In this paper a continuum theory for modeling the behavior of ferromagnetic shape memory alloys is proposed.  The theory is a phase-field theory that contains two intrinsic material length scales associated with the thicknesses of the magnetic domain walls and the martensite twin boundaries.  The theory is used to investigate the low blocking stress that is observed in these materials.  It is postulated that the mechanism for the low blocking stress is the "dissociation" of the magnetic domain wall from its martensite twin boundary, where a strong magnetic field drives the magnetic wall in one direction and a strong shear stress drives the martensite twin in the opposite direction.