Azurite, famous as a blue paint pigment, is also an extraordinary magnet which is heavily influenced by quantum effects. It forms a so-called diamond chain arrangement of spins and has been considered a contender for the first material to show spin-1/3 excitations (spin-1/2 excitations are typically found in magnetic systems). Crucial to understanding azurite's magnetic behaviour is a distinctive 1/3 magnetization phase in which 1/3 of the copper spins are polarized.
We have used Osiris to probe azurite in applied magnetic fields below this plateau phase. The data reveal that additional and unexpected magnetic excitations exist, which challenge the theory of spin-1/3 excitations. Two out of three of the copper atoms are coupled into S=0 dimer pairs while the third spin remains as a monomer and is thus strongly influenced by applied fields. It is the interplay between the dimer and monomer spin-types that gives the unusual dynamics. Another surprise is the importance of field-induced structural distortions which may also be crucial to the unusual excitation spectra.
MC Gibson, KC Rule, DA Tennant (Helmholtz Zentrum, Berlin), S Süllow (Techniches Universität Braunschweig), M Telling (ISIS)