  We show that magnetic susceptibility can reveal spin entanglement between
individual constituents of a solid, while magnetisation describes their local
properties. We then show that these two thermodynamical quantities satisfy
complementary relation in the quantum-mechanical sense. It describes sharing of
(quantum) information in the solid between spin entanglement and local
properties of its individual constituents. Magnetic susceptibility is shown to
be a macroscopic spin entanglement witness that can be applied without complete
knowledge of the specific model (Hamiltonian) of the solid.
