L10 CrPt underlayer thickness effects on FePt film ordering

In this work we have studied the magnetic properties and microstructure of the L1(0) FePt(60 nm)/CrPt bilayer with different CrPt thicknesses from 20 to 120 nm after post-annealing at 400 degrees C. FePt films were deposited coherently on the preordered L1(0) CrPt underlayer due to structural disorder-order transformation with a slight lattice distortion from fcc to fct. The CrPt underlayer ordering clearly increased with thickness and thus coherently induced FePt film ordering. Introducing the ordered equal-atomic CrPt underlayer caused in-plane and out-of plane coercivity and squareness ratio improvement, higher than that for the single layer FePt (H-c similar to 7.3 kOe). The in-plane and out-of-plane coercivity increased with the CrPt underlayer thickness from 7.5 kOe to 10 kOe and from 5 kOe to 8 kOe, respectively. The L1(0) CrPt underlayer also enhanced the FePt layer grain growth with a grain size of around 200-500 nm. This size is much larger than the grain size of the FePt single layer(50 nm) after post-annealing at 400 degrees C. Surface roughness of FePt films from AFM images increased with the CrPt underlayer thickness, possibly causing enhanced ordering of FePt films.