  Magnetic diffractive scattering of coherent metastable helium atomic beams
from CoO(001) surfaces reveal the presence of an anomalous enhancement in the
intensity of the half-order antiferromagnetic diffraction peak over the
temperature range of 250-320K. Electronic cluster calculations based on
Hartree-Fock states with configuration interactions, together with mean-field
and Monte Carlo studies of this system suggest that this behavior is associated
with low-lying surface spin excited states.
