3 research outputs found
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
57Fe Mossbauer spectroscopy and magnetic measurements of oxygen deficient LaFeAsO
We report on the magnetic behavior of oxygen deficient LaFeAsO1-x (x-0.10)
compound, prepared by one-step synthesis, which crystallizes in the tetragonal
(S.G. P4/nmm) structure at room temperature. Resistivity measurements show a
strong anomaly near 150 K, which is ascribed to the spin density wave (SDW)
instability. On the other hand, dc magnetization data shows paramagnetic-like
features down to 5 K, with an effective moment of 0.83 mB/Fe. 57Fe Mossbauer
studies (MS) have been performed at 95 and 200 K. The spectra at both
temperatures are composed of two sub-spectra. At 200 K the major one (88%), is
almost a singlet, and corresponds to those Fe nuclei, which have two oxygen
ions in their close vicinity. The minor one, with a large quadrupole splitting,
corresponds to Fe nuclei, which have vacancies in their immediate neighborhood.
The spectrum at 95 K, exhibits a broadened magnetic split major (84%)
sub-spectrum and a very small magnetic splitting in the minor subspectrum. The
relative intensities of the subspectra facilitate in estimating the actual
amount of oxygen vacancies in the compound to be 7.0(5)%, instead of the
nominal LaFeAsO0.90. These results, when compared with reported 57Fe MS of
non-superconducting LaFeAsO and superconducting LaFeAsO0.9F0.1, confirm that
the studied LaFeAsO0.93 is a superconductivity-magnetism crossover compound of
the newly discovered Fe based superconducting family.Comment: 7 pages text + Figs : Comments/suggestions welcome
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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