  The interplay of charge, spin, orbital and lattice degrees of freedom has
recently received great interest due to its potential to improve the
magnetocaloric effect (MCE) for the purpose of magnetic cooling applications.
Here we propose a new mechanism for a giant inverse MCE in rare-earth
tetraborides, especially for Ho1-xDyxB4 (x = 0.0, 0.5, and 1.0). For x = 0.0,
0.5, and 1.0, the maximum entropy changes of the giant inverse MCE are found to
be 22.7 J/kgK, 19.6 J/kgK, and 19.0 J/kgK with critical fields of 25 kOe, 40
kOe, and 50 kOe, respectively. It is remarkable that such a giant MCE is
realized, even when applying a low magnetic field, which enables a field-tuned
entropy change and brings about a significant advantage for several
applications. For all compounds, we have systematically studied how the entropy
changes as a function of the field and temperature and investigated their
correlation with consecutive double transitions, i.e., the magnetic dipolar
order at T = TN and the quadrupolar order at T = TQ (TQ < TN). We found that
the maximum entropy change occurs at T = TQ and the critical field associated
with the meta-magnetic transition, which is in good agreement with the
experimental data. Thus, we elucidate that this unique behaviour is attributed
to the strong coupling between magnetic dipoles and quadrupoles in the presence
of strong spin-orbit coupling and geometric frustration. Our work offers new
insights into both the academic interest of multipolar degrees of freedom in
magnetic materials and the discovery of giant MCE with various applications for
magnetic cooling systems.
