Physics of Magnetism

SECTION 12.4. COERCIVITY MECHANISMS 113

reduced with respect to the values in the bulk of the material to make a local magnetization

reversal possible. This nucleation of Bloch walls at defects may take place spontaneously

or under the influence of an externally applied negative magnetic field. The field required

for Bloch-wall nucleation, commonly referred to as the nucleation field is often used

to describe the concomitant coercivity Non-uniform processes in which magnetiza­

tion reversal takes place by wall nucleation and propagation dominate in materials with

high magnetocrystalline anisotropy. By analogy with Eq. (12.4.1), an empirical relation of

the type

is often used to describe the nucleation field and the concomitant coercivity The

quantities and are microstructural parameters that determine the relative importance

of the magnetocrystalline anisotropy and the local demagnetizing field, respectively.

In the so-called nucleation-type magnet, the motion of the walls within the grains

is comparatively easy. For obtaining high coercivities, the wall motion must be impeded

by grain boundaries, since otherwise a single nucleated wall would lead to magnetization

reversal of the entire magnet. The possibility of wall pinning at grain boundaries is therefore

considered to be a prerequisite for nucleation-type magnets. Nucleation-type magnets may

be characterized by the following properties: The low-field susceptibility, being a measure

of the reversible displacement of walls, is very large. Magnetic saturation is already reached

in comparatively low fields that are not much larger than the demagnetizing fields For

obtaining the maximum coercivity, a positive saturation field of the order of the

coercive field is required. This necessity finds its origin in the possible persistence of

residual domains of opposite magnetization up to In fields larger than all the

walls will have been removed from the sample, except those walls that cannot be unpinned