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Why Magnets Lose Strength: The Effects of Volume Loss, Geometry, Elevated Temperature, and Demagnetization from External Fields

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In the absence of external influences, a commercially viable magnet alloy should remain magnetic for hundreds of years.  The alloy’s resulting field may degrade slightly from aging effects, but for the most part it should remain an effective magnetic field source.  With that said, most magnets used in real world applications experience many external demagnetizing conditions.  A partially demagnetized magnet may greatly impact operational performance that may result in failures in the field.

Volume Loss
The most prominent performance degradation from field loss is due to a reduction of the magnet’s volume.  Sometimes this can be from mechanical impact, where a portion of the magnet is fractured from the main body, but more often than not, the volume loss is from corrosion.  Usually a loss of performance from corrosion or fracturing is observable and quite obvious, but there are mechanisms that exist where a seemingly intact magnet can partially demagnetize, therefor resulting in performance degradation.

Geometry Contribution
Magnet alloy does not desire to be MAGNETIZED.  When the magnet is magnetized it is put into a higher energy state.  The magnet desires to reduce the energetic state and become demagnetized.  The ability to keep the magnet “magnetized” after the external magnetizing field is removed is what is special about permanent magnet alloys.

Regular steel can become a wonderfully strong magnet while there is an applied magnetizing field, but the steel’s induced field immediately degrades to virtually zero when the external magnetizing field is removed.

Unlike steel, there is a mechanism in the magnetic alloy which allows for the sustained induced magnetism; however, this mechanism has limitations.  The magnet is constantly trying to “self-demagnetize” and the first factor influencing the demagnetization is the geometry of the magnet.

The geometry of the magnet greatly influences the magnet alloy’s ability to tolerate demagnetizing from a number of external and internal influences.  The more ideal the geometry of a magnet, the better resistance to self-demagnetizing, elevated temperatures, and external demagnetizing fields.

The geometry of the magnet can be reduced to a simple ratio, the Magnetic Length / Effective Pole Diameter (L/D).   The Magnetic Length of the magnet is the physical dimension of the magnet in the direction of Magnetization.  The Effective Pole Diameter is the diameter of the pole region or the Equivalent Diameter for a non-circular pole.

The higher the L/D ratio, the better the magnet will resist all forms of demagnetization. This however often results in more magnet volume and higher cost.  Higher L/D ratios will result in higher magnetic performance, but this correlation is not linear and the magnet quickly encounters a point of diminishing returns relative to performance.

(* For magnet geometries which are not discs, the equivalent diameter / area circle can be used in the L/D ratio.  It is necessary that one locates the area of the pole and calculates the diameter of a circle that has an equivalent area.)

Demagnetization from Elevated Temperatures
Magnets exposed to heat can lose strength which cannot be recovered.  Magnets will lose strength as they are heat up, but so long as they are not taken above a certain operating point (maximum operating temperature) the strength will be recovered when they cool back down.  If the magnet is taken above the operating point, a percentage of the magnet’s volume is demagnetized and the magnetic loss cannot be recovered without re-magnetizing the magnet.

The operating point is geometry specific.  Most magnet providers specify a maximum operating temperature for various grades of magnet alloy, but this must be qualified.  The noted upper operating temperature assumes the magnet has an appropriate geometry to tolerate the heat level of the selected alloy grade.  The advertised maximum operating temperature for a particular magnet alloy grade is not always sufficient to ensure elevated temperature performance.  For optimum operational heat resistance the Magnetic Length of the magnet must be sufficiently “long” relative to the area of the pole.

For instance a 0.250” OD x 0.250” Long Neo magnet, oriented and magnetized through the length, has a magnetic length to pole diameter of L/D =  1.  This is a very good L/D ratio and the advertised upper operating temperature may be able to be used as the application’s maximum operating temperature.  However, a magnet that is 0.250” OD x 0.125” long has an L/D = 0.5 and this would indicate that the advertised maximum temperature of the magnet must be adjusted downward or a higher heat grade selected.  Typically L/D ratios over 0.7 are appropriate for the recommended operating temperatures advertised for Neo, but it is best to consult the vendor and execute thermal testing.

Demagnetizing from External Fields
Magnet alloy is magnetized with a sufficiently intense magnetic field that is established in the same direction as the magnet’s orientation.  When a magnetized magnet is exposed to a strong magnetic field that is established in opposition to the magnet’s magnetization, part of the magnet may be demagnetized.  This demagnetization essentially reduces the effective field of the magnet and the magnet’s performance will therefore degrade.

External demagnetizing fields can originate from fields created by electromagnets/coils or other neighboring permanent magnets.  An example of a coil would be a motor application where the dynamic fields created by a coil set interact with fields from permanent magnets to create motion.  An example of a static case of demagnetizing fields from neighboring permanent magnets could be a Halbach Array where some internal magnets have a poor L/D and low Hci and are demagnetized from other array magnets.  A dynamic case could be a permanent magnetic toque coupler which “slips” and like magnet poles rotate over one another.  This partial demagnetizing, with an external field is exacerbated when the magnet is required to operate at high temperatures.  Both the external field and the elevated temperature conspire to demagnetize the magnet alloy.

Intrinsic Coercive Force Hci
The Intrinsic Coercive Force (Hci) indicates the magnet alloy’s ability to withstand heat and demagnetization from external magnetic fields.  A quick review of the table of Neo alloy grades show an increase in operating temperature with an increase in the intrinsic coercive force.  This also holds true for the magnet’s ability to withstand external demagnetizing field.  The higher the intrinsic coercive force, the better the magnet will withstand external demagnetizing fields.

The Intrinsic Coercive Force of a magnet material adds cost and that is why the Hci level should be matched to the application.  A Neo magnet’s Hci is enhanced by adding various materials to the crystal lattice.  The most prominent material is dysprosium which is very expensive.  Also, by adding materials to the lattice, the effective energy of the Neo alloy is reduced.  This means that high grades of Neo with high operating temperatures are very difficult to manufacture, are expensive or not available.  This is why Neo grades of 50 have an Intrinsic Coercive Force of ~ 14 kilo-Oersted and a lower tolerance to heat.

It is best to perform an actual analysis of the magnet’s operating condition to determine the required grade of Neo magnet alloy.  This is especially true when the magnet experiences high temperatures and demagnetizing fields during operation.  One should not rely on advertised temperature performance values from vendors unless they understand their L/D and all external demagnetizing variables.

To learn more about the variables that may contribute to magnetic performance degradation or to discuss your project, contact Dura Magnetics via our website, or speak directly with our specialists at 1-800-497-8182.  We welcome your challenges and pride ourselves on offering industry-leading application specific solutions to all of your magnetic needs.

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