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Effects of Heat on Neodymium Magnets: Reversible vs. Irreversible Field Loss

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A neodymium magnet may be used to supply a triggering field for a magnetic sensor.  At various times of operation, the magnet may be exposed to a temperature which reduces the magnetic field to a level which the sensor will not be triggered.  Other than during the high temperature exposure, the magnet will supply a sufficient field to trigger the sensor.  The magnet does not totally demagnetize nor does it suffer lasting effects from the high temperature exposure.  Repeated exposures are not additive, but the sensor is not triggered during the high temperature phase of operation.  When the magnet is inspected at room temperature, it will measure fine and exhibit no apparent loss.  The Neo magnet alloy was not selected properly for the limited high temperature operation even though it actually survived the high temperature exposures without suffering an irreversible demagnetizing loss.

The magnet described in the Case Study above is said to have experienced a reversible loss.  The magnet only experienced a magnetic field loss that was reversible when it cooled back down.  The magnet was not taken above a temperature threshold where permanent demagnetization occurred such that the loss was not recovered when cooled back to room temperature.  The magnet in the Case Study could be exposed to yet a higher temperature where it experiences a partial, irreversible demagnetization.  At this higher temperature the magnetic field would again be reduced when compared to the starting point at room temperature; however, when the magnet cools back down it will not recover the entire lost field.  A small portion of the magnet has demagnetized and no longer contributes to the net field produced by the magnet.  This loss is considered irreversible.  Higher and higher temperatures further demagnetize greater portions of the magnet.  At some temperature, the magnet will totally demagnetize and no magnet orientation will exist within the magnet.  This temperature is the Curie Temperature, Tc and the magnet will be rendered useless far before this point.  (The Curie Temperature is not the maximum recommended operating temperature.)

Maximum Recommended Operating Temperature:
The maximum recommended operating temperature of a Neo magnet seems like a very simple and straight forward magnet specification; however it is not a simple number and it is dependent upon several variables.

The Neo magnets Intrinsic Coercive Force will dictate much of the magnet’s ability to withstand exposures to elevated operational temperatures.  At face value it seems simple to select a Neo magnet that is advertised to tolerate 120°C of heat for an application that has a maximum exposure potential of 120°C.  The challenge is that the neo magnet’s ability to tolerate heat is also dependent upon the geometry of the magnet.  Most neo magnet suppliers will qualify the recommend maximum operating temperature with a minimum Length / *Diameter (L/D) ratio or a minimum permeance coefficient (Pc).  This helps account for the geometry impact and heat tolerance and it is a good first order estimate of heat tolerance.

(* The diameter is either the actual diameter or the equivalent diameter of the circle having the same pole cross-sectional area of the non-round shape.  This first order estimate (L/D) is reserved for simple geometric shapes.  Magnets which deviate from simple shapes, magnetic arrays, and magnetic assemblies must be evaluated differently to determine upper temperature thresholds.)

Simply put, magnets that are long compared to their pole cross-sections have a better ability to resist demagnetization for high temperature exposures.

The second variable which impacts the maximum recommended operating temperature is the exposure to external demagnetizing fields.  If the magnet is used in a Brushless DC motor (BLDC), it will experience external demagnetizing fields.  This additional form of demagnetization further erodes the heat tolerance of Neo magnet alloy.

The Residual Induction (Br) indicates the available magnetic flux output from the magnet.  The RTC of the Residual Induction (Br) is the % Change of Br / °C and is usually denoted by the symbol (α).  The Intrinsic Coercive Force (Hci) indicates the magnet’s ability to withstand demagnetizing, typically used for demagnetization from heat exposure and from external magnetic fields.  RTC of the Intrinsic Coercive Force (Hci) is the % Change of Hci / °C and is usually denoted by the symbol (β).

The rates of change are treated as being linear between some constrained temperature range.  In all actuality they are a small segment of a temperature response curve for the particular neo grade that can be treated as linear.

Unless a magnet will always exist at or near room temperature, it is imperative that a review is done to ensure the grade is selected properly for the particular application.  Magnets with higher heat tolerance are more costly, but the risk with an improper Neo alloy is a failure in the field.  Also, there may be no elevated heat condition experienced in the application, but what about the manufacturing process?  Thermal curing adhesives or powder-coating may expose the Neo magnets to temperatures where an irreversible loss can occur.  This also includes plastic over-molding, brazing, sterilization, etc.

Consult a Dura Magnetics application technician for assistance to determine the best grade of Neo for your application.

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Did You Know?

When designing near the limits of a magnet’s max operating temperature, knowing your magnet’s load line is extremely important.

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