FePt granular thin films for high density recording

Granular type L10 ordered FePt alloy thin films are considered as one of the promising candidates for perpendicular recording media with the areal density exceeding 1 Terabits/in2, since the large magnetocrystalline anisotropy (Ku=7×107 ergs/cc) of the L10 FePt phase provides thermal stability to the FePt particles as small as 4 nm [1-3]. From the technological point of view, alignment of the c-axis perpendicular to the film plane, reduction of L10 ordering temperature, reduction of particle size, narrow particle size distribution, and the optimization of coercivity to writable values is very important. Recently, extensive studies have been carried out both theoretically [4] and experimentally to establish a way to obtain (001) textured FePt films. These films were grown either epitaxially on a single crystal MgO substrate [5-7] or non-epitaxially on low-cost substrates (glass or thermally oxidized Si) [8-12]. Note that the use of single crystal MgO substrates would not be acceptable for commercial applications because of its high cost. Therefore, the development of L10 FePt granular perpendicular films grown on low-cost substrates is required. However, the control of particle size below 10 nm with narrow particle size distribution and the easy-axis alignment without expensive single crystal substrate remain a challenge [13].

We have carried out systematic studies on the effect of C on the microstructure and magnetic properties of FePt films grown on thermally oxidized Si substrate with a strongly textured MgO intermediate layer. The control of microstructure, degree of L10 order, average FePt particle size, and coercivity were investigated as a function of C content in FePt–C films and their thickness. A MgO layer of 10 nm thickness was deposited on thermally oxidized Si substrates at 100oC by RF sputtering a MgO target. Subsequently, the FePt(4 nm)–C(x vol.%) films and FePt(y nm)–C(50%) films with x = 0 – 50 and y = 4 – 10 were deposited by cosputtering of Fe, Pt and C targets at 500 oC. The nominal thicknesses of the FePt–C and MgO films were adjusted based on the sputtering time, while the C volume fraction in FePt was adjusted based on the precalibrated sputtering rates of FePt and C.


Figure 1 a-Si/MgO/FePt-C thin film and its size distribution and magnetization curve

Figures 1 show the microstructure and magnetic properties of the FePt(y nm)–C(50%) films, respectively. The average FePt particle size decreases to 5.5 nm with increasing FePt thickness to 5 nm and remains almost constant up to 10 nm thick FePt films. The size distribution has been narrowed down (variance = 2.3) with high fraction (~50%) of unique particles. A careful observation of micrographs indicated that the FePt–C films that were thicker than 6 nm consisted of two layers, one grown on MgO underlayer directly and the other grown on the first FePt layer. We have succeeded in fabricating FePt–C nanogranular films with average grain size of 5.5 nm and narrow size distribution on strongly (100) textured MgO intermediate layer that was sputter-deposited on economically viable substrates for the practical application of perpendicular recording media. The nanogranular films consist of well isolated FePt particles of 5.5 nm with narrow size distribution. The coercivity of the films is controllable between 8 and 15 kOe. As the films were grown on amorphous silicon oxide, the same microstructure would be achievable on amorphous soft underlayer for actual perpendicular recording. Thus, we believe that this work has demonstrated a step forward to the application of the L10 FePt phase for perpendicular recording media.

Press Release on 450 Gbit/in2 TAR Recording
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