Magnetization-direction-tunable spin coupling in kagome magnet LiMn6Sn6

Abstract

The LiMn6Sn6-type of RMn6Sn6 kagome magnet, a room temperature intrinsic ferromagnet with a large anomalous Hall conductivity, has recently been shown to have magnetic-field-direction-dependent electronic structure due to the possible interplay between magnetism and band topology. In this work, we demonstrate magnetic moment direction tunable spin coupling from 3D-Ising type in in-plane spin orientation to 3D-Heisenberg type in out-of-plane spin orientation in kagome magnet LiMn6Sn6, which will further enhance the physics and functionality of LiMn6Sn6. Magnetization measurement of LiMn6Sn6 single crystal revealed paramagnetic to ferromagnetic (PM-FM) phase transition at around 381 K and an easy magnetization axis along the ab-plane (H‖ab). The comprehensive analysis of magnetization isotherms measured in the vicinity of Tc yields the asymptotic critical exponents β, γ, and δ as 0.323, 1.169 and 4.72 for H‖ ab, and 0.350, 1.37, and 4.77 for (H‖ c). These critical exponents satisfy the scaling relation and scaling equation of the magnetic state predicted by scaling theory. The critical exponents obtained are consistent with those found using the renormalization group theory approach for a three-dimensional Ising spin system with spin ordered in an easy in-plane axis (H ⊥ c). However, when the H is applied parallel to the c-axis (H‖ c), the spin coupling changes to a three-dimensional Heisenberg type. Consequently, the origin of ferromagnetism and long-range spin interaction are investigated by DFT first-principles calculations. The calculating result reveals that the Mn3-Mn3 interlayer FM coupling mediated by the super-exchange interaction via Sn2 honeycomb layers sandwiched between the two Mn3 layers is responsible for the macroscopic magnetization in LiMn6Sn6. At the same time, the magnetic couplings between Sn2Li and Mn3 layers play an important role in stabilizing the long-range ferromagnetic ordering.