Nonlinear A.C. Susceptibility Response Near the Irreversibility Line

Abstract

The analysis of the response to a.c. magnetic fields has been one of the most popular techniques to study the magnetic properties of conventional type II superconductors. The essence of the method is to immerse the sample in an a.c. magnetic field generated by a coil, and to monitor its response either by the change in the impedance of the same coil, or by the signal induced in a second coil magnetically coupled to the sample. The a.c. field is characterized by its frequency and amplitude. The response of the superconductor can be studied as a function of temperature and the value of a d.c. magnetic field superimposed on the a.c. excitation. Very often, a harmonic a.c. field is used, and both the out of phase and in phase components of the response signal are detected. These components are related to the real and imaginary parts of the samples ac magnetic susceptibility, χ′ and χ″ respectively. (Note: the phase is with respect to the phase of the a.c. field, χ′ measures the amount of shielding and χ″ the amount of energy loss due to the induced current.) If the sample is a type II superconductor, then depending upon the amount of vortex pinning, the sample geometry, the frequency and the sensitivity of measurement, information related to the lower and upper critical fields, the reversible magnetization, the flux flow resistivity and the flux pinning can be obtained. These properties can also be studied by transport and d.c. magnetization, however, a.c. magnetic measurements do not require the attachment of leads to the sample, and the experimental setup, based on phase detection, is simpler than the magnetometers required for d.c. magnetic studies. The price paid for these experimental advantages is a more complex interpretation of the results, which are function of a considerable number of variables.