Magnetic properties of CeRu2Si2 at ultra low temperatures

We simultaneously measured the AC susceptibility and DC magnetization of the heavy fermion compound CeRu2Si2 in various small magnetic fields at temperatures down to 170 μK. The susceptibility and magnetization increased below 50mK followed by the Curie law with the small magnetic moment 0.02μB/Ce. In fields higher than 0.20 mT, the susceptibility showed the peak. The peak temperature (TM) and height shifted higher and lower with increasing the field, respectively. In addition, approaching TM, the static magnetization became a plateau in fields higher than 0.20 mT. These results reflect the spin fluctuation of the itinerant 4f electrons