YRuO3: A quantum weak ferromagnet

The perovskite ${\mathrm{YRuO}}_{3}$ containing low-spin ${d}^{5}\phantom{\rule{4pt}{0ex}}{\mathrm{Ru}}^{3+}$ has a Pnma orthorhombic structure with elongated ${\mathrm{RuO}}_{6}$ octahedra evidencing orbital order and is insulating to low temperatures with a band gap of 70 meV. Canted antiferromagnetic ${\mathrm{Ru}}^{3+}$ spin order is observed below ${T}_{\mathrm{C}}=97\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and magnetization plateaus that emerge below 62 K reveal a quantum weak ferromagnetic state where 1/8 of the spins are reversed, reflecting extreme single-ion anisotropy resulting from the strong spin-orbit coupling of the ${d}^{5}\phantom{\rule{4pt}{0ex}}{\mathrm{Ru}}^{3+}$ ion. Dynamic effects of the reversed spins give rise to an unusual negative AC susceptibility response, and a partial freezing of this motion occurs at 27 K.