Massive Replacement of Cu in FePt-Carbon Ultra-Thin Buffer Layer for Ku Enhancement of FePt Granular Films

The magnetic properties and nanostructure for FePtCu-30 vol% C (metallic part unit is in at%) films and Fe50Pt50-30 vol% B2O3 granular films deposited on FePtCu-30 vol% C buffer layer (BL) with Cu content of 30, 50, and 70 at% were investigated. The followings are confirmed for FePtCu-30 vol% C films: 1) monotonous decrease of saturation magnetization ( ${M} _{\text {s}}^{\text {film}}$ ) from around 470 to 100 emu/cm3 when Cu content increases from 0 to 70 at%; 2) low degree of ordering for granular films with Cu content of more than 50 at%; and 3) significant reduction of lattice constants (a and c), unit cell volume ( ${a} ^{2} {c}$ ) when Cu content increases from 0 to 70 at%, indicating the existence of Cu inside FePt grains. For Fe50Pt50-30 vol% B2O3 granular films deposited on FePtCu-30 vol% C BL: 1) no significant changes in hysteresis for films with BL of various thicknesses and Cu contents; 2) ${M} _{\text {s}}^{\text {film}}$ reduction from 740 to 640 emu/cm3 when Fe35Pt35Cu30-30 vol% C BL thickness increases from 0 to 2 nm and perpendicular magnetic anisotropy of the film peak of $1.7\times 10^{7}$ erg/cm3 at BL thickness of 0.6 nm, which is reflected in ${S} _{\text {in}}$ peak of 0.84; 3) integral intensity of (001) diffraction of the L $1_{0}$ -FePt phase decreases with increasing of FePtCu-C BL thickness until 0.6 nm, indicating the suppression of the existence of magnetic grains with c-axis parallel to the film plane; 4) ${a} ^{2} {c}$ reduction from around 55.8 to 55.1 Å3, suggesting the diffusion of Cu from FePtCu-C BL into FePt grains of FePt-B2O3 to induce large degree of order; and 5) reduction of average grain diameter (GD) from 9.55 to 8.85 nm with increasing Cu content from 0 to 70 at%, suggesting that FePtCu grains of BL act as nucleation sites to suppress lateral growth of FePt grains of FePt-B2O3 film.