Magnetic ground state of the ordered double-perovskite Sr2YbRuO6: Two magnetic transitions

Comprehensive muon-spin-rotation/relaxation (\ensuremath{\mu}SR) and neutron powder-diffraction (NPD) studies supported via bulk measurements have been performed on the ordered double-perovskite ${\mathrm{Sr}}_{2}\mathrm{YbRu}{\mathrm{O}}_{6}$ to investigate the nature of the magnetic ground state. Two sharp transitions at ${T}_{\mathrm{N}1}\ensuremath{\sim}42\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}\ensuremath{\sim}36\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ have been observed in the static and dynamic magnetization measurements, coinciding with the heat-capacity data. In order to confirm the origin of the observed phase transitions and the magnetic ground state, microscopic evidences are presented here. An initial indication of long-range magnetic ordering comes from a sharp drop in the muon initial asymmetry and a peak in the relaxation rate near ${T}_{\mathrm{N}1}$. NPD confirms that the magnetic ground state of ${\mathrm{Sr}}_{2}\mathrm{YbRu}{\mathrm{O}}_{6}$ consists of an antiferromagnetic (AFM) structure with interpenetrating lattices of parallel $\mathrm{Y}{\mathrm{b}}^{3+}$ and $\mathrm{R}{\mathrm{u}}^{5+}$ moments lying in the $ab$ plane and adopting an A-type AFM structure. Intriguingly, a small but remarkable change is observed in the long-range ordering parameters at ${T}_{\mathrm{N}2}$ confirming the presence of a weak spin reorientation (i.e., change in spin configuration) transition of Ru and Yb moments, as well as a change in the magnetic moment evolution of the $\mathrm{Y}{\mathrm{b}}^{3+}$ spins at ${T}_{\mathrm{N}2}$. The temperature-dependent behavior of the $\mathrm{Y}{\mathrm{b}}^{3+}$ and $\mathrm{R}{\mathrm{u}}^{5+}$ moments suggests that the $4d$ electrons of $\mathrm{R}{\mathrm{u}}^{5+}$ play a dominating role in stabilizing the long-range-ordered magnetic ground state in the double-perovskite ${\mathrm{Sr}}_{2}\mathrm{YbRu}{\mathrm{O}}_{6}$ whereas only the $\mathrm{Y}{\mathrm{b}}^{3+}$ moments show an arrest at ${T}_{\mathrm{N}2}$. The observed magnetic structure and the presence of a ferromagnetic interaction between Ru and Yb ions are explained with use of the Goodenough-Kanamori-Anderson rules. Possible reasons for the presence of the second magnetic phase transition and of a compensation point in the magnetization data are linked to competing mechanisms of magnetic anisotropy.