| Magnetic nanoparticles have broad application prospects in magnetic recording, ferrofluids, photocatalysis, micro-and nano-electronics, biotechnology and so on. At the same time, magnetic nanoparticles exhibit novel properties that differ from those of their bulk polycrystalline counterparts. So the investigation on the application and magnetic property of nanoscal magnetic materials has always been one of the high-frofile issue in the areas of materials physics, nanoscience and technology. To the fundmental research, the novel magnetic property of nanoscal magnetic material can be attributed to the finite size effect, surface effect, exchange interaction of crystalline grain and the dipolar interaction between nanoparticles, etc. Specifically, the superparamagnetism will present when the partical size below the critical size of single domain; the enhanced anisotropy and exchange bias effect are derived from the interaction of suiface and core spins; intergranular exchange couping interaction results in the remanence enhancement; strong interpartical dipolar interaction leads to super spin glass, etc. So far there was few experimental reports on systemely revealing the effects of interaction on magnetismof nanoscale magnetic materials, even some opposite results have been reported; The main reason is that the synthesed nanoparticles had broad size distribution and serious aggregation. In this nanoparticle system, it is difficult to distinguish the effects of size effct, exchange interaction and dipolar interaction on magnetism. In this thesis, well-dispersed uniform nanoparticles were synthesized to exclude the effcts of size and distribution on magnetism. By diluting and reducing after diluting, we changed the distance, moment and anisotropy of particles and investigated the effects of surface spin, anisotropy, dipolar interaction and morphology on magnetism.The main research contents are as follows:1. In order to prominently investigate the effects of the surface spin on the magnetic properties, the weak magnetic ZnLao.02Fe1.98O4 nanoparticles were chosen as studying object which benefit to reduce as possibly the effects of interparticle dipolar interaction and crystalline anisotropy energies. By annealing the undiluted and diluted ZnLao.02Fe1.98O4 nanoparticles at different temperatures, we observed the rich variations of magnetic ordering states (superparamagnetism, weak ferromagnetism and paramagnetism). The magnetic properties can be well understood by considering the effects of the surface spin of the magnetic nanoparticles. Our results indicate that in the nano-sized magnets with weak magnetism, the surface spin plays a crucial rule in the magnetic properties.2. The mono-dispersed and uniform CoFe2O4 nanoparticles were synthesized by the thermal decomposition of Fe(acac)3 and Co(acac)2. Then the CoFe2O4 nanoparticles were diluted in amorphous SiO2 matrix with different CoFe2O4 nanoparticles’ concentrations. All samples show the positive or negative exchange bias behavior, indicating the presence of canted spin layer at the CoFe2O4 nanoparticles’surface. The large effective anisotropy constant (3.38×106 erg/cm3) was observed, which can be attributed to the induced surface anisotropy by the canted surface spins. The reduced magnetization (Mr/Ms) was dominated by the interparticle dipolar interaction while the coercivity (Hc) was determined by the synergistic effects of the surface anisotropy, interparticle dipolar interaction and interface effect. By suitably diluting CoFe2O4 in the SiO2 matrix, the high Hc (3056 Oe) and the Mr/Ms (0.63) can be obtained, which is larger than most of those reported before. The present work is meaningful for revealing the underlying mechanism in nano-scaled magnetic system and improving the magnetic performance.3. Well-dispersed uniform cobalt ferrite nanoparticles were synthesized by thermal decomposition of a metal-organic salt in organic solvent with a high boiling point. Some of the nanoparticles were diluted in a SiO2 matrix and then the undiluted and diluted samples were characterized and their magnetic behavior explored in a temperature region 10 K～390 K. The undiluted and diluted samples exhibited maximum coercivity Hc of 23817 and 15056 Oe at 10 K, respectively, which are the highest values reported to date, and the corresponding ratios of remanence (Mr) to saturation (Ms) magnetization (Mr/Ms) were as high as 0.85 and 0.76, respectively. Interestingly, the magnetic properties of the samples changed at 200 K, which was observed in magnetic hysteresis M(H) loops and zero-field cooling curves as well as the temperature dependence of Hc, Mr/Ms, anisotropy, dipolar field, and the magnetic grain size. Below 200 K, both samples have large effective anisotropy, which arises from the surface spins, resulting in large Hc and Mr/Ms. Above 200 K, the effective anisotropy decreases because there is no contribution from surface spins, while the dipolar interaction increases, resulting in small Hc and Mr/Ms. Our results indicate that strong anisotropy and weak dipolar interaction tend to increase Hc and Mr/Ms, and also clarify that the jumps around H= 0 in M(H) loops can be attributed to the reorientation of surface spins. These results further revealed the significant effects of surface spins and dipolar interaction, in different extent with the variation of temperature, on the magnetic properties.4. In the magnetic nanoparticles, the strong dipolar interaction often makes the moments of particles freezing into the superspin glass state, characteristic of the relaxation behavior and the memory effect. The static and dynamic magnetic measurements were performed on the undiluted CoFe2O4 nanoparticles with strong dipolar interaction. The main results are as follows:1) The exchange-coupling between surface spins and core spins enhances the Mr/Ms ratio.2) The anomalous memory effect was observed:it occurred in a broad temperature range below 350 Kwhich can be attributed to the relaxation of surface spins below 200 K and to the relaxation from the several particles with paranellelly alligned moments.5. Using the thermal decomposition of a metal-organic salt, well-dispersed uniform cobalt ferrite (CoFe2O4) nanoparticles with diameters of 9,11,14 and 30 nm are synthesized. Multiple variables, including the interparticle distance, moment and anisotropy, are changed by dilution in a silica matrix and reduction in hydrogen to reveal the intrinsic correlation between the ratio of remanence to saturation magnetization (Mr/Ms) and interparticle dipolar interaction, the strength of which is estimated by the maximum dipolar field Hdip. This correlation has never been systematically investigated experimentally. To prevent particles from agglomerating, their reduction is carried out after dilution. The results reveal that the correlation between Mr/Ms and Hdip roughly follows Mr/Ms∞1/1gHdip independent of the size, distance, moment and anisotropy of the CoFe2O4 nanoparticles. The deviation from Mr/Ms∝1/1gHdip can be attributed to the effects of surface spins for the single phase nanoparticles and to the pinning effect of CoFe2O4 on CoFe2 for the slightly reduced nanoparticles.6. The strength of interparticle dipolar interaction was changed by diluting MnFe2O4 nanoparticles in the SiO2 matrix with moderate and low concentrations. The strong dipolar interaction has been suggested to enhance the blocking temperature TB and suppress the remanence ratio Mr/Ms in many previous reports. Several novel and even opposite phenomena were observed in the diluted MnFe2O4 nanoparticles due to some triangular particles existing in samples. The results in the present work indicate that the particle shape plays a crucial role in the nanoscale magnetic materials. |
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