Microstructure and magnetic characteristics of nanocrystalline Ni_0.5Zn_0.5 ferrite synthesized by a spraying--coprecipitation method

Nanocrystalline Ni_0.5Zn_0.5 ferrite with average grain sizes ranging from 10 to 100~nm is prepared by using a spraying--coprecipitation method. The results indicate that the nanocrystalline Ni_0.5Zn_0.5 ferrite is ferromagnetic without the superparamagnetic phenomenon observed at room temperature. Specific saturation magnetization of nanocrystalline Ni_0.5Zn_0.5 ferrite increases from 40.2 to 75.6 emu/g as grain size increases from 11 to 94nm. Coercivity of nanocrystalline Ni_0.5Zn_0.5 ferrite increases monotonically when d < 62 nm.The relationship between the coercivity and the mean grain size is well fitted into a relation H_\rm c \sim d^3. A theoretically evaluated value of the critical grain size is 141nm larger than the experimental value 62nm for nanocrystalline Ni_0.5Zn_0.5 ferrite. The magnetic behaviour of nanocrystalline Ni_0.5Zn_0.5 ferrite may be explained by using the random anisotropy theory.