Temperature effects on magnets

Curie Temperature Tc

The Curie Temperature (Tc) is the temperature at which the magnet will be completely demagnetized. Even though it may still be a magnetic material, a material that has the ability to retain a magnetic field when magnetized, it would have completely demagnetized. It is important to note that taking a magnet to Tc will demagnetize the magnet, but depending on the process used, heating and cooling rates, and the environment that the magnet is exposed to during the temperature cycle, heating a magnet to Tc can cause structural or mechanical damage.

Maximum Operating Temperature Tmax

One question that is frequently asked is why magnets demagnetize at elevated temperatures even though the magnet's maximum operating temperature was not reached. The maximum operating temperature of a particular magnet is NOT only a function of the material, but also a function of design. This is explained below.

Look at a magnet BH Curve. For example, look at the BH Curve of our N3571 material. This material is rated for use at a maximum operating temperature of 155°C.

On the extreme left and top of the graph there are some numbers. These numbers are to determine the permeance coefficient. Draw a straight line from the origin of the graph (that is from B=0, H=0) all the way to the permeance coefficient number of 1.0. This line is referred to as the load line.

Now let us look at the magnet performance curves. There are two basic curves shown. The curves on the extreme outside with the temperature values adjacent to them are the intrinsic curves. The line that runs from B=12 to H=11.6 and lines lower and parallel to them are the normal curves. We are going to concentrate on the normal curve.

Each intrinsic curve has a normal curve associated with it. It is actually part of the same test. Follow the 20°C intrinsic curve to the normal curve; starting with the intrinsic here is simply to be able to determine the temperature for the normal curve. Observe that the normal curve is a straight line.

Let us follow the next one (60°C). Again, this is a straight line. The 100°C is also a straight line. But the 140°C has a sudden drop in the curve. This is at B=2.2, H=7.4 (approximately). But our load line is above this knee. The magnet will not permanently demagnetize so long as the load line is above the knee.

Let us go to the 180°C curve. The knee occurs rapidly. Our line is BELOW the knee. When the magnet was cooled, the magnet would have lost some of its strength- permanent demagnetization occurred. How much? The magnet cannot climb up a knee � it can only climb up its normal curve if the line is straight. If there is a knee, it will climb up parallel to its original line, but not on the original line. In our case, with the magnet at 180°C, it would have climbed up such that the new curve would intersect the B axis at 8.2 kG. We could expect a loss of 32%.

The maximum operating temperatures are based on a B/H = �rec, in the above example 1.05. If the magnet, however, were operating below that number (lower B/H ratio) in your circuit, you should expect to see demagnetization below the maximum operating temperature.