Thermal, magnetic, and transport properties of single-crystalSr1−xCaxRuO3(0<~x<~1.0)

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Abstract

${\mathrm{SrRuO}}_{3}$ is a highly correlated, narrow $d$-band metal which undergoes a ferromagnetic transition at ${T}_{c}=165\mathrm{K}.$ ${\mathrm{CaRuO}}_{3},$ which is also a highly correlated metal, has the same crystal structure, comparable electrical resistivity and similar effective Ru moment, but it remains paramagnetic at least down to 1 K. High- and low-field magnetization and susceptibility, thermoremanent magnetization, low-temperature heat capacity, electrical resistivity, and Hall effect measurements are presented on as-grown, untwinned, orthorhombic single-crystal samples of ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{RuO}}_{3}$ for the entire concentration range $0<~x<~1.0.$ ${T}_{c}$ is depressed uniformly with increasing $x,$ all the way to $x=1.0,$ with possible spin-glass-type ordering for $x$ close to 1.0. The critical Sr doping of paramagnetic ${\mathrm{CaRuO}}_{3}$ required to cause magnetic correlations among the Ru moments is $\ensuremath{\cong}1\mathrm{at}.%.$ Magnetization to 7 T shows strong hysteresis for mixed $(x>0)$ crystals only, with evidence for a rotation of the easy magnetic axis out of the $\mathrm{ab}$ plane. Low-temperature magnetization in dc fields to 30 T for $x=0$ shows a lack of saturation to the full $S=1$ moment, $2{\ensuremath{\mu}}_{B}/\mathrm{Ru}\mathrm{}\mathrm{atom},$ underscoring the itinerant character of the ferromagnetism. Similar data for $x=1.0$ show it to be a highly exchange enhanced paramagnet, a borderline antiferromagnet or ferromagnet. This is consistent with previous Ru-O in-plane and out-of-plane doping studies. Low-temperature heat capacity $(1<T<20\mathrm{K})$ shows that the mass enhancement ($\ensuremath{\gamma}=29{\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^{2}$ and ${m}^{*}\ensuremath{\approx}3$ for $x=0$) and the Debye temperature (${\ensuremath{\Theta}}_{D}=390\mathrm{K}$ for $x=0$) are nonmonotonically varying with increasing $x.$ The large electrical resistivity suggests these materials are ``bad'' metals, with a mean free path at room temperature $\ensuremath{\approx}10\mathrm{A}$ for $x=0.$ The Hall effect shows a sign reversal for $x=0$ and $x=1.0,$ but not for mixed crystals. The data are compared where it is appropriate to data derived from comparable experiments from polycrystalline samples and from epitaxially grown thin films. The results support the highly electron-correlated nature of ordered magnetism in Ru-based oxides and the results should help to advance our understanding of the transport, magnetic, and thermodynamic properties of bad metals.

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