  We use the Chandra observation of the cold front in the intracluster gas of
A3667 to estimate the magnetic field strength near the front. The front is seen
in the Chandra data as a sharp discontinuity in the gas density which
delineates a large body of dense cool gas moving with the near-sonic velocity
through the less dense, hotter gas. Without a magnetic field, the front should
be quickly disturbed by the Kelvin-Helmholtz instability arising from
tangential motion of gas layers. However, Chandra image shows that the front is
stable within a +-30deg sector in the direction of the cloud motion, beyond
which it gradually disappears. We suggest that the Kelvin-Helmholtz instability
within the +-30deg sector is suppressed by surface tension of the magnetic
field whose field lines are parallel to the front. The required field strength
is B ~ 10 muG. Magnetic field near the front is expected to be stronger and
have very different structure compared to the bulk of the intergalactic medium,
because the field lines are stretched by the tangential gas motions. Such a
magnetic configuration, once formed, would effectively stop the plasma
diffusion and heat conduction across the front, and may inhibit gas mixing
during the subcluster merger. We note that even the increased magnetic field
near the front contributes only 10-20% to the total gas pressure, and therefore
magnetic pressure is unimportant for hydrostatic cluster mass estimates.
