Experimental data shown in Fig. 5(b) were fitted to equation (4), so that β values seen from Table 3 were estimated to be 0.45 for the Y358 sample and 0.51 for the Y123 sample. According to this result, the β value of the Y358 sample is lower than that of the Y123 sample, which is the same as the literature (β = 0.5) [11].