Antiferromagnetic Correlations in Strongly Valence Fluctuating CeIrSn

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 $T_K\sim 480\mathrm{K}$. 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 $T\ll T_K$ result from geometrical frustration.

Figure 1

Ce $3d$ core-level spectrum. Blue curve: as extracted from hard x-ray photoelectron spectroscopy (HAXPES); red curve: full-multiplet configuration-interaction simulation; orange curve: simulation with reduced broadening. The weights of the $4f^0$, $4f^1$, and $4f^2$ configurations in the ground state are also indicated.

Figure 2

Volume magnetostriction coefficient ${\lambda }_V=V^{-1}dV/dB$ for $B\parallel a$, determined from three independent linear magnetostriction measurements (SM). The inset shows the field variation of $BdM/dB$ for $B\parallel a$ at 0.5 K, calculated from the dc magnetization data extracted from Ref. [9].

Figure 3

Linear thermal-expansion coefficient as $\alpha /T$ of CeIrSn along the $a$ (red circles), $c$ (green triangles), and $a^{*}$ (blue squares) axes at zero field. ${\alpha }_a/T$ data for CeRhSn at almost zero pressure and at 0.2 GPa applied along the $a$ axis are extracted from Refs. [13, 19].

Figure 4

(a) Zero field $\mu \mathrm{SR}$ spectra collected at the indicated temperatures. The solid curves are fits to the data by use of Eq. (2). (b) Temperature dependence of the slow (${\lambda }_{\mathrm{ZF}1}$) and the fast (${\lambda }_{\mathrm{ZF}2}$) relaxations. The inset shows the temperature dependence of the initial asymmetry of the slow relaxations ($A_{\mathrm{ZF}1}$) and the fast one ($A_{\mathrm{ZF}2}$).