Magnetic Properties Of 3d Transition-Metals And Their Oxides Nanoparticles

All this time the magnetic nanomaterials have been pursued intensively due to their importance in fundamental research and diverse technical applications. As the typical representatives of magnetic materials, the 3d transition-metals and their oxides nanoparticles were selected as the research object in the present work. Based on the chemical synthesis of samples, their microstructure can be well controlled by tuning the experimental conditions of initial reaction and subsequent annealing. By means of VSM, SQUID, PPMS and ESR characterizations, we made emphasis on the analysis of magnetic properties. The main work has the following three aspects:Ⅰ. Magnetic Properties of Ni, NiO and Ni-NiO NanoparticlesBy means of thermal decomposition, we prepared single-phase spherical Ni nanoparticles （23-114 nm in diameter） that are face-centered cubic in structure. The magnetic properties of the Ni nanoparticles were experimentally as well as theoretically investigated as a function of particle size. By means of thermogravimetric/differential thermal analysis, the Curie temperature TC of the 23, 45,80 and 114 nm Ni particles was found to be 335,346,351 and 354℃, respectively. Based on the size-and-shape dependence model of cohesive energy, a theoretical model is proposed to explain the size dependence of TC. The measurement of magnetic hysteresis loop reveals that the saturation magnetization Ms and remanent magnetization increase and the coercivity decreases monotonously with increasing particle size, indicating a distinct size effect. By adopting a simplified theoretical model, we obtained Ms values that are in good agreement with the experimental ones. Furthermore, with increase of surface-to-volume ratio of Ni nanoparticles due to decrease of particle size, there is increase of the percentage of magnetically inactive layer.Based on the carbon sphere@Ni（OH）₂ composite microsphere prepared by homogeneous precipitation templates method, the porous NiO hollow spheres and Ni-NiO composite nanoparticles could be obtained via the calcining in air and argon atmosphere, respectively. The study found that the hydrothermal reaction temperature, calcination atmosphere, calcination temperature and heating rate have significant impact in the microstructure of samples. The perfect NiO hollow spheres are of uniform size, large surface area and high porosity. The blocking temperature TB, Ms, HC, coercivity enhancement ΔHC and exchange bias HE of Ni-NiO composite nanoparticles are influenced by the particle size, the dual phase scale, and the interface effect. For the optimal Ni-NiO composite nanoparticles, they show exchange bias of 30 O_e and coercivity enhancement of 9 O_e at 5 K, which is due to the weak coupling interaction between ferromagnetic Ni and antiferromagnetic NiO.Ⅱ. Magnetic Properties of Co, CoO and Co-CoO NanoparticlesA simple pyrolysis method has been developed to synthesize microstructure-controlled CoO nanoparticles from cobalt acetylacetonate in oleylamine at or above 200℃. XRD, SEM and HRTEM analyses indicate that the cubic and hexagonal CoO nanoparticles with different morphologies viz, spherical, quasi-cubic and pyramidal could be obtained via varying the precursor concentration, and the average size of hexagonal CoO nanoparticles increases with increasing reaction time or reaction temperature. XPS, TG-DTA and FTIR analyses reveal that the as-synthesized nanoparticles are pure CoO with good thermal stability. Raman and UV-vis absorption spectra show that the optical properties of CoO nanoparticles are of obvious size effect, which revealed their characteristic feature. Whatever the crystal structure and particle shape are, the CoO nanoparticles with sizes of 33,59 and 85 nm exhibit two band gaps, and the corresponding band gap differences are 1.84,1.62 and 1.42 eV, respectively. The pure hexagonal CoO nanoparticles display complete room-temperature paramagnetism, while the CoO nanoparticles that contain cubic phase show interesting magnetic behavior due to intrinsic antiferromagnetic structure and uncompensated surface spins, which were confirmed by VSM and ESR studies.We conducted magnetic study over wurtzite CoO nanocrystals （about 45 nm in size）. The blocking temperature TB and Neel temperature TN were confirmed by comprehensive magnetic characterization. Below TB of～7 K the nanocrystals exhibit coercivity of 400 O_e and exchange bias of 206 O_e ascribable to composition influences of uncompensated surface spins as well as to antiferromagnetic volume phase. The uncompensated magnetic sublattice and the spatial distribution of the anisotropy axis relative to the magnetic field are proposed to be responsible for the distinct electron spin resonance （ESR） lineshape. Based on the temperature dependence of ESR intensity, the accurate TN is found to be 245 K. It is observed that there is anomalous change in resonance field and linewidth around TN.The wurtzite CoO nanoparticles were annealed at 200-400℃ range for 1 h in Ar/H₂. The effects of annealing temperature on structure, morphology and magnetic properties were investigated in detail. As annealing temperatures rise, the obtained samples change from hcp-CoO/fcc-CoO complex phase to fcc-CoO/fcc-Co complex phase to fcc-Co single phase. Especially, the 300℃- and 325℃-annealed samples show the most distinct coexistence of ferromagnetic （FM） Co and antiferromagnetic （AFM） CoO components, and therefore displaying certain exchange bias （HE=284 and 250 O_e） and enhanced coercivity （Hc=1583 and 1148 O_e） at 5 K. The magnetization M, HC, HE and Neel temperature TN exhibit an interesting change rule, which are determined by three factors:size effect, phase composition, and FM-AFM interface coupling effect. Obviously, that adjusting the annealing temperature can control the microstructure of CoO （or Co） particle samples, many superior magnetic properties can be expected.Ⅲ. FeO/Fe₃O₄, MnO/Fe and γ-Fe₂O₃/MnO Binary NanoparticlesBy incorporating the thermal decomposition method into the controllable oxidation process, the core-shell type FeO/Fe₃O₄ nanoparticles can be synthesized. Among them, the average diameter of core is about 8 nm, while the shell thickness is about 3 nm. The core-shell FeO/Fe₃O₄ nanoparticles have Ms of 55.96 emu/g and He of 3 O_e at room temperature.By incorporating the two-step pyrolysis into the Ar/H₂ annealing treatment, the MnO/Fe binary nanoparticles with good crystallinity and controllable microstructure can be obtained easily. The optimal MnO/Fe binary nanoparticles are of average size of<10 nm and show the superparamagnetism at room temperature, and its blocking temperature is 37.6 K. At 5 K, there is weak coupling interaction between antiferromagnetic MnO and ferromagnetic Fe, and thereby giving a certain exchange bias （HE=88 O_e） and coercivity enhancement （ΔHC=41 O_e）.By incorporating the two-step pyrolysis into the Ar annealing treatment, the controlled synthesis of γ-Fe2O3/MnO binary nanoparticles can be realized. The results show that the optimal annealing conditions are 600℃ for 3 h, and the obtained γ-Fe2O3/MnO binary nanoparticles are of room-temperature superparamagnetism. ZFC and TRM curves exhibit a TB of 101 K and a TN of 113 K, respectively. Temperature and cooling field HFC have an important influence on the exchange bias of γ-Fe2O3/MnO binary nanoparticles. At 5 K, the coercivity HC（FC） and the coercivity enhancement ΔHc can reach 4551 O_e and 1173 O_e, respectively, in terms of HFC=50 kO_e; while the exchange bias HE is the biggest 3458 O_e when HFC=60 kO_e. Large HE value indicates that there are many ferromagnetic/antiferromagnetic interfaces in the binary nanoparticles. Just to confirm this conclusion, ESR spectra have found the coexistence of antiferromagnetic phase MnO and ferromagnetic phase in the present system.