The effect of the disorder induced by Cu substitution on the phonon properties of La 1-y A y Mn 1-x Cu x MnO 3 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 A 1-x B x MnO 3 manganites � Colossal MagnetoResistance (CMR) ρ − ρ ∆ ρ = = , ρ 0 = ρ =(H=0) H 0 MR ρ ρ 0 0 r r • MR < 0 and isotropic (no J H dependence) 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 Technological interest By varying x and T manganites show: MagnetoResistance is “Colossal”, • Different typologies of spin ordering because: • Permalloy (Ni/Fe),EuO MR ≈ 2-3% • M-I and PM-FM transitions • Multilayer Cu/Co GMR ≈ 40% • Jahn-Teller distortions and polaronic effects • Manganites CMR ≈ 100% • Charge ordering • Orbital ordering 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: A = rare earth +3 (La, Pr, Y, Nd,…) A 1-x B x MnO 3 B = divalent ion (Ca, Sr, Ba, …) Oxygen Jahn-Teller theorem (1937): Example: La-Ca-Mn-O "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"
Electronic Configuration Neutral Mn electronic configuration = [Ar]3d 5 4s 2 - In AMnO 3 � Mn is trivalent (ionic approx.) � 4 d-electrons will be responsible for its electronic properties (AFM) Mn 3+ in octahedral co-ordination - In BMnO 3 � Mn is tetravalent � 3 d-electrons present (AFM) - For a partial substitution case, A 1-x B x MnO 3 (0 < x < 1), Mn ions are mixed-valent. An amazing thing is that, although AMnO 3 and BMnO 3 manganites are AFM insulators, at some intermediate composition A 1-x B x MnO 3 exhibits CMR :
Rich Phase Diagram by varying x and T, manganites show: x=0, and 1 � AFM insulators x > 0.5 � Charge-Ordering ≈ 0.2 ÷ 0.5 � CMR (not Pr 1-x Ca x MnO 3 ) First explained by Zener (1951), Anderson & Hasegawa in the framework of the: ∀ high T � Paramagnetic Insulator (PI) x Double-Exchange Model
Double-Exchange v L L ( ) ∑ ∑ v = − + + − ⋅ σ + H t c c H . c . J S c c σ σ ij i j H i ab ia ib σ ij , i , ab v v L ∑ + ⋅ J S S AF i j i , j Whitin the solution for two classical spins* � the following relationship holds: t( Θ ) = t cos( Θ /2) Wherever an Mn 3+ and Mn 4+ are in neighbouring Mn sites, there exists the possibility of e g -electron hopping from the lowering T PM � FM ↔ I � M Mn 3+ to the Mn 4+ via the oxygen anion. Two simultaneous electron hops are required � Mn 3+ onto raising H spin alignment � I � M O 2- and O 2- onto Mn 4+ *Anderson and Hasegawa, De Gennes
DE explain qualitatively the experiments, but… Millis et al.: “ Double-Exchange alone One has to consider the el-ph interaction, due in part to the JT splitting of the Mn e g doesn’t explain the resistivity of La 1-x Sr x MnO 3 ” states. Phys. Rev. Lett 74, 5144 (1995): 1 ∑ ∑ ( ) ( ) ˆ ˆ = + + + ab 2 H H g d Q j d k Q j • DE overstimate T c one order of σ σ DE ja jb 2 magnitude(1000-3000 K ) the competition between localisation and DE can be • T dependence of ρ (T) is completely parameterised by an effective el-ph constant: different at T<T c g E λ = = loc • Experimental values for ρ are lerger than kt t eff that predicted by DE theory results are in agreement with experimental data (by Schiffer et al.)
Optical phonons in CMR Manganites Let us consider LaMnO 3 manganite The JT effect distorts the octahedra, and Theory group analysis for the undistorted (cubic) the structure is orthorhombic: perovskite gives the following irreducible 16 (Pnma): D 2h representation: 60 phonons (k=0) are predicted (Iliev,98) Γ = 4F 1u +F 2u O 1h (Pm3m) 3 IR active (F 1u ) 25 IR active � 9B 1u +7B 2u +9B 3u 3 acoustic (F 1u ) 24 Raman � 7A g +5B 1g +7B 2g +5B 3g 1 silent (F 2u ) 8 Silent � A u NO Raman active 3 Acoustic � B 1u +B 2u +B 3u
Phonon Assignment for the undoped LaMnO 3 IR Measurements Raman Spectra Ilev et al., PRB 57, 2872 (1998) De Marzi et al. PRL (98)
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 • M1 corresponds to an A g out-of- phase x-rotation of the oxygen increasing doping � JT reduction � cage “more cubic” structure � extremely • M2 is A 2g (mainly bending) small Raman scattering efficiency � • M3 is B 2g (mainly stretching) difficult measurements AE Pantoja, HJ Trodahl, J. Phys.: Cond. Matt. 13 (2001) 3741
Our samples: polycrystalline La 1-y Sr y Mn 1-x Cu x O 3 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 MnO 6 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 La 1-y Sr y Mn 1-x Cu x O 3 • samples with Cu doping � 0 < x < 0.10 La 1-y Sr y Mn 1-x Cu x O 3 %Mn 4+ • Tc is reduced by B-site substitution: x y T C • Structure rhombohedral R-3c (x-ray 0.00 0.300 32 372 analysis by Sapiña et al.) 0.02 0.274 32 358 0.04 0.248 32 331 • Single phase compounds 0.06 0.222 32 308 0.08 0.196 35 274 0.10 0.170 32 236 • the ratio Mn 4+ /(Mn 4+ +Mn 3+ ) = 0.3 is fixed Table 1: nominal compositions for x, y doping, % of � the effects are not due to DE tetravalent Mn ions, and observed T c [13] mechanism.
Raman spectra of La 1-y Sr y Mn 1-x Cu x O 3 polarized configuration cross-polarized configuration La 1-y Sr y Mn 1-x Cu x O 3 T = 300 K La 1 -y Sr y Mn 1-x Cu x O 3 T = 300 K x y T x y T C C 0.00 0.300 372 0.00 0.300 372 0.02 0.274 358 0.02 0.274 358 0.04 0.248 331 0.04 0.248 331 0.06 0.222 308 0.06 0.222 308 0.08 0.196 274 0.08 0.196 274 0.10 0.170 236 0.10 0.170 236 Raman Intensity Raman Intensity 0 200 400 600 800 1000 0 200 400 600 800 -1 ) Raman Shift (cm -1 ) Raman Shift (cm - three peaks are well evident at • the first peak at about 200 cm -1 completely about 200, 400, and 600 cm -1 - shoulder at around 600 cm -1 disappear for x=0.00 disappears - a strong background signal is present
Data analysis • Spectra were fitted in the 130-900 cm -1 range with six lorentzian oscillators • An example of the best fit curve and the LSM6 julio different components is shown for the Raman Intensity (a.u.) x=0.10 sample; • four peaks were found at 180-215, 430, cm -1 , 498, and 670 and a broad background with a maximum at 450 cm -1 . 0 200 400 600 800 -1 ) Raman Shift (cm
Data analysis Granado et al., PRB 58, 11435 (98): 1200 A 1100 La 1-y Sr y Mn 1-x Cu x O 3 1g E g 1000 A g 900 B 2g 800 700 600 -1 ) 500 Frequency (cm 400 300 220 210 200 190 ω 1 � A 1g N.B. disappears for ε ⊥ ε 180 0 0.02 0.04 0.06 0.08 0.10 i u X doping 6 )= A 1g + 2A 1u + 3A 2u +4E g +5E u +3A 2g ω 2 � E g Γ (D 3d Why didn’t Granado et ω 3 � E g (?) al . see ω 3 and ω 4 ? 8 IR active � 3A 2u +5E u ω 4 � E g (?) 5 Raman � 1A 1g +4E g � polishing effect
Analysis of the A 1g mode Raman shift of A 1g is unusual. infact Tolerance factor + r r d Sr 87 is lighter than La 139 � shift toward lower ω! = − = A O A O t ( ) + 2 d 2 r r − B O B O But we observe just the opposite � - t<0.925 orthorhombic; 0.925<t<0.1 rhombohedric; t=1 cubic A 1g is not an external - since r(Sr 2+ ) > r(La 3+ ) � mode when x increases (and y decreases) � <r A > decreases - Moreover, <r B > is increased by Cu substitution Infact, A 1g involves the motion of the oxygen cage * • band shift is a x=0.10 215 linear function -1 ) 210 Raman Shift of the A 1g mode (cm of t 205 � ω 1 is sensitive to JT distortions • increasing x 200 causes the 195 (that are modified by Sr doping) system to be 190 more distorted x=0.00 185 0,968 0,970 0,972 0,974 0,976 0,978 0,980 Tolerance Factor * Irwin et al. PRB 59 , 9362 (1999)
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