Large Ferrite Permanent Magnet
https://www.mlmagnet.com/product/ferrite-ring-shape-magnets/round-ring-ferrite-large-ferrite-permanent-magnet.html
Round ring ferrite large ferrite permanent magnet, is the larger-selling product in our ferrite permanent magnet series. It is more widely used size for all kinds of inductors, transformers, motors, and generators. The Round Ring Ferrite Large is a permanent magnet made of round ferrite, which has good anti-corrosion properties and good temperature stability.