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Low Temperature Demagnetization of a Ferrite Magnet
【Reversible Change and Irreversible Change of the Operating Point of a Ferrite Magnet】
It is necessary to choose a material with high iHc and design the permeance modulus as high as possible because the demagnetizing factor of the B-H Curve increases according to the temperature change when using the magnet at a low temperature. Big demagnetization shall be shown by magnetizing an anisotropy ferrite magnet with low permeance modulus, cooling to a temperature of around -40℃, and returning to a normal temperature. The graph of the demagnetization is shown in the drawing at right.
【How to calculate the demagnetization value by the irreversible change of a ferrite magnet】
If the operating point at a normal temperature (+20℃) is Point a1, and this is higher than the critical point (permeance modulus Pc1), a1, the operating point moves to a2 according to the drop in the temperature (reversible change). When the permeance modulus is small (Pc2) and the operating point at a normal temperature (+20℃) is near the critical point (b1), b1, the operating point moves to b2 according to the drop in the temperature. So the operating point that was at a position higher than the critical point at a normal temperature drops to a position lower than the critical point according to the drop in the temperature. It does not return to the original position (b1), even if the temperature returns to the normal temperature (irreversible change).
