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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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. - PowerPoint PPT PresentationTRANSCRIPT
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
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
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
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