Effects of Cr and SiN contents on the microstructure and magnetic grain interactions of nanocomposite FePtCr-SiN thin films

We have fabricated granular nanocomposite [(FePt)_100-x-Cr_x]_100-δ-(SiN)_δ thin films with x=0-25 at.% and δ=0-30 vol.% on natural-oxidized silicon substrate by dc and radio-frequency magnetron cosputtering of FePt, Cr, and Si₃N₄ targets. We annealed the as-deposited film in vacuum at 600°C and then quenched it with ice water in order to transform the soft magnetic face-centered cubic γ-FePt phase to the hard magnetic face-centered tetragonal γ₁-FePt phase (L1₀ phase). Transmission electron microscopy observation indicated that Cr and SiN can restrain the grain growth of magnetic grains, and that the structure of the film is a nonmagnetic SiN matrix with FePtCr particles dispersed in it. Average grain size of the magnetic particles decreased as Cr or SiN content increased. Energy dispersed spectrum (EDS) analysis showed that Cr exists mainly in the grain surface area (between the grain boundary and the inner grain) of the magnetic grains. The magnetic grains are isolated by SiN and magnetic grain interactions are reduced as Cr or SiN content is increased. Increasing the SiN volume fraction increases the thermal stability of the film. We found the [(FePt)₉₀-Cr₁₀]₈₅-(SiN)₁₅ film, annealed at 600°C for 30 min, is suitable for high-density magnetic recording. Average grain size of the FePtCr in this film is about 9.5 nm. Its in-plane coercivity H_c// is 3.7 kOe, saturation magnetization M_s is 450 emu/cm³, and in-plane squareness S_{// is about 0.75.}