  Five characteristic temperatures of TM = 148 K, TN = 142 K, Tt = 138 K, Tf ~
125 K and Tg ~ 50 K were found by the measurements of the magnetization curves
at various temperature. The spontaneous magnetization appears below TM. It
increases up to M_{S} simeq 2 times 10^{-4} mu_{B} at Tt and then decreases
steeply below Tt, which qualitatively agrees with the temperature dependence of
magnetization obtained under field-cooling (FC) condition. On the other hand,
the slope of the magnetization curve, namely the magnetic susceptibility, drops
below TN, which coincides with the temperature dependence of magnetization
obtained under zero-FC condition, although the magnetization curves were
obtained under FC condition. The temperature dependence of the spontaneous
magnetization shows a minimum at Tf and a drop at Tg although there is no
anomaly in the temperature dependence of FC or ZFC magnetization.
