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.
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