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The « charge ordered » state in La 1-x Sr x MnO 3 x=1/8 and x=0.15 probed by spin waves

The « charge ordered » state in La 1-x Sr x MnO 3 x=1/8 and x=0.15 probed by spin waves. M. Hennion.

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The « charge ordered » state in La 1-x Sr x MnO 3 x=1/8 and x=0.15 probed by spin waves

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  1. The « charge ordered » state in La1-xSrxMnO3 x=1/8 and x=0.15 probed by spin waves M. Hennion P. Reutler (Orsay D. Shulyatev (MISIS) Y. Mukovskii (MISIS) F. Moussa (LLB) S. Petit (LLB) A. Ivanov (ILL)

  2. Outline IIntroduction: -Phase diagram of La1-xSrxMnO3 around x=1/8: TC and TCO IISpin-waves in thequasi-metallic state TC>T>TCO, x=0.125,0.15 -Large q-range and small q-range - Comparison with calculated spin-waves in a proposed model III– Spin waves in the « charge-ordered state »T<TCO , x=0.125, x=0.15 -Large q-range and small q-range -Comparison with calculated spin-waves in a proposed model IV-Conclusion

  3. Evolution of the magnetic couplings with x LaMnO3, orbital ordering Phase diagram J. Deisenhoffer x(Sr)=x (holes) x=0.125 and x=0.15 La1-xCaxMnO3 (Hennion et al. PRL 94 (2005) Jc Jc Ja,b=Ja=Jb meV x (holes) Feiner and Oles Physica B (1999)

  4. x=0.15 Tco Tc Tco Tc T(K) Resistivity versus T x=0.125 Bebenin et al PRB (2000) TC=180KTCO=159K TC=230KTCO=200K TCO : strong change of lattice parameters, new superstructures (h,k,l/4) Yamada et al. PRL (1999) TCO : No change of lattice parameters, new superstructures (h,k,l/4)

  5. La7/8Sr1/8MnO3Spin waves at 13K , q//[100] F. Moussa et al. PRB 67 (2003)

  6. Spin waves in the quasi-metallic state Sr x=0.15; Tc=230K Sr x=1/8; Tc=180K (1) (2) (3) (4) Two q-regimes: underlying 4a period. same energy values for the levels M. Hennion et al. PRB 73 (2006)

  7. Dir [111] X=1/8 Jc=0 X=0.15 Jc=Jab/3 With a Heisenberg model: EB100=8SJa,b EB001=8SJc EB111=2EB100+EB001 Jc0 !! : 2D 2x(1) 2x(2) 2x(3) 2x(4) Withw=Dq2 D[100]=D[010]=D[110] D[011]=1/2(D[010]+ D(001]) D[111]=1/3(D[100]+ D[010]+D[001]) 2D!! Small q: isotropic large q: anisotropic

  8. Calculation of spin-waves in a Heisenberg model with first neighbor (cf S. Petit, poster) , 2 indep. param (a, b) plane T=0 J inside clusters, J’ inside boundaries J ’’ between the 2 regions The clusters (x=0.15), due to their anisotropic coupling (orbital ordering), are hole-poor [1,0] In large q range: standing spin waves imply confinement into 2D limited-size clusters (cf Hendriksen et al. 1993), suggest 2 ferro regions or charge segregation In small q range, the q-dispersion implies long-range ferromagnetism: ferro coupled spins connected at the boundaries of the clusters (and through the clusters?)

  9. Spin waves in the « charge-ordered » state, small q-range x(Sr)=0.125 T=150K T=150K gap at q~0.125 (rlu) in all sym. directions

  10. x(Sr)=0.15 gap at q=0.125 rlu along [100] and [110]

  11. Spin waves in the « charge-ordered » state, large q-range Sr x=0.15 Sr x=1/8

  12. Dir [100] X=1/8 and x=0.15 Dir [111]: At large q, Jc=Jab/5 At T=14K x=0.125 D isotr. (70meV*A2) but Jc~0 , x=0.15: D isotr. (78±2meV*A2 )but Jc=Jab/5 For x=0.15, along [100] , gaps and modulations of F spin wave dispersion: 4a periodicity along a and b

  13. Spin wave calculationsS. Petit (cf Poster) 1) 2) 3) J inside,J’ across lines, l=J’/J=0.2

  14. Spin wave calculationsfor nickelates or cuprates (2D, AF with antiphase) Kruger and Scheidl PRB 67 (2003) Spacing p=3 Spacing p=4 Spacing p=3 Spacing p=4 Spacing p=5 La 1.79Sr 0.31 Ni04 Bourges et al. PRL (2002) l=J’/J

  15. model of orthogonal stripe superstructure in (a, b) plane for x(Sr)=0.15 Additional proof : static superstructure: 1/4,1/4,0 ? (never reported before), Magnetic? Nuclear? a key for solving Pb of stripes organization along c: Disordered?: lines (1/4,1/4,z) Ordered? Which order?

  16. ConclusionVery original observations for spin waves in the range where Jc << JabFor x=0.15, we can interpret nearly quantitatively the spin waves in a picture of superstructure of orthogonal stripes in (a,b) plane: first observation of stripes in a Ferro compound.Temperature evolution: fluctuating stripes which become stabilizedFor x=0.125 where strong lattice effects exist at TCO (favored by stoechiometry 1/8 ?), small differences observed with x=0.15. magnon-phonon coupling ? Isotropy of the small q-range, Organization of stripes superstructure along c, needs for a full characterisation of (1/4,1/4,z) TTCO T<TCO

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