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Research 6: J Stephen
Citation preview
a b
Jibu Stephen a,b,c, Grant Williams b,c, Ben Ruck b,c, K. Rutherford a
Industrial Research, Lower Hutt, New Zealand The MacDiarmid InstituteVictoria University of Wellington
Outline[1] Double Perovskite Ba2FeMoO6
[2] ASD & Magnetisation[3] Magnetoresistance[4] Thermopower
c
The effect of electronic doping on the magnetisation and thermopower properties of Ba2FeMoO6
• Ba2FeMoO6 (BFMO)/ Sr2FeMoO6 (SFMO) – tetragonal - alternating Fe and Mo surrounded by oxygen octahedra.
• Ba/Sr occupy every hole created by 8 BO6 octahedra
Double perovskites
Crystal structure of ordered double perovskite: A2BB’O6
Ba/Sr Fe Mo O
• Half metallic band structure – conduction electrons 100% spin polarised
• The Fe3+ 3d5 electrons are localised and the Mo5+ 4d1 electrons are itinerant.
Double perovskites
E
N(E)NM
EF
N(E)
E
FM
EF
EEF
N(E)HM
Double perovskites
Intergrain-type tunneling magnetoresistance
High ferromagnetic ordering temperature (Tc) (~310 K for BFMO)
Large MR + high Tc - promising applications: Read head for magnetic hard disk, magnetic sensors and magnetic RAM.
R
BNo applied field
HM HMI
R
BApplied field
HM HMI
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 1200C in air• Regrind, repellet and sinter at 1150C in 5%
H2/95% N2 several times
• XRD on sample at each stage to check phase homogeneity
ASD and Magnetisation
-6 -4 -2 0 2 4 6
-4
-3
-2
-1
0
1
2
3
4
5 K
Disordered
Ordered
M (m
B/f
.u.)
Field (T)
ASD and Magnetisation
Low ASD
Antiferromagnetic coupling model: Fe3+ 3d5 - Mo5+ 4d1
= 5 – 1 = 4μB/f.u.
ASDM s 90.4
MagnetoresistanceMR = Δρ/ρ = [(ρ(0) - ρ(H))/ρ(0)]
Magnetoresistance model in polycrystalline samples,
∆ρ/ρ = −P2mgb(H)2 / (1 + P2mgb(H)2 )
P - degree of spin polarisationmgb(H) - magnetisation from a disordered region near the grain boundaries H - applied field.
E. K. Hemery, G. V. M. Williams, and H. J. Trodahl, Physica B 390, 175 (2007).
mb mgb mgb mb
ThermopowerThermoelectric power, S=ΔV/ΔT
Intragrain phenomena rather than intergrain
ThermopowerElectron diffusion – Mott equation
Phonon drag – at low temperatures (ata fraction of the Debye temperature)
Sd
Thermopower – Electron doped
Thermopower – Electron doped
Hemery et al., PRB 74, 054423 (2006)
Thermopower – near Tc
Tc
Magnetothermopower
Magnetothermopower
Magnetothermopower
Conclusions
• Electron doping of BFMO does not have a significant impact on ASD
• Thermopower of BFMO changes with La doping
• BFMO exhibits magnetothermopower
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