Magnetic Properties Of Cobalt Nanoparticles Research Articles

Properties of polycrystalline nanoparticles with uniaxial and cubic types of magnetic anisotropy of individual grains

The influence of the crystal structure inhomogeneities on the magnetic properties of cobalt nanoparticles with different aspect ratio and spherical nanoparticles of chromium dioxide, cobalt ferrite and magnetite has been studied by means of numerical simulation. The polycrystalline nanoparticles are modeled by means of subdivision of the nanoparticle volume into tightly bound single-crystal granules with randomly distributed directions of the easy anisotropy axes. The probability of appearance of quasi uniform and vortex states in sufficiently large assemblies of polycrystalline nanoparticles of various types have been calculated depending on the nanoparticle diameter. It is shown that the subdivision of a nanoparticle into single-crystal granules with different orientations of the easy anisotropy axes substantially reduces the effective single-domain diameters for particles with uniaxial type of anisotropy of individual granules. However, for particles with cubic type of magnetic anisotropy the influence of the crystal structure inhomogeneities on the equilibrium properties of the particles is not so important even for magnetically hard cobalt ferrite nanoparticles. It is practically absent for magnetically soft magnetite nanoparticles.

Elaboration, characterization and magnetic properties of cobalt nanoparticles synthesized by ultrasonic spray pyrolysis followed by hydrogen reduction

Nanostructured cobalt microspheres were synthesized by a two-stage method. First, cobalt oxide Co3O4 nanostructured microspheres were synthesized by spray pyrolysis at 700, 800, 900, 1000, and 1100, °C. Second, the nanostructured powders thus obtained were reduced to metal cobalt in a hydrogen atmosphere at 300°C. For a sample obtained at 1000°C, the influence of the reduction temperatures (220, 240, 270, 300, and 350°C) on the metal cobalt powders properties was investigated. All the samples obtained were characterized by means of X-ray diffraction (XRD), coherent length determination and scanning electron microscopy (SEM). In addition, an average size of the particles, a specific surface area, and an effective diameter were determined. Finally, the magnetic properties of the metal cobalt, such as coercive force, saturation, and residual magnetization were measured by vibration sample magnetometer (VSM). The dependence on synthesis conditions of nanostructured metal cobalt microspheres properties was highlighted.

Chemical synthesis of low-coercivity, silica/Co composites for high-frequency magnetic components

The paper reports correlation between processing, structural and magnetic properties of cobalt nanoparticles dispersed in an insulating oxide matrix. High-temperature, chemical decomposition approach was utilized to achieve highly-dispersed, ultra-fine cobalt nanoparticles in silica matrix. Synthesis parameters such as precursor to passivating agent ratio were designed to control the metal-to-oxide ratio so as to promote magnetic interactions between the metal particles while keeping the cobalt particle size in sub-nanometer. High-resolution TEM and XRD studies were carried out to demonstrate face-centered phase of single-crystal cobalt particles of size ∼8–10 nm monodispersed in thin silica matrix. The particle size variation as a function of precursor to passivating agent ratio was also studied to yield process conditions for ultra-fine nanoparticles. Composites with sub-10 nm metal particles showed superior soft magnetic properties with a lowest coercivity of 10 Oe. The reduction in coercivity with such fine particles is attributed to the transition to superparamagnetic regime. Larger particles showed higher saturation magnetization and higher coercivity of 100 Oe. The coercivity decreased with further increase in particle size from ∼50 nm to 80 nm because of the transition from single domain to multi-domain structures.

One-pot laser-assisted synthesis of porous carbon with embedded magnetic cobalt nanoparticles.

A novel one-pot laser-assisted approach is reported herein for the synthesis of ordered carbons with embedded cobalt nanoparticles. The process is based on a UV pulsed laser exposure of an ethanolic solution consisting of green carbon precursors, a structure directing agent and a cobalt salt. Very short irradiation times (5 to 30 min) are only required to polymerize and cross-link carbon precursors (i.e. phloroglucinol and glyoxylic acid) independent of a catalyst presence. The influence of three metallic salts (acetate, nitrate and chloride) on the phenolic resin and carbon characteristics (structure, texture and particle size/distribution) was systematically studied. When exposed to UV laser, the metallic salt exhibited a strong influence on the particle size and distribution in the carbon matrix rather than on the textural carbon properties. Using cobalt acetate, very small (3.5 nm) and uniformly dispersed particles were obtained by this simple, fast and green one-pot synthesis approach. An original combined (13)C CP-MAS and DP-DEC solid state NMR spectroscopy analysis allowed to determine the structure of phenolic resins as well as the location of the cobalt salt in the resin. Complementarily, the (1)H solid-state and relaxation NMR provided unique insights into the rigidity (cross-linking) of the phenolic resin and dispersion of the cobalt salt. The magnetic properties of cobalt nanoparticles were found to be size-dependent: large Co nanoparticles (∼50 nm) behave as bulk Co whereas small Co nanoparticles are superparamagnetic.

Enhanced magnetic properties of cobalt nanoparticles on FeMn films

Monodisperse cobalt nanoparticles are synthesized by the thermal decomposition of Co 2(CO) 8 in phenyl ether and subsequently deposited on antiferromagnetic (AFM) FeMn films and glass substrates, respectively. Magnetic measurement shows that the as-prepared Co nanoparticles are superparamagnetic and can be transformed into ferromagnetic (FM) through thermal treatment. While keeping monodisperse, the annealed FM Co nanoparticles on AFM FeMn films show a much larger coercivity than the ones on glass substrates due to FM/AFM exchange coupling. Accordingly, we propose a convenient method to enhance magnetic properties of nanoparticles.

The influence of silica matrix on the crystal structure and high frequency performance of cobalt nanoparticles

Cobalt–SiO 2 nanocomposites were successfully synthesized by chemical precipitation combined H 2 reduction method. The influence of SiO 2 matrix on the crystal structure and high frequency magnetic properties of cobalt nanoparticles was investigated. Amorphous SiO 2 layer, which is adhered to Co particle surface through Si–O–Co bonds, inhibits the growth of the nanoparticles and stabilizes the face-centered-cubic (FCC) structured Co. The FCC-cobalt/SiO 2 nanocomposites exhibit soft magnetic behavior with saturation magnetization ( M s ) about 125 emu/g and coercivity ( H c ) smaller than 100 Oe. The permeability spectra of Co/SiO 2 nanocomposites depend on the SiO 2 contents. When the SiO 2 content is greater than 8 wt%, the permeability spectra demonstrate notable high frequency characteristic, in which the real part μ′ of the permeability is almost independent of frequency and the imaginary part μ″ remains extremely low in high frequency range.

Preparation and magnetic properties of cobalt nanoparticles with dendrimers as templates

A series of cobalt (Co) nanoparticles were synthesized by employing dendrimers with a trimesyl core as templates and sodium borohydride as a reducing agent. Extensive characterizations of the produced materials were studied using TEM, selected area electron diffraction, energy-dispersive X-ray spectroscopy, FT-IR spectra, TGA, and XPS. And their magnetic properties, including hysteresis loop, field-cooled and zero-field-cooled curves, were investigated by superconducting quantum interference magnetometer. The nanoparticles exhibit supermagnetism at room temperature, accompanied by magnetic hysteresis below the blocking temperature. Co nanoparticles exhibited an excellent T 1 MRI enhancement effect and can be a potential MRI contrast agent.

Stability and Magnetic Properties of Cobalt Nanoparticles Covered by a Polymer Chitozan Molecule

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Structural and magnetic properties of cobalt nanoparticles encased in siliceous shells

Stable colloidal dispersions of cobalt nanoparticles were prepared by refluxing toluene solutions of dicobalt octacarbonyl in the presence of poly(dimethylsiloxane-b-(methylvinylsiloxane-co-methyl(2-trimethoxysilethyl)siloxane)) (PDMS-b-(PMVS-co-PMTMS)) dispersion stabilizers. The nanoparticles coated with the polysiloxane copolymer were subsequently heated at 600 or 700 °C with the goal of forming siliceous shells on the cobalt surfaces to protect them from oxidation. The thermolysis processes at 600 and 700 °C produced increases in the cobalt specific saturation magnetizations (σs) from 48 to 141 and 146 emu g−1 of cobalt respectively. The siliceous coatings provided oxidative protection under ambient conditions for long periods. However, mechanically grinding the thermally treated nanoparticles led to decreases in σs upon aging in air. Particle size analyses indicated an increase in average particle size in the materials heated at 600 °C, but the materials that were heated at 700 °C retained their smal...

Magnetic properties of cobalt nanoparticles obtained by ion implantation into amorphous silica

Magnetic characterization of cobalt nanoparticles obtained by 35keV Co+ ion implantation into amorphous SiO2 was carried out. Implantation fluences from 1.5 to 10×1016atoms/cm2 were used to control the average size of the nanoparticles. Transmission electron microscopy measurements revealed sizes ranging from ca. 2.5 to 10nm, depending on the implantation fluence. The magnetic properties were determined using a superconducting quantum interference device, with measurements carried out as a function of temperature and applied magnetic field. The results show that magnetic properties are dependent on the dimensions of the nanoparticles, with an increase of the blocking temperature, anisotropy constant, and relative remanent magnetization, and a decrease of the saturation magnetization field decreasing for larger particles.

Synthesis and magnetic properties of cobalt nanocubes

We report on the synthesis, structure, and magnetic properties of cobalt nanoparticles with cubic shape. The nanoparticles with an average cube-edge length of about $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were prepared by wet-chemical processing and characterized by electron microscopy, x-ray diffraction, and magnetometry. The x-ray data indicate a mixture of hcp Co and $ϵ\text{\ensuremath{-}}\mathrm{Co}$. From the magnetization measurements we deduce on the presence of a ferromagnet/antiferromagnet $\mathrm{Co}∕\mathrm{Co}\mathrm{O}$ interface, which gives rise to a pronounced exchange-bias effect. Between $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the coercivity ${H}_{C}$ increases by almost a factor of 6, whereas the exchange-bias field ${H}_{E}$ takes on a value of about $380\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$ at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and vanishes above $75\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The variation of ${H}_{C}$ with temperature cannot be understood in terms of the various conventional types of magnetic anisotropy.

The influence of various coatings on the electronic, magnetic, and geometric properties of cobalt nanoparticles (invited)

From the results reported here for Co nanoparticles coated with 3-(N,N-dimethyl-dodecylammonium)- propanesulfonate (SB12), Cu, or Au, and from experimental and theoretical results published by several other groups there is strong evidence that the various coatings (organic as well as inorganic) not just influence but even determine the properties of small metallic nanoparticles. In an empirical manner, the core-coating interaction is already used to influence the size and the shape of nanoparticles. Based on previously published results and some experiments, in this paper the influence is described that various coatings have on the geometric, electronic, and magnetic properties of cobalt nanoparticles with diameters smaller than 10nm. The results indicate that there is an interdependence of various properties (e.g., size and electronic properties of a particle with the same coating) so that is seems to be difficult to vary one property in a systematic way without changing others.

Chemical Control of Structural and Magnetic Properties of Cobalt Nanoparticles

Oxide-free cobalt nanoparticles (NPs) of 2.5−3 nm diameter have been synthesized in the presence of di-isobutyl aluminum hydride (DiBAH), a potential alumina source, from two different precursors: Co(η3C8H13)(η4C8H12) and Co[N(SiMe3)2]2. The NPs display very different structural and magnetic features, which are related to the difference in chemical environment at the NPs surface.

Synthesis and magnetic behavior of self-assembled Co nanorods and nanoballs

ε-cobalt nanoparticles with different shapes and sizes, e.g., rod-shaped Co (2×25 nm) and ball-shaped Co (diameter 4–9 nm), were synthesized by the rapid pyrolysis of octacarbonyldicobalt in o-dichlorobenzene solution in the presence of the surfactant, oleic acid. The size, size distribution, and the shape of the nanoparticles were influenced by changing the preparation conditions, such as the molar ratio of the surfactant (oleic acid) to reactant (cobalt carbonyl). The effects of the preparation conditions and annealing temperature on the shape and magnetic properties of cobalt nanoparticles have been investigated. It was found that as-synthesized rod-shaped cobalt nanoparticles (2×25 nm) show paramagnetic properties, but as-synthesized ball-shaped cobalt nanoparticles exhibit superparamagnetic behavior. After postannealing at 773 K for 30 min, the rod-shaped Co nanoparticles formed hcp structure and show ferromagnetic properties.

Self-assembly and magnetic properties of cobalt nanoparticles

Two- and three-dimensional superlattices of passivated cobalt nanoparticles were formed by a self-assembly technique. The size and stabilization of the cobalt nanoparticles are controlled by using the combination of oleic acid and triphenylphosphine. The cobalt nanoparticles are stable for at least 90 days without oxidation at room temperature under ambient conditions. The magnetic properties of the cobalt nanoparticles in different forms are compared, which provides helpful information on the magnetostatic interaction of the nanoparticles.

Ordered arrays and magnetic properties of cobalt nanoparticles

Monodisperse cobalt nanoparticles are produced by high-temperature thermal decomposition method. The influence of the size of nanoparticles, evaporation temperature, organic solvent, and concentration on the formation of the cobalt nanoparticles array is studied by transmission electron microscopy and ultraviolet absorption. Preliminary results of the cobalt nonoparticles magnetic properties are presented. These results lay a good foundation for the further study of the physical properties and the application in nanoscale devices of magnetic nanoparticles.

Preparation, structure, and magnetic properties of cobalt nanoparticles in carbon fibers

Samples of Co clusters embedded in a carbon fiber matrix have been prepared using a heat-treatment method of cellulose fibers with ion-absorbed cobalt cations. Depending on the heat-treatment temperature used, different cluster sizes have been obtained, ranging from superparamagnetic clusters (average size 10 nm) to ferromagnetic nanoparticles (size between 30 and 200 nm). The formation of graphite planes surrounding the nanoparticles has been observed.

Temperature Dependence of Morphology and Magnetic Properties of Cobalt Nanoparticles Prepared by an Inverse Micelle Technique

We have fabricated cobalt nanoparticles using sodium borohydride reduction of cobalt chloride in a didodecyldimethylammonium bromide (DDAB)/toluene inverse micelle solution. The particle morphology changed from single particles to clusters as we increased the reaction temperature. Intracluster dipole−dipole interaction increased the blocking temperature and reduced the effective magnetic moment per cluster.

Synthesis, Characterization, and Magnetic Properties of Cobalt Nanoparticles from an Organometallic Precursor

THF solutions of the new organometallic precursor Co(η3-C8H13)(η4-C8H12) react with 3 bar dihydrogen in the presence of polyvinylpyrrolidone (PVP) at three different temperatures, namely, 0, 20, and 60 °C to give colloidal solutions containing fcc cobalt particles of respectively ≤1 nm (Coll. 1) and ca. 1.5 nm (Coll. 2 and Coll. 3) mean sizes, as evidenced by HREM analysis. All colloids react with carbon monoxide to give new colloids (Coll. 1‘−3‘) displaying broad CO stretches near 2000 and 1890 cm-1 on their infrared spectra recorded in KBr disks. The magnetic behavior of the colloids was studied by SQUID techniques. Coll. 1−3 display a typical superparamagnetic behavior with blocking temperatures in the range 9−10 K. From magnetization studies above and below the blocking temperature, it was possible to determine that Coll. 3 shows a mean size of 1.6 nm and a very narrow size distribution. Because of a magnetic surface effect, the magnetic moment per atom (μCo = 1.94 ± 0.04 μB) appears significantly larger than the value admitted for bulk cobalt. In contrast, Coll. 2‘ shows a weak magnetization and disappearance of the blocking temperature as a result of CO coordination.