How a magnetic field interacts with a ferromagnetic
material is essential to the proper function of many apparatus. The
properties which describe the interaction of a ferromagnetic material
with magnetism include:
- Coercive Force
- Saturation Magnetization
Permeability is a measure analogous
to electrical conductivity: it measures the ability of a material to
conduct magnetic flux. It is measured by the slope of the magnetization
curve plotting magnetic flux in a material vs. magnetizing force.
Materials with high permeability are used for focusing and concentrating
magnetic forces in electromagnets, transformers, and electric motors.
In many cases where the function of a device depends on its
permeability, materials must be tested to ensure adequate permeability.
High permeability materials like Mu-metal and Permalloy, which have an
approximately 80:20 ratio of iron to nickel, are also useful for
magnetic shielding enclosure construction, where they form the path of
least resistance for magnetic flux to travel around an enclosed volume.
This reduces significantly the amount of magnetic flux that crosses the
shielded boundary. In cases where very strong magnets need to be
shielded, or where a region must be made free of all magnetic
influences, including the earth’s ubiquitous magnetic field, multiple
shielding enclosures are nested within each other to increase shielding
efficiency. Shielding is much more effective if the shielding
high-permeability material is divided into several separated layers,
rather than thickening a single layer. Permeability is strongly affected
by alloy chemistry and grain structure, and can be affected by
annealing, heat treatment or cold working, and other modifications.
Coercive force or Coercivity
is a measure of how permanent a magnet is, ie. what amount of magnetic
force it takes to reverse its magnetization. Materials with high
coercive force require high magnetizing H fields to become magnetized or
demagnetized. Coercive force is defined as that amount of magnetizing
H-field needed to return the magnetization B of a magnet to zero, after
it has been magnetized. Coercive force also varies with material
composition and grain structure, and may be modified by annealing or
Remanence: When a ferromagnetic
material is magnetized in one direction, it will not relax back to zero
magnetization (B) when the imposed magnetizing field (H) is removed.
Remanence is a measure of how strong a magnetization remains in a
material after an externally applied magnetizing force is removed
remains in a material after an externally applied magnetizing force is
High values of remanence and coercivity are sought
for permanent magnets, so that the magnets will retain a strong field
and not be demagnetized in response to external magnetic interactions.
Materials with high remanence and coercivity are known as magnetically
“hard”, and are contrasted against materials with low coercivity and
remanence which are known as magnetically “soft”. Magnetically soft
materials are preferred for transformer cores and inductors because they
require less energy to magnetize and demagnetize. This is desirable in
transformers, inductors, and other AC magnetic devices because less
energy is irretrievably lost as heat in the process of re-orienting the
magnetic domains, which is done once every cycle of the applied AC
power. Consequently, such transformers are more efficient.
Coercivity and remanence of some common permanent magnet materials are detailed in the below table. Besides coercivity and remanence, a quality factor for permanent magnets is the quantity (BB0/μ0)max.
A high value for this quantity implies that the required magnetic flux
can be obtained with a smaller volume of the material, making the device
lighter and more compact.
Saturation Magnetization: When the magnetizing force applied to a sample is increased, there comes a point when it cannot be magnetized further. All the magnetic domains in the material are then aligned maximally with the applied magnetic field.
This is referred to as saturation.
Permalloy: Hc: 4 A/m Ms: 10.8 MA/m
Iron: Hc: 80 A/m Ms: 21.5 MA/m
Supermalloy: Hc: 0.2 A/m Ms: 8 MA/m
Supermalloy is a specially processed 80% nickel-iron alloy. It is manufactured to develop the ultimate in high initial permeability and low losses. Initial permeability ranges from 40,000 to 100,000 while the coercive force is about 1/3 that of Square Permalloy 80. Supermalloy is very useful in ultra- sensitive transformers, especially pulse transformers, and ultra-sensitive magnetic amplifiers where low loss is mandatory.