Coercivity of barium ferrite particles with superparamagnetic coating

• Published in: Japanese Journal of Applied Physics 1997-08, Vol.36 (8), p.5091-5096
• Main Authors: TUNG, I.-C, YANG, J.-S, CHIN, T.-S
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Fine-particle systems; Magnetic properties and materials; Physics; Small particles and nanoscale materials; Studies of specific magnetic materials
• ISSN: 0021-4922; 1347-4065
• DOI: 10.1143/jjap.36.5091

The coercivity mechanism of submicron barium ferrite (BF) particles with a superparamagnetic overcoat (i.e. δ- FeOOH or Fe 3 O 4 fine particles) was investigated by experimental observation and micromagnetics based on the two-component Stoner-Wohlfarth model with interfacial coupling. For overcoating microcrystalline δ- FeOOH, BF particles were first coated with an Fe 2+ solution at certain concentrations, followed by the addition of NaOH ( a q ) , and finally oxidized by 35 wt% H 2 O 2( a q ) . For Fe 3 O 4 coating, BF particles were coated with a mixed solution of Fe 3+ and Fe 2+ first at certain concentrations, followed by NaOH ( a q ) . The results showed that the coercivity of BF particles decreased greatly while their saturation magnetizations were maintained at high levels. When Fe 3 O 4 nanoparticles were used as the coating material, the saturation magnetization of the coated BF stayed at about the same level as that of the pure BF, 70 emu/g. The analytical calculation was successful in explaining quantitatively the effect of the coating, showing that the amount of coercivity decrement was related well to the uniaxial anisotropy constants of δ- FeOOH or Fe 3 O 4 fine particles on the surface of BF. In addition, the superparamagnetic coating was found to behave ferrimagnetically during magnetization reversal. The larger the volume of coating, the lower the coercivity. Moreover, the weaker exchange coupling between the barium ferrite and δ- FeOOH or Fe 3 O 4 further depresses the coercivity for the coated BF with thicker coatings.