To get the best air-gap magnetic field intensity for the same type and same size magnetic circuit, you should choose the most suitable magnetic materials for coupling. The permanent magnetic materials transferred torque must include the following properties:
1. High residual magnetic flux density to get strong magnetic force and magnetic torque.
2. High coercivity (Br) to avoid demagnetized under high working temperature; that is to say: magnet needs high magnetic energy product (BHmax)
3. Excellent temperature stability that material has capacity to keep magnetic force stability.

Permanent magnet refers to hard magnet with high coercive force, high remanence, which can keep constant magnetic force for the long term once magnetized. It can be divided into two types:
1. Metal alloy magnets: Nd2Fe14B, SmCo, and AlNiCo
2. Ferrite/Ceramic

All above permanent magnets can be used on magnetic coupling. But neodymium iron boron is the most widely used magnetic material due to high magnetic energy product (BHmax) and intrinsic coercive force. When under high working temperature (above 200 degrees Celsius), you can use smco magnet which can reach 250 to 300 degrees Celsius.

From strong to weak magnetic force, magnets can be listed as below:
Sintered NdFeb ＞ Sm2Co17 ＞ Sm1Co5 ＞ Bonded NdFeB ＞ AlNiCo ＞ Ferrite

To choose magnetic material when designing magnetic coupling, you can follow the below referential principle:

1. Good quality magnet material to keep enough air-gap magnetic field in the coupling air gap
2. Working under special condition to keep magnetic force stability
3. Good mechanical property for easy machining and assembly
4. High performance cost ratio