Crystal and magnetic structure of antiferromagnetic MnPtPd.

We have investigated the crystal and magnetic structure of MnPtPd alloy using powder x-ray and neutron diffraction experiments. This compound is believed to belong to the Heusler family having crystal symmetry I4/mmm (TiAl-type). However, in this work we found that the Pd and Pt atoms are disordered and thus MnPtPd crystallizes in the L1 structure having P4/mmm symmetry (CuAu-I type) like MnPt and MnPd binary alloys. The lattice constants are a  =  2.86 Å and c  =  3.62 Å at room temperature. MnPtPd has a collinear antiferromagnetic spin structure below the Néel temperature T   =  866 K, where Mn moments of ~4 µ lie in the ab-plane. We observed a strong change in the lattice parameters near T . The sample exhibits metallic behaviour, where electrical resistivity and carrier concentration are of the order of 10 Ω cm and 10 cm, respectively.