Th. Herrmannsdorfer

The High Field Project at Dresden/Rossendorf: A Pulsed 100 T/10 ms Laboratory at an Infrared Free-Electron-Laser Facility

This article describes the project to build a pulsed magnetic field user laboratory at the Forschungszentrum Rossendorf near Dresden. Using a 50 MJ/24 kV capacitor bank, pulsed fields and rise times of 100 T/10 ms, 70 T/100 ms, and 60 T/1 s should be achieved. The laboratory will be built next to a free-electron-laser-facility for the middle and far infrared (5 to 150 µm, 2 ps, cw). We describe the work which has been performed until now to start the construction of the laboratory in 2003: coil concepts and computer simulations, materials development for the high field coils, and design of the capacitor bank modules. In addition, a pilot laboratory has been set up where fields up to 62 T/15 ms have been obtained with a 1 MJ/10 kV capacitor module. It is used to gain experience in the operation of such a facility and to test various parts of it. In this test laboratory special devices have been developed for measurements of magnetization and magnetoresistance, and have been successfully used to investigate various materials including semiconductors and Heavy Fermion compounds. In particular, metamagnetic transitions in intermetallic compounds and the irreversibility field of a high-Tc superconductor have been determined. Shubnikov–de Haas oscillations have been observed in the semimetallic compound CeBiPt. Resistance relaxation has been observed to start less than 1second after the field pulse. It could be shown for the first time that nuclear magnetic resonance (NMR) is detectable in pulsed fields.

Transport and magnetic properties of mixed valent SmB6

Abstract We present the results of transport and magnetic measurements performed on two intermediate valent SmB6 single crystals. The received results above 15 K are in agreement with the results obtained by other groups. At the lowest temperatures, however, the resistivity, magnetization, and susceptibility seem to be strongly influenced by the presence of impurities in the investigated samples.

The Origin of the Magnetic Channel of the Thermal Boundary Resistance Between Liquid 3He and Ag Sinters at Very Low Temperatures

At temperatures below about 10mK, the temperature dependence of the thermal boundary resistance between liquid3He and metal sinters deviates significantly from the behaviour expected from the acoustic mismatch theory (RK∝ T−3). This behaviour is explained in terms of the existence of a magnetic channel for the coupling between the3He quasiparticles and the sinter; the origin of this coupling, however, is unknown so far. In our studies of the magnetic properties of Ag sinters made of powders of submicron grain size we have detected and identified a magnetically ordering subsystem which, although it contributes only a few ppms to the total mass, dominates the magnetic properties of the sinters. We present the results of our investigation of the magnetic properties of these Ag sinters which strongly deviate from the behaviour of bulk silver, and discuss a possible mechanism how a magnetically ordered subsystem may open up the magnetic channel of the thermal boundary resistance to liquid3He at very low temperatures. The main conclusion of our study is that our results should allow the tuning of the thermal boundary resistance between liquid3He and sintered metal heat exchangers at very low temperatures.

Magnetic properties of Ag sinters and their possible impact on the coupling to liquid 3He at very low temperatures

We report on measurements of the magnetization of metal sinters made of different Ag powders with average (nominal) grain sizes of 40nm and 70nm at1.6K≤T≤250K and0≤B≤6T. Besides the dominating diamagnetism of Ag, the sinters show unexpected magnetic properties; e.g. once exposed to a magnetic field B, the sinters keep a permanent magnetic moment (in zero external field) which strongly depends on the magnitude of B. The possible impact of the unexpected magnetic properties of the Ag sinters on various experimental observations at very low temperatures, e.g. the magnetic field dependence of the thermal boundary resistance between an Ag sinter and3He at millikelvin temperatures is discussed.

Magnetism of submicron Ag powder : its impact on the Kapitza-resistance and its applicability for thermometry at very low temperatures

We have studied the magnetic properties of submicron Ag powder (400Å and 700Å) in a magnetometer at 1.5K≤T≤300K and in B≤6T. Once exposed to a magnetic field, the sinter keeps a temperature dependent magnetic moment whose magnitude strongly depends on the history of the sample. The dependence of the moment on the applied magnetic field is discussed in relation to the field dependence of the boundary resistance between an Ag sinter and liquid3He.-Doped with a small amount of magnetic impurities, the sinter may be used as an extremely sensitive, continuously working thermometer; its susceptibility exactly follows the differential Brillouin function down to at least 200μK. The most important advantage of this sinter is its simultaneous applicability as thermometer and as heat exchanger in helium experiments.