Study of the Nd(0.7)A(0.3)Mn(1-x)B(x)O(3) (A = Ph, Cd; B = Fe, Co, Ni; x =0, 0.1) phases: Synthesis, characterization, and magnetic properties

The Nd(0.7)A(0.3)Mn(1-x)B(x)O(3) (A = Pb, Cd; B = Fe, Co, Ni; x = 0, 0.1) c ompounds have been synthesized using the sol-gel method and characterized b y redox titration and X-ray diffraction. The replacement of manganese ions by Fe, Co or Ni implies a decrease in the ferromagnetic (or metal-insulator ) transition temperature. The ZFC magnetization drops to zero at low temper atures, showing a general weakening of the ferromagnetic DE interactions an d an increasing contribution of the AFM ones arising by superexchange. Simi lar effects have been observed in the cadmium phases but substitutions of F e, Co and Ni cations do not modify the magnetic behavior of these phases. T he difference observed between the para-ferromagnetic transition temperatur e, T-c, and the metal-insulator one could be explained because of the appea rance of magnetically inhomogeneous regions. Values of magnetoresistance ar ound 70% for the lead compounds have been obtained. From ESR measurements a t room temperature for all of the samples, different line widths related to the kind of metals present in the cell and the corresponding T-c have been obtained. The line width values are higher for the cadmium phases than tho se for the lead ones, and within each family, it increases as follows: Ni < Fe < Co.