  The manifold of energetically degenerate configurations arising from
competing interactions in frustrated magnets gives rise to an enhanced entropy
at lowest temperatures, which can be utilized for adiabatic demagnetization
refrigeration (ADR). We review structural and magnetic properties of various
Yb- and Gd-based oxides featuring frustration related to different triangular
moment configurations and (in some cases) structural randomness. In comparison
to paramagnetic hydrated salts, which have traditionally been employed for
mK-ADR, these novel ADR materials enable cooling to temperatures several times
lower than the magnetic interaction strength, significantly enhancing the
entropy density and cooling power at a given target temperature. A further
advantage is their chemical stability, allowing for a much simpler ADR pill
design and ultra-high vacuum applications. For the temperature range between
0.02 and 2 K, a systematic comparison of the field-induced entropy density
change is provided, that illustrates the advantages of frustrated magnets for
low-temperature ADR.
