Spin reorientation and magnetoelastic coupling in Tb6Fe1−xCoxBi2 (x = 0, 0.125, 0.25, and 0.375) alloy system

Tb6FeBi2 adopts a noncentrosymmetric crystal structure and orders ferromagnetically at TC1 = 250 K with an additional magnetic transition at TC2 = 60 K. The low temperature magnetoelastic response in this material is strong, and is enhanced by cobalt substitution. Here, the temperature dependence of the atomic and magnetic structure of Tb6Fe1−xCoxBi2 (x = 0, 0.125, 0.25, and 0.375) is reported from powder X-ray diffraction (XRD) and powder neutron diffraction (PND) measurements. Below the Néel temperature a ferrimagnetic ordering between the terbium and iron moments exists in all compounds studied. Related to the enhanced magnetostructural response, the Co-doped compounds undergo a crystallographic phase transition below about 60 K. This transition also involves a canting of the magnetic moments away from the c-axis. The structural transition is sluggish and not fully completed in the parent Tb6FeBi2 compound, where a mixture of monoclinic and hexagonal phases is identified below 60 K. The spin reorientation transition is discussed in terms of competing exchange interactions and magnetocrystalline anisotropies of the two Tb sites and Fe/Co sublattices.