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Samples of maghemite and cobalt-ferrite nanoparticles ͑sizes, 3-10 nm͒ were prepared by cross-linking sulfonated polystyrene resin with aqueous solutions of ͑1͒ FeCl 2 , ͑2͒ 80% FeCl 2 + 20% CoCl 2 , ͑3͒ FeCl 3 , and ͑4͒ 80% FeCl 3 + 20% CoCl 2 by volume. Chemical analysis, x-ray powder-diffraction, and 57 Fe Mössbauer spectroscopic measurements show that samples 1 and 3 consist of ␥-Fe 2 O 3 nanoparticles ͑sizes, ϳ10 and 3 nm͒ and sample 2 and 4 consist of Co x Fe 3−x O 4 nanoparticles ͑sizes, ϳ10 and 4 nm͒. The temperature dependence of the zero-field-cooled and field-cooled magnetizations at low temperatures, together with a magnetic hysteresis in the M versus H data below blocking temperatures, demonstrate superparamagnetic behavior. The introduction of Co in the iron oxide-resin matrix results in an increase in the blocking temperature of nanoparticles.

The low-temperature dynamics of a magnetic nanoparticle system ͑␥-Fe 2 O 3 -alginate nanocomposite with average particle size around 4 nm͒ have been studied by superconducting quantum interference device measurements. Using different temperature and field protocols, memory phenomena in the dc magnetization and magnetic relaxation have been observed at temperatures below its blocking temperature T B = 37 K. However, aging experiments show an absence of any waiting time dependence in the magnetization relaxation. These observations indicate that the dynamics of this nanoparticle system are governed by a wide distribution of particle relaxation times which arise from the distribution of particle sizes and weak interparticle interactions.

Articles you may be interested inDistinguishing magnetic particle size of iron oxide nanoparticles with first-order reversal curves Effect of size, composition, and morphology on magnetic performance: First-order reversal curves evaluation of iron oxide nanoparticles J. Appl. Phys. 115, 044314 (2014); 10.1063/1.4863543Role of inhomogeneous cation distribution in magnetic enhancement of nanosized Ni0.35Zn0.65Fe2O4: A structural, magnetic, and hyperfine study Synthesis and magnetic properties of gold coated iron oxide nanoparticles Nanoparticles of ␥-Fe 2 O 3 ͑size 2 -3 nm͒ were precipitated in alginate hydrogels by cross-linking sodium alginate with Fe ions in a methanol-water solution. The zero-field-cooled and field-cooled magnetization measurements between 5 and 350 K and the hysteresis in the M vs H relation below the blocking temperature indicate superparamagnetic behavior. The temperature dependence of the coercive field is not consistent with the T 1/2 behavior predicted by Néel and Brown for the noninteracting particles. The average diameter of the nanoparticles determined from the magnetic data is consistently larger than the corresponding particle size determined by x-ray diffraction, perhaps due to interparticle magnetic interactions.

Fe 2 O 3 magnetic nanoparticles ranging in average diameter from 2 to 4 nm were precipitated within an alginate hydrogel and characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, and SQUID magnetometry. Regardless of the initial Fe valence state of the starting chloride salt, Mössbauer spectroscopy confirmed that -Fe 2 O 3 was the only phase present. As expected, the nanoparticles exhibited superparamagnetic behavior with the magnetic moments becoming frozen with decreasing temperature as evidenced by a bifurcation in the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations and a hysteresis in the -vs.-curves. The values of effective magnetic anisotropy ( 10 6 ergs/cm 3 ) determined from the differences between the ZFC and FC magnetizations were found to be an order of magnitude larger than the magneto-crystalline anisotropy for bulk -Fe 2 O 3 , and are probably the result of surface and particle size dependent effects. Likewise, the nanoparticle size distributions as deduced from the blocking temperature distribution function ( ) based on fits to the difference in the ZFC and FC magnetization curves as well as from fits of the -vs.-curves with a Langevin function in the superparamagnetic regime indicate fairly broad distributions of particle sizes with the particle sizes being comparable to those deduced from XRD measurements. The smaller saturated magnetization values found for these nanoparticles than the bulk value combined with the non-zero slope of the high-field magnetization data suggests that these nanoparticles have a non-negligible surface layer of non-collinear spins surrounding a ferrimagnetically ordered -Fe 2 O 3 core.

In this investigation, a new soft magnetic material (iron with 5 wt% aluminum) has been developed using powder metallurgy processing. The microstructure and the magnetic properties of this new P/M alloy have been characterized at both room and elevated temperatures (up to 500 C). The influence of post-sintering (after initial processing) on the porosity and magnetic properties of this material has also been examined.Test results show that the room temperature soft magnetic properties of this alloy are comparable to other commercially available soft magnetic materials such as P/M pure Fe, Fe-Si, Fe-P, etc. Post-sintering at 1316 C resulted in significant grain growth and lower porosity with more rounded pore morphology and improved the magnetic properties. While the magnetic induction of the alloy was essentially constant from room temperature to 500 C, the coercivity of the material decreased significantly at elevated temperature. This new P/M alloy may be a suitable soft magnetic material for high temperature (up to 500 C) applications.

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