Exchange Springs in -FePt(110)/ -FePt Bilayer Films

To investigate the effect of hard/soft exchange spring on magnetic properties, L1₀-FePt/A1-FePt bilayer films are fabricated on MgO(110) substrates by using magnetron sputtering. The hard layer annealed at a moderate temperature of 500 °C showed an unseparated granular morphology with an almost full coverage, in spite of an incomplete A1 → L1₀ transition. The c-axis (easy axis) of L1₀-FePt preferred to align in-plane rather than off-plane. For a system of L1₀-FePt (10 nm)/A1-FePt (20 nm), the magnetization jumped abruptly at 1.4 kOe (=nucleation field of the soft layer) and -7 kOe (=switching field of the hard layer), respectively. However, a perfectly rectangle-like magnetization curve with a coercivity as high as ~10 kOe was observed for a system of L1₀-FePt (30 nm)/A1-FePt (20 nm). This indicates that the exchange length might be able to be affected by grain growth in an incompletely A1 → L1₀ transited FePt film. According to the experimental results, exchange length of the annealed FePt (30 nm) film should exceed 10 nm, which is much larger than that (5 nm) of an ideal L1₀-FePt. These bilayer films are suitable for fabricating magnetic force microscopy tips to measure different magnetic objects.