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Magnetic Term Hard-Magnetic Materials
Video Soft and Hard Magnetic Materials

For understanding the hard magnetic materials, we have to know certain terms. They are as follows:

  • Coercivity: The capability of a ferromagnetic material to hold up (resist) a peripheral magnetic field without getting demagnetized.
  • Retentivity (Br): It is the amount of magnetism that a ferromagnetic material can maintain even after the magnetic field is decreased to zero.
  • Permeability: It is used to determine how a material reacts to the applied magnetic field.
  • Magnetic materials are mainly classified (based on the magnitude of coercive force) into two sub domains – hard magnetic materials and soft magnetic materials,

Now, we can define hard magnetic materials. These materials are really hard in the basis that it is very difficult to get magnetised. The reason is that the domain walls are motionless owing to crystal defects and imperfections.

But if it gets magnetised, it will be permanently magnetised. That is why; it is also called as permanent magnetic material. They have coercive force greater than 10kA/m and have high retentivity. When we expose a hard magnet to an external magnetic field for the first time, the domain grows and rotates to align with the applied field at the saturation magnetization. After that, the field is removed. As a result, the magnetization is somewhat reverted but it does not track the magnetization curve any longer. A certain amount of energy (Br) is stored in the magnet and it becomes permanently magnetized.

Properties of Hard Magnetic Materials

  • Utmost retentivity and coercivity.
  • Value of energy product (BH) will be large.
  • The shape of BH loop is nearly rectangle.
  • High hysteresis loop.
  • Small initial permeability.

The properties of some important permanent magnetic material are shown in the table below.

Hard magnetic materials Coercivity (Am-1) Retentivity (T)  BHmax(Jm-1)
Alnico 5 (Alcomax) (51Fe, 24 Co,14 Ni, 8Al, 3Cu) 44,000 1.25 36,000
Alnico 2 (55Fe, 12Co, 17Ni, 10Al, 6Cu) 44,800 0.7 13,600
Chrome steel (98Fe, 0.9Cr, 0.6 C, 0.4Mn) 4,000 1.0 1,600
Oxide (57Fe, 28 O, 15Co) 72,000 0.2 4,800

Some important hard magnetic materials are the following:

The carbon steel have large hysteresis loop. Due to any shock or vibration, they lose their magnetic properties rapidly. But tungsten steel, chromium steel and cobalt steel have high energy product.

It is made up of aluminium, nickel and cobalt to boost to improve the magnetic properties. Alnico 5 is the most important material used to create permanent magnet. The BH product is 36000 Jm-3. It is used in high temperature operation.

Rare-Earth Alloys:
SmCo5, Sm2Co17, NdFeB etc.

Hard Ferrites or Ceramic magnets (like Barium Ferrites):
These materials can be powdered and used as a binder in plastics. The plastics made by this method are called plastic magnet.

Bonded Magnets:
It is used in DC motors, Stepper motors etc.

Nanocrystalline hard magnet (Nd-Fe-B Alloys):
The small size and weight of these material make it to use in medical devices, thin motors etc.

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