Domains, which are groups of spins

Domains, which are groups of spins all pointing in thesame direction and acting cooperatively are separated bydomain walls, which have a characteristic width and energyassociated with their formation and existence. The motion ofdomain walls is a primary means of reversing magnetization.Experimental investigation of the dependence of coercivity onparticle size showed a behavior similar to that schematicallyillustrated in Fig. 1 [23]. Size reduction in magnetic materials(multi-domain materials) resulting in the formation of singledomainparticles also gives rise to the phenomenon ofsuperparamagnetism. Briefly, superparamagnetism occurs when thermal fluctuations or an applied field can easily movethe magnetic moments of the nanoparticle away from the easyaxis, the preferred crystallographic axes for the magneticmoment to point along. Each particle behaves like aparamagnetic atom, but with a giant magnetic moment, asthere is still a well-defined magnetic order in eachnanoparticle [25,31]. Superparamagnetic materials areintrinsically nonmagnetic but can be readily magnetized in thepresence of an external magnetic field. The critical radius rCfor different particles differ based on shape, temperature andcrystalline magnetoanisotropy [