| Multiferroics have attracted immense interest not only for their fascinating physical properties but also for the potential applications. Perovskite-type manganites RMnO3(R=Tb,Dy) is a prototypical multiferroic material, in which the ferroelectric polarization mainly originates from the spiral spin order of Mn3+ion. However, recent studies revealed that the rare earth spins also play substantial roles in modulating polarization. Therefore, in this dissertation, in order to systemically study the exchange interactions between rare earth sublattice and transition metal sublattic, we focused on the Dy2NiMnO6and DyMnO3compounds containing two interacting magnetic sublattices, and carried out a series of investigations. Details are as follows:In chapter one:we firstly introduce the basic crystalline structure and electrical structure of perovskite manganites, the concepts of superexchange and double exchange interaction. Based on these theories, the recent studies of the structure and properties of DyMnO3and Dy2NiMnO6have been reviewed.In chapter two:generally, the magnetic behaviors of ferromagnets can be described by the Weiss’s molecular field theory and the Curie-Weiss law. However, for the compounds containing two interacting magnetic sublattices, the internal molecular field is not a uniform magnetic field, and the Curie-Weiss law is no longer suitable. Therefore, we extended the Weiss’s molecular field theory, and obtained a series of expressions to describe the relationship of the magnetization with temperature and magnetic field. Moreover, these expressions are used to simulate the magnetization of Gd2NiMnO6, and the fitting results are satisfactory.In chapter three:in general, the downward deviation of the inverse susceptibility from the Curie-Weiss behavior above Tc is considered as a criterion of the Griffiths phase. Based on this criterion, a Griffiths phase is found in bulk Tb2NiMnO6, while the existence of the non-Griffiths phase has been confirmed in bulk La2NiMnO6. According to chapter two, it is worth noting that the Curie-Weiss law is only suitable for La2NiMnO6, and it could not describe the paramagnetic behavior of the compounds containing two interacting magnetic sublattices. Therefore, we reexamined the magnetic behaviors of Dy2NiMnO6by using the expressions in chapter two. A non-Griffiths phase is found in bulk Dy2NiMnO6, and there is a size-induced transition from the non-Griffiths phase to the Griffiths phase. These feature are well consistent with that in La2NiMnO6. Besides, the electron spin resonance intensity and g factor clearly reveal that the AFM interactions are suppressed in the nanosized Dy2NiMnO6, accompanied with an enhancement of FM tendency, which support the size-induced phase transition revealed by the magnetization measurement.In chapter four:the structure and magnetic properties of orthorhombic DyMnO3nanoparticles with different particle sizes are investigated. It is found that all the lattice parameters a, b, and c gradually decrease, and the orthorhombic distortion increases with the decrease of the particle size. Raman spectroscopy also indicates an increase of the orthorhombic distortion. Magnetic measurements reveal that the antiferromagnetic interaction is weakened with decreasing particle size, due to the increase of the octahedral distortion, which induces a decrease of Mn-O-Mn bond angle. All the nanoparticle samples show a hysteresis behavior and the coercive field HC increases monotonically, indicating a weak ferromagnetic character due to the surface ferromagnetism. Above a critical field H*, DyMnO3undergo a first-order metamagnetic transition from antiferromagnetic to ferromagnetic at4K. The critical field H*remains almost constant for all the samples, indicating that size reduction has little effect on the Dy sublattice. The magnetization M(4K,5T) shows a non-monotonic variation with decreasing particle size, which can be understood by a modified core-shell model, in which the FM ordering (Dy sublattice) and AFM ordering (Mn sublattice) coexist in the core. The opposite effect on Dy and Mn sublattices with reduction in the particle size results in the non-monotonic variation of M(4K,5T).In chapter five:the structure and magnetic properties of orthorhombic DyFe0.8Mn0.2O3nanoparticles with different particle sizes are investigated. It is found that all the lattice parameters a, b, and c gradually increase with the decrease of the particle size. Raman spectroscopy shows a red shift of all the Raman modes with decreasing grain size, indicating an increase of the lattice parameters. Besides, the reduction of Raman intensity and the broadening of Raman peak indicate that there are more disordered crystal structure in nanosized sample. Magnetic measurements reveal that the spin reorientation transition exists in all samples. With decreasing particle size, the spin reorientation transition shifts to lower temperature and becomes more broad, which results from the weakening of the Dy-Fe interaction due to more disordered crystal structure in nanosized sample. |
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