COERCIVITY OF BARIUM FERRITE PARTICLES WITH SUPERPARAMAGNETIC COATING

The coercivity mechanism of submicron barium ferrite (BF) particles wi th a superparamagnetic overcoat (i.e. delta-FeOOH or Fe3O4 fine partic les) was investigated by experimental observation and micromagnetics b ased on the two-component Stoner-Wohlfarth model with interfacial coup ling. For overcoating microcrystalline delta-FeOOH, BF particles were first coated with an Fe2+ solution at certain concentrations, followed by the addition of NaOH(aq), and finally oxidized by 35wt% H2O2(aq). For Fe3O4 coating, BF particles were coated with a mixed solution of F e3+ and Fe2+ first at certain concentrations, followed by NaOH(aq). Th e results showed that the coercivity of BF particles decreased greatly while their saturation magnetizations were maintained at high levels. When Fe3O4 nanoparticles were used as the coating material, the satur ation magnetization of the coated BF stayed at about the same level as that of the pure BF, 70 emu/g. The analytical calculation was success ful in explaining quantitatively the effect of the coating, showing th at the amount of coercivity decrement was related well to the uniaxial anisotropy constants of delta-FeOOH or Fe3O4 fine particles on the su rface of BF. In addition. the superparamagnetic coating was found to b ehave ferrimagnetically during magnetization reversal. The larger the volume of coating, the lower the coercivity. Moreover, the weaker exch ange coupling between the barium ferrite and delta-FeOOH or Fe3O4 furt her depresses the coercivity for the coated BF with thicker coatings.