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What is Magnetic Coercivity and Why Does it Matter?

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Coercivity of Neodymium Magnets

Permanent magnets radiate magnetic fields and do not require an outside source of magnetism. Their materials will take on the properties of a strong magnetic field when exposed to it. The magnet continues to emit a magnetic field after the initial field is withdrawn.

Resistance to demagnetization is one of the most useful properties for a magnetic material. A high-quality permanent magnet should emit a high magnetic field with a low mass. It should be stable against things that would demagnetize it.

Remanence and coercivity tell how much a material can resist demagnetization. A material that is still very magnetic after the magnetizing field withdraws has high remanence. Coercivity measures how much magnetic intensity is needed to demagnetize a magnet.

Neodymium (NdFeB) magnets are the most powerful permanent magnets available. The coercivity of these magnets will drop notably at higher temperatures. The low Curie point of the NdFeB phase limits their use at high temperatures. Developing high performance magnets requires understanding and managing coercivity.

Coercivity in a magnetic material is a measure of whether it is able to resist an outside magnetic field without losing magnetism. This describes the magnetic properties of a material in a useful way. Another way to understand coercivity is as the resistance of a material to demagnetization.

There are different types of coercivity. The measurement you need will depend on the threshold for demagnetization:

  • Normal coercivity, Hcb, is the magnetic field required to reduce the magnetic flux to zero.
  • Intrinsic coercivity, Hcj, is the magnetic field required to reduce the magnetization to zero.

Hard and Soft Magnetic Materials

Hard magnets have high remanence and high coercivity. Their magnetic fields do not dissipate under normal conditions.

For hard magnets, the inherited magnetic field is continuous and will be permanent. It will not get weaker with regular use. A rise in temperature or deliberate demagnetizing of the material can weaken the magnet.

The distinction between normal and intrinsic coercivity is minimal in soft magnetic materials. This distinction may be significant in hard magnets. The strongest rare earth magnets lose very little of the magnetization at Hcb.

Measuring Magnetic Coercivity

Permanent magnets have a high coercivity. These hold on to their magnetic fields most of the time unless they are intentionally demagnetized.

To find the coercivity of a magnetic material, you will need to measure the strength of two magnetic fields. The strength of the magnetic material’s field is denoted as B when fully magnetized. The strength of the opposing field required to reduce B to zero is H.

These magnetic fields are usually measured in Oersted units (Oe), or in amperes/meter (A/m). 1 Oe is equal to 79.57747 A/m.

Coercivity is denoted by HC. It can be measured by empirical measurement or mathematical analysis.

The coercivity of a magnetic material is expressed by the magnetization curve. This may also be called a magnetic hysteresis loop. The hysteresis loop shows how the external magnetizing force and the induced magnetic flux density are related. This gives you data about a material’s response to a magnetic field.

Applications of magnetic coercivity

Magnets are used in many ways, under various constraints and conditions. Hard magnets with strong coercivity are used as permanent magnets.

When a magnet must hold on to its magnetism in the presence of a high magnetic field, it will need high coercivity. Magnets with strong coercive force can resist opposing magnetic fields from cyclic events. This makes them ideal for use in cars and industrial electric motors.

They will also handle harmonic frequencies and noise much better than magnets with lower coercivity. Hard magnets may also be found in hard drives, audio speakers, and electric generators.

Soft magnetic materials have weaker coercivity. This makes them better for things like electric motors and power supply transformers. Soft magnets are best when they need to rapidly reverse polarities. They’re used for things where they must constantly magnetize and demagnetize.

Soft magnets are used in motor laminations like cores of electric motors’ stators and rotors. Here, they will change their polarity in numerous cycles-per-second as they revolve around a magnetic field. The magnetic field is created by an electromagnet or permanent magnet inside the motor.

Coercivity is a key property of permanent magnets. Understanding coercivity helps us to understand the applications and limitations of rare earth magnets. Knowledge of coercivity will enhance technology that benefits the many industries relying on NdFeB magnets.