Final Report

The power for generating high magnetic fields comes directly from the line. The primary power of 16 MVA is provided by a transformer from 10 kV to 2 840V; this can be switched either in series, parallel or antiparallel. The ac-current is rectified by two bridges with 6 thyristors that are regulated by varying the ignition angle α. The maximum dc-power is 10 MW over 1s, or 5 MW for 2s or 1 MW for 10s. With this thyristor regulator the current versus time profile can be freely chosen with 20 points to define the profile. In particular, field plateaus are possible which are important for measurements on metallic samples with minimized eddy current. With this system a quasistatic field of up to 40T is available. The antiparallel circuit allows also a bipolar pulse which is especially valuable for a full characterisation of magnetic materials. There are now three different systems available:
- Low temperature system: this consist of the 40T magnet from Metis, a flow cryostat from Crygenics (1.5K up to 300K) with a sample space of 12mm. In this cryostat a pick-up system was installed in order to allow magnetization measurements.
- High temperature system: This consist of a 35T magnet with a 25 mm bore. In this bore a vacuum isolated furnace was developed; inside is a pick-up system which allows magnetization measurements from room temperature up to 500°C.
- Test system: this is a room temperature system with a self made magnet which generates fields up to 25T in a bore of 25mm. The purpose of this system is to test new measuring methods.
- For the new two coil system a self made 30 T magnet with a large bore of 58 mm was constructed. Inside of this magnet the two coil system can be tested but also new techniques such as modulation method can be tested.
All together experiments between 1.5K and 800K in high magnetic fields are possible now:
The system can be used for measuring the hysteresis loop (magnetization) going over a broad temperature range (from 1.5 K up to 800 K). These measurements can be done in quasi-static fields up to 40T however also with varying field sweep rates dH/dt. This is especially interesting in order to investigate magnetic viscosity effects. Additional the magneto-resistance and the magnetostriction can be measured between 1.5 K and 300 K. We are studying two main topics:
i) Hard magnetic materials - including time dependent effects.
ii) 3d-4f compounds with critical fields. By measuring the critical field the exchange parameter can be determined in a direct manner.
First studies on the magnetic viscosity of SmCo_5-xCu_x were performed. As an example we measured the temperature dependence of the coercive field of a 2/17 based permanent magnet in comparison with data as obtained in our fast pulsed field magnetometer. The values as obtained with the Austromag quasi-static system measured with a linear dH/dt of 67T/s were below those obtained in a short pulsed field magnetometer (sinusoidal pulse; pulse duration about 10ms; about 2000T/s); this is a consequence of the magnetic viscosity of this sample. From the different values of the coercivity measured under varying field sweep rates dH/dt the magnetic viscosity parameter could be estimated. Additionally a program was started measuring the magnetostriction coefficients on technical permanent magnets. First results were obtained at room temperature on a technical anisotropic barium ferrite (supplied by Schramberg A.G.). On the aligned material the different magnetostriction coefficients λ_ij could be determined - as there are: λ_pc: H perpendicular and DMS measurement parallel to the c-axis; λ_cc: H and DMS measurement direction parallel to the c-axis; λ_pp: H and DMS measurement direction perpendicular to the c-axis; λ_cp: H parallel and DMS direction perpendicular to the c-axis.