Magnetic excitations in a metallic spin chain Ti₄MnBi₂

One-dimensional spin chain systems provide platforms to explore, both experimentally and theoretically, a wealth of novel quantum phenomena. So far, almost all spin chain systems studied have been insulating. We investigated a new metallic compound, Ti₄MnBi₂, whose structure consists of chains of S = 1/2 Mn ions that extend along the c-axis. In Ti₄MnBi₂, the bands involving the Mn d_x²_-y² and d_xz orbitals are spin-polarized and AFM order is predicted, consistent with the inferred Weiss temperature of -9 K and broad peaks in the specific heat and magnetic susceptibility. We have performed inelastic neutron scattering measurements on single-crystal Ti₄MnBi₂, finding a dispersionless magnetic excitation mode around ~0.9 meV. There is no evidence for spin waves for T<T_N, or other dispersing excitations at any temperature. With decreasing temperature, there is a marked increase in the scattered intensity that increases with decreasing wave vector Q. While indicating that the excitations are magnetic, this Q-dependence is much stronger than would be expected from the Mn²⁺ form factor. We propose that clusters of spins are becoming increasingly correlated as the temperature decreases, although their fluctuations persist within the ordered state to the lowest temperatures.