LOCAL PROBE STUDIES IN MANGANITES AND COMPLEX OXIDES V.S. Amaral 1, A.M.L. Lopes 2, J.P. Araújo 3, P.B. Tavares 4, T.M. Mendonça 3,5, J. S. Amaral 1, J.N. Gonçalves 1,5, J.G. Correia 5,6 and IS390 Collaboration 1 Dep. Física and CICECO, Univ. Aveiro, Aveiro, Portugal; 2 CFNUL, Univ. Lisboa, Lisboa, Portugal; 3 Dep. Física and IFIMUP, Univ. Porto, Porto, Portugal; 4 CQ-VR, UTAD, Vila Real, Portugal; 5 CERN EP, CH 1211 Geneve 23, Switzerland; 6 ITN, E.N. 10, P-2685 Sacavém, Portugal

OUTLINE Manganites and intrinsic complexity Charge-ordering scenarios, phase separation Multiferroic systems Local Probe Studies Hyperfine technique: Perturbed Angular Correlations Probing electric fields in Pr 1-x Ca x MnO 3 system Phase separation, polaron dynamics in LaMnO 3+  and La 1-x Ca x MnO 3 Multiferroics: RMnO 3 Other oxide systems Perspectives

Manganites Electron doping (charge and orbital) Structure Magnetic Interactions AMnO 3 ( A=La, Pr, Nd &Ca, Sr...) base structure perovkite Mixed valence: Mn 3+ : Mn 4+ Doping with divalent ions (R-D)MnO 3 Mn 3+  3d 4 La 3+ O 2- egeg t 2g  crystal field splitting JT effect If only Mn 3+  collective JT distortion O Introduction to Manganites

Pr 1-x Ca x MnO 3 Magnetic - Electric Phase Diagram CO/OO signatures Small Ionic radii Pr 3+ =1.30 Å, Ca 2+ =1.34 Å -Orthorhombic structure for all x Small overlap between Mn & O orbitals -Insulator for all x Charge Order is stabilized over a broad range of compositions Introduction to Manganites Mn 3+ /Mn 4+ localization in a ordered way

Phase separation and complexity Atomic-scale coexistence Renner et al, Nature 416, 518 (2002) Bi 0.24 Ca 0.76 MnO 3 Charge contrast: occupied and unnocupied states Clusters with CE type arrangement in the paramagnetic phase Ma et al, Phys Rev. Lett 95, (2005) (La,Pr) 5/8 Ca 3/8 MnO 3 20x10 nm

OUTLINE Manganites and intrinsic complexity Charge-ordering scenarios, phase separation Multiferroic systems Local Probe Studies Hyperfine technique: Perturbed Angular Correlations Probing electric fields in Pr 1-x Ca x MnO 3 system Phase separation, polaron dynamics in LaMnO 3+  and La 1-x Ca x MnO 3 Multiferroics: RMnO 3 Other oxide systems Perspectives

Multiferroic materials Inversion symmetry breaking (charge-orbital related) Dislocated spin density waves Magneto-electric coupling Nature Materials 3, 164, (2004) Tb/YMn 2 O 5 Nature, 429, 392 (2004) Magnetic field induces a sign reversal of the electric polarization

Multiferroic manganites Van Aken, thesis univ. Groningen Perovskite phases octahedra Hexagonal phase Trigonal bipyramid AMnO 3

Ferroelectricity due to Charge Ordering in Pr 1-x Ca x MnO 3 New paradigm for ferroelectrics, but it has been hard to prove experimentally that electric polarization exists in CO Pr 1-x Ca x MnO 3 (due to finite conductivity) New scenario for the Charge Ordered State Theoretical work predicts the possibility of local electric dipole moments in CO manganites Ferroelectricity Inversion symmetry is broken = Site centred COBond centred CO + D.Efremov et al, Nature Materials, 3,853 (2004) J. Van den Brink, D.I. Khomskii, J. Phys. Cond. Matt 20, (2008) Charge ordered manganites

OUTLINE Manganites and intrinsic complexity Charge-ordering scenarios, phase separation Multiferroic systems Local Probe Studies Hyperfine technique: Perturbed Angular Correlations Probing electric fields in Pr 1-x Ca x MnO 3 system Phase separation, polaron dynamics in LaMnO 3+  and La 1-x Ca x MnO 3 Multiferroics: RMnO 3 Other oxide systems Perspectives

Two-Photon PERTURBED ANGULAR CORRELATION Hyperfine splitting  Electric field gradient / Magnetic hyperfine field V zz - EFG principal component  - Asymmetry parameter B hf - Magnetic hyperfine field |I i, m i 〉 , Q L2L2 |I, m 〉 |I f, m f 〉 11 22 L1L1  EE EiEi E EfEf Local Probe Studies / Technique Time dependence gives access to splitting of hyperfine levels Probe nucleus: Q – quadrupolar moment  - magnetic moment

Two-Photon PERTURBED ANGULAR CORRELATION Samples implanted with radioactive 111m ISOLDE/CERN E=60 keV, Dose < at/cm 2 111m Cd: I=5/2 and t 1/2 = 85 ns Q=0.83 b and  =  n Samples T=700 ºC in air CdPr/Ca Pr/Ca site Local Probe Studies / Technique

PROPOSAL TO THE ISOLDE COMMITTEE – INTC/P132 Add1 IS390-Studies of Colossal Magnetoresistive Oxides with Radioactive Isotopes Aveiro 1, Dublin 2, Leuven 3, Lisboa 4, Moscow 5, Orsay 6, Porto 7, Sacavém 8, Stuttgart 9, Tokyo 10, Tsukuba 11 Vila Real 12 and the ISOLDE/CERN 13 Collaboration Spokesman: V. S. Amaral Contact person: J.G. Correia E. Alves 8, T. Agne 13, J.S. Amaral 1, V.S. Amaral 1, J.P. Araújo 7, J. M. D. Coey 2, N. A. Babushkina 5, J.G. Correia 8,13, H.-U. Habermeier 9, A. L. Lopes 1, A.A.Lourenço 1, J.G. Marques 4,8, T. M. Mendonça 7, M. S. Reis 1, E. Rita 8, J.C. Soares 4, J.B. Sousa 7, R. Suryanarayanan 6, P.B. Tavares 12, Y. Tokura 10,11, Y. Tomioka 11, A. Vantomme 3, J.M. Vieira 1 and U. Wahl 8.

OUTLINE Manganites and intrinsic complexity Charge-ordering scenarios, phase separation Multiferroic systems Local Probe Studies Hyperfine technique: Perturbed Angular Correlations Probing electric fields in Pr 1-x Ca x MnO 3 system Phase separation, polaron dynamics in LaMnO 3+  and La 1-x Ca x MnO 3 Multiferroics: RMnO 3 Other oxide systems Perspectives

x = 0.25 (NO ≠ Temperature ≠ Ca (x) RT EXPERIMENTAL RESULTS Pr 1-x Ca x MnO 3 PAC Anisotropy Functions Local Probe Studies / Pr 1-x Ca x MnO 3 system

@ 896K V zz increases with decreasing T due to atomic vibrations (rms displacements) High temperature linear slope ~ -1.5(1)  K -1 Temperature dependence for samples outside CO region EFG principal component V zz Local Probe Studies / Pr 1-x Ca x MnO 3 system

Samples within CO region Anomalous V zz (T) dependence approaching CO transition Charge order driven by the softening of a vibration mode? Softening of vibration modes? Same high temperature linear slope ~ -1.5(2)  K -1 Local Probe Studies / Pr 1-x Ca x MnO 3 system

T CO CO region, V zz vs Temperature T*T* Local Probe Studies / Pr 1-x Ca x MnO 3 system Sharp increase of V T< T CO T* (85 to 67 V zz /Å -2 in 2K)

T CO CO region, V zz vs Temperature T*T*T CO T*T* T*T* Local Probe Studies / Pr 1-x Ca x MnO 3 system Sharp increase of V T< T CO T*

EFG and magnetic susceptibility Local Probe Studies / Pr 1-x Ca x MnO 3 system

From NMR and PAC studies in Ferroelectrics Susceptibility Increase Local Spontaneous Polarization TCTC EFG sensitive to atomic vibrations and critical fluctuations Local Probe Studies / Pr 1-x Ca x MnO 3 system

Landau Theory: first order phase transitions T0T0 TcTc T<T c T>T c To model V zz (T) Local Probe Studies / Pr 1-x Ca x MnO 3 system

compatible with first-order dielectric phase transition below T co Results Electric susceptibility / spontaneous polarization below T C x=0.35 ->T EDO =206 K, x=0.4 ->T EDO =218 K, x=0.85 -> T EDO =112 K Local Probe Studies / Pr 1-x Ca x MnO 3 system A.M.L. Lopes et al., Phys. Rev. Lett. 100, (2008)

Piezoresponse Force Microscopy-hysteresis loop at room temperature: The piezoelectric contrast points to the existence of a local polar state Piezoelectric Force Microscopy in Pr 1-x Ca x MnO 3 system A. L. Kholkin, I.K. Bdikin, CICECO; Univ. Aveiro Local Ferroelectric response in Pr 0.6 Ca 0.4 MnO 3 single crystal

OUTLINE Manganites and intrinsic complexity Charge-ordering scenarios, phase separation Multiferroic systems Local Probe Studies Hyperfine technique: Perturbed Angular Correlations Probing electric fields in Pr 1-x Ca x MnO 3 system Phase separation, polaron dynamics in LaMnO 3+  and La 1-x Ca x MnO 3 Multiferroics: RMnO 3 Other oxide systems Perspectives

LaMnO 3+  Orthorhombic (O’) Strong JT distortion Antiferromagnetic Rhombohedric (R) No collective JT distortion Ferromagnetic  Mn 3+ / Mn 4+ MIX-VALENCE Only Mn 3+  collective JT distortion Doping Mn  / Mn 4+ 2  MIX-VALENCE (La 3/(3+  ) Mn 3/(3+  ) O 3 )

PAC anisotropy RT  and V zz as a function of  Orthorrombic  Rhombohedric LaMnO 3+    

LaMnO 3.12 Polarons ? Polaron nature and evolution? Polarons responsible for FM INSULATOR state? Ferromagnetic Insulator T<T C Rhombohedric  Orthorhombic phase transition ~ RT Phase coexistence in a broad range of T

PAC anisotropy ≠ T Orth Rho Orth+Rho 318 K 197 K Rho crystallographic phase (x-ray) Orth crystallographic Phase (x-ray) MHF for T<T C (T C =145 K) Coexistence of two local environments for all T Macroscopic Rho + Orth phase coexistence (x-ray)

% of u and d local environments RhoOrt % Ort High T  X-rays do not see distorted phase Random distributed polaron clusters Strong JT distortions in the Rho. Phase Polarons clusters start to T*>776 K R+O x-ray Smooth variation f u (and f d ) Percolation threshold Percolation of the local environments 31% Below percolation threshold distortions accommodate in a weakly JT distorted phase  and V zz vs T

Attenuation parameters for u and d env. e e T< T C u and d phase FM  dominant phase has an ultra slow dynamics imposing an insulator behavior in the system E a =0.31 eV Arrhenius plot of d E a ~ Polaron binding energy Thermal activated polaron hopping Polaron residence time   rd ~0.5  s Ultra slow polaron dynamics Small mobility of charge carriers

Lightly doped La-Ca manganite The same physics? R to O* transitions on cooling Intermediate JT- orbital ordering transition to O structure J. B. Goodenough, 2003

La 0.95 Ca 0.05 MnO 3 T>900K One fraction; low  T<900K Two fractions: high/low Vzz Change ratio at T~750K Only minoritary fraction sensitive to JT transition T=900K R to O* transition

La 0.95 Ca 0.05 MnO 3 PAC very sensitive to charge distribution changes: Disproportionation?

OUTLINE Manganites and intrinsic complexity Charge-ordering scenarios, phase separation Multiferroic systems Local Probe Studies Hyperfine technique: Perturbed Angular Correlations Probing electric fields in Pr 1-x Ca x MnO 3 system Phase separation, polaron dynamics in LaMnO 3+  and La 1-x Ca x MnO 3 Multiferroics: RMnO 3 Other oxide systems Perspectives

RMnO 3 vs. R ionic radius Single phase RMnO 3 samples (excepting LuMnO 3 with traces of Lu 2 O 3 ) Lu 2 O 3

YMnO 3 vs. Temperature Paramagnetic Ferroelectric AFM FE Paraelectric Two EFG’s present in YMnO 3 samples EFG1 f 1 ~70% w0~120 Mrad/s EFG2 f 2 ~30% w0~180 Mrad/s Hexagonal structure

EuMnO 3 vs. Temperature Two EFG’s present in EuMnO 3 samples EFG1 f 1 ~35-40% w0~120 Mrad/s EFG2 f 2 ~60-65% w0~180 Mrad/s Inversion of the main EFG below 20K AFM Paramagnetic and Paraelectric Orthorhombic structure

OUTLINE Manganites and intrinsic complexity Charge-ordering scenarios, phase separation Multiferroic systems Local Probe Studies Hyperfine technique: Perturbed Angular Correlations Probing electric fields in Pr 1-x Ca x MnO 3 system Phase separation, polaron dynamics in LaMnO 3+  and La 1-x Ca x MnO 3 Multiferroics: RMnO 3 Other oxide systems Perspectives

AgCrO 2 ( 111 In probe): a new multiferroic 294.5K 48.6K 21.4K 19.2K

Ag 2 NiO 2 and analogous: Orbital physics vs Charge Order Like AgCrO Ag and 111 Cd/In probes for the 2 sites: more complete mapping of charge distributions Valence(charge) changes in Ag and Ni avoiding Jahn-Teller distortion

CONCLUSIONS - Combined with other techniques and theoretical modeling, PAC brings new insights on current cutting edge research on solid state physics: correlated electrons systems. -The availability of different probes at Isolde allows tackling different situations: chemical compatibility and environments.