10 research outputs found
Intrinsic magnetic properties of NdFeAsOF superconductor from local and global measurements
Magneto-optical imaging was used to study the local magnetization in
polycrystalline NdFeAsOF (NFAOF). Individual crystallites up to
in size could be mapped at various
temperatures. The in-grain, persistent current density is about
A/cm and the magnetic relaxation rate in a remanent state peaks at about
K. By comparison with with the total magnetization measured in a
bar-shaped, dense, polycrystalline sample, we suggest that
NdFeAsOF is similar to a layered high-, compound such as
BiSrCaCuO and exhibits a crossover in the
vortex structure. The 2D Ginzburg parameter is about
implying electromagnetic anisotropy as large as . Below
, the static and dynamic behaviors are consistent with collective
pinning and creep
Single crystal of superconducting SmFeAsO1-xFy grown at high pressure
Single crystals of SmFeAsO1-xFy of a size up to 120 micrometers have been
grown from NaCl/KCl flux at a pressure of 30 kbar and temperature of 1350-1450
C using the cubic anvil high-pressure technique. The superconducting transition
temperature of the obtained single crystals varies between 45 and 53 K.Obtained
crystals are characterized by a full diamagnetic response in low magnetic
fields and by a high critical current density in high magnetic fields.
Structural refinement has been performed on single crystal. Differential
thermal analysis investigations at 1 bar Ar pressure show decomposition of
SmFeAsO1-xFy at 1302 C.Comment: 12 pages, 3 tables, 6 figure
Combined microstructural and magneto optical study of current flow in polycrystalline forms of Nd and Sm Fe-oxypnictides
In order to understand why the inter- and intra-granular current densities of
polycrystalline superconducting oxypnictides differ by three orders of
magnitude, we have conducted combined magneto-optical and microstructural
examinations of representative randomly oriented polycrystalline Nd and Sm
single-layer oxypnictides. Magneto optical images show that the highest Jc
values are observed within single grains oriented with their c axes
perpendicular to the observation plane, implying that the intragranular current
is anisotropic. The much lower intergranular Jc is at least partially due to
many extrinsic factors, because cracks and a ubiquitous wetting As-Fe phase are
found at many grain boundaries. However, some grain boundaries are structurally
clean under high resolution transmission electron microscopy examination.
Because the whole-sample global Jc(5K) values of the two samples examined are
1000-4000 A/cm2, some 10-40 times that found in random, polycrystalline
YBa2Cu3O7-x, it appears that the dominant obstruction to intergranular current
flow of many present samples is extrinsic, though some intrinsic limitation of
current flow across grain boundaries cannot yet be ruled out.Comment: Submitted to SUS
Evidence for Supercurrent Connectivity in Conglomerate Particles in NdFeAsO1-d
Here we use global and local magnetometry and Hall probe imaging to
investigate the electromagnetic connectivity of the superconducting current
path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High
resolution transmission electron microscopy and scanning electron microscopy
show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around
individual particles, and second phase neodymium oxide which may be responsible
for the large paramagnetic background at high field and at high temperatures.
From global magnetometry and electrical transport measurements it is clear
that there is a small supercurrent flowing on macroscopic sample dimensions
(mm), with a lower bound for the average (over this length scale) critical
current density of the order of 103 A/cm2. From magnetometry of powder samples
and local Hall probe imaging of a single large conglomerate particle ~120
microns it is clear that on smaller scales, there is better current
connectivity with a critical current density of the order of 5 x 104 A/cm2. We
find enhanced flux creep around the second peak anomaly in the magnetisation
curve and an irreversibility line significantly below Hc2(T) as determined by
ac calorimetry.Comment: 11 pages, 4 figure
Structural and critical current properties in polycrystalline SmO1-xFxFeAs
A series of polycrystalline SmO1-xFxFeAs bulks (x=0.15, 0.2, 0.3 and 0.4)
were prepared by the conventional solid state reaction. Resistivity,
susceptibility, magnetic hysteresis, critical current density and
microstructure of these samples have been investigated. It is found that
critical transition temperature Tc increases steadily with increasing fluorine
content, with the highest onset Tc=53 K at x=0.4. On the other hand, the
superconductivity seems correlated with lattice constants; that is, Tc rises
with the shrinkage of a-axis while resistivity increases with the enlargement
of c-axis. A global critical current density of 1.1x10^4 A/cm^2 at 5 K in self
field was achieved in the purest sample. A method of characterization of
inter-grain current density is proposed. This method gives an inter-grain Jc of
3.6x10^3 A/cm^2 at 5 K in self field, in contrast to the intra-grain Jc of 10^6
A/cm^2. The effect of composition gradients on the inter-grain Jc in
SmO1-xFxFeAs is also discussed.Comment: 18 pages, 7 figure
Superconducting properties of SmO1-xFxFeAs wires with Tc = 52 K prepared by the powder-in-tube method
We demonstrate that Ta sheathed SmO1-xFxFeAs wires were successfully
fabricated by the powder-in-tube (PIT) method for the first time. Structural
analysis by mean of x-ray diffraction shows that the main phase of SmO1-xFxFeAs
was obtained by this synthesis method. The transition temperature of the
SmO0.65F0.35FeAs wires was confirmed to be as high as 52 K. Based on
magnetization measurements, it is found that a globe current can flow on
macroscopic sample dimensions with Jc of ~3.9x10^3 A/cm^2 at 5 K and self
field, while a high Jc about 2x10^5 A/cm^2 is observed within the grains,
suggesting that a significant improvement in the globle Jc is possible. It
should be noted that the Jc exhibits a very weak field dependence behavior.
Furthermore, the upper critical fields (Hc2) determined according to the
Werthamer-Helfand-Hohenberg formula are (T= 0 K) = 120 T, indicating a very
encouraging application of the new superconductors.Comment: 14 pages, 6 figure
High-pressure growth of fluorine-free SmFeAsO superconducting single crystals
Superconducting single crystals of SmFeAsO without fluorine doping
were grown at a pressure of 3.3 GPa and a temperature of 1350-1450 C by
using the self-flux method. Plate-like single crystals of a few-150 m in
their lateral size were obtained. Single crystals showed the superconducting
transition at about 53.5 K with a narrow transition width of 0.5 K. The
synchrotron-irradiated x-ray diffractometry patterns and
high-angle-annular-dark-field scanning transmission electron microscopy images
point to the high quality of the crystals with a well-defined layered
tetragonal structure. The chemical compositions of the crystals were estimated
by using the electron-probe x-ray microanalysis.Comment: 12 pages, 3 figure
Evidence for two distinct scales of current flow in polycrystalline Sm and Nd iron oxypnictides
Early studies have found quasi-reversible magnetization curves in
polycrystalline bulk rare-earth iron oxypnictides that suggest either
wide-spread obstacles to intergranular current or very weak vortex pinning. In
the present study of polycrystalline samarium and neodymium rare-earth iron
oxypnictide samples made by high pressure synthesis, the hysteretic
magnetization is significantly enhanced. Magneto optical imaging and study of
the field dependence of the remanent magnetization as a function of particle
size both show that global currents over the whole sample do exist but that the
intergranular and intragranular current densities have distinctively different
temperature dependences and differ in magnitude by about 1000. Assuming that
the highest current density loops are restricted to circulation only within
grains leads to values of ~5 MA/cm2 at 5 K and self field, while whole-sample
current densities, though two orders of magnitude lower are 1000-10000 A/cm2,
some two orders of magnitude higher than in random polycrystalline cuprates. We
cannot yet be certain whether this large difference in global and intragrain
current density is intrinsic to the oxypnictides or due to extrinsic barriers
to current flow, because the samples contain significant second phase, some of
which wets the grain boundaries and produces evidences of SNS proximity effect
in the whole sample critical current.Comment: 28 pages, 14 figure