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14.00 o8 j stephen

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Research 6: J Stephen

Research 6: J Stephen

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  • 1. The effect of electronic doping on themagnetisation and thermopowerproperties of Ba2FeMoO6 Jibu Stephen a,b,c, Grant Williams b,c, Ben Ruck b,c, K. Rutherford a Industrial Research, Lower Hutt, New Zealand The MacDiarmid Institute Victoria University of Wellington Outline [1] Double Perovskite Ba2FeMoO6 [2] ASD & Magnetisation [3] Magnetoresistance [4] Thermopower a b c
  • 2. Double perovskites • Ba2FeMoO6 (BFMO)/ Sr2FeMoO6 (SFMO) – tetragonal - alternating Fe and Mo surrounded by oxygen octahedra. • Ba/Sr occupy every hole created by 8 BO6 octahedra Crystal structure of ordered double perovskite: A2BB’O6 Ba/Sr Fe Mo O
  • 3. Double perovskites• Half metallic band structure – conduction electrons 100% spin polarised• The Fe3+ 3d5 electrons are localised and the Mo5+ 4d1 electrons are itinerant.E E E EF EF EF N(E) N(E) N(E)NM FM HM
  • 4. Double perovskitesIntergrain-type tunnelingmagnetoresistanceHigh ferromagnetic orderingtemperature (Tc) (~310 K for HM I HM HM I HMBFMO) Large MR + high Tc -promising applications: Readhead for magnetic hard disk,magnetic sensors and R Rmagnetic RAM. B No applied field B Applied field
  • 5. Synthesis• Solid state synthesis• Mix appropriate proportions of A(NO3)2 (A = Sr, Ba), Fe2O3 and MoO3 to satisfy stoichiometry.• Grind, denitrate and press into pellets• Sinter at 1200 C in air• Regrind, repellet and sinter at 1150 C in 5% H2/95% N2 several times• XRD on sample at each stage to check phase homogeneity
  • 6. ASD and Magnetisation 4 5K Ordered 3 2 1 /f.u.) Disordered 0 B M ( -1 -2 -3 -4 -6 -4 -2 0 2 4 6 Field (T)
  • 7. ASD and Magnetisation Antiferromagnetic coupling model: Fe3+ 3d5 - Mo5+ 4d1 = 5 – 1 = 4μB/f.u. Ms 4.0 9 ASD Low ASD
  • 8. MagnetoresistanceMR = Δρ/ρ = [(ρ(0) - ρ(H))/ρ(0)]Magnetoresistance model in polycrystallinesamples, ∆ρ/ρ = −P2mgb(H)2 / (1 + P2mgb(H)2 )P - degree of spin polarisationmgb(H) - magnetisation from a disordered regionnear the grain boundariesH - applied field. mb mgb mgb mb E. K. Hemery, G. V. M. Williams, and H. J. Trodahl, Physica B 390, 175 (2007).
  • 9. ThermopowerThermoelectric power, S=ΔV/ΔTIntragrain phenomenarather than intergrain
  • 10. ThermopowerElectron diffusion –Mott equation SdPhonon drag – at lowtemperatures (ata fraction of the Debyetemperature)
  • 11. Thermopower – Electron doped
  • 12. Thermopower – Electron doped Hemery et al., PRB 74, 054423 (2006)
  • 13. Thermopower – near Tc Tc
  • 14. Magnetothermopower
  • 15. Magnetothermopower
  • 16. Magnetothermopower
  • 17. Conclusions• Electron doping of BFMO does not have a significant impact on ASD• Thermopower of BFMO changes with La doping• BFMO exhibits magnetothermopower