Research 6: J Stephen

1. The effect of electronic doping on the
magnetisation and thermopower
properties 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

3. 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

4. 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

5. Double perovskites
Intergrain-type tunneling
magnetoresistance
High ferromagnetic ordering
temperature (Tc) (~310 K for
HM I HM HM I HM
BFMO)
 Large MR + high Tc -
promising applications: Read
head for magnetic hard disk,
magnetic sensors and R R
magnetic RAM. B
No applied field B
Applied field

6. 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

7. 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)

8. ASD and Magnetisation
Antiferromagnetic coupling
model: Fe3+ 3d5 - Mo5+ 4d1
= 5 – 1 = 4μB/f.u.
Ms 4.0 9 ASD
Low ASD

9. Magnetoresistance
MR = 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 polarisation
mgb(H) - magnetisation from a disordered region
near the grain boundaries
H - applied field.
mb mgb mgb mb
E. K. Hemery, G. V. M. Williams, and H. J. Trodahl, Physica B 390, 175 (2007).

10. Thermopower
Thermoelectric power,
S=ΔV/ΔT
Intragrain phenomena
rather than intergrain

11. Thermopower
Electron diffusion –
Mott equation
Sd
Phonon drag – at low
temperatures (at
a fraction of the Debye
temperature)

12. Thermopower – Electron doped

13. Thermopower – Electron doped
Hemery et al., PRB 74, 054423 (2006)

14. Thermopower – near Tc
Tc

15. Magnetothermopower

16. Magnetothermopower

17. Magnetothermopower

18. Conclusions
• Electron doping of BFMO does not
have a significant impact on ASD
• Thermopower of BFMO changes with
La doping
• BFMO exhibits magnetothermopower