Magnetics

Temperature Effects

Magnetic properties of all the permanent materials change with temperature. Remanence, and thus the magnetic flux a magnet produces, decreases in all magnetic materials with increasing temperature. Ultimately, at certain, for each material characteristic temperature the material becomes non-magnetic. This temperature is known as Curie temperature (Tc).

Figure 1 illustrates the temperature behaviour of remanence. Usually the temperature dependence of remanence,  alpha = dB/dT, is given with one value, which is usually valid between 20C to 100C. This is strictly speaking not true, but the temperature dependence increases with increasing temperature. Table 2 shows for a number of materials typical values for Tc and alpha.

        Fig 1 Temperature dependence of Remanence

For coercivity the behaviour is a bit more complicated. In some materials, like NdFeB and SmCo, coercivity decreases with increasing temperature. In some, like hard ferrite, coercivity increases with increasing temperature. In some, like Alnico, the temperature dependence may vary from slightly positive to slightly negative, depending on alloying and heat treatment. As for remanence, the temperature dependence of coecivity is not constant. Usually diHc/dT = beta is given as a constant within certain temperature range (20-100C). Again Table 2 shows typical values of beta for a number of materials.

As long as the magnet is in closed circuit (no risk for demagnetisation), these temperature effects are reversible: This means that after heating to higher temperature and cooling back to the initial temperature the magnet will show same magnetic properties as in the beginning.

With decreasing temperatures in all the other materials but hard ferrite both the remanence and coercivity increase and the materials became more stable. For hard ferrite the coercivity decreases with decreasing temperature and there is risk for demagnetisation already at -40C. For NdFeB magnets the application limit is at appr. -120 C. Only SmCo magnets can be used in cryogenic temperatures.