Nature of the Spin Dynamics and 1/3 Magnetization Plateau in Azurite

We present a specific heat and inelastic neutron scattering study in magnetic fields up into the 1/3 magnetization plateau phase of the diamond chain compound azurite Cu₃(CO₃)₂(OH)₂. We establish that the magnetization plateau is a dimer-monomer state, i.e., consisting of a chain of S=1/2 monomers, which are separated by S=0 dimers on the diamond chain backbone. The effective spin couplings J_mono/k_B=10.1(2)  K and J_dimer/k_B=1.8(1)  K are derived from the monomer and dimer dispersions. They are associated to microscopic couplings J₁/k_B=1(2)  K, J₂/k_B=55(5)  K and a ferromagnetic J₃/k_B=-20(5)  K, possibly as result of d_z² orbitals in the Cu-O bonds providing superexchange (SE) pathways with J_SE=6.5  K.