Magnetic Properties

The magnetic properties of carbon-coated iron and iron carbide nanocapsules, synthesized by arc discharge of carbon rods, were studied [596]. These nanocapsules consist of a-Fe, y-Fe, and Fe3C. Their coercive force was larger than that of bulk a-iron, being ascribed to the small particle size.

Iron particles encaged in carbon nanocapsules were produced by the carbon arc-discharge method [597]. Soot, collarette, and cathode samples were characterized by Moss-bauer spectroscopy and magnetic measurements in the temperature range 4.2-300 K. Different iron phases and iron-carbon solid solutions were detected. The Einstein model was used to evaluate the coupling constant between the particles and their environment, yielding values of the order 1-10 N/m. The superparamagnetism presented only if the particles presented a blocking temperature above 300 K.

Synthesis, crystal structures, and magnetic properties of Co particles encapsulated in carbon nanocapsules were studied [598]. Cobalt particles grown were in an fcc phase, with a trace amount of hcp Co. Thickness and structure of outer carbon layers could be controlled by varying the relative area of a Co-packed hole drilled in the graphite rod. Temperature dependence of the measured Ms was consistent with that for fcc Co. The highest value of Ms of 160 Am2/kg at room temperature, nearly the same value for bulk fcc Co, was obtained for Co particles covered with thin carbon layers.

Magnetic properties of graphitically encapsulated nickel nanocrystals synthesized via a modified tungsten arc-discharge method were investigated [599]. By virtue of the protective graphitic coating, these nanocrystals were stable against environmental degradation, including extended exposure to strong acids. The magnetic properties of the encapsulated particles were characterized with regard to the nanoscale nature of the particles and the influence of the graphitic coating. The Curie temperature of graphitically encapsulated Ni nanocrystals was the same as that of micro-crystalline Ni. The saturation magnetization, remanent magnetization, and coercivity of these particles were reduced, for a range of temperatures. The unique features were compared with those of unencapsulated nanocrystalline and coarse microcrystalline nickel particles.

The structure and magnetic properties of carbon coated nickel and cobalt nanocrystals synthesized in a special low carbon to metal ratio arc chamber were reported [600, 601]. Magnetization measurements as a function of temperature in the range 20-900 °C gave a Curie temperature equal to that of bulk metal within experimental error. Upon heating and recooling of the particles, a large magnetization as high as 57% of bulk Co and 53% of bulk Ni was obtained. The dependence of room temperature saturation magnetization, remanent magnetization, and coercive field of the particles on annealing temperature was reported. These data were described by transition of particles from single domain to multidomain as a result of particle growth due to annealing. The particle size distribution measurements showed a lognormal behavior, indicating substantial particle size growth due to annealing.

Carbon-coated cobalt nanocrystals were prepared by a modified method of arc discharge under a He gas blow around a graphite rod cathode [602]. The magnetic properties of these particles after acid treatment were measured. Large shifts of Raman peaks were observed upon acid treatment, reflecting the structural change of the carbon shell induced by the acid treatment. The blocking temperature of the sample was estimated to be 373 °C.

Shell-core structures of Fe(C), Co(C), and Fe-Co(C) nanocapsules, prepared by an arc-discharge process in a mixture of methane and helium, were demonstrated by means of HRTEM [119-121, 123]. These nanoscale magnetic cores are protected by graphite shells. The ZFC magnetization of Fe-Co(C) nanocapsules, displaying different characteristics in three temperature ranges, can be well interpreted in terms of the unblocking of magnetization of small single-domain particles and the depinning of large multidomain particles. The saturation magnetization of these nanocapsules decreased monotonically, while the coercivity decreased significantly with increasing temperature. Figure 21 represents magnetization curves at room temperature of Fe-Co(C) nanocapsules [120]. Figure 22 shows ZFC and (B = 0.01 T)

Figure 21. Magnetization curves at room temperature of Fe-Co(C) nanocapsules. Samples a-f correspond to 10 wt%, 20 wt%, 30 wt%, 45 wt%, 60 wt%, and 80 wt% Co content in master alloys, respectively. After [120], X. L. Dong et al., J. Appl. Phys. 86, 670l (1999). © 1999, American Institute of Physics.

Figure 22. Zero-field-cooled (ZFC) and (B = 0.01 T) field-cooled (FC) magnetization curves of Fe-Co(C) nanocapsules. After [121], Z. D. Zhang et al., J. Phys. Cond. Matter 13, 1921 (2001). © 2001, IOP Publishing.

Figure 21. Magnetization curves at room temperature of Fe-Co(C) nanocapsules. Samples a-f correspond to 10 wt%, 20 wt%, 30 wt%, 45 wt%, 60 wt%, and 80 wt% Co content in master alloys, respectively. After [120], X. L. Dong et al., J. Appl. Phys. 86, 670l (1999). © 1999, American Institute of Physics.

Figure 22. Zero-field-cooled (ZFC) and (B = 0.01 T) field-cooled (FC) magnetization curves of Fe-Co(C) nanocapsules. After [121], Z. D. Zhang et al., J. Phys. Cond. Matter 13, 1921 (2001). © 2001, IOP Publishing.

FC magnetization curves of Fe-Co(C) nanocapsules [121]. The results could be used to compare with those of ultrafine Ni [603], Fe-Ni [604], and Fe-Co particles [605].

Carbon-encapsulated Ni nanoparticles were synthesized using a modified arc-discharge reactor under methane atmosphere [118, 606, 607]. The average particle size was revealed to be typically 10.5 nm with a spherical shape. Superparam-agnetic property studies indicated that the blocking temperature (TB) was around 115 K at an applied field of 0.1 T. Below Tb , the temperature dependence of the coercivity was given by Hc = Hci[1 — (T/TB)1/2], with Hci approximate to 500 Oe. Above TB, the magnetization M(H, T) can be described by the classical Langevin function L using the relationship M/Ms(T = 0) = coth (fH/kBT) — kBT/fH. The particle size can be inferred from the Langevin fit (particle moment f) and the blocking temperature theory (TB), with values slightly larger than the HRTEM observations. It was suggested that these assemblies of carbon-encapsulated Ni nanoparticles had typical single-domain, field-dependent superparamagnetic relaxation properties.

The magnetic properties of boron-oxide-encapsulated magnetic nanocapsules fabricated by arc discharge in dib-orane (B2H6) atmosphere were investigated in the temperature range from 4.2 to 300 K [122-124]. The saturation magnetizations of Fe(B) and Co(B) nanocapsules decreased monotonically with increasing temperature. The coercivities of the Fe(B) nanocapsules are almost two orders of magnitude higher than that of bulk Fe. The loop shift in the hysteresis loop the Fe(B) and Co(B) nanocapsules indicated the existence of the exchange bias between ferromagnetic and antiferromagnetic components. The Fe3O4 or Co3O4 phase in the shells contributed to the exchange bias as antiferromagnetic components. The very wide energy barrier distribution existed in these boron-oxide-encapsulated nanocapsules.

After passivation, the Fe3C particles prepared by laser-induced pyrolysis of Fe(CO)5 and C2H4 exhibited a high saturation magnetization of 132 Am2/kg compared to that of the a-Fe particles, 95 Am2/kg [151]. Thin amorphous carbon layers, formed on the surfaces of the Fe3C particles, inhibited oxidation of the Fe3C, resulting in the high saturation magnetization achieved by Fe3C particles. The role the thin carbon layer formed on the iron carbonitride particle surface played in the oxidation behavior and in the enhancement of the magnetic properties was studied [152]. A carbon layer (1-2 nm) protected the particles effectively from reaction of the iron carbonitride with oxygen, and the iron carboni-tride particles exhibited a high saturation magnetization of 142 Am2/kg.

Ferromagnetic nanocrystallites of carbon- and CxN-coat-ed iron and its compounds were synthesized by a modified ac arc method in N2 buffer gas [608, 609]. The metal to carbon rate of the particles, ranging from 73.5 to 80.5 wt%, was controlled by the pressure of the buffer gas charging from 0.06 to 0.01 MPa. Typical sizes of these particles were 10-30 nm. The magnetic properties of these particles were measured.

Glass-metal nanocomposite powders in the Fe/SiO2 and Ni/SiO2 systems were prepared by the sol-gel technique followed by reduction treatment [610]. Bulk nanocomposites were then fabricated by hot pressing these powders. The metal particle diameters ranged from 8.9 to 14.8 nm. The materials showed enhanced coercivities. The Mossbauer spectra of Fe/SiO2 samples were comprised of a ferromagnetic component superposed on a superparamagnetic doublet.

Nanocomposites of y-Fe2O3 in a silica matrix were prepared by the sol-gel method using tetramethylorthosilicate as a precursor of silica and introducing iron as Fe(NO3)3 with Fe/Si ratios of 2, 5, 10, and 20% [611, 612]. Superparamagnetic Fe3+ oxide nanoparticles with a narrow size distribution, dispersed over the amorphous silica matrix, were present in Fe2O3-SiO2 nanocomposites (9-33 wt% Fe2O3), prepared by a sol-gel method [613, 614]. Superparamagnetic magnetization curves were filled by a Langevin function considering a log-normal particle size distribution. The specific Faraday rotation spectrum in a magnetic nanocomposite of y-Fe2O3/SiO2 exhibited a narrow peak centered around 765 nm, reaching a value of 110 degrees/cm and an absorption coefficient of 64 cm-1 [612].

Iron particles having diameters around 8 nm and loosely packed with nanosized copper particles were prepared by a sol-gel route [615]. The samples exhibited coercivities in the range 200 to 500 Oe that were typical of single-domain iron grains. The Mossbauer spectrum was consistent with the presence of a-Fe particles in the system. A finite value of the isomer shift was obtained, which was ascribed to possible electron transfer between the iron atoms and the surrounding copper matrix.

Mn0 5Zn05Fe2O4 ferrite nanoparticles (<100 nm) in SiO2 matrix were prepared by the sol-gel method [616]. The nanoparticles showed superparamagnetic behavior when the particle size was below 20 nm, which was confirmed by Moss-bauer spectroscopy. The average particle size in the super-paramagnetic state was also estimated from the low-field magnetization measurement by considering the samples as consisting of noninteracting single domain particles.

The phase formation of nanocrystalline NiFe2O4 particles involved the nucleation of Fe3O4 in amorphous silica at the initial stage of mechanical activation, followed by the growth of nickel ferrite by incorporation of Ni2+ cations into Fe3O4 [617]. Their magnetic anisotropy, surface spin disorder, and cation distribution were investigated by considering both the strain imposed by silica matrix and the buffer effect during mechanical activation.

The room temperature magnetic properties of Ir-Co and La-Zn substituted Ba-ferrite powders, prepared for the sol-gel method, were investigated [618]. Saturation magnetization increased with La-Zn substitution in contrast to Ir-Co substitution. La-Zn mixtures were the least effective in reducing coercivity, while Ir-Co led to a fast reduction at low levels of substitution.

Co65C35 nanocomposite film was prepared by pulsed filtered vacuum arc deposition [619]. The as-deposited film was found to be amorphous and ferromagnetically soft. No obvious magnetic domain structures can be observed in the MFM image, indicating the low anisotropy in the amorphous film. After annealing at a temperature between about 300 and 350 °C, the as-deposited amorphous films went through a metastable stage in which a cobalt carbide phase and hcp crystalline cobalt coexisted. Upon increasing the annealing temperature to 400 °C, the carbide phase decomposed into hcp crystalline cobalt nanograins and graphitelike carbon.

Polyelectrolyte multilayer capsules were introduced as versatile magnetic carrier systems [620]. Superparamagnetic magnetite was mounted to the multilayer shell itself or was a component of the capsule interior. The polyelectrolyte multilayer was formed at different (decomposable) colloidal templates (e.g., melamine formaldehyde resin, glutaralde-hyde fixed red blood cells, emulsion oil droplets).

The catalytic synthesis of carbon nanotubes filled with long continuous cobalt nanowires by a simple chemical process and a following treatment with HCl were reported [621]. The average diameter of the multiwalled nanotubes was about 40 nm. The nanowires were a few micrometers long with a diameter of 20 nm. The cobalt nanowires, encapsulated in the carbon nanotubes, had a fcc structure. The filled nanotubes can be separated by using a permanent magnet.

The magnetic properties of boron nitride nanocapsules with iron oxide nanoparticles, fabricated by an arc-discharge method, were investigated [622]. The iron oxide nano-particles of 20 nm size were encapsulated by boron nitride sheets of 4 nm width. Magnetization of the BN nanocapsules showed paramagnetism and the initial iron oxides showed ferromagnetism, suggesting the transformation into super-paramagnetism by separating the iron oxide nanoparticles with BN sheets.

Efficient synthetic strategies were developed for single molecular magnets (i.e., the design of new magnetic poly-oxometalate clusters) [623-637]. Polyoxometalate chemistry served as an inexhaustible source for molecular models for different reasons, especially due to their versatile redox chemistry. Referring to molecular magnets, paramagnetic centers can be embedded in a structure-determining dia-magnetic polyoxometalate (linker) framework giving rise to properties which were intermediate between simple param-agnetism and bulk magnetism. For example, in {Mo57M6}-type cluster systems, it was even possible to place (or exchange) stepwise different (para)magnetic centers M like Fe2+/3+ and V4+O2- in the respective linker positions, thus allowing some control over the cluster's magnetic properties or even the tuning of these.

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