Morphology, structure, and magnetic properties of <i>L</i> 10-FePt/Tb bilayers upon post-annealing

L 1 0 -FePt is the most important alloy in modern high-density heat-assisted magnetic recording (HAMR) media. In order to increase its bit stability and reach higher bit density, an increase of coercivity HC is needed. In conventional granular HAMR media, this is achieved by using a matrix material that isolates the magnetic grains from each other. We report on an approach that is similar, but aims to increase HC via the exchange coupling between heavy rare earth (RE) elements and 3d-transition metals in addition to the isolation of grains. The RE-covered, partially conjoined islands of FePt are heat-treated to help the diffusion of the RE element along the trenches between the islands, similar to the grain boundary diffusion process used in bulk NdFeB magnets. In this work, we show the effects that this type of treatment has on L10-FePt films of 20 nm thickness with and without a Tb cover layer. It is shown that while a strong increase in coercivity of up to +180 % was achieved in some samples, it could not be traced to the presence of the RE element between the grains, but predominantly to inherent initial differences in the microstructure of the investigated sample pieces.