Switching of magnetic coupling by a structural symmetry change near the Mott transition in Ca2-xSrxRuO4

We studied the structural, magnetic, and transport properties of the quasi-two-dimensional Mott transition system ${\mathrm{Ca}}_{2-x}{\mathrm{Sr}}_x{\mathrm{RuO}}_4.$ In the vicinity of the metal-nonmetal (M-NM) transition at $x\simeq 0.2,$ we found a structural transition accompanied by a structural symmetry change with the instability point at $x_{\mathrm{c}}\simeq \mathrm{0.5.}$ The critical change across the structural transition in the temperature dependence of the susceptibility indicates a crossover of the metallic state, most likely from the nearly antiferromagnetic state next to the M-NM transition to the nearly ferromagnetic state around $x_{\mathrm{c}}.$ The latter evolves into the spin-triplet superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ $\mathrm{}(x=2\mathrm{})\mathrm{}$ with enhanced paramagnetism. We argue that the competition between the structural instability and the ferromagnetism results in such a structural symmetry change with orbital degeneracy lifting, which induces the switching of magnetic coupling. In addition, a changeover from metallic to nonmetallic behavior was observed across the structural transition in the out-of-plane resistivity, which reveals highly anisotropic transport due to the quasi-two-dimensional electronic structure.