Ferromagnetic Ordering Along the Hard Axis in the Kondo Lattice YbIr₃Ge₇

Ferromagnetic Kondo lattice compounds are far less common than their antiferromagnetic analogs. In this Rapid Communication, we report the discovery of a ferromagnetic Kondo lattice compound, YbIr₃Ge₇. As in almost all ferromagnetic Kondo lattice systems, YbIr₃Ge₇ shows magnetic order with moments aligned orthogonal to the crystal electric field (CEF) easy axis. YbIr₃Ge₇ is unique in that it is the only member of this class of compounds that crystallizes in a rhombohedral structure with a trigonal point symmetry of the magnetic site, and it lacks broken inversion symmetry at the local moment site. The ac magnetic susceptibility, magnetization, and specific heat measurements show that YbIr₃Ge₇ has a Kondo temperature TK≈14 K and a Curie temperature T_{C}=2.4K. Ferromagnetic order occurs along the crystallographic [100] hard CEF axis despite the large CEF anisotropy of the ground-state Kramers doublet with a saturation moment along [001] almost four times larger than the one along [100]. This implies that a mechanism which considers the anisotropy in the exchange interaction to explain the hard-axis ordering is unlikely. On the other hand, the broad second-order phase transition at T_{C} favors a fluctuation-induced mechanism. © 2019 American Physical Society.