What is the most indispensable part of a magnetic stirrer?

Yes! Magnetic agitator. Magnetic agitators are usually made of polytetrafluoroethylene (PTFE, F4). Why are they made of polytetrafluoroethylene? Because of its high temperature resistance: use temperature - 200 to + 250 C; low temperature resistance: - 196 C can be maintained at 5%; corrosion resistance: strong acid, strong alkali, aqua regia and various organic solvents; insulation resistance: dielectric properties are independent of temperature and frequency; high lubrication: the lowest friction coefficient in solid materials; non-adherence: non-adherence to any substance; non-toxic.

But some bad vendors use plastics to make it. Although the cost is very low and the price is very cheap, the damage speed is very fast, and it is not resistant to high temperature and corrosion. It is very likely that the liquid you stir will be polluted, which will have a great impact on your experiment.