  The maximum coercivity that can be achieved for a given hard magnetic alloy
is estimated by computing the energy barrier for the nucleation of a reversed
domain in an idealized microstructure without any structural defects and
without any soft magnetic secondary phases. For
Sm$_{1-z}$Zr$_z$(Fe$_{1-y}$Co$_y$)$_{12-x}$Ti$_x$ based alloys, which are
considered an alternative to Nd$_2$Fe$_{14}$B magnets with lower rare-earth
content, the coercive field of a small magnetic cube is reduced to 60 percent
of the anisotropy field at room temperature and to 50 percent of the anisotropy
field at elevated temperature (473K). This decrease of the coercive field is
caused by misorientation, demagnetizing fields and thermal fluctuations.
