Control of Microstructure, Texture, and Magnetic Properties of L10 FePt Granular Magnetic Recording Media

Heat Assisted Magnetic Recording (HAMR) is the leading candidate for the next generation hard disk drive applications. HAMR currently utilizes high anisotropy L10 ordered FePt granular media for thermal stability in combination with a heating laser for writing process. Its performance is strongly affected by magnetic and thermal characters of the FePt granular recording layer, which consist of nano-sized FePt grains segregated by amorphous boundary materials. A good understanding and control of L10 ordering, ordering orientation, microstructure, and thermal conduction of these films are of importance to optimize their magnetic and thermal properties for the intended storage density of HAMR. This research aims at exploring approaches which allow better understanding of atomic ordering, formation mechanism of ordering related defects, and thermal transport at the grain scale level. New insight obtained gives us directions to investigate novel methods and facilitating underlayers which may enable control of the microstructure, texture, and magnetic properties in L10 FePt granular thin films.;The varying oxide content fabrication technique was first introduced to overcome grain separation in FePt-SiOx thin films at high oxide volume fraction. The FePt media were sputtered in multiple layers with decreasing amount of oxide along the film growth direction. The media sputtered in this manner offer "tall and slim" columnar grains, and enhanced coercivity which is otherwise difficult utilizing a single oxide content method.;A technique based on x-ray diffraction (XRD) was proposed to understand the variation of the grain-to-grain L10 ordering in FePt media. The relative angular position of the FePt (001) and (002) peaks in combination with peak profile fitting can be used to identify the possible ways that FePt grains may be ordered.;The electron microscopy analysis using convergent beam electron diffraction (CBED) and high resolution transmission electron microscopy (HRTEM) was performed on current prototype HAMR media, which utilize MgO underlayers for epitaxial growth of FePt films. We found that the extended growth of FePt grains on top of MgO grain boundaries is responsible for forming L10 in-plane and/or multi-variants, which is detrimental to medium DC noise. The results also showed that the widely used MgO underlayer has a minimal impact on controlling FePt grain size. The study underscores the need of finding new underlayers and/or interlayers which can enforce grain-to-grain coherent growth with FePt recording layers in order to eliminate ordering imperfections.;In an attempt to obtain grain-to-grain epitaxial growth, new underlayer systems based on RuAl and Mo were explored to achieve control of FePt media microstructure and ordering orientation. An advantage of the RuAl underlayers is their small grain size and dome surfaced topology, which can be manipulated through sputtering conditions. The RuAl underlayers also seem to promote grain matching with FePt-SiOx composite films, which would allow grain size control of the FePt-SiOx media but evidently degrade coercivities due to strong Al interdiffusion into FePt. A TiN or MgO interlayer functions as a barrier to minimize this diffusion. Compared to the MgO underlayer, a domelike RuAl underlayer and a proper MgO interlayer in combination with a high sputtering pressure of FePt-SiOx succeeded in making media which have better isolated grains of 10.1-nm average size, columnar growth, and a coercivity of 17 kOe.;Mo underlayers were successfully developed to have strong (200) texture and relatively small grains of about 9 nm. Mo was found to define the vertical coherence growth and grain size for FePt films. A large lattice mismatch and inter-diffusion between FePt and Mo layers were overcome by insertion of a thin (1 nm) MgO interlayer. FePt-C composite thin films deposited on MgO/Mo layer structures show smaller grain size and lower in-plane variants than on MgO underlayers. With the use of this new Mo underlayer system, the resulting FePt granular film achieves a coercivity of 32 kOe with 8.8 % in-plane variants and grain size as small as 6.6 nm.;An experimental model system which is based on a stack of FePt/C multilayers was developed to estimate in-plane thermal conductivity kth of FePt-C granular media. The experimental data suggest a strong impact of FePt grain size and C grain boundary thickness on the in-plane thermal conduction of the FePt-C media.