Magnetic hysteresis and coercivity mechanism of a permanent magnet have been long discussed, and their understandings have been significantly
deepened during the last decade. Magnetization reversal in a permanent magnet proceeds with thermally activated nucleation and domain wall depinning
processes followed by magnetic domain expansions. To treat theoretically the thermally activated magnetization reversal process, atomistic spin model and
continuum calculations have been performed. Through these multi–scale calculations, the physical meanings of fluctuation field and activation volume which
have been used phenomenologically become very clear. Experimental approaches for both microscopic and macroscopic thermally activated magnetization
reversal processes and magnetic domain growths have been also performed. It is expected that these theoretical and experimental multi–scale studies will be
integrated to fully understand the magnetic hysteresis and coercivity mechanism with the aid of data science technologies in the future.