Measurements of impurity diffusion coefficients in liquid metals show an important scattering. As this coefficient is very small, even weak convection significantly enhances mass transport and leads to an overestimate of the measured coefficient. We propose here a measurement method using the braking effect of a uniform magnetic field in liquid metals and semiconductors. We used a shear cell, where both solutal and thermal convection are precisely controlled. Magnetic fields up to 0.75 T were applied during diffusion experiments with the Sn-SnIn(1%at.) and Sn-SnBi(0.5%at.) couples. The theoretical braking laws of convection were verified and values consistent with previous microgravity experiments were found.