Research On Thermal Stability Of Magnetic Tunnel Junction Structured CoFe/AlO_x/CoFe Multilayer Films

Reliability and life time of sensors based on giant magnetoresistance multilayers are restricted by the thermal stability of the materials. Many studies have been done on the thermal stability of magnetoresistance multilayers and related exchange coupled bilayers. In this thesis, the /Seed Ta/ Ir20Mn80/Co75Fe25/AlOx/Co75Fe25/Cap Ta/ magnetic tunnel junction multilayer was deposited by high vacuum magnetron sputtering. The effect of annealing on the magnetization process in the magnetic tunnel junctions has been studied by VSM, TEM and atomic force microscope. The thermal relaxation of the magnetic tunnel junction has also been studied and special attention has been paid on the mechanism of the thermal stability.The magnetization reversal of the pinned layer in both as-deposited and annealed magnetic tunnel junctions shows an obvious training effect caused by the realignment of spins at the interface of the ferromagnetic/antiferromagnetic layers. As the magnetic field sweep rate increases, the coercivity increases and the leading mechanism of the magnetization reversal in pinned layer varies from domain wall moving into the magnetic domain nucleation. The exchange bias increases during annealing due to the enhancement of unidirectional anisotropy of antiferromagnetic layer. The relaxation effect appears in the pinned ferromagnetic layer of both annealed and as-deposited films while holding the films in a negative saturation field of the ferromagnetic layer which is caused by thermally activated reversal of antiferromagnetic moments overcoming certain energe barriers. The slight growth of the grain size and the reduction of the density of defects during annealing result in the increasement of energe barriers of magnetization reversal, so the relaxation timeτD increases and the thermal stability has been improved. The exchange bias and the coercivity of the pinned layer decrease monotonously with the temperature increasing. The coercivity of the free layer increases with the temperature increasing up to 373 K and as the temperature increases further it reduces sharply. Hold the film at its negative saturation at various temperatures, the hysteresis loop of the pinned ferromagnetic layer shifts towards the zero field and the exchange bias decreases monotonously. Increasing the temperature enhances the decay of the exchange bias field. The variation of the thermal stability caused by of temperature can be contributed to the alteration reversal mechanism with diffrent energe barrier distributions at various temperatures.