Coercivity is the resistance of a magnetic material to an opposing external magnetic field. There are two different kinds of Coercivity- Hcb and Hcj.
The higher the Coercivity number, the higher the magnet’s resistance to the opposing field.
Hcj vs Hcb
Hcb is Normal Coercivity and Hcj is Intrinsic Coercivity.
The Difference between Hcj and Hcb
Hcb – is called Normal Coercivity. It is the point on the BH curve where a magnet’s internal field is completely canceled out by the opposing field. In other words, the net flux density is now zero because the opposing field is equal and opposite the magnet’s own field. At this point, the demagnetization is still recoverable because the magnet has not lost its polarity.
If the magnet has not been taken below the “knee” the magnet’s field will recover completely after the opposing field has been removed. If the magnet has been taken below the knee, it will have to be re-magnetized.
Hcj – is known as the Intrinsic Coercivity. It is the point where the magnet’s polarization has been reduced to zero. At this point, the magnetic domains have lost their polarity –or anisotropy- and in the case of a sintered NdFeB magnet -or any other typical ferromagnet- they can be re-magnetized if there has been no physical damage to the magnet.
Coercive Force
Coercive Force is another way of saying Coercivity. The terms are used interchangeably.
Units of Coercivity
Coercivity is expressed in the SI system as A/m or kA/m. That is Amperes-per-meter or kilo-Amperes-per-meter. In the cgs system, it is expressed as Oe (Oersteds) or kOe (kiloOersteds).
Coercivity on the BH Curve
Coercivity is always studied on the second quadrant of the BH curve. This is the quadrant where we can study the behavior of a magnet that is being demagnetized.
Even though the second quadrant is governed by negative “x” values, Coercivity is always expressed as a positive number because it expresses the absolute value of the demagnetizing field.
The BH curve is also known as the Hysteresis curve. Scientists and engineers use the Hysteresis curve to study a magnet’s response to external magnetic fields and it describes in graphical form how a magnet responds to an external field.
A magnet initially responds to external fields by becoming magnetized to saturation- where it cannot support any greater magnetic field. Then, when an opposing field becomes large enough, it can totally cancel out the magnet’s own field.
Coercivity of Hard Magnets and Soft Magnets
Materials with a high coercive force are hard magnetic materials and materials with low coercive force are soft magnetic materials. The difference in values between hard and soft magnets is very high. It’s measured in orders of magnitude.
Figure 1. Comparison of Small Coercive Force of Soft Ferromagnetic Materials with the Large Coercive Force of Hard Ferromagnetic Materials
Figure 2. Coercivity of Soft- and Hard Magnetic Materials. The difference between soft magnetic Silicon Steel and hard magnetic NdFeB is over 4 orders of magnitude.
Temperature Effects
The Coercivity of magnets has a temperature-dependent component. When manufacturers prepare a demagnetization curve for a magnetic material, it is common for them to prepare the curve for multiple temperatures. There is a curve for each of several temperatures defining the magnet’s resistance to demagnetization at each temperature within its expected working range.
Figure 3. Colored lines represent Hcj curves for several temperatures ranging from 20-200oC. Permeance Coefficients are shown around the top and left perimeter lines. This BH curve is for grade 48UH.
Effects of Geometry
The geometry of a magnet can affect its response to an opposing magnetic field. This property is called the Permeance Coefficient and designated as Pc. Pc is shown above in Figure 3. It is based on the relationship between the length and width/diameter of the magnet.
The Practical Importance of Coercivity
Magnets work in many different circumstances, and some work harder than others. A magnet with strong coercive force will withstand opposing magnetic fields from cyclic events such as seen in automotive and industrial electric motors. It will also withstand harmonic frequencies and noise much better than a magnet with lower Coercivity.
Magnetic materials with strong Coercivity (i.e. Neodymium magnets) are called hard magnets and are typically used as permanent magnets, and magnets with weak Coercivity are called soft magnets. Soft magnets are often motor laminations that will change their polarity many cycles-per-second as they revolve around the magnetic field of a permanent magnet or an electromagnet in a motor.