Columnar Structure in FePt–C Granular Media for Heat-Assisted Magnetic Recording

We attempted a 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 by 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 nanogranular films with an average grain size of 7.8 nm, a thickness of 10 nm, and a perpendicular coercivity of 40 kOe were obtained.