Fabrication of high coercive Nd–Fe–B based thin films through annealing Nd–Fe–B/Nd–Fe multilayers

The stack structures of Ta (50 nm)/[Nd–Fe–B (Fe-rich) (7.3 nm)/Nd–Fe (Nd-rich) (6 nm)]10/Nd–Fe–B (Fe-rich) (t nm)/Ta (20 nm) (t=3, 7, 10 and 13) multilayers were fabricated by magnetron sputtering at room temperature. The annealed films exhibit the better perpendicular magnetic performance and the coercivity perpendicular to the film plane shows a clear dependence on the thickness of Nd–Fe–B (Fe-rich) capping layer after annealing at 600 °C for 10 min. The maximum coercivity obtained is about 21 kOe at a value of t=10 nm. The cross-sectional transmission electron microscopy (TEM) analyses indicate that the annealed film presents a heterogeneous structure: the large grains with certain preferred orientation in the inner part and the random nano-grains in the outward part. The plane view TEM image shows the amorphous grain boundaries among the hard magnetic grains. A partial preferred orientated structure was constructed and calculated by micromagnetic simulation (LLG simulator), in which the calculated magnetic hysteresis loops present a better perpendicular magnetic performance in agreement with the experimental results.