Stepper motor demagnetization

Hope the community can help make some suggestions...

We have a small device using a 8mm diameter micro stepper motor out in the field, and some have come back demagnetized. I'm under the impression that demagnetization can happen if a stepper is disassembled (this is not the case for us), or if there is an over current (which we need to test for), or if there the motor is subjected to a large magnetic field.

Unfortunately, we don't have a good sense of scale, and most stepper motor advice comes from people with experience with much larger magnet motors. Are smaller motors more susceptible to demagnetization, and why? Looking for root causes of the failures.

Are there other possible sources of failure?
How does heat affect the magnets? Just wondering.

UPDATE: Oh, I see. I'm wondering what heat thresholds are bad in particular. We actually move the motor extremely slowly , so it's doubtful that heat would be a factor, but in any case, are there recommendations for temperature maximums for stepper motors?

From the wiki:

Temperature sensitivity varies, but when a magnet is heated to a temperature known as the Curie point, it loses all of its magnetism, even after cooling below that temperature. The magnets can often be remagnetized, however.

Additionally, some magnets are brittle and can fracture at high temperatures.

The maximum usable temperature is highest for alnico magnets at over 540 °C (1,000 °F), around 300 °C (570 °F) for ferrite and SmCo, about 140 °C (280 °F) for NIB and lower for flexible ceramics, but the exact numbers depend on the grade of material.

Thanks for the information - from what I see here, this seems unlikely, since the device is in contact with the skin (handheld), and even through plastic, i doubt it would get this hot due to internal power sources.

Is the high current problem actually the current causing a high temperature? Or is does the current cause the electromagnet to work against the permanent magnet causing demagnetization?
if the field is strong enough and opposite to the magnet that would demagnetize it. I've seen this effect in magnetizing machines where you feed an item in a particular direction magnetizes,,, then if you feed the item in the opposite way it demagnetizes.
Maybe ....lock the rotor and apply a field of opposite polarity...??
That's what I mean, I'm not sure that's even possible without serious magnetic field in the environment. Can it be done within the motor realistically? and the motor is very small...
I've heard of perm magnetic motors becoming demagnetized...never seen it. I worked in a power steering manufacturing facility where they used brushless DC motors.,... thousands per day and never saw one.
I would think a small experiment would be easy enough to set up...maybe add some heat to loosen those magnetic domains
and apply a reverse field with a locked rotor...
If I remember correctly the manufacturing of magnets is done with the aid of heat....heat.... apply field.... cool down remove field...
8mm motor ....that is small!
I would be surprised that you wouldn't be able to demagnetize it...whether that's the root cause is another story...but it might be a broad movement in the right direction
Thanks. We've been doing these types of experiments without any luck so far. I suppose creating the right conditions is the key. We'll raise the temperature to something within reason (since this device is handheld) and try to apply current while the motor is out of step. Any other suggestions are appreciated, and any other knowledge about demagnetization is also appreciated.