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Spin structure and dynamics in the half-doped cobaltate La 1.5 Sr 0.5 CoO 4

Spin structure and dynamics in the half-doped cobaltate La 1.5 Sr 0.5 CoO 4. Igor A. Zaliznyak Brookhaven National Laboratory. Collaboration J. Tranquada BNL G. Gu BNL R. Erwin NIST CNR S.-H. Lee NIST CNR Y. Moritomo CIRSE Nagoya Univ. Outline.

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Spin structure and dynamics in the half-doped cobaltate La 1.5 Sr 0.5 CoO 4

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  1. Spin structure and dynamics in the half-doped cobaltate La1.5Sr0.5CoO4 Igor A. Zaliznyak Brookhaven National Laboratory Collaboration • J. Tranquada BNL • G. Gu BNL • R. Erwin NIST CNR • S.-H. Lee NIST CNR • Y. Moritomo CIRSE Nagoya Univ.

  2. Outline • Crystal structure of La1.5Sr0.5CoO4 and electronic properties of Co2+/Co3+ ions in it • Charge and spin order at half-doping • neutron-scattering signatures of charge and spin order • sample dependence of the short-range order • Spin-freezing transition: critical slowing down of the spin dynamics • Low-energy excitations in La1.5Sr0.5CoO4 • magnons • RIP:optic phonon, magnetic continuum? • Summary

  3. Perfect “checkerboard” superstructure corresponds to a twice larger unit cell 2ax 2axc, with space group F4/mmm. Crystal structure of the layered perovskite cobaltate around half-doping La1.5Sr0.5CoO4 always (at all T) remains in “high-temperature tetragonal” (HTT) phase Space group I4/mmm, lattice spacings a≈3.83Ǻ, c≈12.5Ǻ

  4. La1.5Sr0.5CoO4: bulk properties. Moritomo et al (1997) Resistivity: activation behavior, Ea ~ 6000 K Susceptibilivity: anisotropic, spin-glass-like behavior T~30 K J~250-450 K D~400-900 K

  5. Charge and orbital order at half-doping Possible checkerboard fillings of the eg levels on a square lattice Out of plane (3z2-r2) In-plane (x2-y2) In-plane “zig-zag” (3x2-r2) / (3y2-r2)

  6. Electronic structure of Co2+/Co3+ ions in La1.5Sr0.5CoO4 Co2+(3d7) S=3/2 eg t2g Co3+(3d6) S=0 S=1 S=2 eg t2g

  7. c ab= 3.5(3)a 2 Al(111) Al(200) c = 0.62(6)c c c c c c c c c c Charge order in La1.5Sr0.5CoO4: neutron diffuse elastic scattering Short-range “charge glass” order, I. Zaliznyak, et. al., PRL (2000), PRB (2001)

  8. Co2+ Co3+ x z Spin-entropy driven melting of the charge order in La1.5Sr0.5CoO4: neutron diffuse elastic scattering x=0.011(1) lu, z=0.0068(4) lu Melting of the short-range “charge glass” order, I. Zaliznyak, et. al., PRB (2001)

  9. J2 J1 Charge order and a spin system Strong single-ion anisotropy D~500 K quenches Co3+ spin at low T S z = ±3/2 Co2+ S=3/2 2D S z = ±1/2 Co3+ S=1 or S=2 S z = ±1 D S z = 0 Co2+ form a square-lattice AFM with almost critical frustration, J1~2J2

  10. ab=14.5(5)a 2 7 5 3 1 Co2+ 2 4 6 8 “Co3+” Spin order in La1.5Sr0.5CoO4: magnetic elastic neutron scattering Q=(0.258(1),0,1), in I4/mmm Q = (h,h,1) T=10K m m m m Q = (0.258,0.258,l) T=6K c=0.85(5)c Lattice-Lorentzian scattering function I. A. Zaliznyak and S.-H. Lee in “Modern Techniques for Characterizing Magnetic Materials”, ed. Y. Zhu (Kluwer)

  11. Magnetic elastic scattering from the frozen spin structure in La1.5Sr0.5CoO4. Intensity map, calculated from the fit T=6 K Al(200) Al(111) Al(111) Al(200) Lattice-Lorentzian scattering from a damped spin spiral in the a-b plane gives perfect fit to the measured intensity

  12. Universal or sample-dependent? Sample #2, by G. Gu, m≈6g Sample #1, by Y. Moritomo, m≈0.5g

  13. ab= 3.4(6)a 2 Charge-order scattering from big new sample #2 x=0.011(1) lu, z=0.0068(4) lu, zLa/Sr=0.0010(1) lu c = 0.2c(fixed) Al(111) Al(200)

  14. ab=14.5(5)a 2 ab=23.0(5)a 2 Magnetic scattering from two samples T=8K Q = (0.256,0.256,1)

  15. ab8 a 2 c0.5c ab4 a 2 c0.2c Melting of the frozen spin order. BT2&BT4, Ef=14.7 meV, 60’-20’-20’-100’. 50K 38K ab15 a 2 c0.9c 6K

  16. ~30K? Temperature evolution of the magnetic scattering: raw data. BT2&BT4, Ef=14.7 meV, 60’-20’-20’-100’. SPINS, Ef=3.7 meV, 40’-60’-60’-240’. ~40K? ~40K? ~30K? Where is the spin-ordering transition?

  17. E~(T-Tc) E~ 0+(T-Tc) E~T E~ 0+T Slowing down of the spin fluctuations: is there a criticality? Although the critical behaviorE~(T-Tc),=3.0(3)is not ruled out, log(E)is surprisingly linear inlog(T):E~Twith ~8(!?). log(E)~log(T) ? log(E)~log(T) ?

  18. Spin dynamics: acoustic magnons

  19. RIP: scattering at higher energy: phonon, magnetic continuum? phonon magnetic continuum?

  20. RIP, dynamics in La1.5Sr0.5CoO4: acoustic magnons, optic phonon, magnetic continuum?

  21. Summary • A short-range checkerboard charge order yields a peculiar spin system in La1.5Sr0.5CoO4 • A short-range, incommensurate spin order results from the frustration and the lattice distortion • the incommensurability and the correlation length are slightly sample dependent • Static spin ordering: a spin-freezing transition at Ts≈ 30 K • relaxation rate vanishes • correlation length saturates • Dynamics at low E is dominated by a well-defined, strong band of acoustic magnons • crosses an optic phonon at 15 meV – interaction? • Continuum magnetic scattering at 20 meV < E < 30 meV?

  22. Exchange modulation by superlattice distortion Heisenberg spin Hamiltonian + Superlattice distortion (eg) = Modulated-exchange Hamiltonian

  23. Spin-spiral ground state better adapts to distortion Harmonics at nQc are generated in spin distribution, To the leading order, As a result, the MF ground state energy of a spin spiral is lowered • In the presence of a superlattice distortion in the crystal antiferromagnetism may loose to a competing near-by spiral state I. A. Zaliznyak, Phys. Rev. B (2003).

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