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