Magnetic, Transport and Thermal Properties of La 0.67 Pb 0.33 (Mn 1-x Co x )O y M. MIHALIK, V. KAVEČANSKÝ, S. MAŤAŠ, M. ZENTKOVÁ Institute of Experimental Physics, SAS, Košice, Slovak Republic AND J. AMMER, K. KELLNER, G. GRITZNER Institute for Chemical Technology of Inorganic Materials, Johanes Kepler University, Linz Goal: effect of Co doping into Mn sites 0.01  x  0.15 Linzsynthesischaracterization XRD measurements Košice XRD interpretation magnetization, AC susceptibility (MPMS) resistance, MR and heat capacity project: No SK-05/06-KE-005 Appl. Phys. A (2007) Appl. Phys. A (2007) DOI: /s Acta Physica Polonica A (2008)

Introduction Jahn-Teller effect The re-discovery of colossal magnetoresistance (CMR) in hole doped perovskites in the early 1990s gave rise to a new investigation. The ferromagnetic and metallic properties of this type of compound were explained by means of the double exchange (DE) mechanism that involves Mn 3+ – O– Mn 4+ bonds. The substitution of the Mn ions by other transition metal ions it gives rise also to changes in the Mn 3+ : Mn 4+ ratio, with important modifications in the magnetic and transport properties. This simple concept cannot explain all the phenomena, about the metal-insulator (M-I) transision and an additional effects such as lattice distortions, orbital degree of freedom or electron correlation should be consider. Double exchange

Crystal structure R -3 c a = (2) c = (4) (La 0.67 Pb 0.33 )(Mn 0.9 Co 0.1 )O 3 atómxyzocc La Pb Mn Co O

Crystal structure - summary The averaged crystal structure revealed no deformations of the octahedral coordination of Mn/Co – O.The averaged crystal structure revealed no deformations of the octahedral coordination of Mn/Co – O. Average bond valence sum for the Mn/Co site decreases and the average bond valence sum for the La/Pb site increases with increasing Co content. Such behavior may be accounted for by stresses and strain within the crystal lattice.Average bond valence sum for the Mn/Co site decreases and the average bond valence sum for the La/Pb site increases with increasing Co content. Such behavior may be accounted for by stresses and strain within the crystal lattice. We believe that induced strain by the substitution is compensated by rotation of the Mn/Co - O6 octahedrons.We believe that induced strain by the substitution is compensated by rotation of the Mn/Co - O6 octahedrons.

Magnetic properties

x T C [K] (µ 0 H < 0.1 mT) Θ [K] (µ 0 H = 5.0 mT) 338* 325* µ eff [µ B ] (µ 0 H = 5.0 mT) 6.33* 6.40* T JT [K] (µ 0 H = 5.0 mT) - * µ s [µ B ] (T = 1.8 K) µ rem [µ B ] (T = 1.8 K) µ 0 H c [mT] (T = 1.8 K) Magnetic properties

Magnetic properties - summary T JT decreases with doping - the Jahn-Teller effect is reduced with Co-doping.T JT decreases with doping - the Jahn-Teller effect is reduced with Co-doping. The ferromagnetic character of the un-doped compound still remains after Co- doping for whole concentration range.The ferromagnetic character of the un-doped compound still remains after Co- doping for whole concentration range. The Curie temperature T C, the paramagnetic Curie temperature Θ, the effective magnetic moment µ eff and the saturated magnetization µ s decrease with increasing Co-doping.The Curie temperature T C, the paramagnetic Curie temperature Θ, the effective magnetic moment µ eff and the saturated magnetization µ s decrease with increasing Co-doping. The decrease in the magnetic characteristics indicates antiferromagnetic coupling between Mn and Co ions and/or a dilution effect by Co 3+ ions, which are mostly in the low spin state with S = 0.The decrease in the magnetic characteristics indicates antiferromagnetic coupling between Mn and Co ions and/or a dilution effect by Co 3+ ions, which are mostly in the low spin state with S = 0. The Co 3+ ionreduce the population of hoping electrons and available hoping sites; DE is then progressively suppressed, weakening the ferromagnetism.The Co 3+ ion reduce the population of hoping electrons and available hoping sites; DE is then progressively suppressed, weakening the ferromagnetism. Remanent magnetization µ rem and the coercive field H c increase with Co-doping, which can be related to highly anisotropic nature of Co, or to defects introduced by doping.Remanent magnetization µ rem and the coercive field H c increase with Co-doping, which can be related to highly anisotropic nature of Co, or to defects introduced by doping.

x T C [K] (µ 0 H = 0.0 T) T C [K] (µ 0 H = 1.0 T) T * [K] (µ 0 H = 0.0 T) T * [K] (µ 0 H = 1.0 T) T p [K] (µ 0 H = 0.0 T) T p [K] (µ 0 H = 1.0 T) Resistance and magnetoresistance properties B.C. Zhao, et al., Phys. Rev. B (2005)

Resistance and magnetoresistance properties

The ferromagnetic transition is accompanied by an anomaly in electrical resistance for all compounds.The ferromagnetic transition is accompanied by an anomaly in electrical resistance for all compounds. The metal - insulator transitions do not coincide with the relevant Curie temperatures.The metal - insulator transitions do not coincide with the relevant Curie temperatures. The “extrinsic” part of the resistivity is responsible for broad maxima in the ρ(T), which are due to I-M transitions.The “extrinsic” part of the resistivity is responsible for broad maxima in the ρ(T), which are due to I-M transitions. DE suppression induced weakening the metallic behavior of the samples.DE suppression induced weakening the metallic behavior of the samples. The intermediate state below the re-entrant transition is characterized not only by the tunneling intergrain mechanism but also the observed enhancement of resistivity arises from the transition of orbibal order-disorder states (field dependence of T*) and/or from an effective Coulomb barrier of electrostatic origin.The intermediate state below the re-entrant transition is characterized not only by the tunneling intergrain mechanism but also the observed enhancement of resistivity arises from the transition of orbibal order-disorder states (field dependence of T*) and/or from an effective Coulomb barrier of electrostatic origin. The high-temperature I-M transition at Tp, observed for all compounds decreases with Co-doping and the re-entrant temperature T*, observed at low temperatures, increases with Co-doping.The high-temperature I-M transition at Tp, observed for all compounds decreases with Co-doping and the re-entrant temperature T*, observed at low temperatures, increases with Co-doping. The applied magnetic field smears out the anomaly at T C, increases both T p and T C but on the other hand decreases T*.The applied magnetic field smears out the anomaly at T C, increases both T p and T C but on the other hand decreases T*. All studied samples show large magnetoresistance.All studied samples show large magnetoresistance. Resistance and magnetoresistance - summary