions of moments were inferred
based on the measured data and the proposed model
Unconventional electronic Raman spectra of borocarbide superconductors
Borocarbide superconductors, which are thought to be conventional BCS-type
superconductors, are not so conventional in several electronic Raman
properties. Anisotropic gap-like features and finite scattering strength below
the gap were observed for the NiBC ( = Lu, Y) systems in our
previous study. The effects of Co-doping on the 2 gap-like features and
the finite scattering strength below and above the gap are studied in = Lu
(B = B) system. In superconducting states, Co-doping strongly suppresses
the 2 peak in both B and B symmetries. Raman
cross-section calculation which includes inelastic scattering shows a
relatively good fit to the features above the 2 peak, while it does not
fully account for the features below the peak.Comment: 2 pages, 1 figur
Titanium additions to MgB2 conductors
A series of doping experiments are reported for MgB2 conductors that have
been synthesized using doped boron fibers prepared by chemical vapor
deposition(CVD) methods. Undoped MgB2 samples prepared from CVD prepared fibers
consistently give critical current densities, Jc, in the range of 500,000
A/cm^2 in low field at 5K. These values fall by a factor of about 100 as the
magnetic field increases to 3T. For heavily Ti-doped boron fibers where the
B/Ti ratio is comparable to 1, there is a substantial suppression of both Tc,
superconducting volume fraction, and Jc values. If, however, a sample with a
few percent Ti in B is deposited on a carbon coated SiC substrate and reacted
at 1100 degrees C for 15 min, then Tc is suppressed only a couple of degrees
Kelvin and critical current densities are found to be approximately 2-5 x 10^6
A/cm^2 for superconducting layers ranging from 4-10 micrometers thick. These
materials show Jc values over 10,000 A/cm^2 at 25K and 1.3 T.Comment: 10 pages, 6 figure
Synthesis and Processing of powders and wires
Sintered powders and wires of superconducting have been fabricated
under a variety of conditions in order to determine details of the diffusion of
the into and to study the types of defects that arise during growth.
For samples prepared by exposure of boron to vapor at , the
conversion of particles of less than size particles to is
complete in about . The lattice parameters of the phase determined
from X-ray are independent of the starting stoichiometry and the time of
reaction. Wire segments of with very little porosity have been produced
by reacting diameter boron fibers in an atmosphere of excess
vapor at . Defects in the reacted fibers are predominantly the
voids left as the boron is converted to
Magnetic Phase Diagram of GdNi2B2C: Two-ion Magnetoelasticity and Anisotropic Exchange Couplings
Extensive magnetization and magnetostriction measurements were carried out on
a single crystal of GdNi2B2C along the main tetragonal axes. Within the
paramagnetic phase, the magnetic and strain susceptibilities revealed a weak
anisotropy in the exchange couplings and two-ion tetragonal-preserving
alpha-strain modes. Within the ordered phase, magnetization and
magnetostriction revealed a relatively strong orthorhombic distortion mode and
rich field-temperature phase diagrams. For H//(100) phase diagram, three
field-induced transformations were observed, namely, at: Hd(T), related to the
domain alignment; Hr(T), associated with reorientation of the moment towards
the c-axis; and Hs(T), defining the saturation process wherein the exchange
field is completely counterbalanced. On the other hand, For H//(001) phase
diagram, only two field-induced transformations were observed, namely at: Hr(T)
and Hs(T). For both phase diagrams, Hs(T) follows the relation
Hs[1-(T/Tn)^2]^(1/2)kOe with Hs(T-->0)=128.5(5) kOe and Tn(H=0)=19.5 K. In
contrast, the thermal evolution of Hr(T) along the c-axis (much simpler than
along the a-axis) follows the relation Hr[1-T/Tr]^(1/3) kOe where
Hr(T-->0)=33.5(5) kOe and Tr(H=0)=13.5 K. It is emphasized that the
magnetoelastic interaction and the anisotropic exchange coupling are important
perturbations and therefore should be explicitly considered if a complete
analysis of the magnetic properties of the borocarbides is desired
Effect of pressure on the superconducting transition temperature of doped and neutron-damaged MgB2
Measurements of the superconducting transition temperatures for Al-doped,
C-doped and neutron-damaged-annealed MgB2 samples under pressure up to ~8 kbar
are presented. The dT_c/dP values change systematically with the decrease of
the ambient pressure T_c in a regular fashion. The evolution of the pressure
derivatives can be understood assuming that the change in phonon spectrum is a
dominant contribution to dT_c/dP
Carbon doping of superconducting magnesium diboride
We present details of synthesis optimization and physical properties of
nearly single phase carbon doped MgB2 with a nominal stoichiometry of
Mg(B{0.8}C{0.2})2 synthesized from magnesium and boron carbide (B4C) as
starting materials. The superconducting transition temperature is ~ 22 K (~ 17
K lower than in pure MgB2). The temperature dependence of the upper critical
field is steeper than in pure MgB2 with Hc2(10K) ~ 9 T. Temperature dependent
specific heat data taken in different applied magnetic fields suggest that the
two-gap nature of superconductivity is still preserved for carbon doped MgB2
even with such a heavily suppressed transition temperature. In addition, the
anisotropy ratio of the upper critical field for T/Tc ~ 2/3 is gamma ~ 2. This
value is distinct from 1 (isotropic) and also distinct from 6 (the value found
for pure MgB2).Comment: 11 pages, 13 figures, submitted to Physica
Effects of Boron Purity, Mg Stoichiometry and Carbon Substitution on Properties of Polycrystalline MgB
By synthesizing MgB using boron of different nominal purity we found
values of the residual resistivity ratio () from 4 to
20, which covers almost all values found in literature. To obtain high values
of , high purity reagents are necessary. With the isotopically pure boron
we obtained the highest 20 for the stoichiometric compound. We also
investigated MgB samples with 0.8 1.2. For the range
MgB up to MgB we found average values
of between 14 and 24. For smaller variations in stoichiometry () . All of our data point to the conclusion that high
() and low () are intrinsic
material properties associated with high purity MgB. In addition we have
performed initial work on optimizing the formation of carbon doped MgB
via the use of BC. Nearly single phase material can be formed by reaction
of nominal Mg(BC) for 24 hours at . The
for this composition is between and (depending on
criterion).Comment: accepted to Physica C, special MgB2 issu
Anisotropic magnetization, specific heat and resistivity of RFe2Ge2 single crystals
We have grown RFe2Ge2 single crystals for R = Y and ten members of the
lanthanide series (Pr, Nd, Sm, Gd-Tm, Lu) using Sn flux as the solvent. The
method yields clean, high quality crystal plates as evidenced by residual
resistivities and RRR values in the range of 3-12 uOhm cm and 20-90
respectively. The crystals are also virtually free of magnetic impurities or
secondary phases, allowing the study of the intrinsic anisotropic magnetic
behavior of each compound. Characterization was made with X-Ray diffraction,
and temperature and field dependent magnetization, specific heat and
resistivity. Very strong anisotropies arising mostly from CEF effects were
observed for all magnetic rare earths except Gd. Antiferromagnetic ordering
occurred at temperatures between 16.5 K (Nd) and 1.1 K (Ho) that roughly scale
with the de Gennes factor for the heavy rare earths. For some members there is
also a lower temperature transition associated with changes in the magnetic
structure. Tm did not order down to 0.4 K, and appears to be a van Vleck
paramagnet. All members which ordered above 2 K showed a metamagnetic
transition at 2 K for fields below 70 kOe. The calculated effective moments per
rare earth atom are close to the expected free ion values of R^3+ except for Sm
which displays anomalous behavior in the paramagnetic state. The non-magnetic
members of this series (Y, Lu) are characterized by an unusually large
electronic specific heat coefficient (gamma ~ 60 mJ/mol K^2) and
temperature-independent susceptibility term (chi_0 ~ 0.003 emu/mol), indicative
of a relatively large density of states at the Fermi surface.Comment: 34 pages, 13 figures, 1 table, submitted to J. Magn. Magn. Mate
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