Ferrite, is a ceramic-like material with magnetic properties that are useful in manytypes of electronic devices. Ferrites are hard, brittle, iron containing, and generally grey or black and are polycrystalline. A ferrite is formed by the reaction of ferric oxide (iron oxide or rust) with any of a number of other metals, including magnesium, aluminium, barium, manganese, copper, nickel, cobalt, or even iron itself. The most familiar ferrite is magnetite. Ferrites exhibit a form of magnetism called ferrimagnetism. The most important properties of ferrites include high magnetic permeability and high electrical resistance. Antennas, for example, need to be highly permeable to magnetic fields. High resistance to electricity is desirable in the cores of transformers to reduce eddy currents. Ferrites of a type known as square-loop ferrites can be magnetized in either of two directions by an electric current. This property makes them useful in the memory cores of digital computers, since it enables a tiny ferrite ring to store binary bits of information. Another type of computer memory can be made of certain single-crystal ferrites in which tinymagnetic domains called bubbles can be individually manipulated. A number of ferrites absorb microwave energy in only one direction or orientation; for this reason, they are used in microwave wave-guides.