  We present a measurement of the correlation between the spins of t and tbar
quarks produced in proton-antiproton collisions at the Tevatron Collider at a
center-of-mass energy of 1.96 TeV. We apply a matrix element technique to
dilepton and single-lepton+jets final states in data accumulated with the D0
detector that correspond to an integrated luminosity of 9.7 fb$^{-1}$. The
measured value of the correlation coefficient in the off-diagonal basis,
$O_{off} = 0.89 \pm 0.22$ (stat + syst), is in agreement with the standard
model prediction, and represents evidence for a top-antitop quark spin
correlation difference from zero at a level of 4.2 standard deviations.
