Evaluation of magnetic alignment in Nd-Fe-B magnet by magneto-optical domain observation

Magnetic alignment in Nd-Fe-B magnets is important for the application of the magnets as well as magnetic properties such as remanence and intrinsic coercivity. The magnetic orientation of sintered grains in Nd-Fe-B magnets which is produced by powder metallurgy, is influenced by the production process, especially during pressing in the magnetic field as well as by the materials used for the punches and die. The magnetization direction of each sintered grain in Nd-Fe-B was examined and two popularly shaped magnets were evaluated by magnetic domain observations while changing the magnetic properties of the die and punch materials. One was an arch shaped (C-type) magnet applied for a synchronous motor requiring magnetic orientation perpendicular to the horizontal base line to reduce cogging torque. The other was a cylindrical magnet applied for instrument panel equipment of automobile requiring magnetic orientation parallel to the diameter. The experiment shows that the magnetic alignment of both samples was strongly affected by the magnetic properties as well as by the design of the die and punch. The most favorable results were obtained when the top parts of upper and lower punches were substituted from ferromagnetic to non-magnetic material for C-type magnets, and the core rod was substituted from high Bs material to low Bs material for cylindrical magnets. Magnetic flux measurements carried out on the surface of the magnets and numerical calculation analysis of the magnetic field in the cavity of the die set indicate good agreement with magnetic domain observations. (orig.)