The influence of an additional Ag top layer with low surface energy ($\cong$ 1.83 J/m$^{2}$) in films Ag (0; 7.5 nm)/Fe$_{50}$Pt$_{50}$0 (15 nm) on SiO$_2$ (100 nm)/Si(001) substrates on diffusion processes of the $L1_0$-phase formation, its structural and magnetic properties after annealing in vacuum 1.3$\cdot$10$^{3}$ Pa at 300–900°C for 30 s is studied by physical materials science methods. Film compositions are obtained by magnetron deposition on substrate of monocrystalline Si(001) with thermally oxidized layer. As determined, the disordered $A$1-FePt phase is formed in as-deposited films. The ordered $L1_0$-FePt phase formation in Fe$_{50}$Pt$_{50}$ (15 nm) film occurs during annealing at 700°C and is accompanied by a sharp increase of coercivity, which continues to rise after further high-temperature annealing. In film with Ag top layer, temperature of the $L1_0$-FePt phase formation increases to 800°C, but, compared to the Fe$_{50}$Pt$_{50}$ film, the (001) texture increases with direction perpendicularly to substrate.