Mössbauer spectroscopy of spin dynamics in Mn _x Fe _1-x Se _0.85 superconductors: Evidence for an incommensurate-spin-density-wave state

Abstract

In the tetragonal crystalline structure of MnxFe1−xSe0.85, the magnetic state contains low- and high-spin Fe2+, with high-spin numbers equal to that of the combined Mn substitute and Se deficiency atoms. The state is pinned by "spin-hopping" around substitution centers via high-spin↔low-spin conversions. During the structural distortion from tetragonal to orthorhombic, from 90 K to 70 K, the rate of spin conversions increases and the iterant character of the magnetic state is enhanced. In the orthorhombic structure, the spin dynamics evolve into an incommensurate spin-density wave (ISDW). Excitations of the ISDW decrease with temperature and level out across the superconducting phase. The ISDW appears to have more than one oscillation mode and contributions from high-order harmonics.