Magnetic phase transitions in RIrGe2 (R = Tb, Ho) compounds.

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 4i site form a collinear commensurate antiferromagnetic structure (Tb moments lie in bc-plane while those of Ho in ab-plane). The magnetic moments at 4h 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 T-t = 9K turns into a sine modulated one with (k) over right arrow = [0.45, 0, 0]. The Neel temperatures for the different sublattices are: for 4h site similar to 12 K (Tb) and 5 K (Ho) and for 4i site. 10 K (Tb) and 2.3 K (Ho). The above results suggest that interactions between the moments at 4h site are stronger than those for 4i 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. (C) 2009 Elsevier B.V. All rights reserved.