Description General description Stirring bars using rare earth magnets have significantly increased magnetic strength compared to those using Alnico magnets, but more importantly they are almost totally resistant to demagnetisation. Stir bars using rare earth magnets are identified by a carbon black spot which us chemically inert. Other Notes The high magnetic coupling power of rare earth stir bars may cause abrasion of the PTFE coating against the base of the container and where stirring arrays are used, as in multi-well plates, lateral interaction between stirrer bars may be observed.