4 research outputs found
Optical floating zone growth of high-quality Cu2MnAl single crystals
We report the growth of large single-crystals of Cu2MnAl, a ferromagnetic
Heusler compound suitable for polarizing neutron monochromators, by means of
optical floating zone under ultra-high vacuum compatible conditions. Unlike
Bridgman or Czochralsky grown Cu2MnAl, our floating zone grown single-crystals
show highly reproducible magnetic properties and an excellent crystal quality
with a narrow and homogeneous mosaic spread as examined by neutron diffraction.
An investigation of the polarizing properties in neutron scattering suggests a
high polarization efficiency, limited by the relatively small sample dimensions
studied. Our study identifies optical floating zone under ultra-high vacuum
compatible conditions as a highly reproducible method to grow high-quality
single-crystals of Cu2MnAl.Comment: 19 pages, 11 figure
Rotating skyrmion lattices by spin torques and field or temperature gradients
Chiral magnets like MnSi form lattices of skyrmions, i.e. magnetic whirls,
which react sensitively to small electric currents j above a critical current
density jc. The interplay of these currents with tiny gradients of either the
magnetic field or the temperature can induce a rotation of the magnetic pattern
for j>jc. Either a rotation by a finite angle of up to 15 degree or -- for
larger gradients -- a continuous rotation with a finite angular velocity is
induced. We use Landau-Lifshitz-Gilbert equations extended by extra damping
terms in combination with a phenomenological treatment of pinning forces to
develop a theory of the relevant rotational torques. Experimental neutron
scattering data on the angular distribution of skyrmion lattices suggests that
continuously rotating domains are easy to obtain in the presence of remarkably
small currents and temperature gradients.Comment: 12 pages, 10 figure
Laser based spot weld characterization
Spot welding is one of the most important joining technologies, especially in the automotive industry. Hitherto, the quality of spot welded joints is tested mainly by random destructive tests. A nondestructive testing technique offers the benefit of cost reduction of the testing procedure and optimization of the fabrication process, because every joint could be examined. This would lead to a reduced number of spot welded joints, as redundancies could be avoided. In the procedure described here, the spot welded joint between two zinc-coated steel sheets (HX340LAD+Z100MB or HC340LA+ZE 50/50) is heated optically on one side. Laser radiation and flash light are used as heat sources. The melted zone, the so called "weld nugget" provides the mechanical stability of the connection, but also constitutes a thermal bridge between the sheets. Due to the better thermal contact, the spot welded joint reveals a thermal behavior different from the surrounding material, where the heat transfer between the two sheets is much lower. The difference in the transient thermal behavior is measured with time resolved thermography. Hence, the size of the thermal contact between the two sheets is determined, which is directly correlated to the size of the weld nugget, indicating the quality of the spot weld. The method performs well in transmission with laser radiation and flash light. With laser radiation, it works even in reflection geometry, thus offering the possibility of testing with just one-sided accessibility. By using heating with collimated laser radiation, not only contact-free, but also remote testing is feasible. A further convenience compared to similar thermographic approaches is the applicability on bare steel sheets without any optical coating for emissivity correction. For this purpose, a proper way of emissivity correction was established