By investigating the linearly polarized component of a pulsar signal at different radio frequencies it is possible to derive constraints on the parallel component of the magnetic field along the line of sight (los) to the pulsar. In this work we have measured the polarimetric properties of 18 known pulsars in the globular cluster 47 Tucanae and we report for the first time the rotation measure (RM) for 13 of them. Our study led to the discovery of a gradient in the RM values of the pulsars across the cluster. This indicates the presence of significant variations in the magnetic field across the very small angular scales beween the pulsars in 47 Tucanae. We discuss the hypotheses that the magnetic field responsible for the gradient is located inside the globular cluster or in the Galactic disk in the direction of the cluster. However, deeper observations with the MeerKAT and/or the SKA1 radio telescopes are necessary to discriminate among the hypotheses.