(color online) The frequency-dependent ac susceptibility of UGe₂ measured as a function of temperature. The amplitude of ac excitation is 10 Oe, operating at 10 Hz, 100 Hz, 1 kHz, and 10 kHz, respectively. In ac susceptibility measurements, a dc bias field of 1 kOe was always applied. Both dc and ac magnetic fields were parallel with the easy a axis. For comparison, zero-field-cooling magnetization in 100 Oe is also plotted, divided by a factor of 100. The inset shows the frequency dependence of T_X in an Arrhenius plot fitted by a full line.