Controlled Synthesis And Magnetic Properties Of CoO And Ni Nanoparticles

In recent years, with the development of chemical synthesis methods and the improvement of testing means, more and more efforts have been directed toward the magnetic properties of antiferromagnetic CoO nanoparticles. In the current study, however, there have been many controversial interpretations as to the magnetic sources of the CoO nanoparticles. Further, few works were reported on the magnetic properties of hexagonal CoO nanoparticles. At the same time, we see an endless number of reports about the research of traditional ferromagnetic Ni nanoparticles. Some researchers have got the crystalline phase-and morphology-controlled pure Ni nanoparticles by using the reaction of organometallic salt in organic media. But it is a pity, the influence of particle size and shape on the magnetic properties of Ni nanoparticles (such as saturation magnetization Ms, coercivity HC and Curie temperature Tc) has not been subjected to intensive study.Generally, the magnetic property of the nanoparticles with different crystal structures, particle sizes and shapes is not entirely the same. Based on this, we have selected CoO and Ni nanoparticles as the research objects. Firstly, the experiment accomplished the controlled synthesis of magnetic nanoparticles by using the thermal decomposition of organometallic salt in oleylamine. Secondly, this paper explored their magnetic properties deeply and particularly.On the one hand, cubic and hexagonal CoO nanoparticles have been prepared by the thermal decomposition of cobalt(Ⅲ) acetylacetonate (Co(acac)3) in oleylamine (OAm). Under same experimental procedure, three sequential CoO particle samples could be produced by simply changing the precursor concentration (the molar ratio of Co(acac)3and OAm), reaction time and temperature. The crystalline phase, microstructure, chemical stability and magnetic properties of CoO nanoparticles were analyzed by using XRD, SEM, TEM, Raman, FTIR, SQUID and ESR.On the other hand, pure Ni nanoparticles with cubic structure were prepared by the thermal decomposition of nickel(II) acetylacetonate (Ni(acac)2) in oleylamine. The size and shape of particles can be well controlled by changing the precursor concentration (the molar ratio of Ni(acac)2and OAm). Room temperature and5K hysteresis loops were measured by VSM and SQUID, respectively. The Curie temperature of samples was obtained by TG/DTA and VSM instruments, respectively. We present a detailed analysis of the size dependent Tc in terms of two theoretical models. (1)The main results of the cubic CoO nanoparticles are:①The CoO nanoparticles obtained under1/20and1/10precursor concentrations are42nm quasi-cubic particles and74nm spherical particles, respectively. The two samples have a pure face-centered cubic (fcc) structure, and the coated oleylamine prevents CoO particles from being oxidized.②For the42nm and74nm cubic CoO particles, their Neel temperature TN is225K and280K, respectively. At5K, the CoO nanoparticles exhibit anomalous magnetic properties such as large moments, coercivities and loop shifts.③The above results provide evidence for the formation of spin compensated random system in cubic CoO particles. The structurally distorted and magnetically disordered surface layer ferromagnetic phase played an important role in the magnetic behavior of cubic CoO nanoparticles. The smaller is the particle size, the stronger is the contribution of the ferromagnetic phase and the more is the surface layer helpful to enhance the observed coercivity HC and exchange bias HE.(2)The main results of the hexagonal CoO nanoparticles are:①When a solution of≤1/50molar ratio of Co(acac)3and oleylamine was employed, the CoO nanoparticles obtained under different reaction times and temperatures have a pure hexagonal close-packed (hcp) structure. The CoO nanoparticles are of hexagonal pyramid shaped configuration, and the particle size increases with increasing precursor concentration, reaction time and reaction temperature.②For the38nm,49nm,67nm and93nm hexagonal CoO nanoparticles, their blocking temperature TB is6K,9K,10K and11K, respectively. The comprehensive influence between the magnetic volume V and the effective magnetocrystalline anisotropy K leads to the slight change of TB from6K to11K.③The hexagonal CoO nanoparticles exhibit relatively large moments and coercivities accompany with specific loop shifts at5K. The saturation magnetization, coercivity and exchange bias increase monotonically with decreasing particle size, indicating an obvious size effect. These observations can be explained by the multisublattice model, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the total CoO particle.④In the ESR spectra of hexagonal CoO nanoparticles, the uncompensated magnetic sublattice, the nanoparticle magnetic anisotropy and the spatial distribution of the anisotropy axis related to the magnetic field were proposed to be most likely responsible for anomalous ESR behavior. Anomalous changes in resonance field and linewidth were observed near its Neel point. On the basis of the temperature dependence of ESR intensity, the TN values were found to be about225K,255K,285K and298K for the hexagonal CoO nanoparticles with sizes of38nm,49nm,67nm and93nm, respectively. The size dependence of TN showed that the Boltzmann curve could fit the experimental data efficiently.(3)The main results of the cubic Ni nanoparticles are:①The as-prepared Ni nanoparticles have pure fcc structure. As the precursor concentration increased from1/200to1/10, the size of Ni particles observably increases from24nm to200nm, and the shape change from spherical to dendritic and starlike. The organic coating on the particles prevents surface oxidation of Ni nanoparticles, rendering them stable over long periods.②Magnetic measurement reveals that all the Ni nanoparticles are ferromagnetic. The saturation magnetization is size-dependent, with a maximum value of52.01emu/g and82.31emu/g at room temperature and5K, respectively. The coercivity decreases at first and then increases with the increase of particle size and the change of shape, which is attributed to the competition between size effect and shape anisotropy.③All Ni nanoparticles shows an obvious ferromagnetic-paramagnetic transition at their Curie point. The precise Curie temperature is593K,612K,622K,626K and627K for the24nm,50nm,96nm,165nm and200nm cubic Ni nanoparticles, respectively. A theoretical model is proposed to explain the size dependence of Ni nanoparticles Curie temperature.