Critical magnetic fluctuations in the layered ruthenates Ca${}_2$RuO${}_4$ and Ca${}_3$Ru${}_2$O${}_7$

Abstract

Materials realizing the XY model in two dimensions (2D) are sparse. Here we use neutron triple-axis spectroscopy to investigate the critical static and dynamical magnetic fluctuations in the square-lattice antiferromagnets Ca${}_2$RuO${}_4$ and Ca${}_3$Ru${}_2$O${}_7$. We probe the temperature-dependence of the antiferromagnetic (AFM) Bragg-intensity, the $Q$-width, the amplitude, and the energy-width of the magnetic diffuse scattering in vicinity to the Néel temperature $T_N$ to determine the critical behavior of the magnetic order parameter $M$, correlation length $\hat{I}¾$, susceptibility $\ddot{I}‡$, and the characteristic energy $\hat{I}“$ with the corresponding critical exponents ${\hat{I}}^2$, $\hat{I}½$, ${\hat{I}}^3$, and $z$, respectively. We find that the critical behaviors of the single-layer compound Ca${}_2$RuO${}_4$ follow universal scaling laws that are compatible with predictions of the 2D-XY model. The bilayer compound Ca${}_3$Ru${}_2$O${}_7$ is only partly consistent with the 2D-XY theory and best described by the three-dimensional Ising (3D-I) model, which is likely a consequence of the intra-bilayer exchange interactions in combination with an orthorhombic single-ion anisotropy. Hence, our results suggest that layered ruthenates are promising solid-state platforms for research on the 2D-XY model and the effects of 3D interactions and additional spin-space anisotropies on the magnetic fluctuations.

19 pages, 17 figures