Electrical and magnetic properties of the complete solid solution series between SrRuO${}_3$ and LaRhO${}_3$: Filling $t_{2g}$ versus tilting

A complete solid solution series between the $t_{2g}^4$ perovskite ferromagnet SrRuO${}_3$ and the diamagnetic $t_{2g}^6$ perovskite LaRhO${}_3$ has been prepared. The evolution with composition $x$ in (SrRuO${}_3$)${}_{1-x}$(LaRhO${}_3$)${}_x$ of the crystal structure and electrical and magnetic properties has been studied and is reported here. As $x$ increases, the octahedral tilt angle gradually increases, along with the psuedocubic lattice parameter and unit-cell volume. Electrical resistivity measurements reveal a compositionally driven metal-to-insulator transition between $x$ $=$ 0.1 and 0.2. Ferromagnetic ordering gives over to glassy magnetism for $x⩾0.3$, and no magnetic ordering is found above 2 K for $x>0.5$. $M_{\mathrm{sat}}$ and ${\Theta }_{\mathrm{CW}}$ decrease with increasing $x$ and remain constant after $x$ $=$ 0.5. The magnetism appears poised between localized and itinerant behavior and becomes more localized with increasing $x$, as evidenced by the evolution of the Rhodes-Wohlfarth ratio. ${\mu }_{\mathrm{eff}}$ per Ru is equal to the quenched spin-only $S$ value across the entire solid solution. Comparisons with Sr${}_{1-x}$Ca${}_x$RuO${}_3$ reinforce the important role of structural distortions in determining the magnetic ground state. It is suggested that electrical transport and magnetic properties are not strongly coupled in this system.

Figure 1

Orthorhombic perovskite crystal structures of (a) SrRuO${}_3$ and (b) LaRhO${}_3$ viewed down the long $b$ axis. The structures were determined by Rietveld refinement of powder XRD data. LaRhO${}_3$ has significantly more tilting than SrRuO${}_3$, shown by the displacement of the apical O, while the rotation angles are similar. The sphere colors correspond to grey, Sr; black, La; green, Ru; blue, Rh; and orange, O.

Figure 2

High-resolution synchrotron powder x-ray diffraction data and Rietveld refinement for (a) $x$ $=$ 0.5 and (b) $x$ $=$ 0.8 in (SrRuO${}_3$)${}_{1-x}$(LaRhO${}_3$)${}_x$. Data (circles), the Rietveld fit (red lines, $R_{\mathrm{Bragg}}$ $<$ 7 % for all samples), and difference between data and fit are displayed. Vertical lines at the top of the panels indicate expected peak positions.

Figure 3

(a) Unit-cell parameters as a function of composition. (b) Unit-cell volume (circles) and psuedocubic cell parameter (squares) as a function of composition. The lines are cubic and linear fits to the data, demonstrating that the Végard law is followed. (c) Perovskite tilt $\varphi$ and rotation $\theta$ angles as a function of composition. BO${}_6$ octahedra are increasingly tilted with LaRhO${}_3$ substitution, while the rotation angle remains mostly constant.

Figure 4

The dc electrical resistivity $\rho$ as a function of temperature for the Ru-rich samples. A metal-insulator transition occurs as a function of temperature in $x$ $=$ 0.1, while $x$ $=$ 0.2 is insulating for all temperatures measured.

Figure 5

Zero-field-cooled and field-cooled magnetic susceptibility collected under a dc field of 1000 Oe. (a) Samples with $x$ $=$ 0 through $x$ $=$ 0.5 and (b) samples with $x$ $=$ 0.6 through $x$ $=$ 1.

Figure 6

The ac magnetic susceptibility of the $x$ $=$ 0.3 sample as a function of temperature for different frequencies. Data were collected under zero static field using a 5 Oe ac field.

Figure 7

Magnetization as a function of applied dc magnetic field at 2 K. Hysteresis is observed for samples that have magnetic order. Data were acquired in a loop from 0 T, to 5 T, to $-$5 T, and back to 0 T.

Figure 8

Scaled inverse magnetic susceptibility as a function of scaled temperature as described by Eq. (1). The dashed black line represents ideal Curie-Weiss paramagnetism. Data are shown for samples with (a) negative and (b) positive ${\Theta }_{\mathrm{CW}}$.

Figure 9

Rhodes-Wohlfarth ratio $n_{\mathrm{eff}}/M_{\mathrm{sat}}$ as a function of $T_c$. $n_{\mathrm{eff}}$/$M_{\mathrm{sat}}$ $=$ 1 (horizontal red line) indicates localized behavior, with examples from Rhodes and Wohlfarth[39] of CrBr${}_3$, MnSb, etc. Ratios lying on the blue curve correspond to itinerant-electron ferromagnets, with the curve constructed using data from Rhodes and Wohlfarth[39] (ferrous metals and their alloys with one another and with Pd). It is seen that (SrRuO${}_3$)${}_{1-x}$(LaRhO${}_3$)${}_x$ lies between the expectation for local-moment and itinerant-electron behavior.

Figure 10

(a) $\rho$ (300 K), (b) $T_c$ and ${\Theta }_{\mathrm{CW}}$, (c) $M_{\mathrm{sat}}$(2 K, 5 T), and (d) ${\mu }_{\mathrm{eff}}$ as a function of composition. In (a), the black dashed horizontal line at $\rho =1\times {10}^{-2}$ $\Omega$ cm indicates the Mott maximum metallic resistivity. In (d), the colored dashed horizontal lines are the expected $S$-only, $L+S$, and $J$ values for Ru${}^{4+}$. The transition from blue to red indicates the compositionally driven metal-insulator transition, and the entire shaded region corresponds to samples that order magnetically.