(Nd, Eu, Gd) Ba2Cu3Ox (NEG123) bulk shows high irreversibility fields over 14 T at 77.3 K. In order to study the origin of the high irreversibility field, transport measurements for NEG123 bulk were performed as functions of temperature, high magnetic fields (up to 27 T) and field angle. We found that c-axis correlated disorder exists for samples with 3 and 40 mol% NEG211, although their irreversibility fields are different. The enhancements of the irreversibility fields at 77.3 K due to the c-axis correlated disorder are almost the same (about 1 T) for both samples and are in agreement with that of Y 123 bulk with a twin boundary. The partially entangled vortex liquid and the Bose glass phases, which are induced by the c-axis correlated disorder, play an important role in the improvement of the irreversibility field. However, in order to explain the much higher irreversibility field, we should take account not only of the c-axis correlated disorder but also of the other pinning centres such as the nanolamella.
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