Effects of S-Se substitution and magnetic field on magnetic order in Fe0.5Ti(S, Se)2 layered compounds

Powder neutron diffraction and specific heat measurements have been employed to study the evolution of an antiferromagnetic (AFM) structure in the intercalated Fe0.5TiS2−ySey compounds with S-Se substitution and under application of a magnetic field. In Fe0.5TiS2 (y=0), the magnetic structure just below TN≃140 K is incommensurate while it becomes commensurate with further cooling below Tt≃125 K. The presence of two magnetic phase transitions at Tt and TN in Fe0.5TiS2 is confirmed by specific heat measurements. The field-induced AFM-FM transitions occurring in Fe0.5TiS2 within temperature interval Tt0.5 are observed to exhibit an AFM structure with the doubled magnetic unit cell along a and c crystallographic directions of the monoclinic crystal lattice (I12/m1 space group). In the transition region around the critical Se concentration yc≈0.5, the magnetic structure of Fe0.5TiS2−ySey is found to be incommensurate. The appearance of the AFM order with decreasing temperature in Fe0.5TiS2−ySey is accompanied by anisotropic deformations of the crystal lattice. At low temperatures, the Fe magnetic moments in all Fe0.5TiS2−ySey form an angle 14-16° to the c crystallographic direction, which can be ascribed to the crystal field effects and spin-orbit couplings.