Synthesis, Structure And Magnetoelectrical Properties Of Pr_0.75Na_0.25Mn_1-xFe_xO₃

There exist strong couplings and competitions among the spin, charge, orbital, and lattice degrees of freedom, which can induce a wide variety of newly novel phenomena such as insulator-metal transition, orderings and phase separation and metamagnetic phase transition etc by studying of the strongly correlated perovskite manganites. Therefore, many theories and models have been brought forward in order to explain those phenomena, which will arouse the further development of this research field in the future. This probably makes strongly correlated manganites one of the hotspots in the field of condensed matter physics in the 21st century. While the origins of many magnetic phenomena and the relatation among them are still open questions.In this dissertation, based on the preparation of single-phase polycrystalline sample series by a sol-gel technique, the phase-separated Pr0.75Na0.25Mn1-xFexO3 (0≤x≤0.30) at low temperatures are investigated in magnetic fields. XRD (X-ray diffraction), XRF (X-ray fluorescence analysis), SEM (scanning electron microscope) and IR (infrared) spectra were used for microstructure and cationic composition analysises. A PPMS (physics properties measurement system) was used for M-T curves, M-H curves,ρ-T curves,ρ-H curves, ac susceptibility and relaxation effect measurements. The interrelation between charge/orbit ordering and Fe doping was analyzed. At the same time, the phase-separated ground state, the interrelation between the ground state and step-like metamagnetic transition, as well as the mechanisms of magnetic induced metamagnetic transition and the possible quantum spin transition were studied.The single-phase polycrystalline perovskites Pr0.75Na0.25Mn1-xFexO3 (0≤x≤0.30) were prepared for the first time by a sol-gel technique. The analysises of XRF and XRD proved that the volatilization of Na and the precipitation of manganese oxide are avoided by sol-gel technique (which offers shorter sintering time and lower temperature than solid-state reaction method). This is a novelty of this dissertation, which proposed a novel idea to prepare oxide ceramics containing volatile element. The scanning electron microscope (SEM) results reveal that the sol-gel-derived compounds are not as compact as ceramic samples. The analysis of IR shows that the Jahn-Teller distortion of MnO6 octahedron decreases with increasing Fe doping.The relative intensity of the CO peak of Pr_0.75Na_0.25MnO₃ synthesized by sol-gel technique is much stronger than that of the sample with the same nominal structural formula synthesized by solid-state method (the real Pr/Na=0.80:0.20). Simultaneously, the resistivity of the former is much larger than that of the latter, which can be ascribed to two factors: First, Mn4+/Mn3+ ¹:1 in the former, which induces stronger charge-orbit ordering antiferromagnetic (COOAF) phase, and weakens the ferromagnetic (FM) phase competed with COOAF. Second, grain boundary effect caused by the granular structure of the sample synthesized by sol-gel technique offers much insulating grain boundary resistance.The effects of Fe doping at the Mn site upon the charge and the magnetic order in Pr_0.75Na_0.25MnO₃ have been investigated. The low temperatures long-range charge order (CO) is suppressed and the CO temperature shift to lower temperatures with increasing Fe doping. The ferromagnetic fraction increases and resistivity decreases with increasing Fe doping for x≤0.05. However, both of them change with contrary trend. This complex behavior is interpreted in terms of the competition between geometric effect (size of the dopant) and the effect of electronic configuration of the dopant. The geometric effect is dominant for low Fe doping, and the weakening of the DE by Fe doping is dominant for high Fe doping in Pr_0.75Na_0.25Mn_1-xFe_xO₃. This novel competitive mechanism explains the interaction in strongly correlated manganites. The step-like metamagnetic transition at 2 K for the samples with x≤5% is observed and the origin is ascribed to the phase transition from COOAF phase to FM phase in a phase separation scenario. And no step-like metamagnetic transition at 2 K is observed for the samples with x≥0.1 because COOAF phase is suppressed by further Fe doping.The ground state of Pr0.75Na0.25Mn0.9Fe0.1O3 is studied detailedly in order to investigate the ground state of the samples without step-like metamagnetic transition at 2 K. The analysis of data shows that there exist correlated ferromagnetic clusters embedded in an antiferromagnetic insulative matrix. The growth of ferromagnetic clusters with increasing magnetic field was examined. The results indicate that the ferromagnetic clusters are far from filling up the whole sample, even up to H = 5 T. This result contributes to directly support the phase separation scenario that is one of the leading theories to explain the physics of manganites. The sample maintains an insulating state at temperatures below 65 K even for H = 5 T and the transport property is natural for magnetic semiconductors at temperatures above 65 K.A detailed study of step-like metamagnetic transitions at 2 K in polycrystalline Pr0.75Na0.25Mn1-xFexO3 (0≤x≤0.30) is presented. The data shows that there are three phases in this sample series: COOAF,FM,AFII (antiferromagnet II). The fraction of AFII with larger antiferromagnetic exchange interaction than COOAF is proved to be the effective factor of critical fields (Hc) for the first time. A model of anisotropy barrier has been proposed to explain the extremely sharp step-like transitions (width < 0.01 T) in polycrystalline manganites and related phenomenon. Both step-like metamagnetic transition at 2 K (below the spin blocking temperature TB) and gradually changed metamagnetic transition at temperatures above TB originate from COOAF phase. And the different characteristic of the spins at different temperatures is responsible for the difference between them.