Magnets lose their magnetism when something scrambles the tiny aligned regions inside them, called magnetic domains. The main culprits are heat, strong opposing magnetic fields and hard physical shocks. Time alone barely weakens a good permanent magnet, but a few bad habits can.
If your fridge magnet has gone limp, a toolbox magnet will not grab screws, or you have heard that a magnet can wipe a credit card, this guide explains what is actually going on. We cover the physics in plain English, the temperatures at which common magnets fail, what does and does not demagnetize a magnet at home, and how to keep yours strong. The sources are listed at the end, mostly the standard encyclopedia entries on magnetism and the published properties of magnet materials.
Why a Magnet Is Magnetic in the First Place
A permanent magnet is magnetic because, inside it, groups of atoms point their magnetism the same way. According to Wikipedia's article on magnetic domains, a magnetic domain is a region of a material in which the magnetization points in a uniform direction. Iron, cobalt, nickel and their alloys, as well as ferrite, are the materials where this happens.
In an ordinary piece of iron the domains point in many different directions and cancel each other out, so the iron is not a magnet. A permanent magnet is made from a "hard" ferromagnetic material such as alnico or ferrite that is processed in a strong magnetic field during manufacture. That lines up its internal structure and makes it very hard to demagnetize, as Wikipedia's magnet article explains.
That gives us the whole idea of demagnetizing in one sentence: anything that mixes the domains back up weakens the magnet. There are four ways that happens.
1. Heat: The Curie Temperature
Heating a magnet past its Curie temperature destroys its magnetism. Wikipedia defines the Curie temperature as the temperature above which certain materials lose their permanent magnetic properties. The effect was noticed as long ago as 1600, when William Gilbert's De Magnete reported that iron filings heated for a long time were attracted by a lodestone less strongly than before. The temperature is named after Pierre Curie, who showed that magnetism is lost at a critical temperature.
The reason is motion. Heat makes the atoms jiggle, and above the Curie point the jiggling wins and the magnetic moments no longer line up. Wikipedia's magnet article puts it this way: the molecular motion destroys the alignment of the magnetic domains, completely demagnetizing the magnet.
Curie temperatures vary a lot by material. According to Wikipedia's articles on the elements:
- Iron: about 770 °C (1,418 °F).
- Nickel: about 355 °C (671 °F), so bulk nickel is non-magnetic above that temperature.
- Cobalt: about 1,115 °C (2,039 °F).
- Alnico alloys: around 800 °C (1,470 °F), among the highest of any magnetic material.
- Neodymium-iron-boron (NIB) magnets: around 320 °C (608 °F), which is lower than many other magnet types.
The Curie point is not the only temperature that matters. Magnets start losing strength well below it. Wikipedia's neodymium magnet entry notes that while these magnets resist demagnetization well at room temperature, their coercivity (resistance to demagnetization) drops sharply once the temperature rises above about 100 °C (212 °F). Standard grades are rated for use only up to about 80 °C (176 °F), and special grades with terbium or dysprosium go to roughly 230 °C (446 °F). In general the maximum usable temperature is over 540 °C (1,000 °F) for alnico, about 300 °C (570 °F) for ferrite and samarium-cobalt, and only about 140 °C (280 °F) for neodymium, depending on the grade, according to Wikipedia's magnet article.
What that means at home: a neodymium magnet left in a hot car or near an oven can lose strength, and some of that loss is permanent if it was pushed far enough. A ferrite fridge magnet is more forgiving. Heating a magnet past its Curie point can wipe it entirely, though the magnet can often be remagnetized afterward.
2. A Stronger Opposing Magnetic Field
A magnet can be demagnetized by a magnetic field that pushes the opposite way and is strong enough. The threshold is called coercivity. Wikipedia describes it as a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. The higher the coercivity, the tougher the magnet.
Two practical points follow from Wikipedia's magnet article:
- Some demagnetization occurs if any part of the magnet sits in a reverse field above its coercivity. That is why you should not stick two strong magnets together in opposition and leave them that way for years.
- Commercial demagnetizers work by placing the object in an alternating magnetic field stronger than the material's coercivity, then slowly withdrawing it. This is the principle used to demagnetize tools, erase credit cards and hard disks, and degauss old CRT screens.
The old TV and monitor technique is the same idea. Degaussing coils run alternating current that gradually fades to zero, leaving the domains in a scrambled, near-zero state.
Can a magnet wipe your credit card?
It can wipe the magnetic stripe, which is just magnetic material attached to a plastic card, read by swiping it past a head. A strong enough magnet held against it can corrupt the data. The chip on a modern card stores data electronically, not magnetically, so the stripe is the part at risk. Mastercard announced in 2021 that by 2033 none of its credit and debit cards would carry a magnetic stripe.
3. Physical Shocks: Hammering and Dropping
A hard knock can weaken a magnet because it jolts the domain walls loose. The walls that separate domains are normally "pinned" in place. Wikipedia's article on magnetic domains says that heating a magnet, subjecting it to vibration by hammering, or applying a rapidly oscillating field tends to pull the domain walls free from their pinned states, so they settle into a lower-energy, less aligned arrangement. The magnet article lists hammering or jarring as a way to randomize the domains and reduce magnetization, though it also warns that it may cause unacceptable damage.
So dropping a magnet repeatedly is not good for it, and brittle magnets such as neodymium can also chip or crack. Wikipedia adds that some magnets are brittle and can fracture at high temperatures. The effect of a single drop is usually small, but repeated knocks add up.
4. Self-Demagnetization and Bad Shapes
A magnet also generates its own demagnetizing field. Wikipedia explains that when a magnet is magnetized, a field is created inside it that works against its own magnetization. The shape decides how bad it is. A straight iron bar tends to demagnetize itself by its own field, while a horseshoe magnet avoids this by returning the field lines to the opposite pole.
This is the reason behind the old advice to store bar magnets in pairs with opposite poles together, or with a small iron "keeper" across the ends: the keeper closes the loop for the magnetic field lines, so the magnet is less able to fight itself.
What Does Not Weaken a Magnet Much?
- Just sitting there. A good permanent magnet made from a hard material keeps most of its strength for a very long time if it is kept cool, dry and away from opposing fields. Wikipedia's coercivity entry is the reason: high coercivity means resistance to demagnetization.
- Normal use. Picking up paper clips or holding a note to the fridge does not drain a magnet. It is not a battery.
- Cold. Cooling a magnet does not destroy it. Heat is the temperature problem.
How to Keep Your Magnets Strong
- Keep them cool. Avoid hot cars, windowsills in direct summer sun, stoves and dishwashers. For neodymium magnets, stay well below 80 °C unless they are rated higher.
- Do not drop or hammer them. Handle strong magnets carefully; they can also snap together and chip.
- Store them sensibly. Use a keeper on horseshoe and bar magnets, and avoid leaving magnets stuck together in opposition.
- Keep them away from strong magnets and from degaussing or demagnetizing tools, such as an alternating-field demagnetizer.
- Do not rely on magnets to store data: stripe cards, tapes and old disks can be corrupted by strong fields.
If a magnet that matters has weakened, it can often be re-magnetized in a strong field by a specialist, according to Wikipedia's note that magnets "can often be remagnetized." Cheap fridge magnets are not worth the effort.
Frequently Asked Questions
What is the fastest way to demagnetize a magnet?
Heat it above its Curie temperature, or put it in a strong alternating magnetic field and slowly pull it out. Both are industrial methods. Heating is not safe at home for most magnets, especially neodymium ones.
Do magnets lose strength over time?
A good permanent magnet loses very little on its own if kept cool and away from opposing fields. Real-world losses come mostly from heat, shocks and strong opposing fields, not from age.
Does hitting a magnet with a hammer demagnetize it?
It can, because the shock jolts domain walls out of their pinned positions and reduces the magnetization. It can also break the magnet, so it is not a method to use.
Will a phone, microwave or TV ruin my magnets?
Ordinary household electronics rarely produce fields strong enough to exceed a hard magnet's coercivity. Heat from an oven or a hot car is a much bigger risk than a gadget.
Can a demagnetized magnet be magnetized again?
Often yes. A material heated above its Curie point can be cooled in a magnetic field to realign its domains, and a strong enough applied field can also do it. Wikipedia notes that magnets can often be remagnetized after heat damage.
Why do neodymium magnets fail at lower temperatures than alnico?
They are stronger at room temperature but have a lower Curie temperature, about 320 °C versus about 800 °C for alnico, and their coercivity falls as it warms. That is why they are graded by maximum operating temperature.
The Bottom Line
A magnet is only as strong as the order of its domains. Heat, strong opposing fields and hard shocks all scramble that order, while ordinary use and time do very little. Keep your magnets cool, intact and away from strong rivals, and they should last for decades. For more everyday science, see why the sky is blue, why leaves change color in fall and why airplane windows have tiny holes.
Sources
- Wikipedia: Magnet
- Wikipedia: Curie temperature
- Wikipedia: Magnetic domain
- Wikipedia: Coercivity
- Wikipedia: Neodymium magnet
- Wikipedia: Alnico
- Wikipedia: Magnetic stripe card
- Wikipedia: Degaussing
- Wikipedia: Iron, Nickel and Cobalt (Curie point values)
About the author
Written by the Jugad Fact Team. We explain science, history, culture and everyday questions in plain English, check facts against primary sources such as NASA, NOAA and official agencies, and update posts when the facts change. Spot a mistake? Tell us.
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