Does freezing a magnet make it stronger?

The idea that extreme cold can enhance the power of a magnet is a fascinating concept. After all, if excessive heat weakens a magnet, shouldn't intense cold do the opposite, turning an ordinary magnet into a super-powered component?

While this idea taps into a common intuition about material science, the reality is more nuanced. For most common commercial magnets, freezing them will indeed cause a slight change in strength, but it’s rarely a dramatic increase and often involves other risks.

The Science of Temperature and Magnetism

To understand the effect of cold, we must first look at how temperature affects a permanent magnet at the atomic level.

A magnet's strength is derived from the alignment of microscopic magnetic regions called domains. In a permanent magnet, these domains are fixed and aligned in the same direction.

Heating: As a magnet is heated, the thermal energy causes the atoms to vibrate rapidly. These vibrations disrupt the neat alignment of the magnetic domains, weakening the overall magnetic field. If the temperature reaches the Curie Point, the magnet loses all permanent magnetism.

Cooling: When a magnet is cooled, the atomic vibrations slow down. This reduction in thermal agitation generally allows the magnetic domains to maintain their alignment more easily, which can lead to a slight increase in the magnetic moment and, thus, a marginally stronger magnetic field.

The Effect on Common Magnetic Materials

The actual change in strength depends heavily on the specific material of the magnet and the temperature range.

1. Neodymium Magnets (NdFeB)

The most common and powerful magnets today, Neodymium Iron Boron (NdFeB), exhibit a very small negative reversible temperature coefficient (RTC) for their residual induction (Br).

The Result: When cooled down to freezing or even cryogenic temperatures (using liquid nitrogen, for example), a Neodymium magnet will show a small, reversible increase in strength. However, this enhancement is usually only fractional—a few percent—and is rarely enough to justify the complex and expensive process of cryogenic cooling.

2. Ferrite (Ceramic) Magnets

Ferrite magnets behave quite differently. They have a significant positive RTC.

The Result: For a ferrite magnet, decreasing the temperature actually causes its magnetic performance to decrease. If you were to place a typical refrigerator magnet (a ferrite magnet) in the freezer, it would become slightly weaker, not stronger.

3. Alnico and Samarium Cobalt Magnets

These materials, often used in high-temperature applications, have very low RTCs.

The Result: The strength of Alnico and Samarium Cobalt magnets is largely unaffected by temperature changes, including freezing. They are designed for stability.

The Cryogenic Caveat: Risk of Irreversible Damage

While a slight increase in strength is technically possible for Neodymium magnets at extremely low temperatures, taking common commercial magnets down to cryogenic levels comes with a major risk: Irreversible Demagnetization.

Coercivity Drop: The coercive force (H_c), which is the magnet’s resistance to demagnetization, can actually drop sharply at very low temperatures for certain low-grade Neodymium magnets.

Irreversible Loss: If the coercivity drops too much, the internal demagnetizing field of the magnet itself can permanently damage the magnetic structure, leading to a major, non-reversible loss of strength when the magnet returns to room temperature. This makes the initial cooling effort counterproductive.

Conclusion: Don't Bother Putting Your Magnets in the Freezer

In summary, the notion that freezing a magnet dramatically increases its strength is largely a myth.

No Practical Gain: The small strength increase seen in powerful Neodymium magnets is usually negligible for practical applications and doesn't warrant the effort.

Ferrite Weakness: Common ceramic magnets actually become weaker in the cold.

Risk of Damage: Attempting extreme cooling on low-grade magnets can result in permanent damage.

If you need a stronger magnet, the most effective solution is always to upgrade to a higher-grade material (such as a stronger NdFeB grade) or simply use a larger volume of your existing material. Leave the freezing for the ice cream!