Study On The Structure And Magnetic Properties Of FePt/Fe Multilayers

In this work, FePt thin films and FePt/Fe multilayers were prepared by magnetron sputtering and post annealing under high vacuum. The crystal structure, morphology and composition were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy and the magnetic properties have been measured by vibrating sample magntrometer. Ordered hard magnetic FePt phases with fct crystal structure have been successfully prepared, and exchange spring type permanent magnets with high energy product have been obtained.FePt phases in as-deposited FePt thin films and FePt/Fe multilayers is disordered soft magnetic phase with fcc crystal structure. After annealing at 500℃ under high vacuum, disordered soft magnetic phases turn into ordered hard magnetic phases with fct crystal structures. The grain sizes in FePt/Fe multilayers increase and their lattice constant a decreases after annealing, which indicate that Fe rich phases were formed as Fe composition in FePt layer increased.However, after annealing at 700℃, hystersis loop appeared as composed by two phases due to the precipitation of second phase. As the annealing temperature becomes high enough, the grains in FePt/Fe samples growth and the coupling between the soft and hard magnetic phase is not sufficient. Hence, annealing condition is critical in achieve good magnetic properties.In the optimization of hard magnetic FePt layers, it is found that FePt layers with thickness larger than 16nm can form fct structure and possess good hard magnetic properties after annealing.In the optimization of soft magnetic Fe layers, it is found that the lattice constant a decreases as Fe layer thickness increase, until certain point ( 11nm) where fct structures can not be formed.In the study of energy product as a function of Fe layer thickness, it is found that the energy product increases with Fe layer thickness––due to the increase of saturation magnetization––and reaches a maximum, then decrease rapidly––due to the decrease of Hc value. The optimized energy products of FePt/Fe multilayers are about 60% higher than that of FePt thin films. In the study, a roughly linear relationship exists between the lattice constant a of FePt phases and coercivity of FePt/Fe multilayers. Rapid annealing can also form ordered FePt phase with fct structures. However, the grain size is small due to the short annealing times. Ag underlayers under FePt/Fe multilayers can not increase the saturation magnetization of the samples, but it can slow down the phase transition and crystal growth processes.