Ionic Migration due to Sample Surface Charging
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Mobile ions in materials can migrate in electrical fields, which are established by SEM beam due to charging effects. The migration can occur:
Ionic migration effect at charged surfaces of insulator samples in electron microscopy measurements has been explained with Cazaux model . If the surface charges negatively, positive ions will migrate toward the surface.
Jbara et al.  established a simple model to explain the anion migration, underneath metallic coated surfaces, driven by the electrostatic field induced by SEM beam.
Table 1159. Migration observation in SEM measurements.
If the migration of charged ions originates from electrostatic force, then the velocity of ions can be given by,
In this case, the electric field applied to a conductor will exert a force on the activated positve ion in the direction opposite to the electron flow. Note that such electrostatic force exists, no matter if there is an electrical current, as far as there is an electrical potential gradient.
On the other hand, in electromigration process, the rate of momentum exchange between the conducting electrons colliding with the activated metal ions will exert a force on the ions in the direction of electron flow.
 J. Cazaux, Some considerations on the electric field induced in insulators by electron bombardment, J. Appl. Phys. 59, 1418 (1986).