  It is known that orthorhombic RMnO3 multiferroics (R = magnetic rare earth)
with low symmetry exhibit a large rotating magnetocaloric effect because of
their strong magnetocrystalline anisotropy. In this paper, we demonstrate that
the hexagonal ErMnO3 single crystals also unveils a giant rotating
magnetocaloric effect that can be obtained by spinning them in constant
magnetic fields around their a or b axes. When the ErMnO3 crystal is rotated
with the magnetic field initially parallel to the c-axis, the resulting entropy
change reaches maximum values of 7, 17 and 20 J/kg K under a constant magnetic
field of 2, 5 and 7 T, respectively. These values are comparable or even larger
than those shown by some of the best orthorhombic phases. More interestingly,
the generated anisotropic thermal effect is about three times larger than that
exhibited by the hexagonal HoMnO3 single crystal. The enhancement of the
rotating magnetocaloric effect in the hexagonal ErMnO3 compound arises from the
unique features of Er3+ magnetic sublattice. In fact, the Er3+ magnetic moments
located at 2a sites experience a first-order metamagnetic transition close to 3
K along the c-axis resulting in a peaked magnetocaloric effect over a narrower
temperature range. In contrast, the (paramagnetic) behaviour of Er3+ magnetic
moments within the ab-plane, produces a larger magnetocaloric effect over a
wider temperature range. Therefore, the magnetocaloric effect anisotropy is
maximized between the c and the ab-directions, leading to a giant rotating
magnetocaloric effect.
