Fig. 4: Magnetic properties.

a The temperature dependence of magnetic susceptibility product (χ − χ₀) of 1 in FC, ZFC modes and the corresponding (χ − χ₀)⁻¹‒T plots under a 1 T field from 1.8 to 300 K. The black curve represents the best fit for (χ − χ₀) by considering the possible exchange interaction between spins. The black line represents the best fit by the Curie‒Weiss law. A large negative θ_cw = −190 K is observed, suggesting strong AF exchange interactions. b Magnetic-field dependence of magnetisation of 1 at 1.8, 20, 50, and 300 K. There is no hysteresis loop at these temperatures. c Spin orientations of the Cu²⁺ ions and Li⁺@C₆₀^•− superatoms emerging in the 1D ladder-like magnetic chain; the four Cu ions and Li⁺@C₆₀^•− superatoms are aligned in a triangular-like lattice and antiferromagnetically interact with each other.