Spin freezing into a disordered state in CaFeTi2O6 synthesized under high pressure

Abstract

Double perovskites exhibit a variety of interesting phenomena due to numerous combinations of cation orderings and distortions. $\mathrm{CaMnT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$ and $\mathrm{CaFeT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$ are the only two discovered double perovskites that have the columnar type A-site ordering associated with the ${a}^{+}{a}^{+}{c}^{\ensuremath{-}}$ titling system. The former perovskite, synthesized recently, has a polar structure below ${T}_{c}$ and shows an unusual ferroelectricity. The latter, while synthesized decades ago, received less investigation. Here, we have synthesized $\mathrm{CaFeT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$ under high pressure and carried out thorough characterizations through measurements of x-ray powder diffraction (XRD), second harmonic generation (SHG), dc and ac magnetization, M\ossbauer spectra, resistivity, thermoelectric power, and specific heat. Although the structural refinement of the XRD pattern cannot distinguish two possible tetragonal phases, SHG measurements help to rule out the possibility of a polar structure observed in $\mathrm{CaMnT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$. Fitting the paramagnetic susceptibility to a Curie-Weiss law gives a negligible Weiss constant. The weak exchange interaction gives way to a dipole-dipole interaction and single-ion anisotropy. An anomaly at 1.2 K in the specific heat is too broad for a magnetic transition, but suggestive of a spin-ice-like state as in the pyrochlores. M\ossbauer spectroscopy shows two distinct iron sites, as expected for the crystal structure. Both sites are occupied by high-spin $\mathrm{F}{\mathrm{e}}^{2+}$ as indicated by the isomer shift. The sharply different temperature dependence of the quadrupole splitting at the two iron sites can be well explained based on the local structural distortions from the refinement of XRD patterns. The line broadening associated with long spin-relaxation times is observed at 6 K as the spin freezing transition is approached from high temperatures.