Choose the Optimum Magnet Grade: Know Why Magnets Demagnetize

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Factors that Can Demagnetize a Magnet

A common question in the magnet industry is whether or not “permanent” magnets can demagnetize, or lose their strength. While magnets can lose power over time, in the absence of external influences, an industrial magnet alloy should hypothetically remain magnetic for hundreds of years.

However, magnets used in real-world applications experience external demagnetizing conditions. An even partially demagnetized magnet may negatively impact operational performance, resulting in failures in the field.

There are several variables that contribute to magnetic performance degradation. In this article, we will review the major factors that could lead to demagnetization and learn why it is advisable to perform an analysis of the magnet’s operating environment to determine the required grade of magnet alloy for your application.

What Causes Magnetism?

To study demagnetization, let us first review the cause of magnetization. All substances are made of atoms, and every atom contains electrons that carry an electric charge. When an equal number of electrons spin in opposite directions, the substance is less magnetic. Substances with many electrons spinning in the same direction are strongly magnetic. These substances become magnetized when a magnetic object is introduced into the materials’ magnetic field or when an electric current is applied.

When the magnet is magnetized, it enters into a less favorable, higher energy state. The magnet then tends toward reduction of this energetic state and becomes demagnetized over time.

Steel can become a powerfully 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.

Permanent magnet alloys are special because they possess the ability to maintain a state of magnetization after the external magnetizing field is removed. Unlike steel, there is a mechanism in magnetic alloys that allows for sustained induced magnetism; however, this mechanism has limitations. The magnet is constantly trying to self-demagnetize, and the first factor influencing this demagnetization is the geometry of the magnet.

Note that each permanent magnetic alloy has its own unique advantages and disadvantages depending on the application.  Understanding these advantages and disadvantages is critical for operational success and is one of the key areas of expertise at Dura Magnetics.

How Is a Magnet Demagnetized?

A magnet’s inherent properties influences how easily it becomes demagnetized. Demagnetization can also be caused by the alteration of physical properties. Factors that affect the demagnetization of a magnet include:

  • Magnet geometry
  • Volume loss
  • Elevated temperatures
  • Introduction of external fields
  • Intrinsic coercive force

Magnet Geometry

The inherent geometry of a magnet influences the magnet alloy’s ability to tolerate demagnetization caused by external and internal influences. The more ideal the geometry of a magnet per the application, the better the resistance to self-demagnetizing, elevated temperatures, and external demagnetizing fields.

The geometry of a 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 more effectively the magnet will resist demagnetization. Achieving this resistance often requires 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 reaches a point of diminishing returns relative to L/D ratio.

Note: For magnet geometries which are not discs, the equivalent diameter / area circle can be used in the L/D ratio calculation. Simply locate the area of the pole and determine the diameter of a circle with equivalent area.

Volume Loss

The most prominent performance degradation is caused by a reduction of the magnet’s volume. Sometimes this reduction occurs due to mechanical impact, where a portion of the magnet is fractured from the main body; however, volume loss can also be caused by corrosion.

Typically, a loss of performance caused by corrosion or fracturing is visually obvious, but there are instances where a seemingly intact magnet can partially demagnetize, resulting in performance degradation. The Dura Magnetics team offers scientific testing that can diagnose these hidden causes of degradation.

Elevated Temperatures

Magnets exposed to heat above certain levels can irreversibly lose their strength. Strength will inevitably degrade as a magnet is heated, but so long as it is not heated above a certain point (the maximum operating temperature), the strength will be recovered when it has fully cooled down. However, if the magnet is heated above the maximum operating point, a percentage of the magnet’s volume is demagnetized, and the magnetic loss will not be recovered upon cool down. Recovery then requires re-magnetizing the magnet.

The maximum operating point is geometry specific. Most magnet providers specify a maximum operating temperature for various grades of 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. However, 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, or L/D ratio, of the magnet must be sufficiently “long” relative to the area of the pole. For instance, a 0.250” OD x 0.250” long neodymium magnet, oriented and magnetized through the length, has a magnetic length to pole diameter of L/D = 1. This is a highly effective L/D ratio, so the magnet should be capable of performing at the advertised upper operating temperature without irrecoverable loss.

However, a magnet that is 0.250” OD x 0.125” in length has an L/D = 0.5, which indicates 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 neodymium magnets, but consulting an expert and execute thermal testing are recommended.

External Fields

Magnet alloys are magnetized with a sufficiently intense magnetic field established in the same direction as the magnet’s orientation. When a material is exposed to a strong magnetic field that is established in opposition to its magnetic orientation, part of the magnet may become demagnetized. This demagnetization reduces the effective field of the magnet, and the magnet’s performance will 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 interacts with fields from permanent magnets to create motion.

A Halbach Array is an example of a static case of demagnetizing fields from neighboring permanent magnets, where some internal magnets have a poor L/D and low Hci, and are demagnetized from other array magnets (more on Hci below).

A dynamic case of demagnetization could be a permanent magnetic torque coupler, which “slips” with like magnet poles rotating 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 Neodymium alloy grades shows an increase in operating temperature correlating to an increase in intrinsic coercive force. This also holds true for the magnet’s ability to withstand an external demagnetizing field. The higher the Intrinsic Coercive Force, the more effectively the magnet will withstand external demagnetizing fields.

Higher Intrinsic Coercive Force in a magnet material adds cost, therefore the Hci level should be matched specifically to the application. A Neo magnet’s Hci is enhanced by adding various materials to the crystal lattice. The most prominent material added is dysprosium, which can be very expensive. Additionally, by adding materials to the lattice, the effective energy of the Neo alloy is reduced. Thus, high grades of Neo with high operating temperatures can be difficult to manufacture, expensive, or have limited availability. Because of these issues, Neodymium grades of 50 have an Intrinsic Coercive Force of ~ 14 kilo-Oersted and a lower tolerance to heat.

Determine the Optimum Magnet Grade to Reduce Demagnetization

One should not rely on advertised temperature performance values without first understanding the impact of the magnetic length / effective pole diameter ratio (L/D), and the intended operating environment.

To determine the required grade of Neo magnet alloy for your application, it is best to perform an analysis of the magnet’s operating condition. This is especially true when the magnet experiences high temperatures and demagnetizing fields during operation.

In need of expert analysis and performance advice? The Dura Magnetics team welcomes your challenges and offers industry-leading testing and application-specific solutions for your operational requirements.

To learn more about the variables that may contribute to magnetic performance degradation, or to discuss your project, contact Dura Magnetics online or speak directly with our specialists at (800) 492-7939.