# Ferromagnetic ordering along the hard axis in the Kondo lattice YbIr3Ge7

## TL;DR

This paper reports the discovery of a new ferromagnetic Kondo lattice compound, YbIr3Ge7, which uniquely orders ferromagnetically along its hard axis despite strong anisotropy, suggesting a fluctuation-driven mechanism.

## Contribution

It introduces YbIr3Ge7 as the first rhombohedral ferromagnetic Kondo lattice with hard axis ordering and no inversion symmetry at the magnetic site.

## Key findings

- YbIr3Ge7 has TK = 14 K and TC = 2.4 K.
- Ferromagnetic order occurs along the hard CEF axis.
- The ordering mechanism is likely fluctuation-induced.

## Abstract

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.