Nature of the spin dynamics and 1/3 magnetization plateau in azurite

Abstract

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 Cu${}_3$(CO${}_3$)${}_2$(OH)${}_2$. We establish that the magnetization plateau is a dimer-monomer state, {\it 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 $J_1/k_B=1(2)$ K, $J_2/k_B=55(5)$ K and a ferromagnetic $J_3/k_B=-20(5)$ K, possibly as result of $d_{z^2}$ orbitals in the Cu-O bonds providing the superexchange pathways.

5 pages, 4 figures

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