Amazing Oxides - Ferromagnets, Superconductors

Amazing Oxides - Ferromagnets, Superconductors

Electronic Materials and
Extreme Conditions
J. Paul Attfield
School of Chemistry and
Centre for Science at Extreme Conditions (CSEC),
University of Edinburgh
High temperature superconductors (1986-)
Compound
Superconductivity
– correlated motion
of electron pairs below a critical
Nb3Ge
temperature
(Tc), characterised by
o6S8
zeroPbM
electrical
resistance and perfect
diamagnetism;
LiTi2O4
Ba0.6 K0.4 BiO3
* ET =
16
13
136
(ET)2Cu(NCS) 2*
Cs3 C60
High-Tc
era 1986-
S
23
30
Low-Tc
1911-1986
HgBaera
2Ca2Cu3O8+δ
- metals and alloys
Li0.2 HfNCl
- copper
M gB2
Tc (K)
13
34
25
39
oxides (etc)
S
S
S
CMR Manganese oxides (1995-)
La0.7Ca0.3MnO3 - ferromagnetic and conducting
 Colossal Magnetoresistances (CMR) for
sensors, spintronic devices etc.
La0.5Ca0.5MnO3 - nonmagnetic
(antiferromagnetic) and insulating
 localisation and long range order of;
• charges (Mn3+/Mn4+ states),
• d-orbitals (Mn3+Jahn-Teller distortion)
• spins (Mn3+/Mn4+ magnetic moments)
High Pressure Perovskites
SrCrO3
Orbitally
driven phase separation
‘Hard-soft’
synthesis
 SrCrO2.80
 SrCrO2.75
Arevalo et al
Ortega
San Martin et al, PRL 2007
ACIE 2012
BiNiO3 (Kyoto)
Colossal NTE
PbRuO3
Symmetry-reversing orbital transition
Kimber et al, PRL 2009
MnVO3
Helimagnetic A site spin order
Bi0.95La0.05NiO3
Azuma et al, Nature Comm. 2011
Markkula et al, PRB 2011
The Verwey Structure of
Magnetite (Fe3O4)
Mark Senn, Jon Wright & JPA, Nature (2012)
Magnetite and magnetism
biomagnetism
geomagnetism
lodestones
ferrites
compass
spintronics
Low temperature properties – the Verwey transition
• Evidenced by a first order transition in
resistivity, heat capacity and
magnetisation at 125 K
• Complex superstructure
Verwey, E. J. W. (1939). "Electronic
conduction of magnetite (Fe3O4) and its
transition point at low temperatures."
Nature 144: 327-328.
Fe3+[Fe2.5+]2O4 →Fe3+[Fe2+Fe3+]O4
Fe2+
Fe3+
Fe3+
Theoretical approaches:
•
Verwey (1939) Fe2+/Fe3+ charge order (Verwey model, 1946)
•
Order-disorder of 2 electron-B4 tetrahedra (Anderson, 1956)
•
CO from U/W band instability (Cullen & Callen, 1970)
•
Polaron (bi-, molecular-) CO (Mott, Chakraverty, Yamada 1970-1980)
•
Bond-dimerisation (no CO) - Fe25+ dimers (Seo, Khomskii 2002-)
Full structure solution (Senn, Wright, JPA) 2006-2012
Use microcrystals from previous powder (Fe3-3dO4, d < 0.0001 - Prof. J. Honig):
•
•
•
•
•
•
Twinning, multiple scattering, extinction problems reduced by using microcrystallites.
Microcrystal beamline ID11@ESRF - 100 μm focused monochromatic beam.
Hard X-rays (74 keV, λ = 0.16653(1) Å) reduces absorption, accesses high Q.
Magnetic alignment (~1 T field from permanent magnet while cooling through TV)
Refinement software for twinned crystals (SHELXL)
Try many microcrystals – be lucky
h = 50 (hkl) sections
Best microcrystal:

• approx. spherical, ~40 μm
• two twins at 90 K, 89:11
• Cc structure determined using
91,433 unique Bragg intensities
• model uniqueness checked
against 2,000 randomised
starting models
Second best microcrystal: 
• irregular, ~100 μm
• four twins
• refined structure same as above
Electronic order in the Verwey state of magnetite
Fe2+/Fe3+ charge
order to first
approximation
….and orbital order
of Fe2+ states….
….but Fe2+ ions also
weakly bonded to two
neighbours –
trimeron units.
Trimeron order.
Significance?
• Ground state
unexpected, not simple
charge order.
• Prevalence of orbital
molecules (trimerons)?
• Dynamics above 125K?
Thanks
Wei-Tin Chen
Lucy Clark
Shigeto Hirai
Andrea Marcinkova
Mikael Markkula
George Penny
Marek Senn
Alex Sinclair
Congling Yin
Minghui Yang
Angel Arevalo-Lopez
Anna Kusmartseva
Martin Misak
Jenny Rodgers