Neutron diffraction studies of the magnetoelastic compounds Tb5SixGe4−x (x=2.2 and 2.5)

We report the results of a neutron diffraction study, carried out on both single crystalline and polycrystalline samples of Tb{sub 5}Si{sub 2.2}Ge{sub 1.8} and polycrystalline Tb{sub 5}Si{sub 2.5}Ge{sub 1.5}. On cooling, at approximately 120 K, the Tb{sub 5}Si{sub 2.2}Ge{sub 1.8} system undergoes a magnetoelastic transition from a high-temperature monoclinic-paramagnetic to a low-temperature orthorhombic-ferromagnetic structure. Between 120 K and 75 K, the magnetic structure has a net ferromagnetic component along the a axis direction. The moments are slightly canted with respect to the a axis, while the components along the b and c axes are ordered antiferromagnetically. A second magnetic transition occurs at approximately 75 K. Below this temperature, the magnetic structure consists of ferromagnetically aligned {mu}{sub x} and {mu}{sub z} projections of the magnetic moments and an antiferromagnetic arrangement of the {mu}{sub y} moment components. Magnetic structures of Tb{sub 5}Si{sub 2.5}Ge{sub 1.5} are nearly identical to those of Tb{sub 5}Si{sub 2.2}Ge{sub 1.8}.