The double perovskites ${\mathrm{La}}_{2}M{\mathrm{RuO}}_{6+\delta }$ contain $M^{2+}$ ions for $M=\mathrm{Mg},$ Zn, Co, and Ni and for $M=\mathrm{Mn}$ and Fe, $M^{3+}$ ions. Annealed samples have ordered $M^{2+}$ and Ru(IV), but samples with $M^{3+}$ ions are atomically disordered and remain oxidized $(\delta >0)$ after annealing in a ${\mathrm{N}}_2$ atmosphere. A comparison of quenched, air-annealed, and ${\mathrm{N}}_2$-annealed ${\mathrm{La}}_2{\mathrm{MgRuO}}_{6+\delta }$ samples showed a persistence of a few cation vacancies in the presence of oxygen vacancies $(\delta <0).\mathrm{}$ 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 $M^{2+}$ 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 ${\pi }^{*}$ band even in the atomically ordered samples, but strong correlations introduce magnetic transitions among the ${\pi }^{*}$ electrons. The $M=\mathrm{Co}$ and Ni samples exhibit a magnetic transition at some, if not all, of the ${\pi }^{*}$ electrons on the Ru array below a $T_{\mathrm{irr}}$ independent of magnetic ordering on the $M^{2+}$ ions, and below a $T_N\approx 26\mathrm{K}<T_{\mathrm{irr}},$ antiferromagnetic ordering of the $M^{2+}$-ion spins suppresses any spin on the intervening Ru(IV). The $M^{2+}{:e}^2-\mathrm{O}-\mathrm{Ru}\left(\mathrm{IV}\right)-\mathrm{O}{-M}^{2+}{:e}^2$ antiferromagnetic superexchange interaction is stronger than the ferromagnetic $M^{2+}{:e}^2-\mathrm{O}-\mathrm{Ru}(\mathrm{IV}{):e}^0$ interaction because of a weak intraatomic exchange with the ${\pi }^{*}$-electron spins on the Ru(IV) atoms. On the other hand, disordered ${\mathrm{La}}_2{\mathrm{MnRuO}}_{6.17\left(1\right)\mathrm{}}$ is ferromagnetic with a magnetization at 5 K of $2.85{\mu }_B$ per formula unit (f.u.) in a magnetic field of 50 kOe. This finding is interpreted with a model in which the π-bonding orbitals on both the Mn and Ru are coupled to form a common ferromagnetic ${\pi }^{*}$ 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 ${\pi }^{*}$ bandwidth of the perovskite system ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{RuO}}_3$ may increase with $x.\mathrm{}$

• Received 8 October 2003

DOI:https://doi.org/10.1103/PhysRevB.69.094416