  Commercial adiabatic demagnetisation refrigerators still employ the same
hydrated salts that were first introduced over 85 years ago. The inherent
limitations of these insulating magnetocalorics - poor thermal conductivity at
sub-Kelvin temperatures, low entropy density, corrosiveness - can be overcome
by a new generation of rare-earth based metallic magnetocalorics. Here, we
present the metallic magnetocaloric YbNi1.6 Sn as an attractive alternative to
conventional refrigerants. YbNi1.6Sn retains high entropy into the 100 mK
regime and avoids the noble metal constituents of alternative refrigerants.
Demagnetisation tests demonstrate that YbNi1.6Sn enables economical and durable
alternatives to traditional cooling devices for temperatures reaching below 120
mK. We find that the magnetocaloric properties of this material are facilitated
by unusually small Kondo and RKKY interactions, which position YbNi1.6Sn in the
extreme local moment limit on the generalised Kondo lattice phase diagram.
