Magnetovolume and magnetocaloric effects in Er2Fe17

Abstract

Combining different experimental techniques, investigations in hexagonal $P{6}_{3}/mmc$ Er${}_{2}$Fe${}_{17}$ show remarkable magnetovolume anomalies below the Curie temperature, ${T}_{C}$. The spontaneous magnetostriction reaches $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ at 5 K and falls to zero well above ${T}_{C}$, owing to short-range magnetic correlations. Moreover, Er${}_{2}$Fe${}_{17}$ exhibits direct and inverse magnetocaloric effects (MCE) with moderate isothermal magnetic entropy $\ensuremath{\Delta}{S}_{M}$, and adiabatic temperature $\ensuremath{\Delta}{T}_{\mathrm{ad}}$ changes [$\ensuremath{\Delta}{S}_{M}\ensuremath{\sim}\ensuremath{-}4.7$ J(kgK)${}^{\ensuremath{-}1}$ and $\ensuremath{\Delta}{T}_{\mathrm{ad}}\ensuremath{\sim}2.5$ K near the ${T}_{C}$, and $\ensuremath{\Delta}{S}_{M}\ensuremath{\sim}1.3$ J(kgK)${}^{\ensuremath{-}1}$ and $\ensuremath{\Delta}{T}_{\mathrm{ad}}\ensuremath{\sim}\ensuremath{-}0.6$ K at 40 K for $\ensuremath{\Delta}H=80$ kOe, respectively, determined from magnetization measurements]. The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er and Fe sublattice. The main trends found experimentally for the temperature dependence of $\ensuremath{\Delta}{S}_{M}$ and $\ensuremath{\Delta}{T}_{\mathrm{ad}}$ as well as for the atomic magnetic moments are qualitatively well described considering a mean-field Hamiltonian that incorporates both crystalline electric field and exchange interactions. $\ensuremath{\Delta}{S}_{M}(T)$ and $\ensuremath{\Delta}{T}_{\mathrm{ad}}(T)$ curves are essentially zero at $\ensuremath{\sim}$150 K, the temperature where the transition from direct to inverse MCE occurs. A possible interplay between the MCE and the magnetovolume anomalies is also discussed.