25 research outputs found
Rotating sample magnetometer for cryogenic temperatures and high magnetic fields
We report on the design and implementation of a rotating sample magnetometer
(RSM) operating in the variable temperature insert of a cryostat equipped with
a high-field magnet. The limited space and the cryogenic temperatures impose
the most critical design parameters: the small bore size of the magnet requires
a very compact pick-up coil system and the low temperatures demand a very
careful design of the bearings. Despite these difficulties the RSM achieves
excellent resolution at high magnetic field sweep rates, exceeding that of a
typical vibrating sample magnetometer by about a factor of ten. In addition the
gas-flow cryostat and the high-field superconducting magnet provide a
temperature and magnetic field range unprecedented for this type of
magnetometer.Comment: 10 pages, 5 figure
Neutron irradiation of coated conductors
Various commercial coated conductors were irradiated with fast neutrons in
order to introduce randomly distributed, uncorrelated defects which increase
the critical current density, Jc, in a wide temperature and field range. The
Jc-anisotropy is significantly reduced and the angular dependence of Jc does
not obey the anisotropic scaling approach. These defects enhance the
irreversibility line in not fully optimized tapes, but they do not in
state-of-the-art conductors. Neutron irradiation provides a clear distinction
between the low field region, where Jc is limited by the grain boundaries, and
the high field region, where depinning leads to dissipation
Magnetic measurement of the critical current anisotropy in coated conductors
We determine the critical current anisotropy at maximum Lorentz force from
hysteresis loops in a vibrating sample magnetometer. To eliminate the signal of
spurious variable Lorentz force currents it is sufficient to cut the sample to
a specific length, which is calculated from the position dependent sensitivity
of the instrument. The procedure increases the resolution of the measurement
and the results compare well to transport data on the same sample. As the
electric field in magnetisation measurements is lower than in transport
experiments the anisotropy at high currents (low temperatures and fields) can
be measured without the need of making current contacts or any special sample
preparation.Comment: 11 pages, 6 figure
Anisotropic critical currents in FeSe0.5Te0.5 films and the influence of neutron irradiation
We report on measurements of the superconducting properties of FeSe05Te05
thin films grown on lanthanum aluminate. The films have high transition
temperatures (above 19 K) and sharp resistive transitions in fields up to 15 T.
The temperature dependence of the upper critical field and the irreversibility
lines are steep and anisotropic, as recently reported for single crystals. The
critical current densities, assessed by magnetization measurements in a vector
VSM, were found to be well above 10^9 Am-2 at low temperatures. In all samples,
the critical current as a function of field orientation has a maximum, when the
field is oriented parallel to the film surface. The maximum indicates the
presence of correlated pinning centers. A minimum occurs in three films, when
the field is applied perpendicular to the film plane. In the forth film,
instead, a local maximum caused by c-axis correlated pinning centers was found
at this orientation. The irradiation of two films with fast neutrons did not
change the properties drastically, where a maximum enhancement of the critical
current by a factor of two was found
A Novel Magnetoscan Setup
Due to a modification in the original magnetoscan setup, a significant
improvement in resolution was obtained. The paper focuses on experimental
results which should support the idea of the new setup using two magnets with
opposite direction of magnetization. This contribution to the characterization
techniques of melt-grown bulk superconductors should promote the easy
installation of this technique in industry. The improved magnetoscan technique
may further help to investigate growth-induced inhomogeneities of the
top-seeded-melt-growth process in the submillimeter range, and it offers new
possibilities to the characterization of smaller structures such as
superconducting films or coated conductors.Comment: 7 pages, 7 figure