Magnetic field-driven phase switching in the antiferromagnetic Mott insulator Ca${}_3$(Ru${}_{0.99}$Ti${}_{0.01}$)${}_2$O${}_7$

Abstract

A bandwidth-controlled antiferromagnetic Mott-insulating phase in Ca${}_3$(Ru${}_{1-x}$Ti${}_x$)${}_2$O${}_7$ is realized through isovalent substitution at the Ru site. For a dilute substitution with only 1% Ti, the Mott insulator ground state remains nearly degenerate with the ground state of pristine Ca${}_3$Ru${}_2$O${}_7$, where the Ru moments are ferromagnetically aligned within the metallic RuO${}_2$ bilayers stacked in an antiferromagnetic fashion. The exceptionally shallow free energy landscape of this doped compound arises from intertwined electron-electron and electron-lattice interactions. This makes its magnetic and transport properties highly sensitive to external perturbations. We systematically investigated magnetic-field-induced phase switching in Ca${}_3$(Ru${}_{0.99}$Ti${}_{0.01}$)${}_2$O${}_7$ to explore its magnetic $H$-$T$ phase diagram. With the field applied along the easy $b$-axis, parallel to the antiferromagnetic moments, the magnetization exhibits a first-order spin-flop transition at $\approx$ 6 T, indicating reorientation of the Ru moments perpendicular to the field. The transition is accompanied by a decrease in the electrical resistance, but the spin-flop phase remains insulating. Above 10.5 T, all Ru moments align with the $b$-axis, resulting in a forced ferromagnetic metallic phase. In contrast, neither spin-flop nor forced-ferromagnetic phases are observed up to 14 T, when the field is applied along the $a$-axis. While the electronic kinetic energy and the electron-lattice coupling contribute to the free-energy balance of this system, the resulting $H$-$T$ phase diagram is remarkably simple and closely resembles that of a canonical anisotropic antiferromagnet, albeit with substantially renormalized critical fields.

10 pages, 7 figures

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