Reentrant magnetism at the borderline between long-range antiferromagnetic order and spin-glass behavior in the B -site disordered perovskite system Ca2−xSrxFeRuO6

We report on the coexistence of magnetic order and disorder in the atomically disordered double perovskites $\mathrm{C}{\mathrm{a}}_{2}\mathrm{FeRu}{\mathrm{O}}_{6}$ and $\mathrm{CaSrFeRu}{\mathrm{O}}_{6}$. Powder x-ray and neutron diffraction were used to investigate the crystal structure and magnetic ordering of these oxides. Both compounds are described by the orthorhombic space group Pbnm down to 3 K, where the $B$ site is found to be statistically occupied by $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{R}{\mathrm{u}}^{5+}$ ions. The compound $\mathrm{C}{\mathrm{a}}_{2}\mathrm{FeRu}{\mathrm{O}}_{6}$ shows a $G$-type antiferromagnetic ordering at ${T}_{N}\ensuremath{\approx}220\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where the moments are aligned parallel to the $c$ axis. The exchange of Ca by Sr suppresses long-range ordering in this system with the consequence that $\mathrm{CaSrFeRu}{\mathrm{O}}_{6}$ shows a diffuse scattering pattern, indicating only the presence of a short-range order of the magnetic moments. M\"ossbauer measurements additionally reveal the coexistence of a long-range ordered and paramagnetic phase in $\mathrm{C}{\mathrm{a}}_{2}\mathrm{FeRu}{\mathrm{O}}_{6}$, and spin-glass behavior in $\mathrm{CaSrFeRu}{\mathrm{O}}_{6}$. The random occupancy of iron and ruthenium atoms at the $B$ site gives rise to locally varying competing magnetic exchange interactions, which favors the emergence of reentrant magnetism with a spin-glass-like transition at ${T}_{f}\ensuremath{\approx}87\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{C}{\mathrm{a}}_{2}\mathrm{FeRu}{\mathrm{O}}_{6}$ and a spin-glass transition at $\ensuremath{\sim}65\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{CaSrFeRu}{\mathrm{O}}_{6}$, as evidenced by frequency dependent ac susceptibility measurements. Our results are an interesting example for crossing the borderline between antiferromagnetism and spin-glass behavior in a $3d\text{\ensuremath{-}}4d$ hybrid perovskite system by modifying the structural distortion associated to the tolerance factor of the perovskite structure rather than changing the concentration of magnetic ions.