Magnetic, Phonon, and Optical Properties of Pure and Doped Ba2FeReO6 and Sr2CrReO6-Bulk Materials and Nanoparticles.

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 Ba₂FeReO₆ and Sr₂CrReO₆-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 Sr₂CrReO₆ 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.