${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$: A local moment antiferromagnetic metal with extremely low ordering temperature

${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$ is an antiferromagnetic metal with a remarkably low ordering temperature $T_{\mathrm{N}}=0.63\mathrm{K}$, while most Ce-based magnets order between 2 and 15 K. Thermodynamic and transport properties as a function of magnetic field or pressure do not show signatures of Kondo correlations, interaction competition, or frustration, as had been observed in a few antiferromagnets with comparably low or lower $T_{\mathrm{N}}$. The averaged Weiss temperature measured below 10 K is comparable to $T_{\mathrm{N}}$, suggesting that the Ruderman-Kittel-Kasuya-Yosida exchange coupling is very weak in this material. The unusually low $T_{\mathrm{N}}$ in ${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$ can therefore be attributed to the large Ce-Ce bond length of about 5.7 Å, which is about 1.5 Å larger than in the most Ce-based intermetallic systems.

Figure 1

(a) Room temperature (symbols) and calculated (red line) powder x-ray diffraction patterns of ${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$, together with the expected peak positions (blue vertical lines) for space group $R\overline{3}c$ and lattice parameters $a=7.8915\left(8\right)\AA$ and $c=20.788\left(6\right)\AA$. Violet curve is the difference between data and calculated patterns. Inset: crystal picture on millimeter-scaled paper. (b) Zero-field resistivity with current parallel to the [100] axis. (c) Inverse magnetic susceptibility $H/M$ vs $T$ for magnetic field $H\parallel \left[001\right]$ (blue), [100] (red), and a polycrystalline average (violet). Solid lines are high-temperature Curie-Weiss fits, with Weiss temperatures of $-360$ K, $-20$ K, and $-100$ K for $H\parallel \left[001\right]$, [100], and the polycrystalline average, respectively (see text). Left inset: the low-temperature $H/M$ vs $T$ with a linear fit for the polycrystalline average. Right inset: the low-temperature $M/H$ vs $T$ for $H\parallel \left[100\right]$.

Figure 2

${\mathrm{CeIr}}_3{\mathrm{Ge}}_{7}M$ vs $H$ isotherms for $T=0.5\mathrm{K}$ (full symbols) and 1.8 K (open symbols) for $H\parallel \left[001\right]$ (blue circles) and $H\parallel \left[100\right]$ (red squares).

Figure 3

(a) Left axis: magnetic susceptibility $M/H$ vs $T$. Right axis: $\mathrm{d}\left(MT\right)/\mathrm{d}T\text{vs}T$ (solid line) for ${\mu }_{0}H=0.01\mathrm{T}$. (b) Left axis: specific heat $C_p$ vs $T$ (symbols) for different magnetic fields. The vertical dashed line through (a) to (b) marks $T_{\mathrm{N}}$ at zero field. (c) Temperature-dependent specific heat for different pressures. (d) Left axis: magnetic contribution to the specific heat $C_{\text{mag}}/T \mathit{\text{T}}$. Right axis: magnetic entropy $S_{\text{mag}}$ vs $T$. $C_{\text{mag}}/T=C_p/T({\mathrm{CeIr}}_3{\mathrm{Ge}}_7$) - $C_p/T({\mathrm{YIr}}_3{\mathrm{Ge}}_7$), where ${\mathrm{YIr}}_3{\mathrm{Ge}}_7$ is a nonmagnetic analog.

Figure 4

(a) $T-H$ phase diagram of ${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$ at ambient pressure with $H\parallel \left[100\right]$. (b) Non-Kondo Ce compounds showing their ordering temperatures with respect to the shortest Ce-Ce bond distances [15, 16, 17, 18, 19, 20, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59]. Blue symbols represent ferromagnetic (FM) ordering temperatures, black symbols represent AFM ordering temperatures, and the red star indicates ${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$. The legend for the symbols is given in the Appendix in Table 2.