Abstract
On the basis of a microscopic model and employing Green's function technique, the effects of temperature, size, and ion doping on the magnetization and phonon energy of the A(1g) mode in double perovskites Ba(2)FeReO(6) and Sr(2)CrReO(6)-both in bulk and nanoscale samples-are investigated for the first time. The Curie temperature T(C) and magnetization M decrease as nanoparticle size is reduced. Doping with rare-earth ions such as Sm, Nd, or La at the Ba or Sr sites further reduces M. This behavior originates from the compressive strain induced by the smaller ionic radii of the dopant ions compared to the host ions. As a result, the antiferromagnetic superexchange interaction between Fe or Cr and Re ions is enhanced, along with an increase in the magnetic moment of the Re ion. The dependence of the band gap energy of Sr(2)CrReO(6) on temperature, size, and doping is also studied. Near the magnetic-phase-transition temperature T(C), anomalies in phonon energy and damping indicate strong spin-phonon coupling. The theoretical calculations show good qualitative agreement with experimental data.