With reference hall effect IC sensor (hopefully on topic as normal EE work) used together with these silver color magnet, as this example of 20 by 10 mm from ebay,
Do they have different formulation (chemical composition) with different property, strength of magnetic field and cost?
What are the popular (most used, low cost, widely available) formula? What is the likley formula as in the photo?
How is 'radiated' (field caused by magnet with 0 to 30mm) magnetic field strength measured? Unit and measurement procedure (distance, orientation). For antenna and loud speaker, they produce sound or wave over distance. We measure them by signal level and directional property diagram. What is the equivalent for these magnet? What typical mag field in G or T at distance 1 to 20mm for the 10 by 20mm magnetic as photo? Rough range needed for choosing sensor chip which rated at mv output per G or T.
How is the magnet shape, rectangle, round, etc., affect the 'radiated' field strength?
How is the magnet shaped? Is is like ferrite coil core, that can practically make into any shape and size? Is that by 'mold', pressed or machined?
For photo magnet, can the pole be made, North South at top and bottom of the 'flat biggest area side' or NS has to be at the two ends (20mm biggest length in this photo).