TEM specimen is immersed in the magnetic field. This can be done by using
a long specimen rod that is inserted either into the pole piece gap (side-entry goniometer stage)
or from the top of the lens (top-entry goniometer stage). The former is the most common method and especially offers more flexibility for
in situ experiments (heating, cooling, straining, etc.), while the latter is less affected
by external influences such as variation of room temperature, air pressure, and
The Eucentric position also means that the specimen height is adjusted to be on the tilt axis of the goniometer stage. This can be done, for instance, in FEI TEMs (with side-entry goniometer stage) by using the alpha wobbler, L2, to tilt the sample back and forth and adjusting the “Z axis” up or down to minimize image movement.
Crystalline specimens have to be tilted in a goniometer in TEM in order to:
i) Observe lattice fringes and crystal structures.
ii) Determine the crystal orientation.
iii) Observe diffraction contrast of lattice defects with certain Bragg reflections or known orientation.
iv) Determine the Burgers vector of lattice defects.
Note that, historically, mechanical instability of the specimen stage had been one of the major problems behind the resolution improvement.
The angle between the zone-axes of a crystal can be read from the goniometer stage, providing additional important information for crystallographic analysis.