A method of preparing a Cr<3+>-doped CoFe2O4 high-density magnetic-recording material belongs to the technical field of preparation of magnetic-recording materials. In the method, Cr<3+> is doped into
metal oxides through a hydrothermal method, so that size of product particles is changed. In a
spinel ferrite structure, iron ions are subjected to
crystal field to cause
quenching of orbital
angular momentum, which leads to low magnetic
crystal anisotropy and a low coercive force. As a result, by means of the Cr<3+>, of which the orbital
angular momentum is not quenched, for
doping the CoFe2O4, the CoFe2O4 is improved in magnetic
crystal anisotropy. By means of the hydrothermal method, the Cr<3+> is doped with the CoFe2O4, so that two methods of size controlling and element
doping are combined, thereby further increasing the coercive force of the magnetic medium. During the process of
doping the Cr<3+>, with increase of the doping amount of the Cr<3+>, the product is gradually increased in magnetic
crystal anisotropy, and meanwhile, the size of the particles is gradually changed, and when the size is in single-domain critical size range (about 40-100 nm), the product reaches a
peak value in the coercive force. The method can further increase the coercive force of the magnetic medium, and is economical and convenient and is easy to popularize.