Formation of $L1_{0}$ -FePt(001) Ultra-Thin Films With Flat Surfaces Using VC and VN Underlayers

FePt alloy films of 2 to 10 nm thicknesses are prepared on (001) single-crystal underlayers of MgO, VN, and VC by employing a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. MgO, VN, and VC crystals are selected as underlayer materials, since the crystals have the same NaCl-type crystal structure and similar lattice constants of about 0.42 nm but different surface energies. The single-crystal underlayers are prepared by RF sputtering on the single-crystal substrates of MgO and SrTiO
₃
at 600 °C. The influences of underlayer material and FePt film thickness on the
$L1_{0}$
ordering degree, the magnetic properties, and the surface morphology are systematically investigated. Epitaxial films involving
$L1_{0}$
(001) crystal structure with the
$c$
-axis normal to the substrate surface are formed on all the underlayers. As the film thickness decreases, the ratio of out-of-plane to in-plane lattice constant
$c/a$
decreases and the ordering degree
$S$
increases. Stronger perpendicular magnetic anisotropy is observed for an FePt film with a higher
$S$
value. The film thickness trends of
$c/a$
,
$S$
, and magnetic properties are similar to those for FePt films thicker than 5 nm formed on MgO underlayers. However, when the thickness is decreased to 2 nm, de-wetting of deposited atoms occurs on MgO underlayers during annealing at 600 °C. The surfaces of 2-nm-thick films formed on MgO underlayers are composed of isolated islands. On the contrary, continuous surfaces are realized for the investigated film thickness range of 2–10 nm in the cases of VC and VN underlayers. The surface roughness of FePt film formed on VN underlayer (
$R_{a}$
: 0.1–0.2 nm) is lower than that of film formed on VC underlayer (
$R_{a}$
: 0.1–0.4 nm). This paper shows that the employment of a VN underlayer is effective in the preparation of
$L1_{0}$
ordered FePt ultra-thin film with flat surface.