FAQ - Frequently Asked Questions (and their answers!)

It depends on various factors:

• the temperature type of the magnet
• the magnet shape
• the positioning of magnets in a group

Most of our magnets (temperature type N) lose part of their magnetisation permanently at 80°C.
Vigorous cooling down (e.g. in liquid nitrogen) does not harm the magnet.

If you heat up a type N neodymium magnet over 80°C - its so-called maximum working temperature - it loses part of its magnetisation. Thereafter, it adheres less strongly to an iron plate, for instance, even when the magnet is cooled down again. At a temperature of 310°C (the so called "Curie temperature") or more, there is no remanence left.

For applications over 80°C we offer special graded magnet types with higher working temperatures (see table with all raw materials sorted by temperature).

An overview of the various temperature types (from page Physical magnet data).

Temperature type	Max. working temperature	Curie temperature
N	80°C	310°C
M	100°C	340°C
H	120°C	340°C
SH	150°C	340°C
UH	180°C	350°C
EH	200°C	350°C

The maximum working temperatures in this table are only reference points. If you need strong magnets with even higher working temperatures, you should get samarium-cobalt magnets (SmCo) (currently not in our assortment).

Below, we will discuss the temperature losses in greater detail.

Types of temperature losses (= loss of magnetisation due to high temperature)

We distinguish between reversible, irreversible and permanent losses.

Reversible temperature loss:

The magnet is less magnetic as long as it is hot. Once it is cooled down, it regains its original strength. It makes no difference how often the magnet is heated up and cooled down.

Irreversible loss:

After heating up above the maximum working temperature and cooling down, the magnet is weakened permanently. Repeated heating at the same temperature does not amplify irreversible losses. Remagnetising an irreversibly weakened magnet through a strong enough external magnetic field can give it its original strength back.

If the temperature rises above 900°C, however, the grain structure of sintered neodymium magnets starts to change. At that point, we talk about permanent losses. Remagnetising is no longer possible.

Duration of heating

The duration of heating has only a minimal influence on the strength of losses when it comes to irreversible losses, given that the temperature was the same everywhere on the inside of the magnet. When heating up a thick magnet for a short time, the outside temperature is much higher than the maximum core temperature inside the magnet. In that case, the temperature losses are dependent on the position.

Magnet shape and position at temperature losses

If heating leads to irreversible losses, depends not only on the temperature type of a magnet but also on its shape and its positioning in a group of magnets or ferromagnetic material. The maximum working temperatures in the table above are therefore only reference points. Depending on the positioning and shape, it is possible that a magnet of a certain temperature type suffers already irreversible losses at lower temperatures.

There is a rule for the dependence of temperature losses on the magnet shape: A very flat (flatness = diameter divided by height) magnet that is polarised along its shortest dimension suffers already irreversible losses at temperatures below the maximum working temperature. If the ratio of diameter to height is less than 4, however, the magnet can be heated up above the maximum working temperature without losing its magnetisation.

Examples of actual maximum working temperatures of stand-alone disc magnets:

The more a magnet in a certain position is exposed to a reverse field, the lower is its actual maximum working temperature.

The smallest temperature losses occur in arrangements where a magnet is magnetically "short-circuited" in a magnetic circuit (analogue to an electric circuit), because there is no reverse field in the magnet. In reality however, this arrangement is rare.

Does immersion in liquid nitrogen damage super magnets?

Immersion in liquid nitrogen at a temperature of -196°C (77 K) does not damage neodymium magnets.

When cooling down to this temperature, the coercive forces H_c of all types rise to four or five times the room temperature. The remanence fields B_r of all types increase between room temperature and approx. -133°C (140 K) by approx. 10% and then decrease rapidly. At about -196°C they reach room temperature.