Effects of machining on magnetic properties of Nd-Fe-B system sintered magnets

Various Nd-Fe-B sintered magnets of different sizes were prepared by mechanical cutting and by the Bond method. Magnetic properties were measured as a function of the S/V value, which is the ratio of surface area to volume of the sample. For samples with S /V⩽1.10 mm^-1, the effects of machining on magnetic properties were small, while for those with S/V ⩽1.9 mm^-1, local deterioration in magnetization (kink) appearing in the low negative fields on the demagnetizing curve was detected. Such kink remains when heat treatment is performed, but disappears when Nd metal is sputter-deposited on the surface of the magnet and then heat treatment is applied. The values of H_k, (BH)_max, and SQ decrease with the increase of the ratio of S/V. For samples with 1.0⩽S/V⩽1.9 mm^-1 after machining, it was found that the magnetic properties of these samples were improved by heat treatment. In samples with a large S/V value, significant Barkhausen jumps were observed at room temperature