  We formulated a technique which combines the first-principles, micromagnetic
and microstructural calculations and allows us to study the nature of
hysteretic phenomena in hard magnets. Two distinct sources of coercivity in
polytwinned CoPt type magnets, domain wall pinning at antiphase boundaries and
splitting at twin boundaries, are illustrated for a realistic microstructure.
Methodology of multiscale modelling of hysteretic phenomena in nanoscale
magnets is discussed.
