Alnico6 ring,  diametrally magnetized.  super accurate.

Alnico alloys basically consist of aluminum, nickel, cobalt, copper, iron and titanium. In some grades cobalt and/or titanium are omitted. the alloys may contain additions of silicon, columbium, zirconium or other elements which enhance heat treatment response of one of the magnetic characteristics.

Alnico alloys are formed by casting or powder metallurgical processes

Alnico magnets are extremely difficult to machine as they are very hard and brittle.