Influence of thermal history on crystal structure, microstructure and magnetic properties of TbFe6Ge6 (II)

The relation between crystal structure, microstructure and magnetic structure in TbFe6Ge6 is derived on the basis of neutron powder diffraction for samples annealed at 925, 800 (part I) and 650 degrees C. All samples display a density modulation of only the guest subsystem (Tb/Tb* and Ge5/Ge5*) described by the wave vector q = (1/2, 1/2, 0) of the orthohexagonal YCo6Ge6 structure. The occupancy of the Tb site is 1 for the sample with T-an - 925 degrees C and decreases to 0.830(6) for the sample with T-an = 650 degrees C, At high temperatures (HT) T-1 < T < T-N (T-N approximate to 450 K, T-1 = 9.5 K) the Fe moments order with the wave vector q(1) = (0 1 0) and have an antiferromagnetic collinear arrangement ( + - + - . .) along the x-axis, The Tb magnetic order sets in at T-1 = 9.5 K and is associated with two wave vectors q(1) = (0 1 0) (antiferromagnetic, AF) and q(2) = (0 0 0) (ferromagnetic, F). The Tb F-component is parallel to x, while the Tb AF-component is confined to the (x 0 z)-plane. However, their relative amount depends strongly on the thermal history and this in turn influences the Fe ordering. The largest Tb F-component is observed for the sample with T-an - 925 degrees C and it induces a reorientation of the Fe moments below T-1 which at 1.4 K are tilted away from the x-axis by an angle phi(xy) = 25.0(6)degrees. The 1.4 K ordered moment values are 2.22(4) mu(beta)/Fe and 8.0(1) mu(beta)/Tb. The coupling within the Tb[1 0 0]-layers stacked along x is F. The canted ferrimagnetic structure has moments in general directions and comprises four sublattices, two Fe AF and two Tb ferrimagnetic. For sample with T-an = 650 degrees C the Tb AF-component is dominant, leading to a braking of the F coupling within the Tb[1 0 0]-layers. The Fe moments preserve the HT arrangement below T-1. The 1.4 K ordered moment values are 2.29(6) mu(beta)/Fe, 9.5(2) mu(beta)/Tb (x = 0) and 4.1(2) mu(beta)/Tb (x = 0.5). The canted magnetic structure comprises six sublattices, two Fe AF and four Tb ferrimagnetic, all within the (x 0 z) plane. The change of the Tb atoms distribution and coherency of the guest subsystem (peak broadening) are coupled in two ways with the magnetic ordering. They change the total magnetic structure and determine the correlation length of the long-range order of the Tb moments. (C) 1998 Elsevier Science B.V. All rights reserved.