| As a research hotspot in the field of condensed matter physics and spintronics, the ABO3(A is rare earth, B is transition metal such as Mn) manganite is one of the typical strongly correlated electron systems, and it provides a good platform to enrich magnetoelectronics and spintronics. Due to the interplay between charges, spin, orbital and lattice degrees of freedom, which gives rise to several bizarre effects, such as: magnetic phase transition, magnetoelectric effect, charge ordering and phase separation, the manganite has a rich physical phase diagram. Especially because of the addition of the rare earth elements, the more complex interactions between d-f orbital, electron and magnetic moment were induced, so it is particularly important to study the magnetic properties of the manganites for a deep understanding of the various magnetic coupling interactions and the development of new spintronic materials and devices. In this thesis, we investigated the doping effects on the crystal structure and the magnetic properties of the hexagonal and the orthogonal perovskite rare earth manganites, and studied the doping effects on the physical properties such as the interaction of the magnetic coupling, magnetic frustration, phase change and the ordering. The main contents of this thesis can be summarized as follows:1. The single crystals of hexagonal manganites with high quality were grown by float-zone technique using an image furnace. The polycrystalline sample of La0.27Nd0.4Ca0.33MnO3with orthorhombic structure was prepared by the sol-gel reaction method.2. The crystal structure and magnetic properties of the hexagonal Y0.95Dy0.05MnO3single crystal are investigated in the steady and pulsed high magnetic field. The results show that the substituting nonmagnetic Y3+ions by magnetic Dy3+ions has an important effect on the magnetization behavior of the compound. Compared with pure YMnO3, by doping at the Y site, the magnetic geometrically frustration effect is greatly suppressed, which is attributed to the doping induced magnetic coupling interactions between4f-4f,4f-3d and3d-3d coexist in the Y0.95Dy0.05MnO3single crystal, and the competition between them leads to the complicated magnetic structure and makes the magnetic frustration less stable.3. We investigated the crystal structure and magnetic properties of the hexagonal Yo.95Euo.o5Mn03single crystal. The refinement of single crystal XRD show that Eu doping leads to the expansion of the MnO5dipyramids along the c axis and the contraction of the Mn trimerization in the ab planes. Magnetization measurements show that Eu doping makes the magnetic anisotropy changed. Meanwhile, the spin glass behavior and a large zero-field cooled exchange bias effect are observed in the doped system. The exchange bias field is as large as735Oe at2K. We propose that the presence of the spin glass state is attributed to the disorder caused by Eu doping and the intrinsic nature of the magnetic frustration of the sample. The observed zero-field cooled exchange bias effect can be explained in terms of the exchange coupling interactions between the ferromagnetic spin glass phase and the antiferromagnetic spin frustration phase. While doping induced the expansion of the MnO5dipyramids along the c axis and the contraction of the Mn trimerization in the ab planes finally lead to the magnetic anisotropy has changed.4. The single crystals of hexagonal manganites YMn1-xAxO3were grown by float-zone technique, the doping effects and the modulation of the magnetic frustration are investigated. Compared with pure YMnO3, by doping Al at the Mn site, the frustration effect is greatly suppressed and the Curie-Weiss temperature θCW shows dopant-dependent, meanwhile the antiferromagnetic transition temperature is also suppressed, which is attributed to the change of the Mn-O-Mn bond angle and the broken antiferromagnetic Mn-Mn coupling.5. The valence state of the Mn ion in the hexagonal YMn0.9Cu0.1O3single crystal was analysed by X-ray photoelectron spectroscopy (XPS) measurement. Results show that there are Mn2+. Mn3+and Mn4+ions coexist in the YMn0.9Cu0.1O3single crystal. In addition, the magnetization measurements suggest that the substituting Mn3+ions by magnetic Cu2+ions has little effect on the antiferromagnetic transition temperature, but the antiferromagnetic Mn3+-Mn3+coupling interaction is greatly weakened and the magnetic anisotropy in the low field has changed.6. The polycrystalline sample of La0.27Nd0.4Ca0.33MnO3with orthorhombic structure was prepared by the sol-gel reaction method, and the field-induced metamagnetic properties were investigated. Results show that the substituting La by Nd has an important effect on the magnetization behavior of the compound. On one hand, the substituting forms a field-induced metamagnetic transition, which may originate from the competition and subtle balance between the energy gain of the charge carriers in terms of the double-exchange interaction and the coulomb and magnetic energy gain due to the charge ordered lattice; On the other hand, the doping of Nd results in an partially irreversible CO-FM transition and lattice strain, which suggest the strong spin-lattice coupling in the system. |
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