TbMgNi 4- x Co x -(H,D) 2 System. II: Correlation between Structural and Magnetic Properties.

The magnetic properties of TbMgNi 4- x Co x intermetallic compounds and selected hydrides and deuterides of this system have been studied by various techniques, including magnetic measurements, in situ X-ray and neutron powder diffraction. The intermetallic compounds crystallize in a SnMgCu 4 -type structure and magnetically order below a Curie temperature ( T C ), which increases exponentially with the Co content. This can be due to the ordering of the Co sublattice. On the other hand, the insertion of D or H in TbMgNiCo 3 strongly decreases T C . The X-ray diffraction measurements versus temperature reveal cell volume minima at T C for the compounds with x = 1-3 without any hints of the structure change. The analysis of the neutron diffraction patterns for the intermetallics with x = 2 and 3 indicates a slightly canted ferrimagnetic structure below T C . The Tb moments refined at 16 K are 4.1(2) μ B /Tb for x = 2, and 6.2(1) μ B /Tb for x = 3, which are smaller than the free ion value (9.5 μ B /Tb). This reduction can be due to the influence of temperature but also reveals the crystal field effect. As Ni and Co occupy statistically the same Wyckoff site, an average Ni/Co moment was refined, leading to 1.7(2) μ B /atom for x = 2 and 1.8(1) μ B /atom for x = 3 at 16 K. This moment is slightly canted compared to the Tb moment.