Observation of isotropic-dipolar to isotropic-Heisenberg crossover in Co- and Ni-substituted manganites

Abstract

High-precision ac susceptibility data have been taken on the La_0.7Pb_0.3Mn_1-y(Co, Ni)_yO₃ (y=0, 0.1, 0.2 and 0.3) manganite system over a wide range of amplitudes and frequencies of the ac driving field in a temperature range that embraces the critical region near the ferromagnetic (FM)-paramagnetic (PM) phase transition (occurring at the Curie point T_C). Elaborate data analysis was performed that (i) enabled the first observation of a crossover from a three-dimensional (3D; d=3) isotropic long-range dipolar asymptotic critical behavior to a d=3 isotropic short-range Heisenberg critical regime as the temperature is raised from T_C in the compositions y≠0 (no such crossover is observed in the parent compound, y=0) and (ii) brought out clearly the importance of dipole-dipole interactions between the e_g electron spins and/or between e_g-t_2g electron spins in establishing long-range FM order in the insulating state. The final charge and spin states of Co and Ni ions, substituting for the Mn³⁺ and/or Mn⁴⁺ ions, are arrived at by using a scenario of substitution that is consistent not only with the present results but also with the previously published structural, thermo-gravimetric, bulk magnetization, dc magnetic susceptibility and electrical resistivity data on the same system. The marked similarity seen between the magnetic behavior of the manganite system in question and the quenched random-exchange ferromagnets, within and outside the critical region, suggests that the percolation model forms an adequate description of the FM metal-to-PM insulator transition.