Complex spin structure in co-trimer-chain Li2Co3Se4O12

Complex magnetic materials are extremely attractive for revealing unconventional spin states and novel magnetic excitations. Here, we report the structural, thermodynamic, and magnetic properties of a novel magnetic material Li 2 Co 3 Se 4 O 12 based on x-ray and neutron diffraction, specific heat, magnetization, and x-ray photoelectron spectroscopy measurements. X-ray and neutron diffraction refinements reveal two Co sites (Co (de Jongh, 1990 (1)) and Co (Vasiliev et al., 2021 (2))) even though both are in the octahedral environment. While they are not connected along the b and c directions, these octahedra are edge-shared forming the Co (Vasiliev et al., 2021 (2)) – Co (de Jongh, 1990 (1)) – Co (Vasiliev et al., 2021 (2)) trimer chain along the a direction. The magnetic susceptibility exhibits the Curie-Weiss (CW) temperature dependence at high temperatures (above ~50 K) with the negative CW temperature, a dip centered at T ⁎ ~ 8.0 K, and an antiferromagnetic transition at T N = 3.3 K. The specific heat confirms that there is a phase transition at T N and a hump at T ⁎ . The long-range magnetic transition at T N implies that, in addition to the intra-chain interaction, there is strong inter-chain interaction, which is likely due to polarized SeO 3 bridging between chains. Single crystal neutron diffraction refinement reveals a complex magnetic structure with the angle between Co (de Jongh, 1990 (1)) and Co (Vasiliev et al., 2021 (2)) moments ~105°. Within the Co (Vasiliev et al., 2021 (2)) – Co (de Jongh, 1990 (1)) – Co (Vasiliev et al., 2021 (2)) trimer, two Co (Vasiliev et al., 2021 (2)) moments are parallelly aligned. Surprisingly, the Co (de Jongh, 1990 (1)) moment (1.92μ B ) is only half of the Co (Vasiliev et al., 2021 (2)) moment (3.96μ B ). There is likely the spin-state change for Co (de Jongh, 1990 (1)) from the high-spin state at T > T ⁎ to the low-spin state at T < T ⁎ , causing a dip in the magnetic susceptibility and a hump in the specific heat. When the magnetic field is applied, multiple metamagnetic transitions are found in all directions, implying field-driven magnetic excitations. Our results demonstrate rich magnetic properties of Li 2 Co 3 Se 4 O 12 that are sensitive to the external stimuli such as the magnetic field. • X-ray and neutron diffraction reveals two Co sites with edge-shared Co (Vasiliev et al., 2021 (2))–Co (de Jongh, 1990 (1)) Co (Vasiliev et al., 2021 (2)) trimer chains in Li₂Co₃Se₄O₁₂. • Magnetic susceptibility and specific heat measurements uncover an antiferromagnetic transition at 3.3 K and a spin-state crossover near 8 K. • Single-crystal neutron diffraction reveals a complex magnetic structure with two Co moments, where Co (de Jongh, 1990 (1)) moment is only half of the Co (Vasiliev et al., 2021 (2)) moment.