The effect of the disorder induced by Cu substitution on ...
Transcript of The effect of the disorder induced by Cu substitution on ...
The effect of the disorder induced by Cu substitution on the phonon properties of
La1-yAyMn1-xCuxMnO3 manganites
Gianluca De Marzi
Outline • Colossal MagnetoResistance and applications
• Crystalline structure, Electronic configuration, Phase diagram
• Some hints on theoretical models
• Optical phonon in manganites
• Experimental: the role of Cu B-site doping on vibrational properties:
- Raman spectra, data analysis
• Conclusions and perspectives
Colossal Magnetoresistance Manganites
interest on A1-xBxMnO3 manganites Colossal MagnetoResistance (CMR)
0
0
0 ρρρ
ρρ −
=∆
= HMR , ρ0 = ρ=(H=0)
MR < 0 and isotropic (no dependence) HJrr
•
MR up to 100% (La-Ca-Mn-O at 77 K)*
*Jin et al., Science 264, 413 (1994)
Scientific and Technological interest in CMR Scientific interest By varying x and T manganites show:
• Different typologies of spin ordering
• M-I and PM-FM transitions
• Jahn-Teller distortions and polaronic
effects
• Charge ordering
• Orbital ordering
Technological interest MagnetoResistance is “Colossal”,
because:
• Permalloy (Ni/Fe),EuO MR≈2-3%
• Multilayer Cu/Co GMR≈40%
• Manganites CMR≈100%
Applications: Read/write magnetic memory devices (bobine and cassette tapes, Digital Audio Tape, etc.), and system sensible to both T and H.
Crystalline Structure
Chemical formula: A1-xBxMnO3
Jahn-Teller theorem (1937): "any non-linear molecular system in a degenerate electronic state will be unstable and will undergo distortion to form a system of lower symmetry and lower energy thereby removing the degeneracy"
Example: La-Ca-Mn-O
A = rare earth +3 (La, Pr, Y, Nd,…) B = divalent ion (Ca, Sr, Ba, …) Oxygen
Electronic Configuration Neutral Mn electronic configuration = [Ar]3d54s2
- In AMnO3 Mn is trivalent (ionic approx.) 4 d-electrons will be responsible for its electronic
properties (AFM)
Mn3+ in octahedral co-ordination
- In BMnO3 Mn is tetravalent 3 d-electrons present (AFM)
- For a partial substitution case, A1-xBxMnO3 (0 < x < 1), Mn ions are mixed-valent. An amazing thing is that, although AMnO3 and BMnO3 manganites are AFM insulators,
at some intermediate composition A1-xBxMnO3 exhibits CMR :
Rich Phase Diagram
high T Paramagnetic Insulator (PI) x∀
by varying x and T, manganites show:
x=0, and 1 AFM insulators
x > 0.5 Charge-Ordering
≈ 0.2 ÷ 0.5 CMR (not Pr1-xCaxMnO3)
First explained by Zener (1951), Anderson &
Hasegawa in the framework of the:
Double-Exchange Model
Double-Exchange
Wherever an Mn3+ and Mn4+ are in neighbouring Mn sites,
there exists the possibility of eg-electron hopping from the
Mn3+ to the Mn4+ via the oxygen anion.
Two simultaneous electron hops are required Mn3+ onto
O2- and O2- onto Mn4+
( ) ∑∑ ++ ⋅−+−=L
abiibiaabiH
L
ijjiij ccSJcHcctH
,,
.. σσ
σσvv
∑ ⋅+L
jijiAF SSJ
,
vv
Whitin the solution for two classical spins*
the following relationship holds:
t(Θ) = t cos(Θ/2)
lowering T PM FM ↔ I M
raising H spin alignment I M *Anderson and Hasegawa, De Gennes
DE explain qualitatively the experiments, but… Millis et al.: “Double-Exchange alone
doesn’t explain the resistivity of La1-xSrxMnO3” Phys. Rev. Lett 74, 5144 (1995):
• DE overstimate Tc one order of
magnitude(1000-3000 K )
• T dependence of ρ(T) is completely
different at T<Tc
• Experimental values for ρ are lerger than
that predicted by DE theory
One has to consider the el-ph interaction, due in part to the JT splitting of the Mn eg
states.
( ) ( )∑∑ ++= + jQkdjQdgHH jbab
jaDE2ˆ
21ˆ
σσ
the competition between localisation and DE can be
parameterised by an effective el-ph constant:
eff
loc
tE
ktg
==λ
results are in agreement with experimental
data (by Schiffer et al.)
Optical phonons in CMR Manganites Let us consider LaMnO3 manganite Theory group analysis for the undistorted (cubic)
perovskite gives the following irreducible
representation:
Γ = 4F1u+F2u O1h (Pm3m)
The JT effect distorts the octahedra, and
the structure is orthorhombic:
D2h16(Pnma):
60 phonons (k=0) are predicted (Iliev,98)
25 IR active 9B1u+7B2u+9B3u 24 Raman 7Ag+5B1g+7B2g+5B3g 8 Silent Au 3 Acoustic B1u+B2u+B3u
3 IR active (F1u) 3 acoustic (F1u) 1 silent (F2u) NO Raman active
Phonon Assignment for the undoped LaMnO3
IR Measurements
De Marzi et al. PRL (98)
Raman Spectra
Ilev et al., PRB 57, 2872 (1998)
Raman measurements on doped manganites Common features in Raman spectra: maxima are mainly located at three intervals:
• 180-300 cm-1 M1
• 400-520 M2
• 580-680 M3
but…phonon assignment is still controversial
increasing doping JT reduction
“more cubic” structure extremely
small Raman scattering efficiency difficult measurements
• M1 corresponds to an Ag out-of-
phase x-rotation of the oxygen
cage • M2 is A2g (mainly bending) • M3 is B2g (mainly stretching)
AE Pantoja, HJ Trodahl, J. Phys.: Cond. Matt. 13 (2001) 3741
Our samples: polycrystalline La1-ySryMn1-xCuxO3
Cu substitution at the B sites of the perovskite structure strongly influence:
• Transition temperature Tc (Sapiña et al.)
• the Mn-O-Mn angles of the MnO6 octahedra
• structural disorder
Therefore:
phononic properties of manganites are modified by B-site doping
aim: to study the evolution of phonons as function of B-site doping
The idea is that a MI transition can occur when the octahedral is forced to be undistorted and the Mn-O-Mn angles tends towards 180° , and this can be obtained by changing the average dimension of the atom at the A and/or B site.
Our samples: polycrystalline La1-ySryMn1-xCuxO3
• samples with Cu doping 0 < x < 0.10
• Tc is reduced by B-site substitution:
• Structure rhombohedral R-3c (x-ray
analysis by Sapiña et al.)
• Single phase compounds
• the ratio Mn4+/(Mn4++Mn3+) = 0.3 is fixed the effects are not due to DE
mechanism.
La1-ySryMn1-xCuxO3 x
y
%Mn4+ TC
0.00 0.02 0.04 0.06 0.08 0.10
0.300 0.274 0.248 0.222 0.196 0.170
32 32 32 32 35 32
372 358 331 308 274 236
Table 1: nominal compositions for x, y doping, % of tetravalent Mn ions, and observed Tc [13]
Raman spectra of La1-ySryMn1-xCuxO3
polarized configuration
0 200 400 600 800 1000
Ram
an In
tens
ity
Raman Shift (cm- 1)
La1-y SryMn1-xCuxO3 T = 300 K
x y TC
0.00 0.300 372 0.02 0.274 358 0.04 0.248 331 0.06 0.222 308 0.08 0.196 274 0.10 0.170 236
- three peaks are well evident at
about 200, 400, and 600 cm-1
- shoulder at around 600 cm-1 disappear for x=0.00
- a strong background signal is present
cross-polarized configuration
0 200 400 600 800
Ram
an In
tens
ity
Raman Shift (cm-1)
La1-y
SryMn
1-xCu
xO
3 T = 300 K
x y TC
0.00 0.300 372 0.02 0.274 358 0.04 0.248 331 0.06 0.222 308 0.08 0.196 274 0.10 0.170 236
• the first peak at about 200 cm-1 completely
disappears
Data analysis
0 200 400 600 800
Ram
an In
tens
ity (a
.u.)
Raman Shift (cm-1)
LSM6 julio
• Spectra were fitted in the 130-900 cm-1
range with six lorentzian oscillators
• An example of the best fit curve and the
different components is shown for the
x=0.10 sample;
• four peaks were found at 180-215, 430,
498, and 670 cm-1, and a broad
background with a maximum at 450 cm-1.
Data analysis
0 0.02 0.04 0.06 0.08 0.10180
190
200
210
220
300400500600700800900
100011001200
La1-ySryMn1-xCuxO3
Freq
uenc
y (c
m-1
)
X doping
A1g
Eg A
g B
2g
Γ(D3d
6)= A1g+ 2A1u+ 3A2u+4Eg+5Eu+3A2g
8 IR active 3A2u+5Eu 5 Raman 1A1g+4Eg
Granado et al., PRB 58, 11435 (98):
ω1 A1g N.B. disappears for ui εε ⊥ ω2 Eg ω3 Eg (?) ω4 Eg (?)
Why didn’t Granado etal. see ω3 and ω4 ?
polishing effect
Analysis of the A1g mode
Raman shift of A1g is unusual. infact
Sr87 is lighter than La139 shift toward lower ω!
ω1 is sensitive to JT distortions
(that are modified by Sr doping)
* Irwin et al. PRB 59, 9362 (1999)
Tolerance factor
( )OB
OA
OB
OA
rrr
dd
t+
==−
−
22r +
- t<0.925 orthorhombic; 0.925<t<0.1 rhombohedric; t=1 cubic
- since r(Sr2+) > r(La3+)
when x increases (and y decreases) <rA> decreases
- Moreover, <rB> is increased by Cu substitution
0,968 0,970 0,972 0,974 0,976 0,978 0,980
185
190
195
200
205
210
215
x=0.00
x=0.10
Ram
an S
hift
of th
e A 1g
mod
e (c
m-1
)
Tolerance Factor
But we observe just the opposite A1g is not an external mode Infact, A1g involves the motion of the oxygen cage *
• band shift is alinear functionof t
• increasing xcauses thesystem to bemore distorted
More Analysis…
Data analysis has still not been concluded assignment of higher frequencies peaks is unresolved:
• is the ω4 mode one of the frozen phonon related to the JT distortions ? (Dediu et al.)
• ω3 and ω4: Second order Raman scattering?
• ω3 and ω4 related to the bending and stretching modes of the MnO6/CuO6 octahedra?
Best fit are now being performed with:
( ) ( )[ ]( )
+−+
Γ+Γ
+= ∑=
n
i i
iiAAnS1
22222221
γωωω
ωγω
ωωω
Bose-
Einstein factor
Odd lorentzian
“collision-dominated” low-frequency response
associated with diffusive hopping of the carriers
Conclusions and Perspectives
• Raman spectra of La1-ySryMn1-xCuxO3
polycrystalline manganites, at T=300 K and
in the 100-1000 cm-1 range, both
and
ui εε ⊥
ui εε have been measured;
- three peaks are well evident at about 200, 400, and 600 cm-1
- shoulder at around 600 cm-1 disappear for x=0.00 ω1 A1g out-of-phase rotation of (Mn/Cu)O6
εε ⊥ N.B. disappears for uiω2 Eg ω3 Eg (?) ω4 Eg (?)
• A1g shift shows linear dependence on tolerance factor
• increasing x cause the system to be more distorted
In order to remove such ambiguities on
the ω3 and ω4 assignment, it is necessary to :
• take measurements as a function of T
• extend the measurements to other system
La1-yAyMn1-xMxO3
A=Sr, M= Cr, Zn; A=Ba, M= Zn, Sc further studies are also highly desirable
• measure IR reflectivity spectra (Belgrad)
ellipsometry in the NIR and vis. (Barcelona)