A hotly debated question in the magnet industry: Will permanent magnets demagnetize, i.e. lose their magnetism? Although permanent magnets should theoretically be able to maintain their magnetism for hundreds of years without external influences.
However, magnets used in practical applications will face various complex external environments and cause demagnetization. Now, let me take you through the main factors that may cause demagnetization and how to control them to minimize demagnetization to determine the grade of permanent magnets required for your application.
The Cause of Magnetism.
Magnets can pick up paper clips and drive motors to rotate. The roots of these magical abilities are actually hidden in the microscopic world. Before we begin analyzing demagnetization, let’s review the causes of magnetization.
In materials such as iron, nickel or neodymium, which make up permanent magnets, each atom of the substance contains electrons with an electrical charge. When a magnetic material’s electrons are introduced into the substance’s magnetic field or an electric current is applied, they spin in well-aligned patterns, forming microscopic magnetic regions called “domains.” When these domains are aligned, their collective power creates the magnetic field we perceive. Think of it like a group of people cheering in unison – the louder (or stronger) the cheers, the stronger the collective effect.
But not all materials can maintain this formation. Over time, magnets will tend to reduce this energy state and eventually become demagnetized. For example, when the external magnetic field of pure steel disappears, its induced field will immediately reduce to zero. Only ferromagnetic materials (such as iron, cobalt, or neodymium iron boron) can maintain strong magnetism for a long time because their magnetic domains are “locked” in a fixed position – unless they encounter high temperatures, reverse magnetic fields, or severe impact. This stability is partially reflected in the hysteresis curve (which we will analyze in detail in another guide), which reveals the material’s ability to resist demagnetization.
What is the Demagnetizing Field?
Now, here’s where things get tricky. Even a perfectly magnetized object is in an invisible battle with itself. When the raw magnet material is exposed to an external magnetic field during the manufacturing process, its internal magnetic domains align, creating a strong magnetization (M). However, once the external field is removed, the magnet’s own shape and structure creates an internal opposing magnetic field (Hₑ) that weakens its overall magnetism. The north and south poles inside the material push against each other, fighting against their own alignment. The longer or flatter the magnet, the stronger this self-destructive effect.
Let me put it in a simpler way: hold two identical magnets with their north poles facing each other. They push against each other, right? Now imagine a single bar magnet—its own north and south poles are constantly trying to repel each other from the inside, reducing its net magnetic field strength. This is essentially the effect of the demagnetization field.
The figure below shows a comparison of the magnetic field (magnetic flux density) B, the demagnetization field H, and the magnetization M of a cylindrical bar magnet. The north pole is on the right and the south pole is on the left.
This explains why the magnetic force of magnets in the shape of thin discs of the same material is always weaker than that of long magnets – the former has a larger demagnetization factor and a more obvious self-cancellation effect.
Principles of magnetostatics
According to the magnetostatic equation in Maxwell’s equations, the demagnetization field is a function of the position H ( r ). In the actual magnetization process, the external magnetic field suppresses the demagnetization field, and the magnetic domains are quickly aligned (the magnetic susceptibility χ is very high at this time)
When the magnetization intensity M reaches a certain critical point, the demagnetization field intensity Hₑ begins to compete with the external magnetic field H₆, and the magnetization process becomes difficult.
Adhere to continuous magnetization, and according to Ampere’s law and Gauss’ law, the final actual magnetic field strength is B = μ₀(H + M).
*μ₀ is the vacuum permeability, and M is the magnetization intensity.
The magnetic field strength H consists of two parts:
- External magnetic field (H₆): externally applied magnetizing force
- Demagnetization field (Hₑ): internal reaction force caused by the shape of the material itself
The formula is: H = H₆ – Hₑ
The energy of the demagnetization field is completely determined by the integral of the volume V of the magnet:
The demagnetization field here is like a “rebel molecule” inside the magnet, always trying to offset the external magnetization effect. Its strength is determined by the demagnetization factor (N)* of the material, which is closely related to the shape of the object – the N value of a spherical magnet is 1/3, while a slender bar may be as low as 0.02.
Factors causing magnet demagnetization.
Although permanent magnets can usually maintain their continuous magnetic field for a long time under normal working conditions. However, in reality, permanent magnetic materials will still demagnetize under certain conditions, such as exposure to high temperatures, volume loss caused by collisions, and exposure to conflicting magnetic fields. ——Our testing department has recorded and analyzed hundreds of failed magnets and found that these 6 “invisible killers” are always behind demagnetization.
- High temperature: the “fuse” of magnetic force
Heat has always been the number one enemy of magnetic force. Every magnet has a maximum operating temperature and a Curie temperature. Exceeding either temperature, the magnet may lose some or all of its magnetism. For example, standard neodymium magnets (such as N52) will begin to weaken above 80°C. If the temperature continues to rise, the internal magnetic domains will begin to permanently misalign. Once misaligned, they cannot recover on their own. The limit temperature of samarium cobalt magnets is 350°C. Alnico magnets have the best temperature characteristics of any standard production magnet material and are capable of continuous duty applications with expected temperatures up to 540°C, but are more expensive.
This usually happens in high-speed motors, welding devices, or outdoor equipment exposed to direct sunlight or high temperatures. Last summer, a neodymium magnet rotor of a motor manufacturer in Zhejiang suddenly lost magnetism during testing. After investigation, it turned out that the local temperature soared to 210°C (close to the Curie temperature of neodymium magnets) due to defects in the heat dissipation design.
If the magnet needs to work in a high-temperature environment, be sure to choose a high-temperature grade material, such as N42SH, N35EH or SmCo, which can better withstand high temperatures. (Click to learn more about the characteristics of magnet grades) When selecting magnetic materials, the actual operating temperature should be lower than 80% of the Curie temperature. When real-world applications involve temperature fluctuations, you can’t just look at peak temperatures, but also look at sustained exposure over time. If necessary, use coating protection (such as epoxy resin + nickel copper nickel three-layer plating). - Reverse magnetic field: “shuffler” of magnetic domains
Exposure to an adverse external magnetic field can cause permanent magnets to demagnetize. When there is a strong magnetic field around a magnet that is opposite to its magnetic orientation, the magnetic field will form a demagnetizing force that pulls its internal magnetic domains in a new direction, silently causing the magnet’s performance to degrade. For example, the magnetic field of other nearby magnets, electromagnetic coils, or industrial equipment.
It is therefore very important to store permanent magnets correctly and avoid placing strong magnets too close without proper shielding. High coercivity materials are preferred (such as N52 neodymium magnets with Hcj up to 1114 kA/m). When assembling magnetic components on a daily basis, ensure that the polarity of adjacent magnets is correctly connected in series. Of course, there are special cases – Halbach arrays, whose specific array method will cause static demagnetization fields for adjacent permanent magnets. Permanent magnetic torque couplings “slip” when rotating with magnets of the same pole. Under the combined effect of the relative and high temperature of the rotor, the magnet alloy is extremely susceptible to demagnetization. - Magnetic damage caused by physical impact
Magnets are fragile, especially neodymium magnets. If the magnet is hit by other objects or dropped, it may chip, crack, or break. Even tiny cracks (invisible to the naked eye) can disrupt the internal magnetic alignment and reduce strength.
An auto parts manufacturer once complained that their magnetic oil pan screws frequently failed. We found through scanning electron microscopy that engine vibration caused microcracks inside the magnets – these cracks were like broken ends of magnetic field lines, causing the magnetic field strength to decay by 37% in three months. Later, we added a rubber cushion to the magnetic oil drain screw and the problem was solved. - Magnet Geometry:
The geometry of a magnet can be reduced to a simple ratio – magnetic length / effective pole diameter (L/D). The pole length of a magnet is the physical size of the magnet in the direction of magnetization. The flatter the shape, the closer the internal poles are, and the stronger the self-cancelling effect. The higher the aspect ratio, the stronger the magnet’s ability to resist demagnetization. This is also the reason why magnets of the same material, thin magnets, sharp-angle magnets or smaller magnets are always easier to demagnetize than thick squares. So when designing custom magnets, work with experts who will consider aspect ratios, edge effects, and operating environments. Sometimes, a small change in geometry can greatly improve long-term performance.
A drone factory in Dongguan once used 0.5mm ultra-thin magnetic sheets to reduce weight. As a result, the entire batch of motors lost magnetism due to geomagnetic interference. Later, a 3mm thick multi-pole magnetization solution was used, which only increased the cost by 15%, but the failure rate dropped from 32% to 1.7%.Shape Optimization Guide Shape Applicable Scenarios Demagnetization Factor Range Ring/Arc Motor Rotor, Sensor 0.1-0.3 Aspect Ratio>3:1 Magnetic Bar, Speaker Magnetic Circuit 0.02-0.1 Thin Sheet Closed Magnetic Circuit Application Only 0.7-0.95 - Corrosion and Environmental Exposure
You may not know that a rusty magnet is like a leaky battery – our comparative tests have found that surface rust can cause a neodymium magnet to lose 5-8% of its magnetic flux per year. Especially the salt spray environment in coastal areas can cause oxidation spots to appear on uncoated magnets within three months. That’s why we determine the coating before every customer places an order. Learn about the rich coating options for magnets. - Time and natural decay
Even if you care for your magnets like treasures, time will leave its mark. Most modern magnets (such as neodymium iron boron and samarium cobalt) are very stable, but inferior magnets or old ceramic magnets will gradually lose their magnetism over time – especially if they are not stored properly.
Although the speed is not fast, after 10 to 20 years, the magnetic domains can slowly relax, especially when the magnet is close to its coercivity limit (the point where it can resist the influence of demagnetization).
Demagnetization is a problem that must be faced during the actual use of magnetic products. It may cost time, money and trust. But once you understand the key factors—temperature, external magnetic fields, time, shock, design flaws, corrosion, and handling—you can make more informed decisions about magnet selection and application design.
How to Avoid Magnet Demagnetization?
Let’s be honest, there’s nothing more frustrating than buying a strong, shiny magnet only to find it’s weakened. Once we understand why magnets are demagnetized, the natural question is: how can we prevent this? Store magnets in a dry, protected place. In magnetic engineering, maintaining magnetic field stability is like protecting a precision clock—every step requires precise control. We have combined the International Magnetic Materials Association (IMMA) standards and twenty years of industry experience to sort out this layered protection system.
Temperature control is the first line of defense.
Taking the most common neodymium iron boron (NdFeB) as an example, its Curie temperature is between 80-230℃, but the actual safety threshold needs to be lowered by 20%. This means that in continuous working scenarios, the surface temperature of the magnet should be strictly controlled. If customers plan to use magnets for motors or outdoor sensors, I usually recommend that they choose magnets with high temperature resistance grades such as N42SH or Sm2Co17. An engineer once neglected this margin in the design of the electric vehicle drive system, causing the motor to experience partial demagnetization during continuous climbing tests. By adding alumina ceramic heat sinks and micro-eddy current cooling tubes, the hot spot temperature was eventually reduced by 35%, and the magnetic flux attenuation rate was reduced from 1.2% to 0.3% per month.
Magnetic field protection.
Combating external magnetic field interference requires a two-pronged approach. Coercivity (Hcj), as the “anti-interference gene” of the material, directly determines its survivability. Reference formula:
Critical demagnetization field Hₐ = Hcj × (1 – N) (N is the demagnetization factor, which is related to the shape of the magnet)
For example, the coercivity of N42SH-grade neodymium magnets reaches 955kA/m, which is 42% higher than the basic N42 model. In actual deployment, it is recommended to adopt “three-layer protection”: give priority to high Hcj materials as the core, wrap the outer layer with electrical pure iron magnetic shield, and finally use Halbach array to optimize the magnetic field distribution. This structure is like putting a bulletproof vest on a magnet, and actual measurements can weaken external magnetic field interference by 68%. If there are many motors, coils or other magnets close to each other in your workspace, be sure to shield them well.
The essence of mechanical protection is energy conversion.
When a magnet is impacted, the vibration energy will cause damage in two ways: direct structural damage to produce microcracks, or change the magnetic domain orientation through lattice distortion. Comparative tests in our laboratory show that magnetic components with polyurethane-silicone composite buffer layers have a 53% higher magnetic flux retention rate than the unprotected group in simulated transportation vibration tests. It is important to note that any drilling or cutting operation will create stress concentration areas inside the magnet – just like scratching glass, even a tiny crack can become the starting point for demagnetization.
Environmental isolation.
Environmental corrosion is like chronic poisoning. In a salt spray environment with a humidity of 70%, uncoated neodymium magnets lose 8-12% of their magnetic flux each year. This is not only a problem of surface oxidation, but the corrosion products will penetrate into the interior along the grain boundaries like tree roots, forming a magnetic leakage channel. According to ASTM B117 standard:
| Protection level | Coating type | Salt spray resistance time | Applicable environment |
|---|---|---|---|
| C1 | Nickel plating (Ni) | 24h | Dry indoor environment |
| C4 | Nickel-copper-nickel (Ni-Cu-Ni) | 96h | Conventional industrial environment |
| C5 | Epoxy resin + laser glaze sealing | 240h | Marine/chemical environment |
The use of nickel-copper-nickel three-layer plating combined with laser glaze sealing technology can extend the salt spray resistance time to more than 240 hours, which is particularly suitable for harsh environments such as ship propulsion systems.
Optimize geometric design.
The effect of shape on demagnetization can be quantified by the demagnetization factor: the N value of a slender cylinder is about 0.02, while that of a thin sheet structure may be as high as 0.95. Thin, sharp-cornered, or undersized magnets are more likely to experience rapid magnetic decay under external pressure. This explains why ring magnets of the same material have a lifespan 3-5 times longer than sheet magnets. In the design of magnetic resonance components for medical devices, we often use multi-pole arc arrays with edge fillets of more than 0.3mm, which not only reduces the self-demagnetization effect, but also controls the stress concentration factor within the safety threshold.
Failure diagnosis and repair path.
When a magnet shows signs of demagnetization, don’t rush to sentence it to death! Try our Magnet Resurrection Trilogy. First, use a gauss meter to measure the surface magnetic field strength and compare it with the initial value to determine the decay rate; then use a thermal imager to scan the temperature distribution and locate the overheating area; finally, observe the microstructure through a metallographic microscope. For repairable magnets, the instantaneous strong magnetic field of the pulse magnetizer (recommended ≥3 times the material coercivity) can reorganize more than 90% of the magnetic domain array. Severely oxidized or broken magnets can be made into magnetic powder through the Hydrogen Decrepitation process, which can be used to manufacture isotropic bonded magnets to achieve resource recycling.
Keeping some daily testing equipment on hand to perform daily inspections regularly can nip the risk of demagnetization in the bud:
| Magnet health self-test table | ||
|---|---|---|
| Test items | Qualified standards | Tools |
| Surface temperature | ≤ Curie temperature × 0.8 | Infrared thermometer |
| Magnetic field strength | Decrease rate <15%/year | Gauss meter |
| Structural integrity | No visible cracks/rust spots | 10x magnifying glass |
| Ambient magnetic field | <30% of material Hcj value | Tesla meter |
Remember to save this guide and check it during the next routine review – scientific protection can extend the life of the magnet by 3-5 times! In the absence of obvious cosmetic damage, usually the lost magnetism can be replenished by re-magnetization. Osencmag not only provides simple magnet blocks, but also supports B2B customers to build magnet systems that can run for many years without obvious loss through clever design and suitable materials.
- High coercivity materials, such as Sm2Co17 or N52M/N52H;
- Shielding or soft magnetic backplane;
- Surface coatings such as epoxy, nickel or rubber;
- Physical isolation to prevent mechanical shock;
- Assembly testing under simulated use conditions;
Whether you need customized surface treatment, high temperature resistance level, or safe assembly guidance, we can ensure that your magnets remain strong and reliable no matter where and how they are used.
FAQs
Do wet magnets lose their magnetism?
No, magnets will not lose their magnetism immediately if they get wet or put into water. However, without any protective coating, prolonged exposure to water (especially hot water or salt water) will accelerate the corrosion of magnets. As we mentioned above, when a magnet corrodes, its magnetism will gradually be damaged. As we mentioned above, the magnet’s magnetism will gradually decrease after corrosion.
Do magnets lose strength over time?
All magnets will slowly lose their magnetism over time. However, permanent magnets lose their magnetism very slowly (no more than 1% strength loss in 10 years) without external factors.
What’s the best magnet for high heat?
When your magnets are subject to high heat, material selection is critical. Standard neodymium magnets (such as N35 or N52) begin to lose strength at around 80°C. For high temperature environments, I recommend considering the following three types of magnets:
- Neodymium high temperature grades (such as N42SH, N52H, N48EH) – can withstand temperatures up to 200°C;
- SmCo (Samarium Cobalt) – expensive, but maintains good magnetic properties up to 250–300°C;
- AlNiCo – maintains performance even at 500°C, but has low intrinsic magnetic force;
