We studied structural and magnetic properties of a series of insulating double perovskite compounds, La${}_{2-x}$Sr${}_x$CuRuO${}_6$ (0 $⩽$ $x$ $⩽$ 1), representing doping via $A$-site substitution. The end members La${}_2$CuRuO${}_6$ and LaSrCuRuO${}_6$ form in monoclinic structure while the intermediate Sr doped compounds stabilize in triclinic structure. The Cu and Ru ions sit on alternate $B$ sites of the perovskite lattice with $\sim 15\%$ antisite defects in the undoped sample while the Sr-doped samples show a tendency to higher ordering at $B$ sites. The undoped ($x$ $=$ 0) compound shows a ferrimagnetic-like behavior at low temperatures. In surprising contrast to the usual expectation of an enhancement of ferromagnetic interaction on doping, an antiferromagnetic-like ground state is realized for all doped samples ($x$ $>$ 0). Heat capacity measurements indicate the absence of any long-range magnetic order in any of these compounds. The magnetic relaxation and memory effects observed in all compounds suggest glassy dynamical properties associated with magnetic disorder and frustration. We show that the observed magnetic properties are dominated by the competition between the nearest-neighbor Ru-O-Cu 180${}^{\circ }$ superexchange interaction and the next-nearest-neighbor Ru-O-O-Ru 90${}^{\circ }$ superexchange interaction as well as by the formation of antisite defects with interchanged Cu and Ru positions. Our calculated exchange interaction parameters from first principles calculations for $x$ $=$ 0 and $x$ $=$ 1 support this interpretation.

• Received 14 July 2011

DOI:https://doi.org/10.1103/PhysRevB.86.094421