Low temperature magnetic properties of unconventional superconductor PrOs4Sb12

Measurements of the susceptibility and magnetization of an unconventional superconductor PrOs4Sb12 have been performed in order to study the vortex dynamics. This type of superconductor contains multi-component order parameter. Our results sheds light on the interplay between superconductivity and nuclear magnetism of Pr. These measurements have been performed down to a sub mK. We have not found out any indication of the nuclear ordering. We have observed two small decrease of susceptibility around 560mK, 300mK and a large dia-magnetization at Tc. The magnetization hysteresis loop is measured at cycling fields of +/-700Oe. Avalanche-like anomalies appear around zero field below 150mK. These avalanches become frequent as decreasing temperature. Based on the property of domain wall in time-reversal symmetry(TRS) breaking superconductors, possible origins of these avalanches are discussed.