  The importance of covalent bonding for the magnetism of 3d metal complexes
was first noted by Pauling in 1931. His point became moot, however, with the
success of the ionic picture of Van Vleck, where ligands influence magnetic
electrons of 3d ions mainly through electrostatic fields. Anderson's theory of
spin superexchange later established that covalency is at the heart of
cooperative magnetism in insulators, but its energy scale was believed to be
small compared to other inter-ionic interactions and therefore it was
considered a small perturbation of the ionic picture. This assertion fails
dramatically in copper oxides, which came to prominence following the discovery
of high critical temperature superconductors (HTSC). Magnetic interactions in
cuprates are remarkably strong and are often considered the origin of the
unusually high superconducting transition temperature, Tc. Here we report a
detailed survey of magnetic excitations in the one-dimensional cuprate Sr2CuO3
using inelastic neutron scattering (INS). We show that although the
experimental dynamical spin structure factor is well described by the model
S=1/2 nearest-neighbour Heisenberg Hamiltonian typically used for cuprates, the
magnetic intensity is modified dramatically by strong hybridization of Cu 3d
states with O p states, showing that the ionic picture of localized 3d
Heisenberg spin magnetism is grossly inadequate. Our findings provide a natural
explanation for the puzzle of the missing INS magnetic intensity in cuprates
and have profound implications for understanding current and future
experimental data on these materials.
