The induced effective uniaxial anisotropy of Mn and Co volume-doped acicular γ-Fe₂O₃ particles

The influences of manganese ions on the coercivity and induced uniaxial magnetic anisotropy of Mn and Co volume-doped acicular γ-Fe particles were studied. These particles, with a coercivity of 923 Oe and Co content of 3.5 wt%, were prepared by the usual method but with an extra special heat treatment process, step annealing. The induced effective uniaxial anisotropy constant K_u and the magnetocrystalline anisotropy constant K₁ of the particles were determined by analyzing the torque curves. The activation energy E of these samples was measured and was found to be about 0.68 eV. The higher coercivity and magnetic thermal stability, and the lower temperature coefficient of coercivity of these particles are due mainly to the higher and more stable induced effective uniaxial anisotropy, resulting from the anisotropic arrangement and preferred orientation of cobalt ions promoted by the presence of manganese and the process of step annealing. With the above characteristics and low Co content, these acicular particles could be promising candidates for particulate high-density magnetic recording materials.