Magnets prepared by adhering dysprosium (Dy), terbium (Tb) and their chemical compounds to a sintered body, followed by thermal diffusion along grain boundaries of crystals can suppress use amounts of Dy and Tb for enhancing coercive force (Hc), compare to those of magnets wherein Dy and Tb are added homogeneously to a mother phase thereof, and can maintain a high residual magnetic flux density (Br).
As prior art that utilizes a grain boundary diffusion technique for Dy, Tb and their chemical compounds from the surface of the magnet, patent document No. 1 discloses a magnet wherein Dy utilizing its low vapor pressure is adhered to the surface of the magnet, patent document No. 2 discloses a sintered magnet wherein Dy—F slurry is coated on the sintered body, followed by grain boundary diffusion, patent document No. 3 discloses a magnet wherein a treating solution of Dy—F and Tb—F in a sol-gel state is coated on the magnet and magnet powder, followed by adhering them to the magnet by drying. The magnets disclosed in the prior art are featured by high coercive force and high magnetic residual flux density. Patent document No. 4 discloses a magnet wherein an isotropic magnet powder treated with Dy—F and Tb—F solution is shaped at room temperature, which is featured by high productivity.
Patent document No. 1; Japanese Patent laid-open S61-264157
Patent document No. 2; WO 2006/043348
Patent document No. 3; Japanese Patent Laid-open 2006-66853
Patent document No. 4; Japanese Patent Laid-open 2007-281433
Although magnets having a small thickness prepared by adhering dysprosium (Dy), terbium (Tb) and their chemical compounds to the sintered body, followed by thermal diffusion along the crystalline grain boundaries exhibit remarkable enhancement of magnetic characteristics with respect to thin magnets, little enhancement of magnetic characteristics are observed in thick magnets because thermal diffusion of the elements into the center portion of the magnet. In addition since there is a distribution of coercive force in the inside of the magnets, which must be controlled by amounts of adhering, thermal treatment temperatures and heat treatment time, it is difficult to produce magnets with constant characteristics at one time.
The concentration of RTB elements are substantially constant throughout the magnet, which is composed of crystal grains of magnet powder. The concentration of the elements in a crystal grain is not constant between the surface and the inside.