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We attempted compositionally graded sputtering process to overcome the phase separation in the growth direction in the FePt-C granular film in order to realize the columnar structure of FePt grains with an aspect ratio larger than 1.5. In order to overcome the problem of the formation of a second FePt-C layer for the thickness larger than 6 nm, we deposited the FePt-C film with changing the carbon concentration during the sputtering process. The formation of the second FePt-C layer was suppressed, which led to the growth of columnar grains with an aspect ratio of 1.5. The overall volume fraction of carbon is a decisive factor on the size of the FePt grains. Using the compositionally graded process, FePt-C nano-granular films with the average grain size of 7.8 nm, thickness of 10 nm and perpendicular coercivity of 40 kOe was obtained.

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In order to optimize the nanogranular structure of FePt-C for heat-assisted magnetic recording media, we investigated the influence of MgO underlayers on the growth of FePt grains in the FePt-C layer. The FePt-C layer was deposited by using the alternating sputtering method, by which FePt and FePt-C layers were alternately deposited. To understand the growth mechanism of the FePt-C layer on the MgO underlayers deposited under various conditions, detailed plan-view and cross sectional transmission electron microscopy observations were made for different film thicknesses. We found that columnar FePt grains grow only when the deposition conditions of the MgO underlayer are optimal. Direct TEM observation of the growth process of the FePt-C layer revealed that the number density of nuclei is sufficient in the initial stage of the film deposition; however, coarsening of the grains after grain impingement causes a substantial decrease in the number density of the FePt grains.