MAGNETIZATION AND NEUTRON-DIFFRACTION STUDIES OF THE MAGNETIC ORDER IN THE COMPOUNDS PR2CO12P7, ND2CO12P7, HO2CO12P7 AND LU2CO12P7

The magnetic order of the lanthanoid and cobalt moments in the hexagon al Zr2Fe12P7-type phosphides Ln(2)Co(12)P(7) (Ln=Pr, Nd, Ho and Lu) ha s been investigated by SQUID magnetization measurements and by neutron powder diffraction. The cobalt sublattices order ferromagnetically be low the Curie temperatures of 136(2), 140(2), 152(2) and 150(2) K in t he praseodymium, neodymium, holmium and lutetium compounds, respective ly. The cobalt atoms with tetrahedral and square-pyramidal phosphorus coordination carry magnetic moments of 0.3(1) and 0.9(1) mu(B) respect ively, and they are aligned parallel to the hexagonal axis. Below the second transition temperature T-t=15(1) K the praseodymium moments ord er ferromagnetically with an antiferromagnetic component relative to t he cobalt sublattice and with an angle off thee axis. In contrast, the neodymium and holmium moments do not order at a well-defined transiti on temperature, instead the order is induced gradually by the ferromag netic cobalt sublattice. In both compounds it sets in at about 65 K. T he neodymium moments are antiparallel and the holmium moments are para llel to the cobalt moments, resulting in ferri-and ferromagnetism, res pectively. The magnetic structures of the three compounds are confirme d by temperature and field-dependent SQUID magnetization measurements of powders; for Pr2Co12P7 and Nd2Co12P7 these measurements were also c arried out with oriented single crystals. The experimentally obtained values for the magnetic moments amount to only between 44 and 77% of t he theoretical values and this is rationalized by partial disorder. (C ) 1997 Elsevier Science S.A.