Ruthenium double perovskites: Transport and magnetic properties

R. I. Dass, J. Q. Yan, J. B. Goodenough

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Abstract

The double perovskites La2MRuO6+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 La2MgRuO6+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 Tirr independent of magnetic ordering on the M2+ ions, and below a TN≈26KTirr, 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 La2MnRuO6.17(1) is ferromagnetic with a magnetization at 5 K of 2.85μB 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 Sr1-xCaxRuO3 may increase with x.

Original languageEnglish
JournalPhysical Review B - Condensed Matter and Materials Physics
Volume69
Issue number9
DOIs
StatePublished - 2004
Externally publishedYes

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