Thermal expansion and magnetovolume effects in the heavy-fermion systemCe2RhIn8

Abstract

We present the results of thermal expansion $\ensuremath{\alpha}(T),$ magnetostriction $\ensuremath{\lambda}(H,T),$ and specific-heat ${C}_{p}(T)$ measurements made on the heavy-fermion antiferromagnet ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}.$ The effects of magnetic order are clearly evident as anomalies in $\ensuremath{\alpha}(T)$ at ${T}_{N}=2.8\mathrm{K}$ and at ${T}_{m}=1.65\mathrm{K}.$ ${C}_{p}(T)$ data indicate that the upper transition corresponds to the onset of long-range antiferromagnetic order while the lower transition involves only a subtle rearrangement of the ordered state. Both ${C}_{p}/T$ and $\ensuremath{\alpha}/T$ grow with decreasing temperature below 20 K in a manner consistent with Kondo renormalization. Kondo interactions appear to be responsible for the large electronic Gr\uneisen parameter which extrapolates to ${\ensuremath{\Omega}}_{e}\ensuremath{\approx}48$ as $\stackrel{\ensuremath{\rightarrow}}{T}0,$ while the characteristic Kondo energy is ${T}_{K}\ensuremath{\approx}10\mathrm{K}$ as determined from the ${C}_{p}/T$ ratio at ${T}_{N}.$ Above 20 K $\ensuremath{\alpha}(T)$ is dominated by crystalline-electric-field (CEF) effects. The data are consistent with a CEF level scheme consisting of excited ${\ensuremath{\Gamma}}_{7}^{(1)}$ and ${\ensuremath{\Gamma}}_{6}$ doublets lying $71\ifmmode\pm\else\textpm\fi{}6\mathrm{K}$ and $195\ifmmode\pm\else\textpm\fi{}10\mathrm{K}$ above a ${\ensuremath{\Gamma}}_{7}^{(2)}$ ground state. In the paramagnetic state $(Tg{T}_{N})$ the volume magnetostriction follows a simple scaling law, $\ensuremath{\lambda}\ensuremath{\propto}[{H/(T+T}_{\ensuremath{\lambda}}){]}^{2}.$ The scaling parameter ${T}_{\ensuremath{\lambda}}=5\mathrm{K}$ is consistent with the Kondo temperature determined from $\ensuremath{\alpha}(T)$ and ${C}_{p}(T)$ data. Analysis of $\ensuremath{\lambda}(H,T)$ data also indicates that the magnetic and electronic energy scales associated with the Kondo state in ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}$ are equivalent. These thermodynamic data indicate that the physical properties of ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}$ result from competition between magnetic exchange, Kondo, and CEF interactions.