The magnet of permanent magnet DC motor is hard magnetic material, that is, after the external magnetic field exceeding the coercive force is removed, a large part of magnetism remains. Keep a small part of magnetism as a soft magnetic material, such as an electromagnetic core that does not retain magnetism after power failure. Demagnetization (demagnetization) is weak reverse polarity magnetization again and again.
The magnet of permanent magnet DC motor produces a reverse magnetic field that exceeds the coercivity of the magnet and is less than the original remanence in the working environment. Namely demagnetization.