Effects of grain microstructure on magnetic properties in FePtAg-C media for heat assisted magnetic recording

Abstract

L10 FePt has been proposed as a magnetic medium suitable for heat assisted magnetic recording, owing to its high magnetocrystalline anisotropy and moderate Curie temperature. Carbon has previously been used as a segregant creating small magnetically decoupled grains by sputter deposition at high temperature. Here, we deposit granular, segregated FePt films having a high degree of chemical order, coercive fields of 4.8 T, high remanence, and average grain size of 7.2 nm with 17% size distribution. Magnetic characterization reveals a large intrinsic switching field distribution of 15 kOe. We incorporate the results obtained by transmission electron microscopy, magnetometry, and x-ray diffraction into a micromagnetic model to better understand the origin of the intrinsic switching field distribution. We find that the dominant contributions to the observed magnetic properties are (i) a grain-size dependent distribution in anisotropy field and (ii) small angle variations in perpendicular anisotropy axis orientation. The presence of small superparamagnetic grains, contributions originating from the grain size distribution, as well as from grains with longitudinal or randomly oriented anisotropy axis are found to play a relatively small role. The results help understand the observed magnetization loops used to evaluate FePt media.