Abstract
We report the magnetic structure and the magnetocaloric effect (MCE) of the ternary compound HoAl2Ge2 with a trigonal CaAl2Si2-type crystal structure. A neutron powder diffraction experiment reveals that HoAl2Ge2 exhibits an incommensurate spin density wave (SDW) with a propagation vector k=(0.23,0,0.06). The special arrangement of magnetic moments in HoAl2Ge2 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 −16.1J/kgK under a magnetic field change of 0–70 kOe for an isotropic HoAl2Ge2 powder and it increases to −17.9J/kgK for a single crystal when the magnetic field (H) is applied parallel to the ab plane. A large rotating magnetic-entropy change of −5.1J/kgK for H=20kOe in a HoAl2Ge2 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 RAl2Ge2 (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.
