Promoted columnar grain structure in bilayer FePt:C/FePt:B films

FePt has drawn lots of attention for the next-generation perpendicular magnetic recording media because of its large magnetocrystalline anisotropy. Different segregants have been studied to achieve well-isolated grains of L1₀ FePt films with reduced grain sizes. C addition was reported as a promising segregant to obtain a small grain size of 6 nm with good L1₀ ordering and (001)-orient-ed FePt grains [1]. However, it is quite challenging to achieve columnar grains because extra C segregation forms on the top of FePt grains, leading to a second layer of FePt grains with (111) orientation. Columnar grains with elongated shape along the film thickness enhance the thermal stability and improve SNR. To eliminate the growth of the second layer, film thickness of FePt:C films is limited to less than 7 nm with grain size less than 10 nm. On the other hand, B as a segre-gant was demonstrated to achieve a grain size of 3 nm and good columnar growth with aspect ratio of 4 [2]; however, the B additions deteriorated the ordering of FePt grains, which led to a small coecivity of less than 1 kOe. The deterioration of L1₀ ordering is attributable to the formation of Fe-B bonding in the FePt grains [3].