Influence of Quenched Disorder on the Structural Properties and Spin-Wave Excitations in Ferromagnetic Metallic Manganites (La_(1-x)Nd_x)_(2/3) (Ca_(1-y)Sr_y)_(1/3)MnO_3

摘要

The influence of quenched disorder on the structural properties and on low-temperature field-dependent magnetization, in terms of the spin-wave theory, of (La_(1-x)-Nd_x)_(2/3)(Ca_(1-y)Sr_y)_(1/3)MnO_3 with J1 (x = 0, y = 0); J2 (x = 0.05, y = 0.04); J3 (x = 0.25, y = 0.20); J4 (x = 0.3, y = 0.24); J5 (x = 0.98, y = 0.8) is investigated by characterizing a series of samples with the same A-site cational mean radius and the same average valence of the Mn ion but different A-site ionic radii variance. Our results demonstrate that moderate-small chemical disorder does not affect the crystallographic structure, but enhance the random local distortion of MnO_6 octhaedra. Magnetization measurements show that the Curie temperature (T_C) decreases with disorder. The evolution of the saturation magnetization with the disorder cannot be deduced due to the possible contribution of the Nd spins to the total magnetic moment. The thermal evolution of magnetization in the ferromagnetic phase at low temperature varies as T~(3/2), in accordance with Bloch's law. The spin wave stiffness constant D obtained from the Bloch constant is found to decrease with the increase of disorder. We found that for the range of the study disorder, the magnetic behavior corresponds to a conventional ferromagnet, where T_C should be proportional to D.