How to select and use Strong Magnet together with hall sensor and amplifier?

Question

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,

1. Do they have different formulation (chemical composition) with different property, strength of magnetic field and cost?

2. What are the popular (most used, low cost, widely available) formula? What is the likley formula as in the photo?

3. 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.

4. How is the magnet shape, rectangle, round, etc., affect the 'radiated' field strength?

5. 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?

6. 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).

Answer 1

1. The most popular types of magnets are Neodymium-Iron-Boron (sintered or bonded), Samarium-Cobalt, ferrite (often bonded), Aluminium-Nickle-Cobalt (often called Alnico), and ceramic magnets. They all have different properties in terms of magnetic field strength, resistance to high and/or low temperatures, resistance to demagnetization, cost, etc.
2. The magnet in the photo is most likely a sintered Neodymium-Iron-Boron (NdFeB) magnet with a Nickel-Copper-Nickel coating to help protected it from corrosion. NdFeB magnets are produced in a variety of different strengths and temperature grades. This will be called out as a number and a letter, like 35UH or 50M. The number tells you the energy product of the magnet (which gives you a rough idea of the strength of it) and the letters tell you what the Intrinsic Coercivity of the magnet is (which gives you a rough idea of the maximum temperature the magnet can be used at).
3. The equivalent for a magnet is called flux density and it is measured in Tesla, Gauss or sometimes "lines per square inch." Flux density is a measure of flux per unit area. "Typical" magnetic field strength at a certain distance is going to depend on the specific grade of NdFeB and the thickness of the magnets. Once you have those, you can find calculators online that will give you the answer.
4. Once you have a material picked out, the main things you need to know are shape (square, rectangle, circle, etc), area and thickness of the magnet.
5. Sintered NdFeB magnets are made in the following way: Raw materials are melted down and then formed into a fine powder. The powder is pressed into a shape (this can be done in a couple different ways -axial pressing or isostatic pressing). Then they are sintered (heated). Then they can be and often are machined, which could be slicing a bigger block into smaller parts or grinding in order to meet a certain tolerance. Then they are coated to prevent corrosion. The coating could be an epoxy, Nickel-Copper-Nickel, or some other method. Once this is done, they are magnetized by being placed in a strong magnetic field.

Answer 2

1. Strong magnets come in two general varieties. The most common is Neodymium/Iron/Boron and the less common is Samarium/Cobalt. Less strong ones are usually ferrite materials.

2. Neodymium/Iron/Boron

3. Measure in Gauss, or more commonly Tesla. 1T = 10,000G

4. Speaker magnets are usually either ferrite or neodymium

5. Typically powder, compressed and heated

Answer 3

There are many different grades of all the metals that can be used for magnets. I'd suggest that if this is important for your design needs, that you avoid the ebay route and buy well-specced magnets from a reliable source. For example, http://www.magnetsource.com/Solutions_Pages/NEOMAIN.html will give you great parameters for a variety of Neodymium grades. From there, you can then transform all your units to what they need to be and use magnetic field calculators (or look up the equations). Couple this with poring over the spec sheets for your Hall sensors, and that should help you come up with a viable electro/mechanical/magnetic system that's reliable.

Another good source is https://www.kjmagnetics.com/fieldcalculator.asp