7. Magnetic materials : Permanent Magnet substances

HARD FERROMAGNETIC MATERIALS
(permanent magnet materials)
Assessment of their behavior in actual device

RATIONALE

These materials are used in a very large diversities of applications, which spans practically all the human activity. Like the soft ferromagnetic materials, they are essentially composed of iron atoms but associated with many other atoms to produce special characteristics much different of the soft materials.

Small permeability, large coercitivity and anisotropy characterize their magnetic behavior. Once magnetized they retain a certain level of magnetization in a specific direction, which generates a permanent (remanent) magnetic field. The present industrial permanent magnetic materials fall essentially in 3 groups: ALNICO, FERRITE and NEODYMIUM IRON BORON compounds.  

If the magnetized material is submitted to an applied magnetic field in the opposite direction of the remanent field, it can lose the original magnetization when the applied field intensity reaches a certain value (the coercitivity field). Then it becomes magnetized in the reverse direction. Pursuing the process and changing again the direction of the applied field, it becomes possible to develop a complete cycle of magnetization (the hysteresis cycle).

In term of application of the permanent magnet material through a concrete structure what is important it is not the hysteresis cycle but their behavior of the material in the range in the application without reaching the coercitivity field. If this limit is attained then a catastrophic demagnetization process occurs which will damage irremediably the application.

The MAGNETICA software is offering the possibility to assess the behavior of the permanent magnet materials in the realistically well simulated conditions of the applications. The software provides the magnetic data for a set of typical materials and it offers the possibility to enter easily the data characteristics of new materials. 

The illustration of this approach is shown for the HALHBACH 0.23 tesla magnet of the previous example. This device uses 2 specific materials: Neodymium Iron Boron composite for the inner ring and Ferrite materials for the outer ring.

Each ring is made of 16 bars of materials such as shown in the following figure:

1- single bar 3D schematic

The characteristics of the specific Neodymium Iron Boron composite are the following:

Remanent field : 1.31 Tesla or 13100 Gauss
Intrinsic coercitive field : - 955 kA/m or -12000 Oersteds
Relative dynamic susceptibility
In the principal direction : 0.1
In the transverse direction : 0.2

2- Hysteresis cycle of Neodymium Iron Boron
The functional operating line is the blue line for the applied field in the direction of the initial magnetization (principal or easy direction) and the red line in any transverse direction. 

The characteristics of the specific FERRITE material are the following:

Remanent field : 0.42 tesla or 4200 gauss
Intrinsic coercitive field : -25.5 kA/m or -3200 oersteds
relative dynamic susceptibility
in the principal direction : 0.1
in the transverse direction : 0.2

3- Hysteresis cycle of FERRITE
The functional operating line are the blue line for the applied field in the direction of the initial magnetization (principal or easy direction) and the red line in any transverse direction.