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Doped manganites preparation, structure and magnetic properties

Author: ChenLiang
Tutor: HeJunHui
School: Yangzhou University
Course: Condensed Matter Physics
Keywords: colossal magnetoresistance perovskite structure manganite magnetic
CLC: O482.5
Type: Master's thesis
Year: 2009
Downloads: 145
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Abstract

The perovskite manganites is one kind of typical strongly correlated electron system, which simultaneously has the complex charge, spin, orbitals and lattice degrees of freedom that are strongly coupled to each other, which causes novel electronic, spin transportation characteristic and rich physical phenomena. The perovskite manganites provide a good object for researching these specific attributes. On the other hand, it also provides a broad application prospect for utilizing these characteristics to construct the specific functional materials and the devices.In this dissertation, we select the perovskite manganites La0.67Sr0.33Mn1-xMoxO3(0≤x≤0.06) and La1-xSrxFe0.5Mn0.5O3(0≤x≤1) as the study subjects, and investigate the effects of doping on Mn and La sites, the interactions between different magnetic ions, and the magnetic and transport properties of them. First, the polycrystalline samples have been prepared by standard solid-state reaction. In order to prepare high quality polycrystalline samples, we carried on the optimal design of the presintering, sintering, and temperature processing parameters to find out the appropriate technic conditions. Then we study their crystal structure, microstructure, magnetic, transport properties, and magnetoresistance, and gained several valuable experiment and research results.The dissertation contains five chapters as follows:The first chapter gives a brief review of the magnetoresistance effect of different types of materials, characteristics, and their important value. Then we introduce the research status and the related physical properties of perovskite manganese oxide magnetoresistance materials, such as crystal structure, electronic structure, magnetic structure, magnetic, electrical transport properties, exchange effect, polaron theory and phase separation. Finally, we present the research purpose, ideas, and contents. In chapter two, we describe the preparation methods and processes of polycrystalline samples. Then we introduce the sample testing methods and technologies, such as X-ray diffraction (XRD), scanning electronic microscope (SEM), differential scanning calorimetry (DSC), vibrating sample magnetometer (VSM), the standard four-probe technique and superconducting quantum interference devices (SQUID ) magnetic measurement system.In chapter three, we present the detailed results of the structure, magnetic, magnetic phase transition, electrical transport properties and magnetoresistance effect of the Mo doped La0.67Sr0.33Mn1-xMoxO3 (0≤x≤0.06). The experimental results show that the Mo doping in Mn site lowers the Curie temperature Tc slightly, and induces the cluster spin glass behavior in ferromagnetic state of La0.67Sr0.33MnO3. According to a second-order phase transition model, the critical behavior near Tc is investigated systematically based on the specific heating data. It is noticed that the value of the critical exponentβlocates between the mean-field value (β=0.5) and the three dimensional (3D) Ising model value (β=0.325), which suggests the coexistence of the short-range and long-range ferromagnetic orders around the critical temperature. A significant enhancement for room temperature magnetoresistance has been observed due to Mo doping, which could likely be attributed to the presence of the cluster spin glass state in the Mo doping La0.67Sr0.33MnO3 system. In the low temperature range ( 0.1 < ( T Tc)< 0.5), the electrical resistivity is found to follow the equation 0nρ=ρ+ρnT(2<n<2.5). Both the electron-phonon and electron-magnon scatterings processes dominate the conduction in the ferromagnetic state, while the single magnon scattering becomes more remarkable with increasing temperature. Further more, the samples clearly exhibit the minimum resistivity of zero field and the resistivity is suppressed by applied magnetic field (H=1.5 T), and the temperature corresponding to the resistivity minimum decreases considerably in presence of the applied magnetic field.In chapter four, the evolution behavior of the structure and magnetic properties of Sr doped systems La1-xSrxFe0.5Mn0.5O3 have been investigated. The experimental results show that when the doping level of Sr ions is in the range of 0.25≤x≤0.75, the original ideal perovskite-type structure ( Pm 3m ) has been distorted to a rhombohedral perovskite structure ( R 3c). With the increase of the contents of Sr-doped samples, the lattice constants and unit cell volume decrease, and the average grain size of the polycrystalline samples decrease as well. Further more, the Sr doping in La site induce the changes of valence state of the magnetic ions Fe and Mn, which leads to the changes of the distribution ratio of different types of exchange effect between different valence state of Fe and Mn, and leads to complex evolution behavior of the magnetic properties. The qualitative explanation of magnetization versus external magnetic field in the system La1-xSrxFe0.5Mn0.5O3 has been given based on the exchange interactions among magnetic ions Fe3+( 3d 5 : t 23 g eg 2), Mn3+ and Fe4+( 3d 4 : t 23 g e1g ), and Mn4+( 3d 3 : t 23 g e g0).The fifth chapter gives a brief summary and prospect of the research work.

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