Spin Dynamics and Two-Dimensional Correlations in the FCC Antiferromagnetic Sr$_2$YRuO$_6$

The face-centered cubic lattice lattice of Ru$^{5+}$ spins in the double perovskite Sr$_{2}$YRuO$_{6}$ shows a delicate three dimensional antiferromagnetic (AFM) ground state composed of stacked square AFM layers. We present new inelastic neutron scattering data taken on this state revealing a gapped low-energy excitation band that may be modeled by a simple $J_1-J_2$ interaction scheme allowing quantitative comparison of similar materials. At higher temperatures, the low-energy excitation spectrum is dominated by a quasi-elastic component associated with size fluctuations of two-dimensional AFM clusters that exhibit asymmetric correlations even at low temperatures. Thus, the FCC lattice in general and the double perovskite structure in particular emerge as hosts of both two-dimensional and three-dimensional dynamics resulting from frustration.