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Graphic Analysis of Demagnetization at Low Temperatures

In general, ferrite magnets are affected by temperature changes by as much as ten times compared to cast magnets ; the Br reduces by a factor of about 0.2%/℃ and Hc increases by a factor of about 0.2 to 0.5%/℃.
A change of magnetic characteristics in the ferrite magnet depends largely on the combined influence of various factors including marginal surface migration.
However, the determination of the demagnetization stability of a magnet is essential to magnet circuit design work and our graphical analysis of approximate characteristics changes caused by changes in the temperature is shown in Fig. 2.
This method of analysis is based on the assumption that the B-H curve at an arbitrary temperature has a mathematical relation of 1:1 to that of the same B-H curve at an ambient temperature and, also, on condition that the knee of the curve stays on tangent line S-T.
This approximate analysis method is applicable for an assumption of demagnetization at a low temperature provided that the magnetic circuit is not affected by external magnetic fields in the ambient temperature range.

Accurate data may be obtained only from the actual measurement

Irreversible Demagnetization at Low Temperatures

In Fig. 2, demagnetization at an ambient temperature is shown by curve Br-iHc and at low temperature is shown by curve Br′-iHc′.
When the operating line of the magnet is on O-P1 line, the operating point is P1, However, at a low temperature, the operating point goes to Q1′ causing the flux density to reduce by an amount equivalent to P1-Q1′.
When the magnet brought back to the ambient temperature, the point corresponding to Q1′ becomes R1.
But the operating line of the magnet remains unchanged, and the operating point becomes point Q on the minor loop starting form point R1.
This means that demagnetization occurs in the magnet ;
Operating point goes Q1′ from P1, when the temperature drops below ambient.

Operating point goes to Q from Q1′, when the temperature rises to ambient from a low temperature.

Reversible Demagnetization at Low Temperatures

When the operating line of a magnet is O-P2, the operating point is P2 at an ambient temperature and Q2′ at low temperature.  
But, in this case, the corresponding point after the temperature has been brought back to an ambient temperature exists at point R2 causing the operating point to return to the original point P2.
Thus, no demagnetization will take place.
As mentioned above, the differential in magnetic flux density at a low temperature and at the ambient temperature varies depending on the operating line.
It is essential, in the case of those equipments which incorporate a revolving electric device inducing an external demagnetization field, that an analysis of the demagnetization effects, not only on the initial operating point (operating line), but also on the possible migration of the operating point as the result of the demagnetization field must by made.
This is referred to in the following paragraph.