Ferrimagnetism
The ferrimagnetism (lat .: ferrum iron, from Greek. MAGN ( lithos ): stone from Magne Sien) is a cooperative, magnetic phenomenon by which a ferrimagnetic materials magnetization have. Such substances have a crystal structure in which the magnetic moments of the atoms are alternately aligned antiparallel within certain areas (so-called Weiss areas ) . In contrast to the phenomenon of antiferromagnetism , however, they do not completely cancel each other out because the alignment of the magnetic moments is stronger in one of the two directions.
Ferrimagnetism does not differ from ferromagnetism in the arrangement of the Weiss domains, but in the magnetic order of the elementary magnets , which comes about through an energetically favorable alignment. The magnetizations of two neighboring elementary magnets partially cancel each other out, as a result of which the Weiss regions are magnetized significantly less. The macroscopic behavior is thus a weaker form of ferromagnetism.
Similar behavior to the ferromagnet
A magnetic moment results in each Weiss district. Since these magnetic moments are aligned uniformly in all directions over the entire crystal , the crystal shows no magnetic properties towards the outside. If you bring the crystal into an external magnetic field, it is possible, however, to align all magnetic moments in the manner described above. This results in a macroscopic magnetic moment, which is also retained when the external magnetic field is removed (see hysteresis ). Above a specific temperature for each compound, the Néel temperature (analogue to the Curie temperature of ferromagnetic substances), the entire magnetic structure is lost and the compound shows paramagnetic behavior.
Ferrimagnetic materials
The most frequently used group of materials with ferrimagnetic properties are ferrites , which can be imagined as magnetites , in which the divalent iron is replaced by another divalent metal ion (e.g. copper, nickel, zinc, magnesium, manganese) or mixtures of these is replaced. The trivalent iron of magnetite and the bivalent metal ions then form two lattices, which are separated from each other by the oxygen atoms, which are closely packed together ( spinel structure of ferrites).
Since the ferrites are ceramic oxides and have a high resistance that suppresses eddy currents, they are often used as a magnetic core material in the high frequency range. They should have a slim hysteresis curve that includes a small area. This also allows the magnetic reversal losses (hysteresis losses) to be kept low.
For use as permanent magnets, however, they should have a hysteresis loop that is as rectangular as possible. Such magnets are found in everyday life as holding magnets or field magnets in electrodynamic loudspeakers. Such materials were previously used in core memories (toroidal cores arranged in a matrix) of computers as non-volatile memory components.
Another ferrimagnetic material is cobalt iron stone .
Specialist literature
- Horst Stöcker: Pocket book of physics. 4th edition, Verlag Harry Deutsch, Frankfurt am Main, 2000, ISBN 3-8171-1628-4
- Hans Fischer: Materials in electrical engineering. 2nd edition, Carl Hanser Verlag, Munich Vienna, 1982 ISBN 3-446-13553-7
- Charles Kittel : Solid State Physics. 14th edition, Oldenbourg Verlag, Munich, 2006 ISBN 3-486-57723-9