Abstract

The manganites of the type RMnO3 (R=La, Nd, Pr, Sm) are antiferromagnetic and the end (n= µ) members of the so-called Ruddlesden-Popper series, Rn+1MnnO3n+1. These oxide materials illustrate many interesting properties like colossal magnetoresistance (CMR)1-13, charge ordering (CO)14-21, magnetic field induced structural and ferromagnetic transitions22-24 when R is partially substituted by divalent cation A (=Ca, Sr, Ba, Pb) as R1-xAxMnO3. According our results of resistivity (r) and specific heat (C) on the Pr1-xCaxMnO3 series, we confirmed the Pr0.65Ca0.35MnO3 is the good choice to investigate the interplay of double exchange (DE) interaction and charge(CO)/orbital(OO) ordering.
A systematic study of r, magnetization (M) and C on polycrystalline Pr0.65Ca0.35-xSrxMnO3 (x=0-0.35) perovskite manganites has been reported. The T-x phase diagram presenting their electrical and magnetic properties is prevailed. The Pr0.65Ca0.25Sr0.1MnO3 (for x=0.1) sample is particularly unique showing a CO transition at TCO ~ 200K, an antiferromagnetic (AFM) ordering transition at TN ~ 175K, a metal-insulator (MI) transition at TMI ~ 80K and a ferromagnetic (FM) ordering transition at TC ~ 45K in the absence of magnetic fields. However, the C data of it do not show any anomaly at TMI for MI transition but illustrates a much smaller anomaly than expected one at TC and is suppressed by magnetic fields. This may indicate that the FM ordering in it, commonly related to MI transition, is of meta-stable characteristic and is ascribed to electronic and magnetic instability induced by spin fluctuations. This is established from the T-H phase diagram, as well as the thermal and magnetic hysteresis in r, M and C.

The manganites of the type RMnO3 (R=La, Nd, Pr, Sm) are antiferromagnetic and the end (n= µ) members of the so-called Ruddlesden-Popper series, Rn+1MnnO3n+1. These oxide materials illustrate many interesting properties like colossal magnetoresistance (CMR)1-13, charge ordering (CO)14-21, magnetic field induced structural and ferromagnetic transitions22-24 when R is partially substituted by divalent cation A (=Ca, Sr, Ba, Pb) as R1-xAxMnO3. According our results of resistivity (r) and specific heat (C) on the Pr1-xCaxMnO3 series, we confirmed the Pr0.65Ca0.35MnO3 is the good choice to investigate the interplay of double exchange (DE) interaction and charge(CO)/orbital(OO) ordering.
A systematic study of r, magnetization (M) and C on polycrystalline Pr0.65Ca0.35-xSrxMnO3 (x=0-0.35) perovskite manganites has been reported. The T-x phase diagram presenting their electrical and magnetic properties is prevailed. The Pr0.65Ca0.25Sr0.1MnO3 (for x=0.1) sample is particularly unique showing a CO transition at TCO ~ 200K, an antiferromagnetic (AFM) ordering transition at TN ~ 175K, a metal-insulator (MI) transition at TMI ~ 80K and a ferromagnetic (FM) ordering transition at TC ~ 45K in the absence of magnetic fields. However, the C data of it do not show any anomaly at TMI for MI transition but illustrates a much smaller anomaly than expected one at TC and is suppressed by magnetic fields. This may indicate that the FM ordering in it, commonly related to MI transition, is of meta-stable characteristic and is ascribed to electronic and magnetic instability induced by spin fluctuations. This is established from the T-H phase diagram, as well as the thermal and magnetic hysteresis in r, M and C.

Table of Contents

Abstract …………………………………………………………………………..i
Table of Contents ………………………………………………………………..ii
List of Figures ……………………………………………………………………iv
List of Tables ……………………………………………………………………..ix

Chapter Ⅰ : Introduction
1.1 Physical properties of R1-xAxMnO3…………………………..1
1.2 (Pr, Ca, Sr)MnO3……………………………………………..2

Chapter Ⅱ :Theory
2.1 Double Exchange…………………… ………………………5
2.2 Superexchange Interaction……………………………………7

Chapter Ⅲ : Experimental Details
3.1 Sample preparation……………………………….………….8
3.2 Sample characterization…………………………..………….9
3.3 Electrical resistivity……………………… ………………...11
3.4 High temperature specific heat system………….…………..12
3.5 X-ray absorption near-edge structure spectra (XANES)
technique…………………………………………………….15
3.6 Low temperature specific heat System……………………...16
3.7 Magnetoresistance and Magnetization……………………....18


Chapter Ⅳ : Experimental Results
4.1 Pr1-xCaxMnO3…………………………. ……………………..30
4.2 Pr0.65(Ca0.35-xSrx)MnO3. ……………………………………….30
4.3 Pr0.65Ca0.25Sr0.1MnO3…………………………………………..30

Chapter Ⅴ : Discussion
5.1 Pr0.65(Ca0.35-xSrx)MnO3. ………………………………………86
5.2 Pr0.65Ca0.25Sr0.1MnO3…………………………………………92

Chapter Ⅵ :Conclusions ………………………………………………………103

Reference ………………………………………………………………………...104


List of Figures:

Fig. 3.1. Flowing chart of sample preparation………………………………………19
Fig. 3-2. Flowing chart of operating X-ray diffraction measurement and parameters calculation………………………………………………………………...20
Fig. 3-3. Schematic diagram of resistivity specimen………………………………..21
Fig. 3-4. Flowing chart of resistivity measurement…………………………………22
Fig. 3-5. The structure of sample holder…………………………………………….23
Fig. 3-6. Flowing chart of specific-heat measurement………………………………24
Fig. 3-7. One can slid the inner lens barrel forward or backward to adjust the strength of the modulating light……………………………………………………25
Fig. 3-8. The measured photon flux of 6m-HSGM beamline……………………….26
Fig. 3-9. Schematic diagram of instruments in the Specific-Heat…………………...27
Fig. 3-10. Schematic diagram of the insert of the calorimeter……………………….28
Fig. 4-1. The powder X-ray diffraction patterns for the Pr1-xCaxMnO3 system (x=0.3-0.4)………………………………………………………………...34
Fig. 4-2. The temperature dependence of resistivity of the Pr1-xCaxMnO3 system (x=0.3-0.4)………………………………………………………………...35
Fig. 4-3. The temperature dependence of C of the Pr1-xCaxMnO3 system (x=0.3-0.4)...............................................................................................….36
Fig. 4-4. The powder X-ray diffraction patterns for the Pr0.65(Ca0.35-xSrx)MnO3 system (x=0-0.35)…………………………………………………………………37
Fig. 4-5. Refine of the XRD pattern of Pr0.65Ca0.35MnO3…………………………...38
Fig. 4-6. Refine of the XRD pattern of Pr0.65Ca0.3Sr0.05MnO3 ………………………39
Fig. 4-7. Refine of the XRD pattern of Pr0.65Ca0.275Sr0.075MnO3 …………………….40
Fig. 4-8. Refine of the XRD pattern of Pr0.65Ca0.25Sr0.1MnO3 …………………..…..41
Fig. 4-9. Refine of the XRD pattern of Pr0.65Ca0.225Sr0.125MnO3 …………………...42
Fig. 4-10. Refine of the XRD pattern of Pr0.65Ca0.2Sr0.15MnO3 …………………….43
Fig. 4-11. Refine of the XRD pattern of Pr0.65Ca0.15Sr0.2MnO3 ……………………..44
Fig. 4-12. Refine of the XRD pattern of Pr0.65Ca0.1Sr0.25MnO3 ………………….….45
Fig. 4-13. Refine of the XRD pattern of Pr0.65Ca0.05Sr0.3MnO3 ……………………..46
Fig. 4-14. Refine of the XRD pattern of Pr0.65Sr0.35MnO3 …………………………..47
Fig. 4-15. The lattice parameters a, b, and c* vs. x of the Pr0.65(Ca0..35-xSrx)MnO3 (c*=c/ ) ……………………………………………………………....48
Fig. 4-16. The temperature dependence of resistivity of the Pr0.65(Ca0.35-xSrx)MnO3 (x=0-0.35)system………………………………………………………49
Fig. 4-17. The temperature dependence of specific heat of the Pr0.65(Ca0.35-xSrx)MnO3 (x=0-0.35) system……………………………………………………...50
Fig. 4-18. The profiles of the magnetization vs. temperature of Pr0.65Ca0.35-xSrxMnO3 (x=0, 0.05, 0.1, and0.35)……………………………………………….51
Fig. 4-19. Mn 2p-edge Pr0.65Ca0.35-xSrxMnO3 (x=0, 0.05, 0.1, 0.15, and0.35)………52
Fig. 4-20. Magnetoreistance vs. T of the Pr0.65Ca0.35MnO3 in the range of 0£H£8T for cooling process…………………………………………………………53
Fig. 4-21. Magnetoreistance vs. T of the Pr0.65Ca0.3Sr0.05MnO3 in the range of 0£H£8T for cooling process……………………………………………………..54
Fig. 4-22. Magnetoreistance vs. T of the Pr0.65Ca0.275Sr0.075MnO3 in the range of 0£H£5T for cooling process…………………………………………...55
Fig. 4-23. Magnetoreistance vs. T of the Pr0.65Ca0.25Sr0.1MnO3 in the range of 0£H£8T for cooling process……………………………………………………..56
Fig. 4-24. Magnetoreistance vs. T of the Pr0.65Ca0.225Sr0.125MnO3 in the range of 0£H£5T for cooling process……………………………………………57
Fig. 4-25. Magnetoreistance vs. T of the Pr0.65Ca0.2Sr0.15MnO3 in the range of 0£H£5T for cooling process……………………………………………………58
Fig. 4-26. Magnetoreistance vs. T of the Pr0.65Sr0.35MnO3 in the range of 0£H£5T for cooling process………………………………………………………..59
Fig. 4-27. MR vs. T of the Pr0.65Ca0.35MnO3 for cooling and warming processes (H=0, and 5T)………………………………………………………………..60
Fig. 4-28. MR vs. T of the Pr0.65Ca0.3Sr0.05MnO3 for cooling and warming processes (H=0, 2, 3, and5T)…………………………………………………….61
Fig. 4-29. MR vs. T of the Pr0.65Ca0.275Sr0.075MnO3 for cooling and warming processes (H=1, 2, 3, and5T)……………………………………………………..62
Fig. 4-30. MR vs. T of the Pr0.65Ca0.25Sr0.1MnO3 for cooling and warming processes (H=0, 1, 2, and3T)……………………………………………………..63
Fig. 4-31. MR vs. T of the Pr0.65Ca0.225Sr0.125MnO3 for cooling and warming processes (H=0, 0.5, 1, and 2T)…………………………………………………..64
Fig. 4-32. MR vs. T of the Pr0.65Ca0.2Sr0.15MnO3 for cooling and warming processes (H=0, 0.5, 1, and2T)……………………………………………………65
Fig. 4-33. MR vs. T of the Pr0.65Sr0.35MnO3 for cooling and warming processes (H=0, 0.5, 1, and2T)…………………………………………………………..66
Fig. 4-34. M vs. T of the Pr0.65Ca0.35MnO3 for cooling processes (H=0.1, and 8T)..67
Fig. 4-35. M vs. T of the Pr0.65Ca0.3Sr0.05MnO3 for cooling processes (H=0.1, and 8T)……………………………………………………………………..68
Fig. 4-36. M vs. H of the Pr0.65Ca0.35MnO3 for T=10, 70, and 300K……………….69
Fig. 4-37. M vs. H of the Pr0.65Ca0.3Sr0.05MnO3 for T=10, and 300K………………70
Fig. 4-38. M vs. H of the Pr0.65Ca0.25Sr0.1MnO3 for T=10, and 300K………………71
Fig. 4-39. M and logr vs. H of the Pr0.65Ca0.35MnO3 at T=10K…………………….72
Fig. 4-40. M and logr vs. H of the Pr0.65Ca0.3Sr0.05MnO3 at T=10K………………..73
Fig. 4-41. M and logr vs. H of the Pr0.65Ca0.25Sr0.1MnO3 at T=10K………………..74
Fig. 4-42. The temperature dependence of magnetization of the Pr0.65Sr0.35MnO3 sample for H=0.1, 1, 2, 3, and 5T……………………………………..75
Fig. 4-43. r(H)/r(0) vs. H of the Pr0.65Sr0.35MnO3 for T=290, 295, 300K………….76
Fig. 4-44. The magnetization vs. temperature in the range of 0£H£8T of Pr0.65Ca0.35-xSrxMnO3………………………………………………….77
Fig. 4-45. The magnetization vs. temperature under ZFC and FC under 100Oe of Pr0.65Ca0.35-xSrxMnO3………………………………………………….78
Fig. 4-46. The M vs. T for HDC is 0Oe and HAc is 1Oe and the frequency is 500Hz of Pr0.65Ca0.25Sr0.1MnO3………………………………………………….79
Fig. 4-47. C vs. T under magnetic fields (H=0, 1, 2, 2.5, 3, 4, and 8T) of Pr0.65Ca0.25Sr0.1MnO3………………….………………………………..80
Fig. 4-48. C vs. T at zero-field in the range of 0.6£T£300K of Pr0.65Ca0.25Sr0.1MnO3..
Fig.4-49. C/T vs. T curves for Pr0.65Ca0.25Sr0.1MnO3 sample at different magnetic fields (0-2T). The inset shows the absolute C/T vs. T of Pr0.65Ca0.25Sr0.1MnO3 sample measured by HPTR calorimeter at zero field…………………………………………………………………….81
Fig. 4-50. r(H)/r(0) vs. H bellows 0.3Tesla at the temperature (T=70, 75, 90K) of Pr0.65Ca0.25Sr0.1MnO3…………………………………………………..82
Fig. 4-51. Hydrostatic pressure (P) dependent ac magnetic susceptibility (cac) for Pr0.65Ca0.25Sr0.1MnO3 sample measured at warming temperature from 80 to 280K…………………………………………………………………83
Fig. 5-1. The phase diagram of T vs. x for the Pr0.65Ca0.35-xSrxMnO3 series………...98
Fig. 5-2. The cures of MRRmax vs. x of the Pr0.65Ca0.35-xSrxMnO3 for H=0.1, 0.2, and 0.5T……………………………………………………………………..99
Fig. 5-3. r(H)/r(0) vs. H bellows 0.1Tesla of the x=0.1 sample at 70K and the x=0.35 sample at 295K……………………………………………………….100
Fig. 5-4. MR vs. T of the Pr0.65Ca0.35-xSrxMnO3 (x=0, 0.05, and 0.1) samples for 0T£H£8T………………………………………………………………101
Fig. 5-5. The H-T phase diagram the Pr0.65Ca0.25Sr0.1MnO3 sample………………102

List of Tables:
Table 3.1 General design feature and accessible experiments of 6m-HSGM beam line in SRRC………………………………………………………………….29

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