RF isolators and circulators are passive components essential to modern communication systems. They act as traffic routers for RF signals, moving them to any place in the circuit. They are manufactured using anisotropic ferrite materials, often biased by a permanent magnet, making them non-reciprocal devices.

Circulators are built using the anisotropic magnetic material called ferrite, which has special properties. Anisotropic materials exhibit different electrical characteristics that depend on the direction an electrical signal moves. To build a circulator, a designer needs ferrite and a permanent magnet to generate a rotating magnetic field — the RF signal is driven by the flow of the field. Once the signal is inside the circulator, it only follows the direction of the rotating field, much like a boat floating down a river.  Read more...

Reference: Electronics 360
                   Abe Michelen