The magnetic phase transitions of the ternary compounds TbIrGe2 and HoIrGe2 have been studied by magnetic, specific heat and neutron diffraction measurements. Both compounds crystallize in the orthorhombic YIrGe2 -type structure (space group Immm) in which the rare earth atoms occupy two non-equivalent crystallographic sites. The rare earth magnetic moments at different crystallographic sites order independently with two different types of magnetic ordering. The Tb and Ho moments at 4 i site form a collinear commensurate antiferromagnetic structure (Tb moments lie in bc -plane while those of Ho in ab -plane). The magnetic moments at 4 h site also form a collinear antiferromagnetic structure at low temperature but the orientation of magnetic moments is different: Tb moments are aligned along the c-axis while Ho moments lie in ac -plane (in case of TbIrGe2 the magnetic unit cell is four times larger than the crystallographic one). In TbIrGe2 the magnetic order of moments at 4h site at Tt ¼ 9 K turns into a sine modulated one with k =[0:45; 0; 0]. The Néel temperatures for the different sublattices are: for 4h site (Tb) and 5 K (Ho) and for 4 i site: 10 K (Tb) and 2.3 K (Ho). The above results suggest that interactions between the moments at 4 h site are stronger than those for 4 i site. Each sublattice has an uniaxial antiferromagnetic moment arrangement but different reorientation processes and different ordering temperatures reflect the presence of two distinct order parameters. © 2009 Elsevier B.V. All rights reserved.

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