Observation of multiple peaks in the magnetization curves of NdBa2Cu3O7 single crystals

Induced current densities j(s)(T,B) were measured in a wide temperature (5 less than or equal to T less than or equal to 90 K) and field range (0 less than or equal to mu(0)H(a) less than or equal to 7 T) on single crystals of NdBa2Cu3O7-delta using a superconducting quantum interference device magnetometer. The samples exhibit the fishtail or peak effect at temperatures above 30 K. In a small temperature window between 50 and 74 K, three peaks are seen in the j(s)(T,mu(0)H(a)) curves. The temperature and field behavior of the induced current densities is analyzed using a model describing a magnetization curve as composed of two independent contributions at low and high fields. By means of this analysis, it is demonstrated that the third peak corresponds to the fishtail peak, whereas the position of the second peak is practically independent of temperature. It is shown that the appearance of the second peak is due to the large values of the position of the fishtail peak.