99 research outputs found
High coercivity induced by mechanical milling in cobalt ferrite powders
In this work we report a study of the magnetic behavior of ferrimagnetic
oxide CoFe2O4 treated by mechanical milling with different grinding balls. The
cobalt ferrite nanoparticles were prepared using a simple hydrothermal method
and annealed at 500oC. The non-milled sample presented coercivity of about 1.9
kOe, saturation magnetization of 69.5 emu/g, and a remanence ratio of 0.42.
After milling, two samples attained coercivity of 4.2 and 4.1 kOe, and
saturation magnetization of 67.0 and 71.4 emu/g respectively. The remanence
ratio MR/MS for these samples increase to 0.49 and 0.51, respectively. To
investigate the influence of the microstructure on the magnetic behavior of
these samples, we used X-ray powder diffraction (XPD), transmission electron
microscopy (TEM), and vibrating sample magnetometry (VSM). The XPD analysis by
the Williamson-Hall plot was used to estimate the average crystallite size and
strain induced by mechanical milling in the samples
Muon Spin Rotation Measurement of the Magnetic Field Penetration Depth in Ba(Fe0.93 Co0.07)2 As2 : Evidence for Multiple Superconducting Gaps
We have performed transverse field muon spin rotation measurements of single
crystals of Ba(FeCoAs with the applied magnetic field
along the direction. Fourier transforms of the measured spectra
reveal an anisotropic lineshape characteristic of an Abrikosov vortex lattice.
We have fit the SRSR spectra to a microscopic model in terms of the
penetration depth and the Ginzburg-Landau parameter .
We find that as a function of temperature, the penetration depth varies more
rapidly than in standard weak coupled BCS theory. For this reason we first fit
the temperature dependence to a power law where the power varies from 1.6 to
2.2 as the field changes from 200G to 1000G. Due to the surprisingly strong
field dependence of the power and the superfluid density we proceeded to fit
the temperature dependence to a two gap model, where the size of the two gaps
is field independent. From this model, we obtained gaps of
and , corresponding to roughly 6 meV
and 3 meV respectively
Static magnetic order of SrAOFeAs (A = Sc and V) revealed by local probes
Static magnetic order of quasi two-dimensional FeAs compounds
Sr4A2O6-xFe2As2, with A = Sc and V, has been detected by 57Fe Moessbauer and
muon spin relaxation ({\mu}SR) spectroscopies. The non-superconducting
stoichiometric (x = 0) A = Sc system exhibits a static internal/hyperfine
magnetic field both at the 57Fe and {\mu}+ sites, indicating antiferromagnetic
order of Fe moments below TN = 35 K with ~ 0.1 Bohr magneton per Fe at T = 2 K.
The superconducting and oxygen deficient (x = 0.4) A = V system exhibits a
static internal field only at the {\mu}+ site below TN ~ 40 K, indicating
static magnetic order of V moments co-existing with superconductivity without
freezing of Fe moments. These results suggest that the 42622 FeAs systems
belong to the same paradigm with the 1111 and 122 FeAs systems with respect to
magnetic behavior of Fe moments.Comment: 4 pages 4 figures: for information, contact
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Measurements and analysis of the upper critical field on an underdoped and overdoped compounds
The upper critical field is one of the many non conventional
properties of high- cuprates. It is possible that the
anomalies are due to the presence of inhomogeneities in the local charge
carrier density of the planes. In order to study this point, we
have prepared good quality samples of polycrystalline
using the wet-chemical method, which has demonstrated to produce samples with a
better cation distribution. In particular, we have studied the temperature
dependence of the second critical field, , through the magnetization
measurements on two samples with opposite average carrier concentration
() and nearly the same critical temperature, namely
(underdoped) and (overdoped). The results close to do not
follow the usual Ginzburg-Landau theory and are interpreted by a theory which
takes into account the influence of the inhomogeneities.Comment: Published versio
Radiation-damage produced in BaHfO₃ irradiated with thermal and fast-neutrons
Samples of BaHfO₃ were irradiated with fast and thermal neutrons to produce ^181Hf. Attenuations of the gamma-gamma perturbed angular correlations in ^181Ta were observed, and they may have been caused by the interaction between defects (produced during or after neutron irradiation) and oxygen vacancies. Several defect structures were detected. For one of the structures, an excitation energy of 12 meV was measured, which was attributed to an electron state below the conduction band. The values of the dielectric constant and of the effective electron mass were also deduced
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