P. Reutler (Orsay D. Shulyatev (MISIS) Y. Mukovskii (MISIS)

The « charge ordered » state in La1-xSrxMnO3 x=1/8 and x=0.15 probed by spin waves

F. Moussa (LLB)

S. Petit (LLB)

A. Ivanov

(ILL)

M. Hennion

OutlineI Introduction:

-Phase diagram of La1-xSrxMnO3 around x=1/8: TC and TCO

II Spin-waves in the quasi-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

Feiner and Oles Physica B (1999)

Ja,b=Ja=Jb

Jc

x (holes)

meV

Evolution of the magnetic couplings with xLaMnO3, orbital

ordering

x=0.125 and x=0.15

Phase diagram

La1-xCaxMnO3 (Hennion et al. PRL 94 (2005)

J. Deisenhoffer

x(Sr)=x (holes)

Jc

Resistivity versus T

TC=180K TCO=159KTC=230K TCO=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)

Tco Tc

T(K)

x=0.15

Tco Tc

Bebenin et al PRB (2000) x=0.125

La7/8Sr1/8MnO3 Spin waves at 13K , q//[100] F. Moussa et al.

PRB 67 (2003)

M. Hennion et al. PRB 73 (2006)

Sr x=0.15; Tc=230K

same energy values for the levels

Spin waves in the quasi-metallic state

Sr x=1/8; Tc=180K

(1)

(2)

(3)

(4)

Two q-regimes: underlying 4a period.

Dir [111]

X=1/8 Jc=0 X=0.15 Jc=Jab/3

2x(1)

2x(2)

2x(3)

2x(4)

With a Heisenberg model:

EB100=8SJa,b EB 001=8SJc

EB 111=2EB100+EB

001

Jc0 !! : 2D

With =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

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?)

Calculation of spin-waves in a Heisenberg model with first neighbor (cf S. Petit, poster) , 2 indep. param

The clusters (x=0.15), due to their anisotropic coupling (orbital ordering), are hole-poor

J inside clusters, J’ inside boundaries J ’’ between the 2 regions

(a, b) plane

x(Sr)=0.125

T=150K

T=150K

Spin waves in the « charge-ordered » state, small q-range

gap at q~0.125 (rlu) in all sym. directions

gap at q=0.125 rlu along [100] and [110]

x(Sr)=0.15

Spin waves in the « charge-ordered » state, large q-range

Sr x=1/8

Sr x=0.15

Dir [100] X=1/8 and x=0.15

For x=0.15, along [100] , gaps and modulations of F spin wave dispersion: 4a periodicity along a and b

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

2)1) 3)

Spin wave calculations S. Petit (cf Poster)

J inside,J’ across lines=J’/J=0.2

Bourges et al. PRL (2002)

La 1.79Sr 0.31 Ni04

Spacing p=3

Spacing p=4

Spacing p=5

Spacing p=3 Spacing p=4

Spin wave calculations for nickelates or cuprates (2D, AF with antiphase)

Kruger and Scheidl PRB 67 (2003)

=J’/J

a key for solving Pb of stripes organization along c: Disordered?: lines (1/4,1/4,z)

Ordered? Which order?

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?

Conclusion

Very original observations for spin waves in the range where Jc << Jab

For 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 stabilized

For 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)

TTCO T<TCO