Anomalous slow dynamics in the metallic helimagnet Gd_1-x Y _x

AC-suseptibility measurements were performed in the metallic helimagnet Gd_1-xY_x alloy. A remarkable increase of the imaginary part of the ac-suseptibility was observed in the temperature range of the helimagnetic phase. Moreover, a strong nonlinearity of the magnetization was observed at paramagnetic-helimagnetic transition temperature T_N. On the other hand, These anomalous behavior were not observed in similar rare-earth helimagnets Ho and Ho_1-xY_x. It strongly suggests that the weak magnetic anisotropy of the Gd-moments is responsible for the anomalous slow dynamics in the helimagnetic phase and for the strong nonlinearity of the magnetization at T_N. The slow dynamics may result from a chiral-domain motion, or a long-time variation of the period of the helimagnetic structure.