Exchange Couplings In L1₀-FePt/A1-FePt Bilayer Films

By using the magnetron sputtering, L10-FePt/A1-FePt textured bilayer exchang springs with minimal and maximal demagnetization factors were fabricated on MgO single-crystal substrates with different crystallographic orientations. By changing the annealed temperature(Ta) for hard layer and the thickness of hard layer and soft layer, the properties of hard/soft bilayer films, such as structure, morphology, domain structure, and behaviour in external magnetic field, were investigated. The results are as follows:(1) The(110) superlattice diffraction peak of annealed FePt(10 nm) film on MgO(110) substrate was very weak. This peak enhanced obviously by increasing the thickness of film to 30 nm. The easy magnetization axis aligned in plane. At Ta = 500℃, the transition of A1â†'L10 started. The morphology destroyed if Ta rised. Therefore, the best Ta is 500℃ for fabrication of L10-FePt/A1-FePt exchange springs. The structure and morphology of hard layer was not significantly affected by the covered A1-FePt(20 nm), but the coercive force reduced from 12 kOe to 7 kOe. A phenomenon of double shoulders in magnetization curves was observed obviously in L10-FePt(10 nm)/A1-FePt(20 nm) bilayer film.(2) By using a hard layer with thickness of 30 nm, and change the thickness of soft layer(t = 0 to 40 nm), the coercive force changed significantly. The weak(111) XRD peak for closely packed lattice planes indicated an incomplete(110) epitaxial growth of A1-FePt layer. At Ta = 500℃, the interlayer exchange length was 20 nm < lex < 30 nm for L10-FePt(110)(30 nm)/A1-FePt, but lex < 20 nm for L10-FePt(110)(10 nm)/A1-FePt.(3) The easy magnetization axis aligned vertical to plane if L10-FePt(001)(30 nm, Ta)/A1-FePt(t nm) bilayer films were deposited on MgO(001) substrates. At Ta = 500℃, the soft layer was pinned by hard layer at t = 20 nm, but double shoulders appeared in magnetization curves at t = 30 nm. Therefore, the interlayer exchange length was also 20 nm < lex < 30 nm.