Si-NMR study of antiferromagnetic heavy-fermion compounds ${\mathrm{CePd}}_2{\mathrm{Si}}_2$ and ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$

We report Si-NMR studies on the magnetic property of pressure-induced superconductors ${\mathrm{CePd}}_2{\mathrm{Si}}_2$ and ${\mathrm{CeRh}}_2{\mathrm{Si}}_2,$ which exhibit antiferromagnetic (AF) order with the Néel temperature $T_{\mathrm{N}}=10\mathrm{}$ K and 36 K at ambient pressure, respectively. The NMR results in ${\mathrm{CePd}}_2{\mathrm{Si}}_2$ are consistent with those obtained from the previous neutron-diffraction (ND) experiments. On the other hand, the NMR study in oriented powder ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ has revealed that the spectrum splits into two peaks due to the onset of the AF order with the wave vector of ${\mathit{q}}_1=(1/21/20)\mathrm{}$ below $T_{\mathrm{N}1\mathrm{}}=36\mathrm{}\hspace{0.5em}\mathrm{K}$ and each peak splits further into two peaks below $T_{\mathrm{N}2\mathrm{}}=25\mathrm{}\hspace{0.5em}\mathrm{K}$ due to the formation of AF domains with ${\mathit{q}}_2=(1/21/21/2).$ The saturation moment $M_{\mathrm{AF}}\left(\mathrm{NMR}\right)=0.36\mathrm{}$ and $0.22{\mu }_{\mathrm{B}}$ estimated from NMR are significantly smaller than $M_{\mathrm{AF}}\left(\mathrm{ND}\right)=1.86\mathrm{}$ and $1.69{\mu }_{\mathrm{B}}$ from ND. From this discrepancy in the sizes of $M_{\mathrm{AF}},$ it is proposed that a correlation time in fluctuations of f-electron moments is longer than the characteristic time of observation for thermal neutrons but shorter than that for NMR. This probe dependence of $M_{\mathrm{AF}}$ was observed in the uranium heavy-fermion (HF) compounds ${\mathrm{UPt}}_3$ and ${\mathrm{URu}}_2{\mathrm{Si}}_2.$ From the temperature dependence of the nuclear spin-lattice relaxation rate ${1/T}_1$ in the paramagnetic state, the Kondo temperature $T_{\mathrm{K}}$ in ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ is estimated to be around $\sim 100\hspace{0.5em}\mathrm{K},$ much higher than $T_{\mathrm{K}}\sim 12\hspace{0.5em}\mathrm{K}$ in ${\mathrm{CePd}}_2{\mathrm{Si}}_2.$ The ${1/T}_1$’s in both the compounds decrease markedly below $T_{\mathrm{N}},$ followed by a $T_{1}T=$ constant behavior far below $T_{\mathrm{N}}.$ From the latter result, it is suggested that low-lying excitations at low temperatures are dominated by quasiparticle excitations in the AF ordered state in ${\mathrm{CePd}}_2{\mathrm{Si}}_2$ and ${\mathrm{CeRh}}_2{\mathrm{Si}}_2.$