Effects of isovalent substitution on the structure and properties of Fe0.25Ti(S,Se)2 : Ising-to-Heisenberg crossover

The polycrystalline layered compounds Fe0.25TiS_2-ySey have been synthesized using the solid-state reaction method and characterized by means of x-ray diffraction, magnetization, electrical resistivity, and magnetoresistance measurements. The iron-chalcogen bonds in Fe0.25TiS_2-ySey are found to be noticeably longer than the titanium-chalcogen bonds, and the bond lengths increase when sulfur is replaced by selenium, although the axial ratio $c/a$ remains unchanged. Although all compounds exhibit antiferromagnetic (AFM) order below TN, increasing the selenium content in Fe0.25TiS_2-ySey changes the magnetization behavior from Ising type in selenium-poor compounds ($y<0.5$) to Heisenberg type at $y>0.5$. In compounds with mixed anionic layers, the AFM order presumably coexists with the cluster glass state. At low concentrations of selenium ($y<0.5$), the compounds exhibit irreversible field-induced phase transitions to a metastable ferromagnetic (FM) state, which are accompanied by significant remnant magnetoresistance. Reversal of magnetization in the FM state is accompanied by a large hysteresis Hc (up to ~50 kOe at 2 K). In the Fe0.25TiSe₂ compound ($y=2$) with N\'eel temperature TN~43 K, an anomalous electrical resistivity behavior was detected in the vicinity of 110 K, which is associated with a charge-density-wave-type transition, similar to that observed in nonintercalated TiSe₂.