The complementary techniques of magnetization measurement, positron annihilation and nuclear magnetic resonance spectroscopy indicated that the formation of O vacancy clusters was at the origin of the fishtail anomaly in non-stoichiometric material. In contrast to stoichiometric material, from which the anomaly was intrinsically absent, it could be suppressed in an optimally doped state when vacancies were present. It was therefore concluded that clusters of O vacancies, rather than single vacancies or point defects, were responsible for the anomaly.

Experimental Evidence for Fast Cluster Formation of Chain Oxygen Vacancies in YBa2Cu3O7-δ as the Origin of the Fishtail Anomaly. A.Erb, A.A.Manuel, M.Dhalle, F.Marti, J.Y.Genoud, B.Revaz, A.Junod, D.Vasumathi, S.Ishibashi, A.Shukla, E.Walker, O.Fischer, R.Flükiger, R.Pozzi, M.Mali, D.Brinkmann: Solid State Communications, 1999, 112[5], 245-9