Neutron-powder-diffraction study of the nuclear and magnetic structures of the antiferromagnetic superconductor HoNi2B2C

The nuclear and magnetic structures of HoNi2B2C have been investigated by neutron-powder-diffraction methods at room temperature and at 10, 5.1, and 2.2 K. The compound crystallizes with the symmetry of space group I4/mmm and lattice parameters a=3.5170(1) and c=10.5217(3) , at room temperature. The structure may be regarded as formed of (HoC) layers with the rock salt configuration alternating with (B)(Ni2)(B) blocks having a fluorite-type arrangement of the atoms. No phase transition of the nuclear structure has been observed down to a temperature of 2.2 K. Magnetic peaks begin to appear at about 8 K. The magnetic structure is the superposition of two configurations, one in which ferromagnetic sheets of holmium spins parallel to the a,b plane are coupled antiferromagnetically along the c axis (magnetic space group Pm'mm'), and another in which ferromagnetic planes are rotated away from the antiparallel configuration to give an incommensurate helicoidal structure with a period approximately equal to 12 times the length of the c axis.