  Grain-to-grain Curie temperature (Tc) variation in the media reduces
signal-to-noise ratio due to its contribution in transition jitter noise,
especially when average grain size is pushing down to increase the area storage
capacity. A thermally insulating magnetic grain boundary may suppress such
grain-to-grain Tc variation, especially at small grain size. Here we present an
experimental study on the effect of adding thermally-insulating magnetic oxide,
in particular BaFexOy, as part of the grain boundary materials in granular
FePt-C HAMR media. It is found that the BaFexOy is chemically inert to FePt and
the chemical ordering of FePt-BaFexOy-C media are similar to that of FePt-C
meida. By tuning the volume fraction of BaFexOy and C, well-separated FePt
grains (average grain size = 6.8 nm) surrounded by BaFexOy shell with
perpendicular Hc above 35 kOe can be obtained. Transmission electron microscopy
study with chemical analysis shows that the magnetic oxide appears to be
crystalline and surround the FePt grains with immediate full enclosure.
Magnetic measurements indicate an effective increase of magnetic grain size at
temperatures below FePt Curie temperature. Pulsed laser pump-probe measurement
indicates a measurable reduction of Curie temperature variation for the
FePt-BaFexOy-C media with carefully comparison with the reference FePt-C media.
