Abstract
We report on the out-of- and in-plane resistivity results in underdoped superconducting (SC) NdBa2Cu3O7 − x crystals that show a pronounced upturn in resistivity followed by a SC or metallic state at low temperature for x ⩽ 0.55. The resistivity and the magnetization exhibit an unusual upturn and irreversibility, respectively, at the same temperature, suggesting a peculiar charge localization and a cluster-spin-glass-like behaviour, probably due to magnetic ordering with low-energy magnetic fluctuations. The thermomagnetic and magnetic hysteresis, and relaxation indicate the presence of an actual frozen cluster-spin-glass-like state at low temperature, where spin fluctuations slow down significantly. Our results indicate that the metal-insulator-like upturn in resistivity and thermomagnetic history effects have a common origin.