TY - JOUR
T1 - Ruthenium double perovskites
T2 - Transport and magnetic properties
AU - Dass, R. I.
AU - Yan, J. Q.
AU - Goodenough, J. B.
PY - 2004
Y1 - 2004
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=2142812974&partnerID=8YFLogxK
U2 - 10.1103/PhysRevB.69.094416
DO - 10.1103/PhysRevB.69.094416
M3 - Article
AN - SCOPUS:2142812974
SN - 1098-0121
VL - 69
JO - Physical Review B - Condensed Matter and Materials Physics
JF - Physical Review B - Condensed Matter and Materials Physics
IS - 9
ER -