Ruthenium double perovskites: Transport and magnetic properties

The double perovskites La₂MRuO_6+δ contain M²⁺ ions for M = Mg, Zn, Co, and Ni and for M = Mn and Fe, M³⁺ ions. Annealed samples have ordered M²⁺ and Ru(IV), but samples with M³⁺ ions are atomically disordered and remain oxidized (δ > 0) after annealing in a N₂ atmosphere. A comparison of quenched, air-annealed, and N₂-annealed La₂MgRuO_6+δ samples showed a persistence of a few cation vacancies in the presence of oxygen vacancies (δ < 0). In quenched samples, oxygen vacancies are preferentially located between two Ru atoms, where they form a deep two-electron trap state, whereas the two-electron trap state formed at an oxygen vacancy between an M²⁺ and Ru(IV) is shallow. Magnetic as well as transport data indicate the π-bonding 4d electrons at the low-spin Ru atoms occupy itinerant-electron states of a π* band even in the atomically ordered samples, but strong correlations introduce magnetic transitions among the π* electrons. The M = Co and Ni samples exhibit a magnetic transition at some, if not all, of the π* electrons on the Ru array below a T_irr independent of magnetic ordering on the M²⁺ ions, and below a T_N≈26KT_irr, antiferromagnetic ordering of the M²⁺-ion spins suppresses any spin on the intervening Ru(IV). The M²⁺:e²-O-Ru(IV)-O-M²⁺:e² antiferromagnetic superexchange interaction is stronger than the ferromagnetic M²⁺:e²-O-Ru(IV):e⁰ interaction because of a weak intraatomic exchange with the π*-electron spins on the Ru(IV) atoms. On the other hand, disordered La₂MnRuO_6.17(1) is ferromagnetic with a magnetization at 5 K of 2.85μ_B per formula unit (f.u.) in a magnetic field of 50 kOe. This finding is interpreted with a model in which the p-bonding orbitals on both the Mn and Ru are coupled to form a common ferromagnetic π* band in which only the antibonding electrons are not spin paired. The strong next-nearest-neighbor interaction between Ru atoms made manifest in the ordered double perovskites provides an explanation of why the π* bandwidth of the perovskite system Sr_1-xCa_xRuO₃ may increase with x.