The magnetic circuit is composed of one or more permanent magnets, current carrying wires and soft iron according to a certain shape and size to form a component with specific working air gap magnetic field. Soft iron can be pure iron, low carbon steel, Ni-Fe, Ni-Co alloy and other materials with high permeability. Soft iron, also known as yoke iron, plays a role in controlling magnetic flux direction, increasing local magnetic induction intensity, preventing or reducing magnetic flux leakage, and improving the mechanical strength of the whole component.

Generally, the magnetic state in which a single magnet is located without soft iron is referred to as an open state; when the magnet is in a magnetic flux loop formed by a soft iron, the magnet is said to be in a closed state