Hitachi Metals Ltd (Tokyo, JP) has created an ultra small rare-earth permanent magnet that can avoid degradation in magnetic properties, which is probably caused by the absence of the grain boundary phase on the surface. The magnet can keep good corrosion resistance even under a harsh environment, according to Hitachi Metals scientist Tomoki Fukagawa.
Recently, there are growing demands for ultra small magnets. The demands are escalating not just in the fields of optical pickups and ultra small motors but also in the fields of cardiosurgery and neurosurgery as well. In the fields of these cutting-edge medical treatments, a technique for controlling the direction in which a vascular catheter advances at a branching point of a blood vessel by attaching a small high-performance magnet to the end of the catheter and applying a magnetic field from outside of the patient's body has been researched. On the other hand, in a magnetic induction surgical system, it has been proposed that an ultra small magnet be embedded at a particular location of the body and used as a location marker. The ultra small magnets for use in such applications should have a cylindrical shape with a diameter of 0.3 mm and a length of 2 mm, for example
In U.S. Patent 7,655,325, Hitachi details the method to produce a rare-earth magnet with minimal deterioration in properties at the surface. The rare-earth magnet includes a magnet body made of an R--Fe--B based rare-earth magnet material (where R is at least one rare-earth element) and a metal film that has been deposited on the surface of the magnet body. The magnet further includes a plurality of reaction layers between the magnet body and the metal film. The reaction layers include: a first reaction layer, which contacts at least some of R2Fe14B type crystals, included in the magnet body, and a second reaction layer, which is located between the first reaction layer and the metal film and which has a lower rare-earth element concentration than that of the first reaction layer.
FIG. 1A is a cross-sectional view schematically illustrating the nanostructure of a rare-earth sintered magnet, FIG. 1B is a cross-sectional view of the sintered magnet of which the surface has been subjected to a machining process, and FIG. 1C is a cross-sectional view of a sintered magnet in which a metal film and reaction layers have been formed on its surface.