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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.

EuGa4 hosts a magnetic Eu2+ sublattice surrounded by a network of covalently-bound Ga atoms with the BaAl4 structure type (space group I4/ m m m ). In this study, we present the synthesis and characterization of three new single crystal substitutional series Eu A x 1 − x Ga4 with A ​= ​Ca, Sr, or La. X-ray diffraction and resistivity measurements show that Ca substitution induced a structural phase transition from the tetragonal crystal structure at high temperatures to the monoclinic crystal structure (CaGa4 type, space group C2/m) at low temperatures and suppressed the antiferromagnetic ordering temperature to 8.8 ​K for x ​= ​0.45. Comparatively, La or Sr substitution maintained the tetragonal crystal structure and suppressed the antiferromagnetic ordering temperatures to 6.7 ​K and 1.6 ​K for (A, x) = (La, 0.37) and (Sr, 0.91), respectively. In addition to suppressing the magnetic order, magnetization and specific heat measurements indicate the onset of anisotropic metamagnetic transitions in (La, 0.18), (La, 0.37), and (Sr, 0.63), along with an incommensurate-to-commensurate magnetic transition in (Sr, 0.38). By comparing these effects of doping EuGa4, we show how size, disorder, and charge determine the structure-physical property relations in EuGa4.

The solid solution $\mathrm{Eu}{({\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x})}_{4}$ was grown in single crystal form to reveal a rich variety of crystallographic, magnetic, and electronic properties that differ from the isostructural end compounds ${\mathrm{EuGa}}_{4}$ and ${\mathrm{EuAl}}_{4}$, despite the similar covalent radii and electronic configurations of Ga and Al. Here we report the onset of magnetic spin reorientation and metamagnetic transitions for $x=0--1$ evidenced by magnetization and temperature-dependent specific heat measurements. ${T}_{\mathrm{N}}$ changes nonmonotonously with $x$, and it reaches a maximum around 20 K for $x=0.50$, where the $a$ lattice parameter also shows an extreme (minimum) value. Anomalies in the temperature-dependent resistivity consistent with charge density wave behavior exist only for $x=0.50$ and 1. Density functional theory calculations show increased polarization between the Ga-Al covalent bonds in the $x=0.50$ structure compared to the end compounds, such that crystallographic order and chemical pressure are proposed as the causes of the charge density wave behavior.