  The development of high-density magnetic recording media is limited by the
superparamagnetism in very small ferromagnetic crystals. Hard magnetic
materials with strong perpendicular anisotropy offer stability and high
recording density. To overcome the difficulty of writing media with a large
coercivity, heat assisted magnetic recording (HAMR) has been developed, rapidly
heating the media to the Curie temperature Tc before writing, followed by rapid
cooling. Requirements are a suitable Tc, coupled with anisotropic thermal
conductivity and hard magnetic properties. Here we introduce Rh2CoSb as a new
hard magnet with potential for thin film magnetic recording. A
magnetocrystalline anisotropy of 3.6 MJm-3 is combined with a saturation
magnetization of {\mu}0Ms = 0.52 T at 2 K (2.2 MJm-3 and 0.44 T at
room-temperature). The magnetic hardness parameter of 3.7 at room temperature
is the highest observed for any rare-earth free hard magnet. The anisotropy is
related to an unquenched orbital moment of 0.42 {\mu}B on Co, which is
hybridized with neighbouring Rh atoms with a large spin-orbit interaction.
Moreover, the pronounced temperature-dependence of the anisotropy that follows
from its Tc of 450 K, together with a high thermal conductivity of 20 Wm-1K-1,
makes Rh2CoSb a candidate for development for heat assisted writing with a
recording density in excess of 10 Tb/in2.
