Rare earth-based sintered magnet and permanent magnet synchronous motor therewith

Disclosed is a rare earth-based sintered permanent magnet block used in the rotor of a permanent magnet synchronous motor. The magnet block of the form having an upper surface and a lower surface in parallel is provided in each of the upper and lower surfaces with at least one narrow slit having a specified incision depth, width and length so as to decrease the adverse influences of the eddy currents generated in the magnet block while the running direction of the slits in the upper surface and the running direction of the slits in the lower surface make an angle of at least 10 DEG or, preferably, 90 DEG so that the decrease in the bending strength of the magnet block due to the slits can be minimized without decreasing the desired eddy current-decreasing effect.