Spin gap in quasi-one-dimensional S = 3/2 antiferromagnet CoTi₂O₅

Quasi-one-dimensional (1D) antiferromagnets are known to display intriguing phenomena especially when there is a spin gap in their spin-excitation spectra. Here we demonstrate that a spin gap exists in the quasi-1D Heisenberg antiferromagnet CoTi₂O₅ with highly ordered Co²⁺/Ti⁴⁺ occupation, in which the Co²⁺ ions with S = 3/2 form a 1D spin chain along the a-axis. CoTi₂O₅ undergoes an antiferromagnetic transition at T _N ∼ 24 K and exhibits obvious anisotropic magnetic susceptibility even in the paramagnetic region. Although a gapless magnetic ground state is usually expected in a quasi-1D Heisenberg antiferromagnet with half-integer spins, by analyzing the specific heat, the thermal conductivity, and the spin-lattice relaxation rate (1/T ₁) as a function of temperature, we found that a spin gap is opened in the spin-excitation spectrum of CoTi₂O₅ around T _N, manifested by the rapid decrease of magnetic specific heat to zero, the double-peak characteristic in thermal conductivity, and the exponential decay of 1/T ₁ below T _N. Both the magnetic measurements and the first-principles calculations results indicate that there is spin-orbit coupling in CoTi₂O₅, which induces the magnetic anisotropy in CoTi₂O₅, and then opens the spin gap at low temperature.