Microstructure and Magnetic Properties of FePt-MgO Multilayer Thin Films

Abstract

FePt alloys with the L1{sub o} (CuAuI) ordered structure are important magnetic materials, since their large uniaxial-magnetocrystalline anisotropy (Ku {approx} 7 x 10{sup 6} J/m{sup 3}) provides thermal stability for nanoscale single-domain particles. As-sputtered FePt thin films have a face-centered cubic (FCC) structure and are superparamagnetic at nanometer grain sizes. To increase the magnetocrystalline anisotropy and coercivity requires chemical ordering into the L1{sub o} structure by annealing the material at 550 to 700 C. The degree of ordering and improvement in magnetic properties depends upon the annealing time and temperature, e.g., annealing FePt nanocrystals at 600 C for one hour has produced coercivity greater than 19 kOe. An additional aspect of using FePt as magnetic recording media is the orientation distribution of the axis of easy magnetization. The ideal configuration is a strong crystallographic texture with alignment of the L1{sub o} [001] axis of easy magnetization normal to the substrate. This study involves a series of five FePt-MgO multilayers sputtered at room temperature onto silicon substrates with the following configurations (with X = 2, 2.5, 3, 3.5, and 4 nm): Si+SiO{sub 2}/Seedlayer (MgO, Ag {approx}25 nm)/(MgO 6 nm/FePt X nm){sub 3} MgO 6 nm, all annealed at 700 C formore » 30min. Plan-view samples, produced by ion milling from the substrate side, were characterized with a Philips CM200-FEG and a Philips CM30 equipped with a Gatan imaging filter (GIF) to better understand the magnetic properties through structure-property correlations. For example, the coercivity of the annealed sample with the 2.5 nm FePt repeat was 11.5 kOe whereas for the thicker FePt repeat of 4 nm the coercivity was reduced to 6.5 kOe.« less