What causes stir bars to stop working?

[pangea]

Glas Col Heating Mantle Stir Problem -help please

Found this How it Works Excessive heat causes atoms to move more rapidly, disturbing the magnetic domains. As the atoms are sped up, the percentage of magnetic domains spinning in the same direction decreases. This lack of cohesion weakens the magnetic force and eventually demagnetizes it entirely. In contrast, when a magnet is exposed to extreme cold, the atoms slow down so the magnetic domains are aligned and, in turn, strengthened. Ferromagnetism The way in which specific materials form …