Atomic disorder, magnetic order and phase transitions of TbFe 6Ge 6 studied by X-ray, neutron diffraction and magnetic measurements (I)

Abstract

The crystal structure and the magnetic ordering of TbFe 6Ge 6 were studied by X-ray and neutron powder diffraction and by magnetic measurements on a single crystal. It has been found that TbFe 6Ge 6 forms a superstructure described by the wave vector q = ( 1 2, 1 2, 0 ) of the orthohexagonal YCo 6Ge 6 type of structure ( c, √ 3 a, a) h . High resolution neutron data of an as cast material annealed at 800°C indicate that the major factor leading to the superstructure formation is associated with a density modulation of the Tb/Ge ordering that can be described by a density modulation function with the wave vector q . The Fe ordering is antiferromagnetic between 1.4 and 293 K with the wave vector q 1 = (0 1 0) . The three Fe sites have a collinear moment arrangement along the x axis with the same moment value. The Tb sublattice, as a result of the density modulation, splits into two sites ( Tb(0, y, 1 4 )/ Tb∗( 1 2 , y, 1 4)) with (0.89(1)/0.11(1)) occupancy. Both Tb sites order magnetically below T 1 = 9 K. Their ordering can be described by two wave vectors q 1 = (0 1 0) and q 2 = (0 0 0) , the former giving rise to a collinear antiferromagnetic ordering along z (perpendicular to the Fe moments), the latter to ferromagnetic ordering along x (parallel with the Fe moments). At 1.4 K the Fe moment values are 2.02(4)μ B/Fe atom. Assuming that the two wave vectors correspond to a single magnetic phase, this would lead to the moment values μ Tb = 7.34(8)μ B , μ Tb∗ = 5.1(7)μ B with an angle of ±45° with the z direction in the plane (x 0 z) corresponding to a flopside structure. The nuclear and the magnetic patterns display an anisotropic peak broadening depending on the h k l direction and the annealing temperature. This effect is discussed in terms of microdomain structure associated with fluctuations of density modulation.