AC susceptibility measurements on heavy fermion compounds CeCu₆ and CeNi down to submillikelvin temperatures

We have measured the AC magnetic susceptibility of single crystals of the heavy fermion compounds CeCu₆ and CeNi at temperatures down to 250 μK. The susceptibility of CeCu₆ shows a peak at about 2 mK in a zero magnetic field. The peak height along a-axis is about 10 times as large as along b- and c-axis, and decreases to zero in increasing field up to 11 mT. It is likely that the susceptibility of CeNi along c-axis has three peaks: below 350 μK, at 47 mK and at 0.3 K, respectively. The two peaks at lower temperatures are suppressed by a magnetic field. The newly observed peaks in the susceptibilities for CeCu₆ and CeNi suggest effects of magnetic impurities rather than of magnetic ordering.