Magnetism and spin-gap behaviour in the layered Pr₃ T ₄Al₁₂ (T = Fe, Ru, Os) compounds with the distorted Kagomé lattice

We have investigated the physical and magnetic properties of the distorted Kagomé lattice structure compounds; Pr₃T₄Al₁₂ (T = Fe, Ru, Os) which crystallize in the hexagonal Gd₃Ru₄Al₁₂-type structure with space group P6₃/mmc (No. 194). The three compounds show long-range magnetic orderings of FM type at T _C = 36.5 K (Fe), 39 K (Ru) and 37 K (Os) as indicated by the temperature dependences of magnetic susceptibility, Χ(T), specific heat, C _p (T) and electrical resistivity, ρ(T). Pr₃Ru₄Al₁₂ shows an additional anomaly at 7 K which is associated to possible spin reorientation of some of the Pr moments. We also observed an additional anomaly in Pr₃Fe₄Al₁₂ at 132 K which might be associated to the formation of a spin density wave. The Sommerfeld coefficient, γ extracted from C _p (T) in the temperature range immediately above T _C indicate heavy fermion behaviours in Pr₃T₄Al₁₂ (T = Fe, Ru, Os). ρ(T) of the three compounds show spin-gap behaviours below T _C of comparable energy gaps magnitude.