Magnetic behavior of CeRu2Si2 below millikelvin temperatures

We have measured the AC magnetic susceptibility of the heavy-fermion compou nd CeRu2Si2 down to 150 mu K. In zero field, the susceptibility monotonousl y increased down to the lowest temperature. In 0.27 mT, however, the suscep tibility shows a peak at 510 mu K. With increasing magnetic field, this pea k shifts towards higher temperature and the susceptibility is suppressed. I n addition, we have measured the frequency dependence of the susceptibility between 12 and 160 Wt An internal relaxation time was obtained to be about 11 ms in terms of Casimir and Du Pre thermodynamic theory. (C) 2000 Elsevi er Science B.V. All rights reserved.