High-Field ac Susceptometer Design for Measurements of Superconducting Single Crystals and Results

Abstract

The use of ac induction techniques involving ac mutual inductance bridges and variances thereof have been discussed in detail in Section I of this volume. As has been emphasized in Section I and elsewhere^1–3, a wealth of information can be obtained from measurements of the complex ac magnetic susceptibility, χ^ac(H,T)=χ′(H,T)-iχ″(H,T) where H is the total magnetic field acting on the sample and T its temperature. Examples of the utility of these measurements involving topics of current and significant interest are: (a) the study of the “loss” peak as demonstrated by a peak in plots of χ_H″(T) VS. T and its relationship to the onset of magnetic irreversibility^4,5 and (b) the dependence on the frequency of the ac measuring field of the normal to superconducting transition temperature, T_c, as manifested by the change in χ_H′(T). Studies of this frequency shift allows one to determine pinning energies involved in thermally activated flux motion occuring in single crystals and multigrained bulk samples^1,6.