When we apply an outside magnetic field to a magnetic material, we are forcing it -coercing it- to become magnetized. Now that it is magnetized, how much of an applied field is required to demagnetize the material? When we measure the applied field that either magnetizes or demagnetizes the material, we measure the Coercivity of the material.
Simply put, Coercivity is the resistance of a magnetic material to changes in magnetization. Coercivity is usually referred to as the magnetic field required to demagnetize the material. Why is that? Because it tells us a lot about the magnetic hardness of the material in the same environment as its normal working environment.
We can measure Coercivity by measuring the external magnetic field required to reduce the material's magnetic field to zero. This is the amount of negative (H) required to reduce (B) to zero, so it is the crossing of the horizontal axis to the left of the vertical axis.
Coercivity is closely related to and better understood if you also understand Permeability.
Hard Magnetic Materials and Soft Magnetic Materials
A Hard Magnetic Material maintains its magnetic properties once magnetized and is difficult to demagnetize. A Soft Magnetic Material is relatively easy to demagnetize, and many soft magnetic materials will begin to demagnetize as soon as the applied field is removed.
Figure 1. Comparison of Small Coercive Force of Soft Ferromagnetic Materials with the Large Coercive Force of Hard Ferromagnetic Materials
Coercivity is also known as coercive force. Since the coercive force is much lower for soft magnetic materials, it makes sense to use them for applications where the polarity will be reversed often. The lower Coercivity of soft magnetic materials results in lower losses during operation, so these materials see a lot of usage in transformer cores and motor windings, which change their polarity in response to an applied magnetic field hundreds or more times per second.
The higher Coercivity of hard magnetic materials makes them suitable for use where the applied field is insufficient to demagnetize them. Hard magnetic materials are then used as permanent magnets where they maintain the best utility for magnetic designs.
Figure 2. The Coercivity of any Magnetic Material is Determined By The Field Required to Drive the Magnetization to Zero
Hysteresis Loops of Selected Materials
Let's take a jump from theory to practicality and examine the hysteresis loops of some materials of interest, and see how their Coercivity values compare. The previous figures show that the hysteresis loops of different materials have different shapes, but due to a very large difference in the strength values, the size of the loops is very different.
The hysteresis loop of a soft magnetic material like silicon steel, for example, is a taller, thinner loop than that of a permanent magnet like NdFeB (Neodymium), which has more of a square hysteresis loop, as shown above in Figure 1. But since NdFeB is much stronger, the area inside the loop is much, much bigger.
Another factor that needs to be mentioned is operating temperature. Some magnetic materials excel at high operating temperatures. Samarium cobalt and Alnico are two materials that are very good at temperatures of 200-500°C. For the purposes of this discussion, we are concentrating on lower operating temperatures where the most applications are found that require the strongest magnets. These temperatures are typically in the 25°C to 150°C range where NdFeB magnets are of the greatest interest.
How much bigger is the NdFeB loop than the Silicon steel loop? Here is a table of Coercivity values for selected materials.
Since NdFeB magnets have the highest Coercivity value in the table, these magnets have the highest resistance to demagnetization. They also generally have the highest energy product.
Table 1. Coercivity of Soft- and Hard Magnetic Materials
How Coercivity Affects the Hysteresis Loop
The hysteresis loop of a soft magnetic material may reach values in excess of +/- 1.5 Teslas on their vertical axis, which is very similar in amplitude to the hysteresis loops of hard magnetic materials. But on the y axis, we can see a huge difference. Soft magnetic materials only reach a range of somewhere between +/- .2-1 H on the horizontal axis, while hard magnetic materials may reach upwards of 15.
So when we compare the Hysteresis loop of the two kinds of materials, we are looking at a width of the loop that is up to 10,000 times wider in hard magnetic materials than in soft magnetic materials. This is easy to see in the table above by the Coercivity values, and it is even easier to see in the graph that displays the Hysteresis Graphs for soft- and hard magnetic materials on the same chart.
When looking at a Hysteresis loop for a material, be sure to not only look at the shape of the loop. Always look at the x-axis values (H) and the y-axis values (B). We all like to get our chart on one page, so sometimes the numbers are expanded or compressed to make them fit. The values tell us a lot about the hardness or softness of the material, so be sure to identify the values.
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