When designing microwave and RF components, a non reciprocal device can be obtained by using ferrimagnetic components. Sometimes, it is a good thing for a device to be reciprocal (when the ports of the S parameter matrix are reversible), but in the case of RF devices such as circulators or isolators, it is important for power flow to only move in one direction or to have directional dependence. When directional dependence is present, permeability and permittivity become a tensor rather than a constant and the material is said to be anisotropic.
Ferrimagnets are different than ferromagnets such as iron or steel in the sense that ferrimagnets have high resistivity and directional dependence at mictrowave frequencies. Both are very strongly magnetic.
A circulator is a three port device which can be matched at all ports and lossless at the same time. It can couple power in direction or the other, but not both directions. If the reverse direction is desired, the Scattering matrix can be transposed. For a ferrimagnetic circulator this is achieved by changing the polarity of the magnetic bias field. Most of the time a permanent magnet is used, but an electromagnetic can be used for the circulator to function as an SPDT switch.
An isolator is a two port device which only functions in a single direction. The scattering matrix shown below, implies that the device is nonreciprocal (asymmetric matrix) and lossy due to disobedience to the unitary matrix properties.
An isolator can prevent damage to a high power source by forcing the power to flow only from the source to load. Any reflected power due to an impedance mismatch will be absorbed by the isolator. The two main types of ferrite isolators are resonance and field displacement isolators.
Another two port nonreciprocal RF device is the ferrite phase shifters. Phase shifters are generally used in test and measurement systems or in phased array antennas where the antenna beam can be steered using the device. It is also possible to design a reciprocal phase shifter. In fact most phase shifters are reciprocal in the sense that they provide an equal phase shift in both directions.