Collapse of spin gap by Ru-site substitution in the antiferromagnetic Kondo semiconductor ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$

We investigated the Ce- and Ru-site substitution effects on the spin gap in ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$ by measuring the magnetic and thermal properties of ${\mathrm{Ce}}_{1-y}{\mathrm{Ln}}_y{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$ (Ln = La, Pr, Y) and ${\mathrm{Ce}(\mathrm{Ru}}_{1-x}{\mathrm{T}}_x{)}_2{\mathrm{Al}}_{10}$ (T = Re, Rh) system, respectively. The magnetic susceptibility shows that the itinerant and localized nature is enhanced by Re and Rh doping, respectively. In the latter, the orientation of the magnetic moment in the antiferromagnetic ordered phase suddenly changes from the $c$ to $a$ axis at $x=0.03$. The orientation of the magnetic moment to the $a$ axis is consistent with the large anisotropy of the magnetic susceptibility in the paramagnetic region. The specific heat shows that the exponential temperature dependence, $e^{-\Delta /k_{\mathrm{B}}T}$, and the electronic specific coefficient $\gamma$ is not changed in the Ce-site substituted systems at least up to $y=0.1$ but in the Ru-site substituted systems, the $e^{-\Delta /k_{\mathrm{B}}T}$ dependence disappears, and the $\gamma$ value increases rapidly with $x$. These indicate that although the spin gap is robust against the Ce-site substitution by Ln ion at least up to $y=0.1$, the spin gap is rapidly collapsed by the Ru-site substitution and in place, the conduction electron with heavy effective mass appears at the Fermi level. The spin gap which is formed under the subtle balance between the localized and itinerant nature is easily collapsed by a small amount of Re or Rh doping.

Figure 1

(a) Temperature dependence of $\chi$ along the three crystal axes of ${\mathrm{Ce}(\mathrm{Ru}}_{1-x}{\mathrm{T}}_x{)}_2{\mathrm{Al}}_{10}$ (T = Rh, Re) and ${\mathrm{Ce}}_{0.9}{\mathrm{La}}_{0.1}{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$ measured at $H=1$ T; (b) ${\chi }_a$ and (c) ${\chi }_c$ in an expanded scale at low temperatures. The arrows in Fig. 1 indicate the transition temperatures for T = Rh. The inset in Fig. 1 is the $x$ dependence of the transition temperatures of ${\mathrm{Ce}(\mathrm{Ru}}_{1-x}{\mathrm{Rh}}_x{)}_2{\mathrm{Al}}_{10}$ at $H$ = 0.

Figure 2

Temperature dependence of (a) $C$ and (b) $C_{4f}/T$ of ${\mathrm{Ce}}_{1-y}{\mathrm{Ln}}_y{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$ (Ln = La, Pr,Y), where the result of ${\mathrm{Ce}}_{0.9}{\mathrm{La}}_{0.1}{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$ is cited from Ref. [13].

Figure 3

Temperature dependence of (a) $C$ and (b) $C_{4f}/T$ of ${\mathrm{Ce}(\mathrm{Ru}}_{1-x}{\mathrm{T}}_x{)}_2{\mathrm{Al}}_{10}$ (T = Rh : $x$ = 0.02, 0.03, 0.05, T = Re: $x$ = 0.05).

Figure 4

Temperature dependence of the magnetic entropy, $S_{\mathrm{mag}}$ of (a) ${\mathrm{Ce}}_{1-y}{\mathrm{Ln}}_y{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$ (Ln = La, Pr, Y) and (b) ${\mathrm{Ce}(\mathrm{Ru}}_{1-x}{\mathrm{T}}_x{)}_2{\mathrm{Al}}_{10}$ (T = Rh: $x$ = 0.02, 0.03, 0.05, T = Re: $x$ = 0.05). In Fig. 4, the results of La ($y$ = 0.1 $\sim$ 0.9) are obtained from the specific heat data in Ref. [13] and the arrows for $y$ = 0.5 and 0.7 indicate the transition temperature $T_0$ [13].

Figure 5

$x$ and $y$ dependence of the $\gamma$ value of ${\mathrm{Ce}(\mathrm{Ru}}_{1-x}{\mathrm{T}}_x{)}_2{\mathrm{Al}}_{10}$ (T = Rh, Re) and ${\mathrm{Ce}}_{1-y}{\mathrm{Ln}}_y{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$ (Ln = La, Pr, Y) obtained by subtracting that of ${\mathrm{LaRu}}_2{\mathrm{Al}}_{10}$.