Neutron and X-ray Scattering Studies of Spin, Charge and Orbital Order in TM Oxides

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Neutron and X-ray Scattering Studies ofSpin,
Charge and Orbital Order in TM Oxides
Andrew Boothroyd Department of Physics, Oxford
University
resistivity
La5/3Sr1/3NiO4
magnetization
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Transition Metal Oxide Research in Oxford
Physics Department

• Characterization
• Magnetization
• Transport
• etc

Sample Preparation lab. Single Crystals Dr
Prabhakaran
X-ray scattering Dr Hatton (Durham) Prof Cowley
Muon-spin Rotation Dr Blundell
Neutron Scattering
Dr Boothroyd Dr Coldea Prof Cowley
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Crystal growth floating-zone method
Image furnace (Clarendon Laboratory)
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Oxford single crystals of TM oxides
La0.7Sr0.3MnO3
La5/3Sr1/3NiO4
La3/2Sr1/2CoO4
Na0.7CoO2
CoNb2O6
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Neutron scattering studies of stripe-ordered
nickelates
collaborators
D. Prabhakaran Oxford University Paul Freeman
  Mechthild
Enderle Institut Laue-Langevin Jiri Kulda
Arno Hiess
Louis-Pierre Regnault CEA,
Grenoble, Felix Altorfer Paul-Scherrer
Institut, Switzerland Christof Niedermayer
Chris Frost ISIS Hyungje
Woo Brookhaven National Lab/ISIS Kenji
Nakajima University of Tokyo John Tranquada
Brookhaven National Lab
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Outline

• Overview of stripe phenomena in La2xSrxNiO4
• What can be learned by neutron diffraction ?
• Interesting aspects of magnetic ordering
• Interesting features in magnetic excitation
  spectra

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Stripe order in La2xSrxNiO4
(Tranquada et al, Cheong et al, Yoshizawa et al)
x 0
x 1/4
x 1/3
x 1/2
ideal stripe structures
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Neutron Diffraction
(1) Scattering from nuclei
Q
t
Q ki - kf
Laue Q t
Bragg nl 2d sinq

• Applications
• Crystal structure refinement
• Structural distortions (e.g. charge order)

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Neutron diffraction
(2) Scattering from magnetic moments
V(r) mn.B(r)
Neutrons scatter from component of magnetization
perpendicular to Q
No!

• Applications
• Spin arrangements in ordered phases
• Magnetic form factors

Yes!
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Polarized neutron scattering
Magnetic field defines polarization axis, P
1. P Q
SF Myy Mzz NSF N
Incident neutrons
Scattered neutrons
2. P Q (in plane)
SF Mzz NSF N Myy
Spin-flip (SF)
3. P Q (vertical)
SF Myy NSF N Mzz
Non-spin-flip (NSF)
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Advantages of polarized neutron scatteringfor
studying complex order
1. Distinguishing magnetic and non-magnetic
scattering

• static charge vs magnetic order
• dynamic spin fluctuations vs phonons

La5/3Sr1/3NiO4
2. Separating different magnetic components

• static determining moment directions
• dynamic identifying anisotropy gaps
• and anisotropic fluctuations

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Spin charge order in La3/2Sr1/2NiO4
Freeman et al, Phys. Rev. B 66 (2002)
212405 Kajimoto et al, Phys. Rev. B 67 (2003)
14511
Half-doping
170 K lt T lt 460 K checkerboard charge ordering
charge order
magnetic order
T lt 170 K spin charge ordering Not simple
checkerboard pattern
Correlation length 35 Å
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Models for spin-charge order in La3/2Sr1/2NiO4
Charge peaks at Qco (½, ½) and (e, e),
e 4/9 Magnetic peaks at Qm (½, ½)
(e/2, e/2)
Possible diagonal stripe pattern
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Spin reorientations
La3/2Sr1/2NiO4
f
La1.63Sr0.37NiO4
Similar results for La5/3Sr1/3NiO4 reported by
Lee et al, Phys. Rev. B 63 (2001) 60405(R) TSR
50 K, Df 13 deg
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Overview of spin dynamics in La2xSrxNiO4
e.g. La5/3Sr1/3NiO4
Spin waves
Gap-like features
2 components
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Low energy quasi-1D spin fluctuation in
La5/3Sr1/3NiO4
Diffuse inelastic scattering
Scans along line B
Scans along line A
? Ni3 ions (probably) carry spin S ½

• Consistent with
• quasi-1D AFM chains

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Soft X-ray scattering studies of manganates
collaborators
  Peter Hatton University of Durham Philip
Spencer Stuart
Wilkins Institute of Transuranium
Elements,Karlsruhe, and European Synchrotron
Radiation Facility, Grenoble D. Prabhakaran
Oxford University Steve Collins Daresbury
Laboratory Mark Roper
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X-ray Scattering

• Non-resonant scattering
• X-rays can probe charge density (Thomson) very
  strong!
• X-rays also scatter from spin and orbital
  moments very weak!
• Resonant scattering
• Strong enhancements at atomic absorption edges
• Resonant scattering from spin and orbital
  moments
• Element specific
• Higher-order anomalous scattering processes
  observable

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Examples of X-ray resonant magnetic scattering
Antiferromagnetic order in PrBa2Cu3O6x
Pr LII edge
Cu K edge
J.P. Hill et al, Phys. Rev. B 58 (1998) 11211
J.P. Hill et al, Phys. Rev. B 61 (2000) 1251
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X-ray resonant scattering from Mn
K edge resonance
L edge resonance
4p
3d
EA 0.65 keV l 19 Å
2p
EA 6.55 keV l 1.9 Å

• Probes 3d orbitals directly
• Very large resonant enhancements
• Long wavelengths

1s

• Soft X-rays
• Long period structures only

Indirect probe of 3d magnetism
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Daresbury Laboratory

• 2 GeV Machine
• 5U1 Soft X-ray Undulator
• Good Sample Environment
• Awful Food

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Soft X-ray scattering at 5U1, SRS
Air absorption at 650 eV is severe!
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Soft X-ray resonant scattering from
La22xSr12xMn2O7 (x 0.45)

• Charge order between 120 K and 220 K
• Antiferromagnetic order below 170 K

(001) AFM Bragg peak
S.B. Wilkins et al Phys. Rev. Lett. 90 (2003)
187201
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Orbital ordering in La0.5Sr1.5MnO4
Spin, charge and orbital order below 240
K Jahn-Teller distortion very small
AFM order
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Soft X-ray scattering theory
Theoretical predictions
Soft X-ray resonant scattering At the orbital
ordering Bragg peak
(Castelton Altarelli, Phys. Rev. B 62 (2000)
1033)
(S.B. Wilkins et al, to appear in Phys. Rev Lett.)
No Jahn-Teller distortion
(¼, ¼, 0)
Strong JT distortion
Conclusion scattering is due to combined orbital
ordering and cooperative Jahn-Teller
distortions
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Conclusions
Neutron scattering studies of stripe-ordered
nickelates

• Spin and charge order La3/2Sr1/2NiO4 not
  checkerboard!
• Spin reorientations occur
• Evidence for coupling of spin excitations to
  charge stripes
• AFM spin correlations on the charge stripes

Soft X-ray studies of spin-charge-orbital ordered
manganates

• Large resonant enhancements at L edge
• Can probe ordering of 3d orbitals directly
• Limited to ordering phenomena with period d gt 10
  Å
• Jahn-Teller mechanism important in driving
  orbital order