Page 102 - Maxwell House

82 Chapter 2

1. Low magnetizing field (blue area). Reversible movements of the domain walls occur such
that domains oriented in the general direction of the magnetizing field grow at the expense
of those unfavorably oriented. The magnetic permeability increases almost linear. The
walls return to their original position on the removal of the magnetizing field, and there is
no residual magnetization, or it is very low.
2. Medium magnetizing field (pale green area). Larger movements of domain walls occur,
many of which are irreversible, and the volume of favorably oriented domains is much
increased. The magnetic permeability grow accelerates. On the removal of the field, not all
the walls return to their original positions, and there is a residual magnetization.
3. High magnetizing field (rosy area). Large movements of domain walls occur such that
many are swept out of the specimen completely. The directions of magnetization of the
remaining domains gradually rotate, as the field is increased, until the magnetization is
everywhere parallel to the field, and the material is magnetized to saturation. The magnetic
permeability attains the maximum and starts dropping on the way to saturation and beyond.
On the removal of the field, domain walls partially reappear, and the domain
magnetizations may rotate away from the original field direction. The residual
magnetization has its maximum value.

Table 2.4 lists the relative permeability of some ferromagnetic materials and the magnetic field
strength we need to reach the permeability maximum. The additional information is given for
the electrical conductivity and Curie temperature. All data are not exact and can be used as the
references only. Note that ferromagnetics, being metallic materials, are relatively good
electrical conductors.
Table 2.4
Material Conductivity Maximum Magnetic Curie
(S/m) Relative Field Temperature
Permeability Strength (℃ )
Metglas (7 – 8.2) ∙ 10 1 000 000 0.5 T 350 - 540
3
Pure Iron 10 200 000 1043
7
Mu-metal (nickel- 6 20 000 – 200 0.002 T Up to 860
iron alloy) ~1.7 ∙ 10 000
Cobalt 1.646 ∙ 10 18,000 10 T 1388
7
Cobalt-Iron 2.5 ∙ 10 24,000 900-950
7
Electrical Steel 1.46 ∙ 10 4 000 0.002 T 400 - 500
7
Nickel 1.43 ∙ 10 100 - 600 0.002 T 627
7
Neodymium (6 – 9.1) ∙ 10 1.05 310 - 400
3
Magnet
Samarium-Cobalt 1.2 ∙ 10 1.05 720 - 800
4
Unfortunately, ferromagnetics with high relative permeability is very sensitive to frequency
variations. From the discussion surrounding the complex dielectric and magnetic constant, we
know that any movement of electric or magnetic dipoles cannot be done instantaneously. It
means that the external alternating magnetic field is not able to shifts the boundaries between
domains promptly too. Meanwhile, the quantum by nature force aligning domain spin moment
is relatively robust that leads to the substantial inertia of magnetic moments in ferromagnetics.