Investigation of stability of ordered manganites

Cation-ordered manganites of the PrBaMn2O6 system have been obtained using a two-stage synthesis and characterized with respect to the chemical composition, crystal structure, magnetic and magnetotransport properties, and the stability of the ordered state on heating. The physical properties of the cation ordered PrBaMn2O6 manganites obtained using this method significantly differ from the properties of cation disordered Pr0.50Ba0.50MnO3 synthesized by means of the conventional ceramic technology and depend on the degree of ordering of the Pr3+ and Ba2+ cations. In particular, the cation-disordered Pr0.50Ba0.50MnO3 has a cubic perovskitelike unit cell $$(SG = Pm\bar 3m,Z = 1)$$ , while cation-ordered PrBaMn2O6 has a tetragonal unit cell (SG = P4/mmm, Z = 2). Cation states in the system under study are reversible. The cation-ordered PrBaMn2O6 state remains stable upon heating in an oxidizing medium (P[O2] = 1 bar) up to 1300°C. The ordering of the Pr3+ and Ba2+ cations leads to a significant increase in the critical temperatures of phase transitions. In particular, PrBaMn2O6 with the maximum degree of ordering is a metallic ferromagnet with the Curie temperature T C∼320 K, whereas T C of a fully disordered sample is on the order of 140 K. The samples with intermediate degrees of ordering contain two magnetic phases. Slightly below T C, all such samples exhibit a metal-insulator transition and a peak of the magnetoresistance, which amounts to approximately 10 and 65% in a magnetic field of 9 kOe for the fully ordered PrBaMn2O6 and disordered Pr0.50Ba0.50MnO3, respectively. The results are interpreted in terms of the Goodenough-Kanamori empirical rules for indirect exchange interactions with allowance for the degree of ordering of the Pr3+ and Ba2+ cations.

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