Antiferromagnetic correlations in strongly valence fluctuating CeIrSn

TL;DR

This study reveals unexpected antiferromagnetic correlations at very low temperatures in the valence fluctuating compound CeIrSn, supported by various experimental techniques indicating geometrical frustration effects.

Contribution

It demonstrates the presence of antiferromagnetic correlations in a strongly valence fluctuating Ce compound, challenging typical expectations for such systems.

Findings

Negative in-plane thermal expansion below 2 K

Sign change in magnetostriction before metamagnetic transition

Muon spin relaxation indicates broad distribution of internal magnetic fields

Abstract

CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of $T_{\mathrm{K}}\sim 480$\,K. We report a negative in-plane thermal expansion $\alpha/T$ below 2\,K, which passes through a broad minimum near 0.75\,K. Volume and $a$-axis magnetostriction for $B \parallel a$ are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6\,T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1\,K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at $T\ll T_{\mathrm{K}}$ result from geometrical frustration.