Magnetic ordering of TbFe4Al8 studied by neutron diffraction

The structure and magnetic ordering of the tetragonal TbFe 4 Al 8 compound were studied by neutron diffraction at temperatures between 1.5 and 170 K. The structure refinement in the paramagnetic state confirms the preference of the Fe atoms for the 8(f) positions. Magnetic reflections are only discernable below 110 K. The magnetic ordering is described by two wave vectors: q 1 =0 , and q 2 =(q x , q x , 0) , the latter having a temperature-dependent length. The ordering associated with these two wave vectors occurs simultaneously below 110 K. In the high-temperature range 40–110 K the best model obtained by refining neutron difference diagrams leads for the ordering associated with q 1 =0 to exclusively Tb ferromagnetic contributions with moments in the basal plane. For the ordering associated with q 2 =( q x , q x , 0 ) it leads to a cycloid spiral structure comprising Tb and Fe contributions. The moments of both sublattices rotate within the plane of the wave vector (0 0 1). The value of q x below 8 K is 0.125(1) r.l.u. Symmetry analysis shows that the Fe site splits into two independent sets (orbits) under the action of the in-plane wave vector q 2 =( q x , q x , 0 ). Various models are discussed. For the cycloid spiral, within the 3 σ limit, the phases of the Fe moments of both orbits are zero. The high-temperature structure is a distorted cycloid with elliptical envelope for the Tb sublattice which below 40 K induces a distortion to the Fe cycloid spiral because of the increasing in-plane magnetic field of the in-plane ferromagnetic Tb moment. Below 40 K the q 1 =0 structure comprises ferromagnetic Tb as well as antiferromagnetic Fe contributions in the basal plane.