Search

Ferromagnetic Kondo lattice compounds are far less common than their antiferromagnetic analogs. In this work, we report the discovery of a new ferromagnetic Kondo lattice compound, YbIr3Ge7. Like 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. AC magnetic susceptibility, magnetization, and specific heat measurements show that YbIr3Ge7 has a Kondo temperature TK = 14 K and a Curie temperature TC = 2.4 K. 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 TC favors a fluctuation-induced mechanism.

${\mathrm{CeIr}}_{3}{\mathrm{Ge}}_{7}$ is an antiferromagnetic metal with a remarkably low ordering temperature ${T}_{\mathrm{N}}=0.63\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, while most Ce-based magnets order between 2 and 15 K. Thermodynamic and transport properties as a function of magnetic field or pressure do not show signatures of Kondo correlations, interaction competition, or frustration, as had been observed in a few antiferromagnets with comparably low or lower ${T}_{\mathrm{N}}$. The averaged Weiss temperature measured below 10 K is comparable to ${T}_{\mathrm{N}}$, suggesting that the Ruderman-Kittel-Kasuya-Yosida exchange coupling is very weak in this material. The unusually low ${T}_{\mathrm{N}}$ in ${\mathrm{CeIr}}_{3}{\mathrm{Ge}}_{7}$ can therefore be attributed to the large Ce-Ce bond length of about 5.7 \AA{}, which is about 1.5 \AA{} larger than in the most Ce-based intermetallic systems.