Effect of addition of intermediate Au layer with thickness of 30 nm in nanoscale film composition (NFC) Fe$_{50}$Pt$_{50}$ (15 nm)/Au (30 nm)/Fe$_{50}$Pt$_{50}$ (15 nm) on SiO$_{2}$ (100 nm)/Si(001) substrates on the diffusion-controlled phase-formation processes–transformation of chemically-disordered soft magnetic А1(FePt) phase into chemically-ordered hard magnetic L1$_{0}$(FePt) phase under annealing in a vacuum at temperature of 600°C during various time intervals is investigated by the methods of physical materials science: X-ray phase analysis and magnetic-properties’ measuring on SQUID (superconducting quantum interference device). As revealed, the А1(FePt) → L1$_{0}$(FePt) phase transformation in Fe$_{50}$Pt$_{50}$ (15 nm)/Au (30 nm)/Fe$_{50}$Pt$_{50}$ (15 nm) NFC begins at temperature of 600°C for annealing duration of 30 s, which is lower by 100°C in comparison with Fe$_{50}$Pt$_{50}$ alloy film. During thermal treatment, Au atoms diffuse along grain boundaries of L1$_{0}$(FePt) phase restricting their growth and magnetic interaction. This is accompanied by increasing of coercive force, but isotropy of magnetic properties is saved independently on magnetic-field direction.