Neutron powder diffraction study of the nuclear and magnetic structures of HoNi_1.985Co_0.015B₂C and HoNiBC

The nuclear and magnetic structures of HoNi_1.985Co_0.015B₂C and HoNiBC have been analyzed by neutron powder methods. The first of these materials crystallizes with the symmetry of space group I4/mmm, lattice parameters (at room temperature) a = 3.52011(9), c = 10.5286(3) Å, and is isostructural with the undoped compound HoNi₂B₂C. Cobalt and nickel atoms are disordered over the same crystallographic sites. Well below the Neèl temperature (∼ 8 K), the magnetic moments of the Ho atoms are ordered ferromagnetically in the a, b planes and antiferromagnetically along the c-axis, with a magnitude of the moment of 8.26(6)μ_B. The Co doping, even at such low concentrations, destroys the superconducting behavior of the undoped phase. The compound HoNiBC is also tetragonal (space group P4/nmm), with lattice parameters (at room temperature) a = 3.5631(2), c = 7.5486(6) A, and is isostructural with LuNiBC. Below the Neèl temperature (9.82(8) K), the Ho magnetic moments are ordered ferromagnetically in the bi-layers (HoC)(HoC) with spins lying in the a, b planes, and successive bi-layers are ordered antiferromagnetically along the c-axis. The magnitude of the moments is 6.74(9)μ_B. No structural distortions associated with the magnetic ordering were observed in either compound.