Y Shi

CoFe2O4 nanoparticles prepared by the mechanochemical method

Abstract Cobalt ferrite nanoparticles have been prepared by the combination of chemical precipitation, mechanical alloying and subsequent heat treatment. Sodium chloride was added before milling in order to avoid agglomeration. From XRD measurements it was found that the cobalt ferrite phase could form directly during mechanical milling of the precipitated hydroxide/oxidhydroxide precursor. Long-term milling caused contamination and growth in particle size. In order to reduce these two undesirable effects, a revised processing route, milling at a lower speed for a relatively shorter time and further heat treatment, was adopted. CoFe2O4 nanoparticles were obtained after a simple washing process with deionised water. TEM measurements showed that the nanoparticles had a fairly uniform structure and a mean particle size of approximately 10 nm. Anisotropic nanoparticles were obtained after magnetic annealing at 300°C.

Microstructure and soft magnetic properties of nanocrystalline Fe–Si powders

Abstract In this work, fine Fe–Si powders with a nanocrystalline structure were prepared by mechanical alloying (high-energy ball milling) and subsequent heat treatment (in order to optimize their magnetic properties). Good soft magnetic properties were obtained in mechanically alloyed Fe–Si powders. The Fe 75 Si 25 powder annealed at 450°C possessed a magnetization of 149 Am 2 /kg and a coercivity of 0.2 kA/m. The coercivity model for soft magnetic nanocrystalline materials could be well applied to the Fe–Si powders in this work. The mechanically alloyed Fe–Si possessed significantly higher magnetic permeability than that of commercially available Fe–Si powder. The permeability of the mechanically alloyed Fe 75 Si 25 powder was comparable with that of mechanically alloyed pure Fe powder. Considering of lower density and better chemical stability of Fe–Si, the mechanically alloyed Fe–Si may be interesting for soft magnetic application including magnetic shielding and electromagnetic noise suppression.

Cation migration and magnetic ordering in spinel CoFe2O4 powder: micro-Raman scattering study

Micro-Raman scattering was used to characterize spinel CoFe2O4 at high temperature up to 870 K and under an external magnetic field up to 6.0 kOe. It was found that the rapid increase in the linewidth of the Raman modes was related to the inter-site cation migration starting at ~390 K. A red-shift of the Raman peaks induced by the magnetic ordering was observed upon applying a magnetic field at room temperature. Phase analysis of CoFe2O4 powder was also carried out by means of x-ray diffraction and micro-Raman spectroscopy in this work.

High coercivity in SiO2-doped CoFe2O4 powders and thin films

Mossbauer and magnetic studies have shown that 1–2 wtu200a% of SiO2 could be dissolved in the CoFe2O4 structure. Changes in magnetization and Curie temperature were observed. The presence of SiO2 might enhance magnetic anisotropy. Coercivity values of up to 3.5 kOe were measured for mechanically alloyed CoFe2O4/SiO2 powders. High coercivities were also achieved in SiO2-doped Co–ferrite thin films prepared by sputtering technique. The Co–ferrite thin film deposited on a silicon wafer using a 5 wtu200a% SiO2/Co–ferrite target possessed a coercivity of 7.4 kOe. Spring-magnet behavior was observed, indicating the possible presence of remanence enhancement due to exchange coupling because of nanocrystalline structure.

A structural, magnetic and microwave study on mechanically milled Fe-based alloy powders

Fe90M10 powders with M=Fe, Co, Ni, Si, Al, Gd, Dy and Nd were prepared by mechanical milling. Their structure and magnetic properties were investigated. Microwave measurements were performed on the mechanically milled Fe90M10 powders. The results were compared with those of carbonyl Fe powders and coarse Fe powder. It has been shown that fine nanocrystalline Fe-based alloy powders prepared by mechanical milling are promising for microwave applications.

High coercivity in mechanically alloyed BaFe10Al2O19

Abstract Al-substituted Ba-ferrite in the form of BaFe10Al2O19 was prepared by mechanical alloying and subsequent heat treatment. Their structure and magnetic properties were investigated in comparison with Ba-ferrite without Al substitution. After annealing at 1100°C, a coercivity as high as 9.3xa0kOe was measured, while the coercivity values of the Ba-ferrite samples without Al substitution with a similar particle size were observed in the range of 5–6xa0kOe. A significant reduction of saturation magnetization was observed as compared to pure barium ferrite.

Strong uni-directional anisotropy in disordered NiFe2O4

Abstract High-energy mechanical milling of spinel NiFe2O4 leads to the formation of a disordered wustite-like structure. Cluster glass behavior was found in the Mossbauer study. The investigation suggested ferrimagnetic clusters in an antiferromagnetic matrix. The ferrimagnetic and antiferromagnetic exchange coupling results in a strong uni-directional anisotropy and a coercivity of over 10xa0kOe after magnetic cooling.

Structure and magnetic properties of copper(II) hexacyanoferrate(III) compound

Abstract Molecular magnet copper(II) hexacyanoferrate(III) powder, Cu 1.5 [Fe(CN) 6 ]·6H 2 O was prepared by coprecipitation method where the particle size is in the nanoscale range. It possessed a face-centered cubic (FCC) crystal structure with lattice parameter of around 10xa0A. This ferromagnetic powder has a Curie temperature of 19xa0K. The effects of annealing on its physical as well as magnetic properties were studied. The Fe III –CN–Cu II structure is the major structure in the freeze-dried Fe–Cu cyanide. As the cyanide was annealed at higher temperature, two other structures appeared, namely Fe II –CN–Cu III and Cu II –CN–Fe III as probed by Fourier-transform infrared (FTIR) and Mossbauer techniques. After annealing at 140°C, Fe III –CN–Cu II structure disappeared while Fe II –CN–Cu III structure appeared to be the dominant structure. Complete decomposition of the cyanide phase occurred at around 370°C, whereby nanoscale Fe–Cu particles were found, as well as significant amount of amorphous carbon particles. These soft magnetic Fe–Cu particles have M s value of 32xa0emu/g and coercivity of 0.5xa0kOe.

Strong unidirectional anisotropy in mechanically alloyed spinel ferrites

Cluster glass and relatively high coercivity at low temperatures were found in disordered ultrafine nickel ferrite powders. High-energy mechanical milling of spinel NiFe2O4 led to formation of a wustitelike structure. Our investigation suggested that ferrimagnetic clusters formed in an antiferromagnetic matrix. The strong ferri/antiferromagnetic exchange coupling resulted in a strong unidirectional anisotropy and a coercivity of over 10 kOe at 4.2 K.

Ni/Fe2O3 magnetic composite synthesized by mechanical alloying

Abstract Mechanical alloying of Ni and Fe 2 O 3 resulted in the formation of a disordered structure, when only broadened X-ray diffraction peaks corresponding to wustite appeared. After annealing at 500–900°C, a nanocomposite mixture of ferrite and (Ni,xa0Fe) was formed. The investigation showed that Fe atoms concentrated in the ferrite phase, while the intermetallic structure was Ni-rich.