DIRECT EVIDENCE FOR CRITICAL EXCHANGE COUPLING VIA AL LAYERS IN THE HEAVY-FERMION COMPOUND CEPD2AL3

The long-range planar-type antiferromagnetic ordering in the heavy-fer mion compound CePd2Al3 is shown to critically depend on the exact occu pation and distribution of aluminum atoms over their available lattice sites. A polycrystalline sample (T(N) = 2.7 K) is systematically comp ared with a nonordering single crystal, by means of susceptibility, sp ecific heat and site-specific Al-NQR/NMR experiments. Implications for other Al-containing heavy-fermion systems are discussed.