This is a topic which we addressed in our recent article on the workings of stepper motors, with particular focus on the 4-phase 28BYJ-48 stepper motor and the disadvantages of steppers versus servos.

Exploded view of a 28BYJ-48 stepper motor. ... As an example of a basic form you got a unipolar coil design with a permanent magnetic (PM) core, such as the very common 28BYJ-48.