Advancements in A.C. Magnetic Susceptibility Method at High Pressures

A superior test for superconductivity is the search for a strong diamagnetic transition in the magnetic susceptibility. With the development of high-pressure novel hydrogen rich, high Tc superconducting materials and their smaller sample size, it is needed to develop a more sensitive measurement techniques for detecting the diamagnetic response. With modern measuring equipment and a new coil design, a significant improvement in sensitivity and SNR was obtained with a constant background over the full temperature range. The coil system includes two identically wound pairs of coils, each pair consisting of a primary and a secondary coil wound on top of each other. The primary coil creates an alternating magnetic field and the secondary coil sense the field. The other pair of coils connected series with opposite polarity to reduce the background. Both the excitation and modulation signals are superpositioned and fed into the primary coil. This eliminates the need for a separate coil for the modulation. Then the signal from the pickup coil is analyzed by a lock-in amplifier in dual reference mode. Extensive studies have been performed to find the optimal frequency and amplitude combinations of the excitation and modulation field on both low-temperature and high-temperature superconductors.