Effects of Gd, Co, and Ni doping in La2/3Ca1/3MnO3: Resistivity, thermopower, and paramagnetic resonance

The effect of elemental substitutions on the properties of the ferromagnetic, conducting, highly magnetoresistive compound La2/3Ca1/3MnO3 has been studied. The results of Co doping and Ni doping on the Mn site and Gd doping on the La site are reported. These compounds were investigated by x-ray diffraction, magnetization measurements, resistivity measurements, thermopower measurements, and by paramagnetic resonance. The result of replacing La by Gd atoms is to lower the ferromagnetic (or metal-insulator) transition temperature, an effect which is shown to be due to bond bending caused by the lattice adjusting to the size differential between the La and Gd ions. On the other hand, the reduction of the magnetic transition temperature when Mn ions are replaced with Co or Ni ions is not attributed to changes in the size of the ions. instead, we ascribe the lowering of the ferromagnetic transition temperature to a weakening of the double-exchange interaction between two unlike ions. The resistivity and Seebeck coefficient in these materials have been investigated as a function of elemental substitution. The magnetic polaron theory of Zhang is used, phenomenologically, to provide a quantitative explanation of these transport properties. In addition, the effect of these elemental substitutions on the Linewidths of the paramagnetic resonances is studied and is discussed in terms of exchange narrowing and spin-lattice relaxation.