Fig. 8

a–e Time dependence of field-cooled magnetization. The sample was cooled down to 2 K in the applied field H _FC = 1 kOe, and magnetization was recorded with time t in the zero field. f Two-dimensional plot of cooling field against the mean size < d>, while different colors are used to differentiate the β values, with β ~ 0 (blue color), β ~ 0.5 (green color), and β = 1 (red color)