Relationship between magnetic ordering and gigantic magnetocaloric effect in HoB2 studied by neutron diffraction experiment

Recently, ${\mathrm{HoB}}_{2}$ was discovered to undergo a gigantic magnetic entropy change; the largest $|\mathrm{\ensuremath{\Delta}}{S}_{M}|=40.1\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ ($0.35\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$) in magnetocaloric materials studied for hydrogen liquefaction [P. Baptista de Castro et al., NPG Asia Mater. 12, 35 (2020)]. In this study, in order to investigate the origin of the gigantic $|\mathrm{\ensuremath{\Delta}}{S}_{M}|$ in ${\mathrm{HoB}}_{2}$, we studied the magnetic orderings by neutron diffraction with an enriched powder sample of ${\mathrm{Ho}}^{11}{\mathrm{B}}_{2}$. Our experimental results showed the onset of ferromagnetic long-range order below ${T}_{C}=15\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. For the second phase transition at ${T}_{2}=11\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the spin reorientation occurs, and the temperature dependence of the total magnetic moment of ${\mathrm{Ho}}^{3+}$ shows a kink anomaly, which is typically seen in compounds with quadrupolar ordering. In addition, the temperature dependence of the ferromagnetic short-range order above ${T}_{C}$ is strongly related to that of the magnetic entropy change in ${\mathrm{HoB}}_{2}$. These results infer that the gigantic $|\mathrm{\ensuremath{\Delta}}{S}_{M}|$ in ${\mathrm{HoB}}_{2}$ is caused by (i) the spin reorientation transition, which is likely associated with quadrupolar ordering, giving the additional contribution to $|\mathrm{\ensuremath{\Delta}}{S}_{M}|$ at the lower phase transition temperature, ${T}_{2}=11\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and (ii) the large FM fluctuation above ${T}_{C}$ with origins in the strong suppression of the FM long-range order.