Effect of the A-site Randomness on Magnetotransport Properties of PrBaMn_2O_6 Manganites

摘要

A-site ordered PrBaMn_O_6 manganites were obtained by a two-step synthesis process. The chemical composition, crystal structure, magnetic and magnetotransport properties have been investigated with regard to the degree of ordering of the Pr~(3+) and Ba~(2+) cations. A-site disordered Pr_(0.50)Ba_(0.50)MnO_3 has a cubic perovskite-like unit cell (SG = Pm3m, Z=1), while the A-site ordered PrBaMn_2O_6 has a tetragonal unit cell (SG = P4/ mmm, Z = 2). The A-site ordered state in PrBaMn_2O_6 remains stable on heating to 1300 °C in an oxidizing medium (P[O_2] = 1 bar). The cation ordering leads to a significant increase in the critical temperatures of phase transitions. A-site ordered PrBaMn_2O_6 is a metallic ferromagnet with the T_C ～ 320 K, whereas T_C of a fully disordered sample is ～ 140 K. The samples with intermediate degrees of ordering contain two magnetic phases. Slightly below T_C, all samples exhibit a metal-insulator transition and a peak of the magnetoresistance, amounting to approximately 10 % and 65 % in a magnetic field of 9 kOe for the fully ordered PrBaMn_2O_6 and disordered Pr_(0.50)Ba_(0.50)MnO_3, respectively. The results are interpreted in terms of the degree of ordering of the Pr~(3+) and Ba~(2+) cations.