Direct evidence for critical exchange coupling via Al layers in the heavy-fermion compoundCePd2Al3

The long-range planar-type antiferromagnetic ordering in the heavy-fermion compound ${\mathrm{CePd}}_2$ ${\mathrm{Al}}_3$ is shown to critically depend on the exact occupation and distribution of aluminum atoms over their available lattice sites. A poly-crystalline sample (${\mathit{T}}_{\mathit{N}}$=2.7 K) is systematically compared with a nonordering single crystal, by means of susceptibility, specific heat, and site-specific Al-NQR/NMR experiments. Implications for other Al-containing heavy-fermion systems are discussed.