Magnet Stability:

In the previous Section 3A, we discussed the variation in a magnet's remanence B_r with increasing temperature, showing that its decline is approximately linear and reversible up to some transition level. Once the change becomes non-linear, though, an irreversible loss of B_r has occurred. This transition is associated with the onset of a reversal of the material's magnetization, and is represented by a "knee" in the demagnetization curve. In Section 4A, we introduce a load line, whose intersection with the demagnetization curve identifies a unique operating point (B_m, H_m) for the magnet supplying flux to a specific magnetic circuit. As with the remanence, if the operating point is safely above the "knee" of the demagnetization curve, changes in the magnet's condition will be reversible, but if the operating point falls below the "knee", there will be an irreversible loss of B_m.

The problem is that the position of the "knee" (i.e. the threshold for irreversible loss) is dependent upon temperature, and as the temperature changes, the operating point may fall below the "knee" of the appropriate demagnetization curve.

Let's look at a couple examples using the grade of fully dense anisotropic neodymium-iron-boron whose demagnetization curves at various temperatures were given in Section 3A, and another for the "Ceramic 8" magnet which was described in Section 2A. Now, consider these magnets to be operating in a magnetic circuit with a load line (shown in red) whose slope (÷µ_o) is -1.

Example 1:

In this example, the magnet is cycling between +20^oC and +60^oC. In these (second quadrant) demagnetization curves, there is no "knee" present at +20^oC, but one does appear when the temperature is raised to +60^oC. However, the magnet's operating point at the intersection of the load line and the +60^oC curve is safely above the "knee", so there will be no irreversible loss. This reversible change is illustrated to the right of the B-axis, with magnet flux cycling between point a at +20^oC and point b at +60^oC.

Example 2:

In the second example, the magnet starts again at +20^oC (point a), is heated first to +60^oC (point b), and then further to +100^oC (point c). However, the intersection of the load line and the +100^oC curve is now below the "knee", and an irreversible loss of magnet flux has occurred. This is apparent when the magnet is cooled again to +20^oC (point d)...the magnet's operating point on the load line is no longer on the +20^oC demagnetization curve, but at some point within the major B vs. H curve (the second quadrant of which is the demagnetization curve). The reason for this is simply that the major B vs. H curve represents data measured on a fully magnetized magnet, but with the irreversible loss, the magnet is no longer fully magnetized. It must be re-magnetized to saturate the material's magnetization once again, and to regain operation on the major B vs. H curve. In summary:

• a to b is a linear change and is reversible, as described in Example 1.
• b to c is a non-linear change and an irreversible loss.
• c to d is a linear change and is reversible, but with reduced magnetic properties.
• d to a restores the full magnetic properties by re-magnetizing the magnet.

As the slope of the load line is increased, it is apparent from the shape of these demagnetization curves that the transition temperature to an irreversible loss also increases. Consequently, while a greater load line slope obviously raises the flux delivered by the magnet to the magnetic circuit, it will also help to stabilize the magnet against such thermal effects. A set of temperature characteristics (such as those shown here for a sintered Sm₂Co₁₇ magnet) helps to illustrate this.

Example 3:

When discussing the Change in Coercivity in Section 3A, we noted that while H_ci decreases with temperature both for samarium-cobalt and for neodymium-iron-boron magnets, H_ci increases with temperature in the case of ceramic ferrites (because they are based solely on magnetocrystalline anisotropy). This means that the "knee" of the demagnetization curve can come into play as temperature falls. In this example, the magnet is cycled from +20^oC to -60^oC and back again. The diagram below illustrates that, with a load line slope (÷µ_o) of -1, the transition to an irreversible loss occurs when the temperature falls below about -20^oC.

The sequence of events is:

• a to b is a linear reversible change between +20^oC and -20^oC.
• b to c is a non-linear irreversible change from -20^oC down to -60^oC.
• c to d to e is a linear reversible change from -60^oC back to +20^oC, but with reduced magnetic properties.

Always check the operation of a magnet over its complete temperature range (even a little beyond this range to be safe). If it appears there is irreversible loss, decide whether you will accept this operating condition with reduced but reversible magnetic properties, or whether the magnet should be redesigned to increase the load line slope and stabilize it's operation without degrading the properties.

Copyright © 1999 by Princeton Electro-Technology, Inc.
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