Physical Principles of Electron Microscopy: An Introduction to TEM ...

118 Chapter 4
High-magnification phase-contrast images are particularly valuable for
examining the atomic structure of interfaces within a solid, such as the grain
boundaries within a polycrystalline material or the interface between a
deposited film and its substrate; see Fig. 4-18. Sometimes the lattice fringes
(which represent ordered planes of atoms) extend right up to the boundary,
indicating an abrupt interface. In other cases, they disappear a short distance
from the interface, as the material becomes amorphous. Such amorphous
layers can have a profound effect on the mechanical or electrical properties
of a material. A further type of phase-contrast image occurs in specimens
that are strongly magnetic (ferromagnetic), such as iron, cobalt, and alloys
containing these metals. Again, phase contrast is produced only when the
specimen is defocused. The images are known as Lorentz images; they are
capable of revealing the magnetic-domain structure of the specimen, which
is not necessarily related to its crystallographic structure. But the objective
lens of a modern TEM normally generates a high field that is likely to
change the domain structure, so Lorentz microscopy is usually performed
with the objective turned off or considerably weakened. With only the
intermediate and projector lenses operating, the image magnification is
relatively low.
Although related to the phase of the electron exit wave, these Lorentz
images can again be understood by considering the electron as a particle that
undergoes angular deflection when it travels through the field within each
magnetic domain; see Fig. 4-19a. If the magnetic field has a strength B in the
y-direction, an incident electron traveling with speed v in the z-direction
experiences a force Fx = evB and an acceleration ax = Fx/m in the x-direction.
After traveling through a sample of thickness t in a time T = t/v, the electron
(relativistic mass m) has acquired a lateral velocity vx = axT = (evB/m)(t/v).
Figure 4-19. (a) Magnetic deflection of electrons within adjacent domains of a ferromagnetic
specimen. (b) Lorentz image showing magnetic-domain boundaries in a thin film of cobalt.