Magnetic and transport properties of the spin-state disordered oxide La${}_{0.8}$Sr${}_{0.2}$Co${}_{1-x}$Rh${}_x$O${}_{3-\delta }$

We report measurements and analysis of magnetization, resistivity, and thermopower of polycrystalline samples of the perovskite-type Co/Rh oxide La${}_{0.8}$Sr${}_{0.2}$Co${}_{1-x}$Rh${}_x$O${}_{3-\delta }$. This system constitutes a solid solution for a full range of $x$, in which the crystal structure changes from rhombohedral to orthorhombic symmetry with increasing Rh content $x$. The magnetization data reveal that the magnetic ground state immediately changes upon Rh substitution from ferromagnetic to paramagnetic with increasing $x$ near 0.25, which is close to the structural phase boundary. We find that one substituted Rh ion diminishes the saturation moment by 9 ${\mu }_B$, which implies that one Rh${}^{3+}$ ion makes a few magnetic Co${}^{3+}$ ions nonmagnetic (the low-spin state) and causes disorder in the spin state and the highest occupied orbital. In this disordered composition ($0.05⩽x⩽0.75$), we find that the thermopower is anomalously enhanced below 50 K. In particular, the thermopower of $x=0.5$ is larger by a factor of 10 than those of $x=0$ and 1, and the temperature coefficient reaches 4 $\mu$V/K${}^2$, which is as large as that of heavy-fermion materials such as CeRu${}_2$Si${}_2$.

Figure 1

(a) A typical x-ray-diffraction pattern and (b) the lattice parameters of La${}_{0.8}$Sr${}_{0.2}$Co${}_{1-x}$Rh${}_x$O${}_{3-\delta }$. The crystal structure is rhombohedral for $x⩽0.2$ and orthorhombic for $x⩾0.25$. The inset shows the $x$ dependence of the main peak near $2\theta =15$ deg.

Figure 2

(a) Magnetization $M$ and (b) inverse magnetization $1/M$ of La${}_{0.8}$Sr${}_{0.2}$Co${}_{1-x}$Rh${}_x$O${}_{3-\delta }$. The dotted lines are guides for the eyes and indicate the Curie temperature at $1/M=0$.

Figure 3

Magnetization-field curves for La${}_{0.8}$Sr${}_{0.2}$Co${}_{1-x}$Rh${}_x$O${}_{3-\delta }$ at 5 K.

Figure 4

(a) Resistivity and (b) thermopower for La${}_{0.8}$Sr${}_{0.2}$Co${}_{1-x}$Rh${}_x$O${}_{3-\delta }$.

Figure 5

The thermopower divided by temperature $S/T$ plotted as a function of $1/T$. The arrows represent the degenerate temperature below which the thermopower is roughly linear in temperature.

Figure 6

Phase diagram of La${}_{0.8}$Sr${}_{0.2}$Rh${}_{1-x}$Co${}_x$O${}_{3-\delta }$. $T_{\mathrm{C}}$ and $T_0$ correspond to the Curie temperature determined in Fig. 3 and the degenerate temperature determined in Fig. 5, respectively. The circles indicate the Curie temperature as a bulk transition, and the triangles represent a kink temperature seen in Fig. 2b. In the right axis, the temperature coefficient of the thermopower $S/T$ at 15 K is also plotted.