This is a special ring ferrite magnet, the special place is its magnetisation, the inside of the ring (internal) is all N poles, the outside is all S poles, the outer diameter size of 25mm, hole 19.2mm, thickness of 5mm, widely used in magnetic sensors, radar systems, medical equipment, antenna applications, etc., the following picture is the schematic diagram of its magnetisation;
Radiation-magnetized sintered ferrite ring magnetization picture;