Novel Transport Behaviors Induced By Multi-spin Co-ion In La_2/3Ca_1/3MnO₃ Manganites

The strongly correlated manganites with prevoskite structure have a strong coupling between the spin, orbit, charge and lattice freedom of degree which lead to a series of novel physical behaviors related to the basic conception and important questions in modern physics and materials science, for example, strongly correlated electrons, metal-insulator (M-I) transition, ordering parameters, spintronics and devices physics etc. All these challenge the traditional understanding of physics and information science and will provide newly subject for the physicists, material scientists and information scientists. This probably makes strongly correlated manganites the one of the hotspots in the field of condensed matter physics in the 21^st century. In this paper, two metal-insulator (M-I) transitions induced by multi-spin Co ion, and Co-doping effect on the resistivity minimum behavior as well as the modulation of these abnormal transport properties by the magnetic field in the La_2/3Ca_1/3Mn_1-xCo_xO₃ (0≤x≤0.15) system were systematically studied by XRD structural analysis technique and physical transport measurements. The correlation between these behaviors and spin ordering character was given. And the related abnormal field dependence of low temperature minimum and its physical mechanism were also discussed. The whole content of this thesis consists of six chapters as following:In chapter one, we introduce the history of the study on manganites and give a summary of the research on the manganites' physics properties. The doping effect at Mn site was presented in details, especially in Co doping. It also introduced some related theories and models for colossal magnetoresistivity (CMR) manganites which were used in this thesis. The motivations, significance and main content of the present work are given at the end of this chapter.In chapter two, the main methods and principles in our experiment are involved, including the preparation of samples, structural analysis and physical property measurements such as electron transport, magnetization on the study of CMR manganites.In chapter three, we study systematically the two metal-insulator (M-I) transition properties in La_2/3Ca_1/3Mn_1-xCo_xO₃ (0≤x≤0.15) system by electrical, magnetic transport measurements and XRD structural analysis technique. The results showed that all the samples are good single phase and the lattice parameters decrease with the increasing of doping concentration x. All the samples display a single paramagnetic-ferromagnetic (PM-FM) phase transition, which is the typical character of CMR manganites. The Curie temperature T_C (defined as the inflection point in the M-T curve) shifts to low temperature with the increasing x. In addition, all the samples follow a Curie-Weiss behavior to some extent in the PM region .The results of magnetization show field-cooled (FC) curve didn't coincide with the zero-field-cooled (ZFC) one below T_C, which indicates the coexistence of and competition between ferromagnetic (FM) and antiferromagnetic(AFM) interaction and is a signature of cluster-glass-like(CG) behavior in highly doped Co samples. All these may indicate the substitution of Co could enhance the magnetic disorder in the system and reflect the general intrinsic characteristic. The transport behavior in the higher temperature satisfied with the model of variable-range-hopping (VRH) model very well. The abnormal transport properties were induced by Co doping. In details, the resistivity exhibited two M-I transitions. The high M-I transition peak (T_PH) and low M-I transition peak (T_PL) all shifted to lower temperature with the increasing of x. While the T_PH shifted to high temperature but the T_PL was almost unchanged under the external field, respectively. The corresponding M-I transition peak values of resistivity showed strong dependence on the external field and Co doping concentration x. All the peak values of resistivity increased with the increases of x, and the low temperature peak value seemed more sensitive than that of high temperature peak. Compared with the magnetization properties, it could be concluded that the high-temperature M-I transition peak should be related with the undoped M-I transition, and the abnormal behavior of low-temperature M-I transition peak have some relationship with the oxygen vacancy and high spin state induced by Co³⁺ substituting Mn⁴⁺ in the present system. Corresponding to the double M-I transitions in resistivity, MR also exhibits two peaks, and the coexistence of two MR peaks gives rise to the MR effect in a broad temperature range, which is very useful for practical application. Present study may provide abundant experimental information for understanding the nature of the strongly CMR manganites.In chapter four, the multi-spin Co ion doping effect on low-temperature resistivity minimum for La_2/3Ca_1/3Mn_1-xCo_xO₃ (0≤x≤0.15) were systemically studied, and the correlation between this behavior and the spin ordering characteristic is also given. The novel resistivity minimum appears at low temperature for all samples, which is very sensitive to the content of Co. Moreover the abnormal electrical transport can be tuned strongly by applied magnetic field, which is very different from that of conventional dilute alloys. Combining with the magnetic properties of the samples, we think the abnormal behavior is mainly related to Co substitution. AFM clusters increase as the Co increases, which may result in magnetic disorder in the system, and it is under such strong disorder system that the resistivity upturn can not be returned. On one hand, the magnetic field restrains the spin related scattering, but on the other hand, in such a disorder system in which enhanced e-e interaction exists, the magnetic field may make the abnormal electrical transport more obvious. In order to explain the novel electrical transport we fit the present experimental data in the framework of Kondo-like spin scattering, electron-electron (e-e) and electron-phonon (e-p) interactions. It is seen that all the data are fitted very well. The fitting results show that this kind of low-temperature abnormal transport can be explained by spin disorder scattering (Kondo-like spin scattering), e-e and e-p interactions, and antiferromagnetic clusters on a nano/micro-scale. Combining with the early studies in our group which firstly showed the appearance of Kondo-like scattering in ferromagnetic metal oxides, our work again indicate that the novel resistivity minimum at low temperatures may reflect the general intrinsic character of the strongly correlated electron system. It may play a important role to reveal the nature of the strongly CMR manganites.In chapter five, the modulation of these abnormal transport properties by the magnetic field for the polycrystalline bulk La_2/3Ca_1/3Mn_0.9Co_0.1O₃ sample were systematically studied. We also discussed the abnormal field dependence of low temperature minimum and its physical mechanism. The results showed that besides the first M-I transition at T_PH corresponding to T_C on the electric transport curve, a secondary M-I transition is found at lower temperature T_PL .But the magnetic results indicated the secondary M-I transition peak has an abnormal characteristic with magnetic field-free on temperature dependency. The electrical resistivity minimum phenomenon was observed under various magnetic fields below 50K. Based on the effect of grain boundary, the magnetization results indicate the existence of the spin disorder scattering, including the spin polarization and the grain boundary (GB) tunneling effect, which should reflects the typical intrinsic property in the polycrystalline bulk samples. Moreover, the low field can do a little suppression on the resistivity minimum, but the high field can enhance the resistivity upturn. The different behaviors between the low field and high field may indicate there should exist not only the spin disorder scattering, including the spin polarization and the grain boundary (GB) tunneling effect, but also the enhanced e-e interaction, and all are the origin of the low-temperature resistivity upturn. The resistivity minimum at low temperatures proves the existence of Kondo-like spin disorder scattering and e-e interaction due to the strongly correlated interaction between the electrons. These should be a general character in the bulk manganites.In chapter six, we give a conclusion of the whole thesis and discuss the open questions on the work.