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

Neutron and X-ray Scattering Studies of Spin, Charge and Orbital Order in TM Oxides. Andrew Boothroyd Department of Physics, Oxford University. resistivity. La 5/3 Sr 1/3 NiO 4. magnetization. Transition Metal Oxide Research in Oxford Physics Department. Characterization Magnetization

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

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  1. 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

  2. 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

  3. Crystal growth — floating-zone method Image furnace (Clarendon Laboratory)

  4. Oxford single crystals of TM oxides La0.7Sr0.3MnO3 La5/3Sr1/3NiO4 La3/2Sr1/2CoO4 Na0.7CoO2 CoNb2O6

  5. 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

  6. Outline • Overview of stripe phenomena in La2–xSrxNiO4 • What can be learned by neutron diffraction ? • Interesting aspects of magnetic ordering • Interesting features in magnetic excitation spectra

  7. Stripe order in La2–xSrxNiO4 (Tranquada et al, Cheong et al, Yoshizawa et al) x = 0 x = 1/4 x = 1/3 x = 1/2 ideal stripe structures

  8. 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)

  9. Q m m 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!

  10. z z z y y y Q P x x x Q P P Q 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)

  11. 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

  12. 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 < T < 460 K ‘checkerboard’ charge ordering charge order magnetic order T < 170 K spin & charge ordering Not simple checkerboard pattern • Correlation length » 35 Å

  13. Models for spin-charge order inLa3/2Sr1/2NiO4 • 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:

  14. 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]

  15. Overview of spin dynamics in La2–xSrxNiO4 e.g. La5/3Sr1/3NiO4 Spin waves Gap-like features 2 components

  16. 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

  17. 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 ¢ ¢

  18. 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

  19. Examples of X-ray resonant magnetic scattering Antiferromagnetic order in PrBa2Cu3O6+x 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

  20. 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

  21. Daresbury Laboratory • 2 GeV Machine • 5U1 – Soft X-ray Undulator • Good Sample Environment • Awful Food

  22. Soft X-ray scattering at 5U1, SRS Air absorption at 650 eV is severe!

  23. T = 83 K LIII LII Soft X-ray resonant scattering from La2–2xSr1+2xMn2O7 (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

  24. Orbital ordering in La0.5Sr1.5MnO4 Spin, charge and orbital order below 240 K; Jahn-Teller distortion very small AFM order

  25. 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

  26. 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 > 10 Å • Jahn-Teller mechanism important in driving orbital order

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