A NEUTRON-DIFFRACTION STUDY OF THE MAGNETIC ORDER IN THE THCR2SI2 TYPE PHOSPHIDES PRCO2P2 AND NDCO2P2

The magnetic structures of the compounds LnCo2P2 (Ln = Pr and Nd) were investigated at different temperatures by neutron powder diffractometry and refined by the Rietveld technique. In agreement with the previously determined magnetic susceptibility data the magnetic moments of both the Pr3+ ions and the cobalt atoms of PrCo2P2 were found to order antiferromagentically with different, well-defined Neel temperatures. The magnetic lattices of the two magnetic species have different propagation vectors of k = (0, 0, 1) and k = (0, 0, 1/2) for the Pr3+ ions and the cobalt atoms, respectively. In contrast, in NdCo2P2 only the magnetic Moments of the cobalt atoms order at a well-defined Neel temperature with a propagation vector again of k = (0, 0, 1/2), while the magnetic order of the Nd3+ moments is induced gradually at a lower temperature with the same propagation vector through the magnetic order of the cobalt atoms. The magnetic moments of the cobalt atoms of both compounds are ordered ferromagnetically within the basal planes of the tetragonal magnetic structures and parallel to the c-axis with the stacking sequence + + - -, + + - - along this direction. For the moments of the Pr3+ ions the order is again ferromagnetic within the basal planes and the stacking sequence is + - + - , + - + - . In the magnetic structure of the Nd3+ sublattice no magnetic order (0) was found for every other basal plane; the stacking is of the type 0 - 0 + , 0 - 0 + . At low temperatures the magnetic moments of the cobalt atoms were found to be about mu(exp) = 0.8 mu(B) in both compounds. The experimental moments of the Ln3+ ions were 3.08(2) mu(B) for Pr3+ and 2.78(6) mu(B) for Nd3+, somewhat smaller than the theoretical values of mus = 3.20 mu(B) and 3.27 mu(B), respectively, expected for the Ln3+ ions.