| Since the discovery of the colossal magnetoresistance effect (CMR), the perovskite-like manganites have been studied extensively due to fascinating physical properties derived from the interplay of charge, spin, orbital, and lattice degrees of freedom, as well as their potential applications. As for the exchang bias (EB) effect, not only its potentially technological application in the magnetic devices but also its mechanism has been an attractive issue among topics of interest. The observation of EB effect in the manganites gives new insight into the research on the manganites and provides them more opportunities in the field of application. Though many efforts are contributed to the investigations in the EB effect in the manganites, EB effect in the charge ordered manganites is lack of systemic studies. Moreover, the influence of the size effect on the EB effect and the dynamic properties of the ferromagnetic cluster originating from the size effect remain to be unsolved. Aiming at the problems described above, the dissertion is arranged as follows:In chapter 1, the research history of perovskite-like manganites and the progresses of the EB effect are concisely introduced. It includes the following main points:the crystal structure and the Jahn-Teller effect, CMR effect and the physical mechanisms such as double exchange interaction, superexchange interaction and the polaron effect, charge ordering (CO) and methods to control the CO state, phase separation and the electronic phase separation, the phase diagrams of La1-xCaxMnO3 and other systems, EB effect and the corresponding theoretical models, EB effects in the manganites in the form of film, bulk and nanoparticles, as well as the cluster state in the nanoparticles.In chapter 2, the EB effect of La1-xCaxMnO3 (0.55≤x≤0.95) compounds was investigated. The electronic phase separation and structure inhomogeneities probably lead to the occurrences of the FM phases in the systems with antiferromagnetic (AFM) structures. The dc susceptibility Xdc vs temperature curve shows a low temperature anomaly for all the samples, which is attributed to the appearance of ferromagnetic (FM) components of which the fraction is estimated using the initial magnetization curves. Since the FM and AFM phases coexist in the system, the EB field obtained by measuring the field-cooled hysteresis loop is observed and found to present a maximum about 2200 Oe at x in the region 0.6-0.63, which may be mainly affected by the AFM anisotropy and the exchange energy at the FM-AFM interface. With further doping Ca to x> 0.85, the spin canted G-type AFM phase showing the FM characteristic occurs, increases with increasing Ca content, and coexists with the reduced C-type one. The competition between these two states results in another exchange-bias effect peak near x= 0.90 for the compounds.In chapter 3, how the size effect influences the EB effect in La0.25Ca0.75MnO3 nanoparticles has been systematically studied. It is found that, as the particle size decreases, the uncompensated surface spins play an important role in the observation of EB effect. Since the FM cluster exists in the shell of La0.25Ca0.75MnO3 nanoparticles with diameters ranging from 40 to 350 nm, the magnetic hysteresis loops display horizontal and vertical shifts in field-cooled processes. Moreover, the variations of the exchange bias field (HEB) and the coercivity (HC) with particle size follow non-monotonic dependencies and show maxima for particles with diameter around 80 nm. The peak position for He shifts to larger particle size at higher temperature while that for HEB is unconspicuous, which results from the cooperation of many factors, such as the FM coupling strength in the FM cluster, the AFM anisotropy in the core and so on. Besides, the linear relationship between HEB and vertical magnetization shift (MEB) further indicates that the characteristics of uncompensated spins play an important role in the occurrence of HEB for the manganite.In chapter 4, the dynamic magnetic properties of cluster glass in La0.25Ca0.75MnO3 nanoparticles with average diameter range from 40 nm to 1000 nm have been investigated by measuring the ac susceptibility as a function of temperature at different frequencies and dc magnetic fields (H), respectively. For the samples in the particle size range from 40 to 350 nm, the frequency-dependent Tf, the freezing temperature of the FM clusters at H= 0, is fit to a power lawτ=τ0 (Tf/Tg-1)-zv. The relaxation time constantτ0 obtained by the power law decreases as the particle size increases from 40 to 350 nm, which indicates the decrease in the size of the clusters at the surface of the nanoparticle. In addition, the relationship between H and Tj(H) deviates from the De Almeida-Thouless (AT)-type phase boundary at relatively high fields for the samples with size range from 40 nm to 350 nm. Moreover, for the samples with particle sizes of 40 and 100 nm, To increases with increasing H, which indicates the increasing cluster size. These results may be ascribed to the competition between the influence of Hand the local anisotropy field in the shell spins. In chapter 5, temperature dependences of the resistivity, the magnetization and ultrasonic sound velocity and attenuation were studied for the La1-xCaxMnO3 (0.25≤x≤0.45) compounds, and infrared absorption spectra were measured at different temperatures. The resistivity and magnetization measurements show that the Curie temperature TC coincides with the metal-insulator transition one. Around Tc, both the sound velocity and attenuation display abnormal behaviors, while the stretching mode in the infrared spectrum shows significant frequency shift, and the effective number of carrier increases obviously. Thus, in La1-xCaxMnO3 (0.25≤x≤0.45) system, the strong electron-phonon coupling occurs near Tc, which is closely related to the conductive properties of the system.
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