Structural, magnetic, and transport properties of the giant magnetoresistive perovskites La2/3Ca1/3Mn1-xAlxO3-delta

The effect of the substitution for Mn with Al in the magnetoresistive perovskite La2/3Ca1/3MnO3 has been studied by preparing the series La2/3Ca1/3Mn1-xAlxO3 (x less than or equal to 0.2). A careful study of the magnetic, structural, and transport properties has been carried out by means of electrical resistance, magnetoresistance, ac magnetic susceptibility, x-ray-diffraction, and neutron-diffraction techniques. Up to x=0.05 the Curie temperature (and the associated metal-insulator transition) decreases drastically with Al doping and the magnetoresistive properties do not change very much. For x greater than or equal to 0.1 the lattice spontaneously begins to lose oxygen atoms and for x=0.2, 3% of oxygen vacancies are present. This fact along with the random distribution of the Al atoms makes these compounds rather disordered from a structural and magnetic point of view. However, the magnetoresistance is enhanced, reaching colossal values of 10(7)% at H=12 T at low temperatures for x=0.2.