Incommensurate spin density wave and magnetocaloric effect in the metallic triangular lattice HoAl2Ge2

Abstract

We report the magnetic structure and the magnetocaloric effect (MCE) of the ternary compound $\mathrm{Ho}{\mathrm{Al}}_{2}{\mathrm{Ge}}_{2}$ with a trigonal ${\mathrm{CaAl}}_{2}{\mathrm{Si}}_{2}$-type crystal structure. A neutron powder diffraction experiment reveals that $\mathrm{Ho}{\mathrm{Al}}_{2}{\mathrm{Ge}}_{2}$ exhibits an incommensurate spin density wave (SDW) with a propagation vector $\mathbit{k}=(0.23,\phantom{\rule{0.28em}{0ex}}0,\phantom{\rule{0.28em}{0ex}}0.06)$. The special arrangement of magnetic moments in $\mathrm{Ho}{\mathrm{Al}}_{2}{\mathrm{Ge}}_{2}$ induces interesting physical phenomena and large magnetocaloric effects. The rise in resistivity at low temperatures indicates the effect of the SDW state in the electronic transport. The maximum magnetic-entropy change is $\ensuremath{-}16.1\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{kg}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ under a magnetic field change of 0--70 kOe for an isotropic $\mathrm{Ho}{\mathrm{Al}}_{2}{\mathrm{Ge}}_{2}$ powder and it increases to $\ensuremath{-}17.9\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{kg}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for a single crystal when the magnetic field ($H$) is applied parallel to the $ab$ plane. A large rotating magnetic-entropy change of $\ensuremath{-}5.1\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{kg}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $H=20\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$ in a $\mathrm{Ho}{\mathrm{Al}}_{2}{\mathrm{Ge}}_{2}$ single crystal is obtained, which is closely associated to the magnetic anisotropy of the SDW order and its response to the external magnetic field. We discuss the large MCE in terms of the field-induced metamagnetic transition from the incommensurate SDW order to the ferromagnetic order. Our study establishes the triangular lattice $R{\mathrm{Al}}_{2}{\mathrm{Ge}}_{2}$ ($R$ = rare-earth elements) as a unique family of compounds to explore the existence of the incommensurate spin density waves and the correlated physical properties.