Indranil Das ECMP Division Saha Institute of Nuclear Physics, Kolkata

1. Magnetism and Charge Ordering in Manganite Nanoparticles Indranil Das ECMP Division Saha Institute of Nuclear Physics, Kolkata E-Mail: indranil.das@saha.ac.in

2. Anis Biswas — Ph. D. work Acknowledgements : Plan of talk : Introduction Modification of Charge ordering in Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 nanoparticles Observation of Charge ordering in Pr0.65Ca0.35MnO3 nanoparticles Phase co-existence and Low Field Magnetoresistance in Pr0.65(CaySr1-y)0.35MnO3 nanoparticles

3. 2+ 2- A A R,T B  Mn O  O   B R1-xTxMnO3 O   3+ 3+/4+ Perovskite Structure Ref: Y. Tokura et al, JMMM, 200, 1, 1999 Orthorhombic distortion Mn – O - Mn Super exchange  AFM Double exchange  FM

4. Ref:Rev.Mod.Phys. 73, 583 (2001) Ref:PRB, 60, 12191 (1999) Nd0.5Sr0.5MnO3 Pr0.5Sr0.5MnO3

5. Magnetic Field X-ray irradiation Electric Field Charge ordering in manganites :  Real space ordering of Mn3+, Mn4+  Onsite Coulomb repulsion predominant over kinetic energy of electrons CO Molten state CO State CO Melting by Magnetic Field Large MR Ref: Y. Tokura et al, JMMM, 200, 1, 1999

6. Martensitic transition: • Cooperative motion of atoms resulting in a formation of different crystal structure in parent crystal. • Strain (martensitic strain) developed due to the nucleation of new crystal structure • The nucleation of the martensitic crystal depends on the structure of the martensitic crystal and the structure of parent crystal • The accomodation of martensitic strain is sensitive to grain size. • The grain boundary regions act as barrier to propagate the martensitic strain throughout the parent crystal Podzorov et al. [Phys.Rev.B, 64,R140406 (2001)] Polarized optical microscopy study “Charge order transition is martensitic-like transition” `Reduction of grain size has influence on martensitic transition’ Ref: ‘Martensitic Transformation, Edited by M. Fine, M. Meshi, C. Wayman (Academic, 1978)

7. A)Para to Ferro transition & Ferro to CO [ Pr1-xSrxMnO3, Nd1-xSrxMnO3 ] B) Para to Ferro absent, CO transition present • [ Pr1-xCaxMnO3 ] • Para to CO & CO to FM transition • [ Pr0.65(CaySr1-y)0.35MnO3 ] Systems • XRD • TEM, HRTEM, ED • SEM Characterization • Transport measurement • Magnetotransport measurement • Specific heat measurement • I-V characteristics study • Magnetization measurement (SQUID magnetometer) Study

8. For bulk, TC ~ 270 K, TCO/TN ~ 140 K Pr0.5Sr0.5MnO3 Sample Preparation (A) Bulk (Solid-state reaction at 15000 C for 48 hrs) Pr6O11 + SrCO3 + MnO2Pr0.5r0.5MnO3 (B) Nanoparticles (Sol-gel Technique) (Pr6O11 , MnO2 nitrates) + SrNO3 + Citric acid  Gel Gel  (Decomposed at 1000C)  Porous Powder (annealing at different temperatures for 4 hrs)  sample Single phases are confirmed by XRD

9. For bulk sample, insulator to metal transition coincides with PM to FM transition at ~ 270 K and CO transition occurs below ~ 140 K • for nanoparticles, insulator to metal transition absent • FM to antiferromagnetic CO transition is absent for nanoparticles • Below 140 K, M for 30 nm particle is larger than for 45 nm • M(H) curves show that M for 30 nm particles larger than that for 45 nm at 3. K, however , at 225 K, situation reverses. • No hysteresis in M(H) curve at 3.3 K for nanoparticles.

10. For the bulk sample, MR increases abruptly below ~ 140 K melting of CO state by magnetic field. For nanoparticles, MR increases gradually with lowering T. • At low temperature, MR for 45 nm particles is larger for 30 nm particles. • In high field region, MR for 45 nm particles > MR for 30 nm particles at 3.3 K.

11. Pr0.5Sr0.5MnO3 (average particle size~ 30 nm and 45 nm) • Ferromagnetic transition ~ 270 K (same as the bulk sample) • Charge Order transition is invisible down to 3.3 K. • There may exist a little fraction of charge ordered state at low temperature. This fraction is larger in larger particle sized sample Anis Biswas, I. Das et al., J. Appl. Phys.,98, 124310 (2005)

12. 100 nm 100 nm For bulk, TC ~ 255 K, TCO/TN ~ 160 K Nd0.5Sr0.5MnO3 Nanoparticles are prepared by sol-gel method Single phases are confirmed by XRD SEM micrograph of sample prepared at 8000 C TEM micrograph of sample prepared at 8000 C

13. For nanocrystalline samples, insulator to metal transition temperature (TIM) is lower than bulk. For 55nm particles TIM ~ 150 K and for 30 nm particles, TIM ~ 100 K. For bulk TIM ~ 255 K (coincides with TC) • Up-rise of resistivity with lowering temperature below ~ 30 K • Value of MR increases quite sharply with lowering temperature below ~ 30 K

14. TC ~ 235 K (lower than bulk). TIM does not coincide with TC • No clear signature of FM to antiferromagnetic CO transition

15. No signature of charge ordering in low temperature specific heat data. • Schottky- like anomaly below 15 K • No charge ordering, but hysteresis and negative differential resistance (NDR) in I-V characteristics ! This type of I-V characteristics can be explained by models of self-heating, Ref:: 1. Fisher et al., APL, 88, 152103 (2006). 2. Chen et al., APL, 88, 222513 (2006).

16. Non-linear I-V at low temperature • Differential conductance,G (V) can be well fitted according to Glatzman and Mateev theory (up to v2.5): G(v) = 0 +1v1.33+2v2.5  Inelastic tunneling of electron through localized states in grain boundary region • Low field magnetoresistance due to increase of tunneling probability in the presence of magnetic field

17. Nd0.5Sr0.5MnO3 (average particle size~ 30 nm and 55 nm) • TC ~ 230 K (Lower than bulk) • Charge Order transition is invisible down to 2K • The insulator –metal transition (TIM) does not coincide with TC, it sifts to lower temperature region due to the grain boundary effect Anis Biswas and I. Das (Communicated)

18. Pr1-XCaXMnO3 • In case of Pr1-XCaXMnO3 ,charge ordering occurs for a concentration range 0.3<X<0.5. Charge ordering transition temperaturevaries from 225 K to 240 K depending on the value of X. • C-E type antiferromagnetic transition occurs below charge order transition temperature. A canted anti ferromagnetic state is also observed well below TN for concentration range 0.3<X<0.4. Ref: Urushihara et al., PRB, 51, 14103 (1995) For bulk , TCO ~ 225 K, TN ~ 145 K, TCA ~ 100 K Pr0.65Ca0.35MnO3

19. SAMPLE PREPARATION & CHARACTERIZATION Pr0.65Ca0.35MnO3 (Sol-gel Technique) Pr6O11 , MnO2 , CaCo3 Nitrates + Citric acid  Gel Gel (Decomposed at 1000C) Porous Powder Porous Powder  (Heat treated at 10000 C for 6 hr)  sample Single phases are confirmed by XRD Crystal Structure  Orthorhombic, pbnm symmetry, (a = 5.420Å, b = 5.449Å, c = 7.660Å) Average Particle size of Sample ~ 40 nm(determined by TEM measurement) Average crystallite size ~ 36 nm (determined from FWHM of XRD) Thanks to,P. Roy of TEM facility section, SINP for TEM, Dr. P. V.Satyam and J. Ghatak of IOP, Bhubaneswar for HRTEM

20. Observation of Charge Ordering in Nanoparticle System Pr0.65Ca0.35MnO3 ~ 40 nm • CO has been observed • TCO ~ 225 K (close to bulk) • Super-lattice spots in low temperature ED pattern T=120 K Thanks to Dr. P. V.Satyam and Mr. J. Ghatak of IOP, Bhubaneswar for Low Temperature Electron Diffraction measurements.

21. Tetragonal to Monoclinic Pr0.5Sr0.5MnO3 Invisibility of CO Nd0.5Sr0.5MnO3 Orthorhombic to Monoclinic Orthorhombic to pseudo-tetragonal Pr0.65Ca0.35MnO3 In case of Pr0.65Ca0.35MnO3, the “martensitic” crystal structure is more symmetrical w.r.t the parent crystal structure CO is possible in nanoparticles Martensitic like character of CO transition plays key role Observation of Charge Ordering in Nanoparticle System Anis Biswas and I. Das Phys. Rev. B.,74, 172405 (2006) V J Nanoscale Sci. & Tech., 2006

22. Pr0.65(CaySr1-y)0.35MnO3 • Substitution of Sr in Ca –site, causes the spontaneous destabilization of CO state at low temperature. • Up to y~0.65, TCO >TC • Phase-coexistence occurs over large length scale (~ m) Ref: G.B.Blake et al., PRB, 66, 144412 (2002) • For y~0.6, below 200 K, CO state co-exist with the FM state. • For y~ 0.7, CO transition at ~ 220 K and then co-mixture of CO & FM state below ~100 K. • With decreasing temperature, the fraction of CO state decreases Pr0.65(CaySr1-y)0.35MnO3, y ~ 0.6, 0.7

23. Pr0.65(Ca0.6Sr0.4)0.35MnO3 Bulk Sample • Large hysteresis below TIM Admixture of Co & FM state • TIM almost coincides with Tp LFMR decreases with decrease of T Origin of LFMR is CO melting 

24. Pr0.65(Ca0.6Sr0.4)0.35MnO3 (Nanoparticles) 80 nm • Same trend as bulk 60 nm

25. Pr0.65(Ca0.6Sr0.4)0.35MnO3(~ 60 nm) • TC ~ 215 K, almost same as TIM • Large irreversibility between FCC and FCW curve at H = 5 kOe (similar as bulk sample)due to the coexistence of CO and FM state. • The loop area of M(H) curve decreases with decreasing temperaturethe fraction of CO phase decreases with temperature and at low temperature almost one phase (FM) survives.

26. Pr0.65(Ca0.6Sr0.4)0.35MnO3~ 40 nm • TIM ( ~ 125 K) shifts to the lower temperature region in comparison with the bulk sample • There exist large hysteresis in the temperature dependence of resistivity like the bulk sample  indication of co-existence of FM & CO phase • The hysteresis is also observed between temperature dependence of FC warming and FC cooling susceptibility curve in presence of 5 kOe magnetic field , which is another indication of co-existence of FM & CO phase.

27. Pr0.65(Ca0.6Sr0.4)0.35MnO3~ 40 nm MR vs.H at different T • There exist hysteresis in MR(H) curve at the temperatures below TIM  indication of co-existence of FM & CO phase • Below TIM Low Field MR (LFMR) is quite large and it decreases with decrease of temperature as observed in case of bulk sample  LFMR below TIM arises mainly due to the melting of CO state by magnetic field and decreases with decreasing temperature because the fraction of CO state decreases. Co-existence of CO & FM phases even in 40 nm particles !

28. Pr0.65(Ca0.7Sr0.3)0.35MnO3 (Nanoparticles) • CO transition is observed at ~ 220 K for 70 nm particle sized sample • Transition temperature is almost same as bulk • At low temperature spontaneous destabilization of CO state occurs and as a result co-mixture of CO & FM phase is observed.

29. Pr0.65(Ca0.7Sr0.3)0.35MnO3(Nanoparticles) • There exists hysteresis in MR(H) curve at the temperatures below TIMindication of co-existence of FM & CO phase • Below TIM , LFMR is quite large. • LFMR below TIM arises mainly due to the melting of CO state by magnetic field, with decreasing temperature fraction of CO state decreases  Below TIM LFMR decreases with decreasing of T The admixture of CO & FM state exist below TIM and the origin of large LFMR is the melting of CO state.

30. Pr0.65(CaySr1-y)0.35MnO3 During cooling from room temperature, at transition • Lattice cell volume decreases • Crystal symmetry (Pnma) remain unchanged Ref: PRB, 66, 144412 (2002) Existence of CO state along with FM state possible in nanocrystalline sample

31. Pr0.65(CaySr1-y)0.35MnO3, y ~ 0.6, 0.7(Nanoparticles) • In case of Pr0.65(CaySr1-y)0.35MnO3, the coexistence of FM and CO state has been observed for the nanocrystalline sample of average particle size down to ~ 40 nm below insulator to metal transition temperature. • For y ~ 0.7, clear CO transition is observed at ~ 220 K for 70 nm similar to the bulk sample. • Quite large LFMR at low field has been obtained below TIM for the samples mainly due to the melting of CO state by magnetic field. • As temperature decreases from TIM, the fraction of COphase decreases and LFMR also decreases. • The value of LFMR may be tuned by varying particle size. Anis Biswas and I. Das (Communicated)

32. Summary • Charge ordering can occur in nanoparticles  martensitic like character of transition plays the dominant role in CO • Phase coexistence (CO & FM) is possible even in nanoparticles • Large LFMR can also originate due to CO melting by magnetic field

33. Thank You

35. Other works on PrCa- system: • Transport, magnetotransport and magnetic properties have been studied for Pr0.65Ca0.35(Mn0.9Fe0.1)O3 in both bulk and polycrystalline form CO transition disappears FM transition occurs at ~ 90 K MR is enhanced in nanoparticles and shows ‘–H2’ dependence • A comparative study of MR and magnetocaloric effect have been performed on Nanocrystalline Pr0.65(Ca0.6Sr0.4)0.35MnO3 Similar Magnetic field depedence of MR and ∆S above transition temperature and this similar dependence disappears below the transition temperature (to be communicated)

36. La0.125Ca0.875MnO3 • CO has been observed for x = 0.5 to 0.875 • X=0.875 is a phase boundary between CO & CAF region • For y~0.875, TCO ~ 125 K • For y ~ 0.875, possibility of existence of both CO & CAF phase Ref: S.W. Cheong et al., in Review: Collossal Magnetoresistive Oxides, Ed. By Y. Tokura (Gordon and Breach Science Publishers)

37. La0.125Ca0.875MnO3 • Sample shows insulating behavior in bulk and nanocrystalline form • There exist sharp increase of resistivity below ~ 125 K for the bulk sample. For nanoparticles, resistivity increases not so sharply. • There exist sharp peak in Cp (T) curve for bulk at ~125 KCO For the sample, prepared at 12000 C, peak is broad

38. La0.125Ca0.875MnO3 MR = [R(H)-R(0)]/R(H) (%) • At 50 K and 5 K, for bulk, the value of MR is also quite small • MR is enhanced in nanoparticles For nanoparticles, magnetic field can easily make the electrons itinerant CAF phase dominants Incomplete !!!!!! Magnetization measurements required !!!!