Discuss thermal knockdown and the limitations in various magnet materials.
All magnetic materials are at all times as fully magnetized as their thermal state permits. Prior to “magnetization” their magnetic domains arrange randomly to minimize their internal (and external) energy state. Magnetization rotates magnetic domains into common alignment. Permanent magnets retain this alignment to a degree, depending on their geometry, chemistry and anisotropy mechanisms. Consider anisotropy here as all those things that resist a magnetizing force, and thus a demagnetizing force as well.
Magnetic domains in the center of a magnet support each other, but those domains closer to the sides, ends and edges of the geometry have less support, and some are reversed by the magnet’s own external field, which has a polarity opposite to the internal field. When heat is applied, longer electron orbits cause all domains to weaken to a degree, and those with more exposure to the external field (or are weaker for some other reason) will also reverse.
Thermal knockdown is the process of raising a magnet’s temperature to the temperature expected in the application so any impending change will have occurred prior to installation of the magnet. At elevated temperatures, the demagnetizing force for isolated magnets will be its own self demagnetizing force, so thermal stabilization should be done in a fixture that reproduces the operating permeance coefficient to avoid loss of useful and stable flux levels.
Does the flux density of a magnet increase when it is operating in very low temperatures (e.g. -60 °C)?
Yes, the temperature effect is fairly linear over the range of +/- 100 °C, so electron orbits are shorter and metallic magnets will exhibit an increase in flux density. Ceramic magnets are the exception.
How is a magnet heat stabilized? When should this be done, to what benefit, and exactly what does it do to the magnet?
A magnet is heat stabilized by exposing it to elevated temperatures for a specified amount of time. This is done to prepare for the irreversible losses of magnetism that most magnets experience when exposed to elevated temperatures.
You can think of heat stabilization as insurance against elevated temperatures. We recommend this when the magnets are to regularly see high temperatures during service.
There are two types of magnetic losses when a magnet is heated to elevated temperatures: reversible & irreversible.
Reversible magnetic loss is the weakening of a magnet when heated to elevated temperatures. It is called reversible because the magnet recovers this portion fully upon returning to room temperature.
Irreversible magnetic loss also occurs at elevated temperatures but is not recovered upon return to room temperature. It is a permanent loss, unless the magnet is sent back for remagnetization. This is a one-time-only effect.
An example: A given magnet produces .100 Tesla at room temperature. It is baked at 200 °C. While at that temperature, it only produces .085 Tesla. Upon return to room temperature you measure it and find it now only produces .095 Tesla. The missing .005 Tesla is the irreversible loss. If the magnet was returned to 200 °C, it will still produce .085 Tesla. If it was taken to a higher temperature then it would lose more output.
The amount of irreversible loss depends on a lot of factors, including the type of magnetic material, the shape of the magnet, the temperature it experiences and the amount of time it sees that temperature
Something else to keep in mind about heat stabilization: The magnet usually must be isolated during the process, and should not be stacked when in the oven. This usually means each magnet must be individually handled, which has an associated extra cost.
What happens to a magnet when operating temperatures increase or decrease?
When we speak of temperature increase and decrease we are talking about changes relative to “room temperature” which is just an arbitrary point to start from. To understand temperature effects we need to look at the atomic structure of the elements that make up the alloy. Atoms have a nucleus around which spinning electrons orbit. As temperature increases (from absolute zero), the distance from the nucleus, and other electrons, increases so they follow a longer path and have less influence on each other, and magnetic properties of metallic magnets generally decrease.