Spin-fluctuation effects in ${\mathrm{CeSi}}_{\mathrm{x}}$ alloys: High-field heat-capacity, magnetic susceptibility, and electrical resistivity studies, and a comment on ${\mathrm{TiBe}}_2$

The low-temperature (1.5–20 K) heat capacity in zero and applied fields up to 9.8 T, the magnetization in a field of 1.8 T in the range 1.5–300 K, the low-field ac magnetic susceptibility (50 mK–10 K), and electrical resistivity (2.4–300 K) have been measured for ${\mathrm{CeSi}}_{\mathrm{x}}$ (x=1.83, 1.85, and 1.90) alloys. A contribution of the form $T^3$lnT to the heat capacity, predicted for spin-fluctuation compounds by the paramagnon theory, occurs for all three compounds. In ${\mathrm{CeSi}}_{1.83}$ and ${\mathrm{CeSi}}_{1.85}$ the deviation from $T^3$lnT behavior occurs below approximately 8 and 2.4 K, respectively, due to the onset of magnetic order at 5.5 and 1 K, respectively. The transition of the spin-fluctuation regime to a magnetically ordered state in these two alloys is reported for the first time. In ${\mathrm{CeSi}}_{1.83}$ the entropy associated with magnetic ordering accounts for approximately 17% of the theoretically expected R ln2 entropy; the rest is presumably associated with the spin fluctuations. Spin fluctuations in ${\mathrm{CeSi}}_{1.90}$ are not quenched in fields up to 7.5 T, but the data suggest that the quenching may be starting at 10 T. The heat capacity in applied fields develops a bump which is presumably due to the development of a magnetic moment induced by the field. A similar such bump in ${\mathrm{TiBe}}_2$ may also occur. A gradual progression from paramagnetic spin or valence fluctuations to magnetically ordered Kondo regime takes place with decreasing silicon concentration. An analysis of these data indicate that the ${\mathrm{CeSi}}_{\mathrm{x}}$ alloys are moderately heavy fermions with γ values of 184.6–269.6 mJ/mol Ce ${\mathrm{K}}^2$, $T_s$ (spin fluctuation temperature) values of 28 and 29 K, and Stoner enhancements varying from 135 to 645.