Nonlinear magnetization of Y-Ba-Cu-O crystals

We investigate the nonlinear magnetization M(B) of an YBa2Cu3O7-x crystal in applied superimposed dc and ac magnetic fields B up to about 80 G. M[B(t)] contains both even and odd harmonics of the applied fundamental audio frequency in a restricted temperature range of 1--2 K below Tc. Harmonic amplitudes oscillate with both temperature and dc field. We propose a simple dynamical model based on the observed ''flux creep'' resistance in which temperature and field appear only in the ratio (1-T/Tc)3/2B. The model is in qualitative and semiquantitative agreement with the data