Magnetic hysteresis basically refers to the persistent, atomic alignment of magnetic dipole moments of ferromagnetic materials, originated from the presence of an external magnetic field. Hysteresis is represented by the retention of magnetization after the external field has been removed. The presence of magnetic domains in the soft magnetic material of the lenses may impart their own directionality upon magnetism in a particular polarization of electrical current. A magnetic domain is a physical volume within a magnetic material in which the magnetization is in a uniform direction. Due to the persistent of initial excitation of magnetic field, not all magnetic domains will re-orient in the desired direction by following the change of the electrical current polarization. Therefore, in order to re-orient the directionality of all the magnetic domains in the lens material in the same direction, more energy is needed to drive the soft magnet to saturation.
Hysteresis is an intrinsic property of many magnetic materials and is present throughout the electron optics of electron microscopes, for instance, in the different lens systems, deflection coils, pole piece, and energy filter. The main electromagnetic lenses in a TEM, which can contribute to beam instability during lens mode switching are:
i) Condenser lens system.
ii) Objective lens in TEMs/STEMs, and objective lens in SEMs/STEMs.