  Magnetic and vibrational excitations in SrCu$_2$(BO$_3$)$_2$ are studied
using Raman spectroscopy at hydrostatic pressures up to 34 kbar and
temperatures down to 2.6 K. The frequency of a particular optical phonon, the
so-called pantograph mode, shows a very strong anomalous temperature dependence
below about 40 K. We link the magnitude of the effect to the magnetic exchange
energy on the dimer bonds in the Sutherland-Shastry spin lattice in this
material. The corresponding dimer spin correlations are quantitatively
estimated and found to be strongly pressure dependent. At around P$_2\sim$22
kbar they switch from antiferromagnetic to being predominantly ferromagnetic.

Strongly Correlated Electrons