  Recent spectropolarimetric surveys of main-sequence intermediate-mass stars
have exhibited a dichotomy in the distribution of the observed magnetic field
between the kG dipoles of Ap/Bp stars and the sub-Gauss magnetism of Vega and
Sirius. We would like to test whether this dichotomy is linked to the stability
versus instability of large-scale magnetic configurations in differentially
rotating radiative zones. We computed the axisymmetric magnetic field obtained
from the evolution of a dipolar field threading a differentially rotating
shell. A full parameter study including various density profiles and initial
and boundary conditions was performed with a 2D numerical code. We then focused
on the ratio between the toroidal and poloidal components of the magnetic field
and discuss the stability of the configurations dominated by the toroidal
component using local stability criteria and insights from recent 3D numerical
simulations. The numerical results and a simple model show that the ratio
between the toroidal and the poloidal magnetic fields is highest after an
Alfv\'en crossing time of the initial poloidal field. For high density
contrasts, this ratio converges towards an asymptotic value that can thus be
extrapolated to realistic stellar cases. We then consider the stability of the
magnetic configurations to non-axisymmetric perturbations and find that
configurations dominated by the toroidal component are likely to be unstable if
the shear strength is significantly higher than the poloidal Alfv\'en
frequency. An expression for the critical poloidal field below which magnetic
fields are likely to be unstable is found and is compared to the lower bound of
Ap/Bp magnetic fields.
