  Isotope ratios can be measured in presolar SiC grains from ancient Asymptotic
Giant Branch (AGB) stars at permil-level (0.1\%) precision. Such precise grain
data permit derivation of more stringent constraints and calibrations on mixing
efficiency in AGB models than traditional spectroscopic observations. In this
paper we compare SiC heavy-element isotope ratios to a new series of FRUITY
models that include the effects of mixing triggered by magnetic fields. Based
on 2D and 3D simulations available in the literature, we propose a new
formulation, upon which the general features of mixing induced by magnetic
fields can be derived. The efficiency of such a mixing, on the other hand,
relies on physical quantities whose values are poorly constrained. We present
here our calibration by comparing our model results with the heavy-element
isotope data of presolar SiC grains from AGB stars. We demonstrate that the
isotopic compositions of all measured elements (Ni, Sr, Zr, Mo, Ba) can be
simultaneously fitted by adopting a single magnetic field configuration in our
new FRUITY models.
