| Nanomaterial is a kind of natural or manufactured powders or block containing particles. For50%or more of the particles in the number size distribution, one or more external dimensions is among the size range of1-100nm. Nanomaterial possesses small size effect, surface effect, quantum confinement effect and macroscopic quantum tunneling effect, so this kind of material has a wide application in many fields, such as optical, chemical, ceramics, electronics and medicine, provoking the strong interest of researchers in the world. In addition, due to the improvement of the preparation techniques, characterization methods and theoretical analysis, nanostructured film has also been rapidly developed. Because of their unique advantages in mechanical properties, catalytic properties, optical properties and superconducting properties, the thin film plays a very important role in the fields of scientific research, military, and electronics industry. However, due to the high cost of preparation and harsh conditions, part of the nanomaterial is still in the laboratory stage at present.Multiferroic materials exhibit two or more than two primary ferroic order parameters simultaneously, such as ferromagnetism/anti-ferromagnetism, ferroelectricity/anti-ferroelectricity and ferroelasticity, etc. These ferroic order parameters in the multiferroic materials can be adjusted and controlled by one another. Currently, the study on the multiferroic materials focuses on the point that making ferromagnetism and ferroelectricity coexist in the single-phase material, and to produce a magnetoelectric coupling. The magnetization of the multiferroic materials can be controlled by application of electric fields and vice versa.With the magnetoelectric coupling effect, the multiferroic materials have a vast application prospect in the fields of data accessors, spin quantum devices and detectors, etc. So in recent years, the development and application of multiferroic materials has become a major research in the field of functional materials.So far, scientists have made a lot of experiments and theoretical studies on the magnetism of single-phase multiferroic materials, however, there are still some questions which is not clear, even controversial, such as the original of magnetism in the La-modified PbTiO3(PLT) and K-modified Na0.5Bi0.5TiO3((Na, K)0.5Bi0.5TiO3) ferroelectric system; Whether the magnetic is intrinsic; Which electron plays a main role for the magnetic of the ferroelectric systems; How cation and anion vacancies will impact the magnetic of the two systems; How the applied electric and magnetic fields control magnetic and ferroelectric properties in the material, respectively, etc.These issues still need further research and analysis, in which case we can get a more clear understanding of the magnetic properties of single-phase multiferroic materials. The work is also very helpful for us to find more single-phase multiferroic material and make a better use of them. In addition, the study of organic small molecules magnetic material is another hot issue in the magnetic field. Although there is a lot of achievements, the source of the magnetism of the organic small molecules with non-magnetic metal still need to explore.To solve problems above, this paper obtained the conclusions as follows:1. We find that both Pb0.82La0.18TiO3bulk annealed at high temperature of1200℃and nanocrystalline Pb0.82La0.18TiO3plates annealed at low temperatures (700-1000℃) exhibit ferromagnetism with a Curie temperature of75K. Moreover, nanocrystalline Pb0.82La0.18TiO3plate is still ferromagnetic at room temperature due to the existence of oxygen vacancies at the surface of nanograins. Nanocrystalline Pb0.82La0.18TiO3plate annealed at1000℃for1h exhibits multiferroic near room temperature, where the strong magnetoelectric couplings are also observed.2. After first-principles calculations, we find that for the stoichiometric Pb0.875La0.125TiO3, there is a spin splitting near the Fermi level, indicating the existence of magnetism of0.57μB, which is mainly induced by the polarization of Ti3d electron. The magnetic moment is mainly contributed by Ti atoms surrounding La atom. Ferromagnetic coupling state for the stoichiometric Pb0.875La0.125TiO3supercell system is more stable than the antiferromagnetic state. The introduction of oxygen defect can improve the value of magnetic of the stoichiometric Pb0.875La0.125TiO3supercell system. By comparing the relative defect formation energy, for the five cases we considered, the oxygen atom in the Pbo875La0.12TiO3supercell system at the03position is the easiest to lost, then the system has a magnetic moment of0.74μB. Pb0.875La0.125Ti03supercell system, with two Pb atoms removed, shows magnetism, which is mainly due to the spin polarization of O atom2p electron. For the four cases we considered, Pb0.875La0.125TiO3supercell system, with Pb2atom and Pb4atom removed, has the lowest energy and shows a magnetic moment of0.97μB-For the Pb0.875La0.125TiO3supercell system, with Pb2atom and Pb4atom removed, the introduction of oxygen defect can improve the value of magnetic, and the oxygen atoms at the position of07is easier to lose in the two cases considered, then the system shows a magnetic moment of1.05μB.3. By vacuum annealing experiments, we find that Na0.5Bi0.5Ti03nanocrystals at room temperature shows ferromagnetism, which is induced by cation vacancies. We observed a strong magnetoelectric coupling in the Na0.5Bi0.5Ti03multiferroic material at room temperature. Na0.5Bi0.5TiO3ceramic sample annealed at900℃for1hour shows a significant magnetic dielectric effect. With increasing magnetic field, the dielectric constant of the four frequencies increase, and at lower frequency, the magnetodielectric effect is more intense. In addition, the magnetic dielectric effect has isotropic. We also found that with the increasing DC electric field, strength, the saturation magnetization of the Na0.5Bi0.5TiO3ceramic samples also increased, reflecting the magnetoelectric coupling effect. The LDA+U calculation results on the magnetism of Na0.5Bi0.5TiO3(100) surface shows that, Na vacancies may introduced a ferromagnetic at the surface of Na0.5Bi0.5Ti03.4.(Na,K)0.5Bi0.5TiO3nanocrystals show room-temperature ferromagnetism. In both nanocrystalline ceramic sample and powders of (Na,K)0.5Bi0.5Ti03, with the increase content of potassium, the magnetic moment is gradually weakening. The first-principles LDA+U calculations are used by us to discussed this problem, showing that, stoichiometric(Na2/3K1/3)0.5Bi0.5Tio3is non-magnetic, and with a Na vacancy,(Na2/3K1/3)0.5Bi0.5TiO3shows magnetism, which is introduced by the polarization of the O2p electrons. The results of vacuum annealing experiments also show that the ferromagnetism of (Na2/3K1/3)0.5Bi0.5Tio3nanocrystals at room temperature is induced by cation vacancies. With the increase of the applied magnetic field, the dielectric constants at the four frequencies are increased. Magnetodielectric effect of the sample is more intense at low frequencies and with increasing frequency, the magnetodielectric effect also weakens. In addition, we find that, with increasing DC electric field, the saturation magnetization of (Na,K)0.5Bi0.5Ti03nanocrystalline materials also increases.5. By first-principles calculation method of GGA, we find stoichiometric Alq3molecule and Gaq3molecule are non-magnetic. With a H vacancy, Alq3molecule and Gaq3molecule show magnetism, which is mainly due to the polarization of the C2p electrons.Six cases of the H vacancy introduced in the Alq3molecule and Gaq3molecule are considered, respectively. The magnetic moment in the system is mainly distributed at the C atom close to the H vacancy and when the H vacancy positioned at V111and VH2, the N atom is so close to the H vacancy that the magnetic moments are also distributed on the N atom. The results above proved that the magnetic in the Alq3molecule and Gaq3molecule introduced by H vacancy is local, which is because that molecular bonds are covalent bonds and there is no free electron in the system.By comparing the relative defect formation energy, for the six cases we considered, the H atom located at the position VH3are most likely to lose. The radius of Al atom and Ga atoms are not the same, so the distances from VH3to the Al atom and Ga atom are inconsistent. |
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