Effect of iron doping on the properties of TbBaCo2O5.5 layered perovskite

Abstract

The properties of the TbBa(Co2−xFex−)O5+δ (0<x⩽1) system, exhibiting the giant magnetoresistance effect, were studied by magnetization, thermogravimetric analysis (TGA), and resistivity measurements as well as by Mössbauer spectroscopy. It was found that the properties of this system dramatically change at x>0.1, at which a concentration-dependent orthorhombic–tetragonal transition takes place. The orthorhombic composition with x=0.1 exhibits a transition from an antiferromagnetic to weak ferromagnetic state at a temperature Ti=200 K on warming and at 160 K on cooling, and a transition from a weak ferromagnetic to paramagnetic state at TN=304 K. A metal–insulator transition occurs at TM=348 K on warming. Transformation into a tetragonal phase (x⩾0.12) leads to the disappearance of both the weak ferromagnetic state and the metal–insulator phase transition. Also, the Néel point drops. The orthorhombic and tetragonal phases coexist in the concentrational range (0.1<x<0.12). Mössbauer spectroscopy has shown that Fe3+ ions preferentially occupy two low-symmetry positions that are interpreted to be pyramidal sites with fivefold coordination. Both Mössbauer spectroscopy and TGA data revealed that the increase of Fe content leads to the decrease of oxygen content according to the formula TbBa(Co2−xFex)O5.5−x/2. It is supposed that at x⩾0.12 a change of the type of oxygen vacancy ordering occurs.