Magnetization and neutron diffraction studies of the magnetic order in the compounds Pr 2Co 12P 7, Nd 2Co 12P 7, Ho 2Co 12P 7 and Lu 2Co 12P 7

Abstract

The magnetic order of the lanthanoid and cobalt moments in the hexagonal Zr 2Fe 12P 7-type phosphides Ln 2Co 12P 7 (Ln=Pr, Nd, Ho and Lu) has been investigated by SQUID magnetization measurements and by neutron powder diffraction. The cobalt sublattices order ferromagnetically below the Curie temperatures of 136(2), 140(2), 152(2) and 150(2) K in the praseodymium, neodymium, holmium and lutetium compounds, respectively. The cobalt atoms with tetrahedral and square-pyramidal phosphorus coordination carry magnetic moments of 0.3(1) and 0.9(1) μ B respectively, and they are aligned parallel to the hexagonal axis. Below the second transition temperature T 1 = 15(1) K the praseodymium moments order ferromagnetically with an antiferromagnetic component relative to the cobalt sublattice and with an angle off the c axis. In contrast, the neodymium and holmium moments do not order at a well-defined transition temperature, instead the order is induced gradually by the ferromagnetic cobalt sublattice. In both compounds it sets in at about 65 K. The neodymium moments are antiparallel and the holmium moments are parallel to the cobalt moments, resulting in ferri- and ferromagnetism, respectively. The magnetic structures of the three compounds are confirmed by temperature and field-dependent SQUID magnetization measurements of powders; for Pr 2Co 12P 7 and Nd 2Co 12P 7 these measurements were also carried out with oriented single crystals. The experimentally obtained values t 1, magnetic moments amount to only between 44 and 77% of the theoretical values and this is rationalized by partial disorder.