  The aim of this work is to probe the magnetic field properties in relics and
infall regions of galaxy clusters using Faraday Rotation Measures. We present
Very Large Array multi-frequency observations of seven sources in the region
South-West of the Coma cluster, where the infalling group NGC4839 and the relic
1253+275 are located. The Faraday Rotation Measure maps for the observed
sources are derived and analysed to study the magnetic field in the South-West
region of Coma. We discuss how to interpret the data by comparing observed and
mock rotation measures maps that are produced simulating different
3-dimensional magnetic field models. The magnetic field model that gives the
best fit to the Coma central region underestimates the rotation measure in the
South-West region by a factor ~6, and no significant jump in the rotation
measure data is found at the position of the relic. We explore different
possibilities to reconcile observed and mock rotation measure trends, and
conclude that an amplification of the magnetic field along the South-West
sector is the most plausible solution. Our data together with recent X-ray
estimates of the gas density obtained with Suzaku suggest that a magnetic field
amplification by a factor ~3 is required throughout the entire South-West
region in order to reconcile real and mock rotation measures trends. The
magnetic field in the relic region is inferred to be ~2 microG, consistent with
Inverse Compton limits.
