Neodymium Iron Boron Temperature – Temperature Effects on Neodymium Iron Boron (NdFeB magnets)
Temperature characteristics of NdFeB grades:
The NdFeB magnets are the strongest magnets available. Up to +150 degrees C, they are stronger than the other Rare Earth magnet, SmCo. At around +150 degrees C and above these Neo magnets perform not as strongly as SmCo. The maximum recommended temperature for the NdFeB magnets is +230 degrees C, whereas the SmCo can work at +300 to +350 degrees C.
NdFeB can be used at low temperature but at around 135 Kelvin (-138 degrees C), the direction of magnetisation is said to change from a single axis (easy-axis) to an easy-cone which could cause a fall in output of up to 15% due to this spin reorientation. It is possible to use Neodymium Iron Boron magnets to be used at even colder temperatures but this drop in output will need to be taken into account.
The temperature coefficient of Intrinsic Coercivity (how Hci varies with temperature), b, for Neodymium is approximately -0.6%/degree C (from ambient, but a range of -0.45%/degree C to -0.6%/degree C is possible depending on the Neodymium grade) between +20 and +120 degrees C.
The temperature coefficient of Remanent Induction (how Br varies with temperature), a, for Neodymium is -0.12%/degree C (from ambient, but a range of -0.08%/degree C to -0.12%/degree C is possible depending on the Neodymium grade).
It should be noted that, strictly speaking, these values for a and b actually vary with temperature and use of stated values beyond 20 to 120 degrees C may result in inaccuracies when designing (actual BH curves at temperatures assist in clarifying for design purposes).
There are three effects due to elevated temperatures. A reversible loss is when the output falls with temperature but returns as it cools down (the temperature coefficients reflect this e.g. a twenty degree C rise above ambient causes a drop in magnetic output of N42 of around 20 x 0.12 = 2.4%, which recovers when the temperature returns down to ambient). An irreversible but recoverable loss occurs when the output falls but does not return when the magnet cools down (e.g. the high temperature takes the Intrinsic working point beyond the knee of the Intrinsic curve, causing demagnetisation) but this would be recovered if the Neodymium magnet is remagnetised. For all extents and purposes, this output is lost because the magnet will not be remagnetised during practical application but will demagnetise again if a remagnetised magnet is re-used in the same application (the heat will demagnetise the Neodymium magnet again – the magnetic circuit would have to be improved to fix this problem). When cooled, such a demagnetised magnet will have the original Hci but a lower Br (the Br will have increased by the reversible temperature coefficient amount applied to the reduced high temperature Br). An irreversible irrecoverable loss is due to high temperature causing a permanent structural change in the magnet as is usually because the magnet has been taken far beyond its working temperature. The damage is permanent and remagnetising will not bring the performance back again. If an irreversible but recoverable loss occurs, the magnet is said to be thermally stabilized. If the cooled Neodymium magnet is taken back up to temperature, no further irreversible losses will occur – the only loss is the temporary drop in output due to the reversible losses (a function of the temperature coefficients). By being thermally stabilized, the Neodymium magnetic performance is better predicted even if the drop in output has occurred due to an irreversible but recoverable loss. Some applications may require this thermal stabilization which is usually done by taking to a temperature a few degrees C above the required maximum operating temperature but it may also be performed by applying a demagnetising pulse (this replicates the process by taking the Intrinsic curve beyond the knee of the Intrinsic curve but requires knowing by how much the magnets would demagnetise by through raised temperature but cannot take into account BH curve waveshape variations so has an element of inaccuracy at all times).
It is also generally advised against subjecting magnets to thermal shock (i.e. putting a cold magnet onto an extremely hot surface) as too high a thermal shock could cause the magnets to break.