Magnetic properties and viscosity effect of L10 FePt film with GePt underlayer

We explored the GePt underlayer thickness effect on the magnetic properties and microstructure of L10 FePt film. The GePt underlayer was deposited on quartz at 800 °C and the thickness changed from 10 to 120 nm. The Ge2Pt3 compound was formed and the island-like structure was able to relax the growth stress between Ge2Pt3 and quartz when the thickness is smaller than 60 nm. After FePt deposition, the bilayer was post-annealed at 400 °C. In plane coercivity (Hc) decreased from 14 to 2 kOe when the underlayer thickness increased from 10 to 120 nm. First, we suggested L10 FePt film is higher ordered (S = 0.71-0.93) on the island-like underlayer and the coercivity are larger than 8 kOe. It is because the island-like Ge2Pt3 induced tensile stress on FePt, which enhanced the formation of ordered FePt. The obstacles or pinning sites created by Ge2Pt3 island-like surface morphology makes FePt domain wall motion lagging and the coercivity become larger. But the inter-diffusion effect still exists for film with 10 nm Ge2Pt3 underlayer at 400 °C. Second, with underlayer thickness 120 nm, the Ge atoms diffused into the FePt film that suppressed the ordering with parameter S = 0.49 and the coercivity was 2 kOe.