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B doping effect of perovskite manganites

Author: YingYue
Tutor: ZhangYuHeng
School: University of Science and Technology of China
Course: Condensed Matter Physics
Keywords: University of Science and Technology of China Double exchange interaction Ferromagnetic metal Doping effects PhD thesis Antiferromagnetic interaction Manganese oxide Magnetoresistance effect Magnetization Transport properties of
CLC: O482
Type: PhD thesis
Year: 2007
Downloads: 336
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Abstract


Since the discovery of colossal magnetoresistance （CMR） in manganese perovskites. It has sparked renewed interest in these long-known materials with an eye towards both an understanding of the CMR mechanism and potential applications of CMR in magnetic devices, e.g., read and/or write heads for magnetic disk drives, magnetic refrigeration, magnetic random access memories as well as magnetic field sensors. Excepted for the CMR effect, charge and orbital ordering, and electronic phase separation have been observed in these materials, hence it will give impetus to the progress in many fields of the condense matter physics once the origin of the CMR effect is really elucidated. In this thesis, we investigated the substitution effect on Mn site and the interaction between different elemental ions in manganese.The thesis is divided into four chapters.Chapter one: We present a review of the history and progress of MR effects research. At first, the various MR materials are briefly introduced. Then we lay a major part on the physical properties of CMR manganites, such as the phase structure, magnetic structure, charge and orbital ordering, and electronic phase separation.Chapter two: We investigated the structural, magnetic, and transport properties of polycrystalline doping systems La0.5Sr0.5Mn1-xRuxO3 （ 0≤x≤0.15 ） with ferromagnetic matrix and La0.45Sr0.55Mn1-yRuyO3 （0≤x≤0.50 ） with antiferromagnetic matrix. X-ray photoelectron spectroscopy shows that Ru ions exist mainly in the form of Ru4+ while there also exist a small quantity of Ru5+. For the lowdoping samples （0<x（y）< 0.15（0.20））, the ferromagnetism is enhanced and Curie temperature Tc rises with increasing Ru doping level, but Tc is much higher than the temperature TIM corresponding to insulating-metallic transition. For the highdoping samples （0.30≤y≤0.50）, oppositely the ferromagnetism is suppressed andthe insulating property is enhanced with further increasing Ru doping level. These results show that there is an exchange interaction between Mn3+ and Ru4+ (Ru5+). The electron spin resonance spectra clearly and directly inspect that the interaction between Mn3+ and Ru4+ (Ru5+) is ferromagnetic. Furthermore, the electron spin resonance spectra also show that a Ru-Ru antiferromagnetic interaction takes place inthe high doping region （y=0.40, 0.50）. We explained the disagreement between Tc and TIM.Chapter three: we designed a system La1-xSrxMn1-xRuxO3, in which LaMnO3 was consistently Sr and Ru ions. Thus, Mn ions will always maintain three-valence Mn3+ and Ru ions will exist as four-valence Ru4+ in this system. It will avoid the produce of Mn4+. Thus we can directly study the interaction between Mn3+ and Ru4+. The structural, magnetic and transport properties of polycrystalline La1-xSrxMn1-xRuxO3 （0≤x≤0.80） were investigated. It is found that the interaction between Mn3+ and Ru4+ is double exchange and it coexists with the FM interaction between Ru4+ ions.Chapter four: The structural, magnetic, and transport properties of the polycrystalline doping systems Nd0.7Sr0.3Mn1-xGaxO3 （0≤x≤0.20） were investigated. For the parent and low-doping samples （0≤x≤0.11）, they show a long-rangeferromagnetic state below Tc and the Griffiths singularity above Tc. When x furtherexceeds 0.12, a transition from the metal ferromagnetic to the insulating cluster-spin-glass state as a function of Ga doping takes places at low temperatures. The hole localization induced by the Ga doping is responsible for this transition.

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