APPARATUS FOR COMPLEX AC MAGNETIC-SUSCEPTIBILITY MEASUREMENTS IN THE RADIOFREQUENCY RANGE

Complex AC magnetic susceptibility measurement is one of the standard methods used to obtain information on the magnetic properties of HTSC materials. Usually the measurements are performed in the frequency ran ge from a few tens of hertz to a few megahertz with the well known loc k-in technique. Much less experimental information has been collected in the RF range. A dedicated instrument to measure the AC magnetic sus ceptibility (chi', chi'') and harmonic generations of high temperature superconductors in the frequency range 20-500 MHz is described. In th e case of linear susceptibility complex values (chi', chi'') are measu red in frequency bands centred around the values of (30 + n x 60) MHz while at harmonics the amplitudes are determined in the whole frequenc y range. A calibration procedure to obtain correct values of (chi', ch i'') from the two components detected by lock-in of the RF signal pick ed up by a low inductance coil containing the sample is worked out. Th e dependences on frequency, temperature and static magnetic field of t he complex AC susceptibility were measured for high temperature superc onducting YBaCuO ceramics. Construction details, calibration methods a nd data evaluation are described.