Ferrite Materials and Devices

Ferrite is a generic name for compounds which contain Fe and O atoms as well as doping materials used to alter its thermal, magnetic and electrical properties. The common factor in all of the compounds is that they contain unpaired electron spins in the outer shells of some of the atoms. These spinning electrons are randomly oriented usually and so the material is magnetically neutral.

A typical electron may be represented as shown.

If the ferrite material is placed in a static (DC) magnetic field as might be produced by a hard magnet or electromagnet then the spinning electrons will have the spin axis aligned with the applied magnetic field. This happens because the action of spinning causes the electron to have its own magnetic field.

Typically then in an externally magnetized ferrite all the spin axes are aligned.

If the ferrite is placed in a position where it is influenced by an alternating magnetic field; such as a microwave of RF signal, then it will start to precess around the precession axis. This is the same effect as when a spinning top is pushed from the side or a gyroscope under the influence of gravity.

If the alternating magnetic field is in the same direction as the precession then energy will be added to the precessional motion. If the rotation of the alternating field is opposite to the precession then it will give up energy to the field. There are two effects which can happen depending on the direction of the circularly polarized alternating field. These effects show themselves as a different permeabilities of the material. The effective permeability of the ferrite therefore depends on the applied static field level, H dc, the frequency of the alternating signal, w , and the direction of its circular polarization; +ve is clockwise and –ve is anticlockwise. The simple characteristic is shown below.

As indicated the static field and the frequency can be interchanged on the graph without really affecting the effects. There is however a resonance shown where m + crosses unity again. At magnetic resonance there is a lot of energy absorbed from the signal and devices designed at this field and frequency will exhibit large losses.

Devices

Within waveguide there are planes where the alternating magnetic field can be shown to be circularly polarized. The diagram indicates that a rectangular waveguide operating in the TE 10 mode exhibits circular polarization in the region between ¼ and 1/ 3 of the broad dimension as the wave travels to the right. (If the wave travelled to the left the polarizations would be reversed.)

By placing a ferrite slab on the walls or fully across the waveguide in these planes a range of devices can be created.

If the ferrite is placed on one side of the waveguide only then a non-reciprocal device is created. When propagation is from the other end then the ferrite is in a region of opposite circular polarization. This causes the permeability and so the phase shift to be different.

If the magnetic field is further adjusted the waveguide can be made to cut off in one direction. This creates a component called an isolator which will allow a signal to pass in one direction but not in the other.

If a symmetrical structure is used then the phase shift should be the same when propagation occurs from either direction because each piece of ferrite will see either +CP or –CP in each direction.

It is possible to create other devices once the basic principles have been understood. In these different shapes of ferrite are used or the static magnetic fields are produced by different means. In the latching phase shifter the DC magnetic field is produced by a wire carrying a large current. If the toroid is made from “soft” ferrite it will “remember” and become magnetized by a pulse of the magnetizing current. It can be re-magnetized to produce a different phase shift if necessary.

Other ferrite devices you might want to read about are circulators and other forms of isolators and limiters.