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- Science Learning Hub - Magnetism
- LiveScience - What is Magnetism? | Magnetic Fields and Magnetic Force
- Rensselaer Polytechnic Institute - Introduction to Magnetism and Induced Currents
- Khan Academy - Introduction to magnetism (video)
- North Eastern university - College of Engineering - Magic of Magnetism
- Chemistry LibreTexts - Magnetism
- Academia - A Study on Classes of Magnetism
- University of Saskatchewan Pressbooks - Introduction to Electricity, Magnetism, and Circuits - Magnetism and Its Historical Discoveries
- University College London - Earth Sciences - Magnetism
- University of Minnesota - College of Science and Engineering - Classes of Magnetic Materials
- National Geographic - Magnetism
- NASA - Magnetism
In substances known as antiferromagnets, the mutual forces between pairs of adjacent atomic dipoles are caused by exchange interactions, but the forces between adjacent atomic dipoles have signs opposite those in ferromagnets. As a result, adjacent dipoles tend to line up antiparallel to each other instead of parallel. At high temperatures the material is paramagnetic, but below a certain characteristic temperature the dipoles are aligned in an ordered and antiparallel manner. The transition temperature Tn is known as the Néel temperature, after the French physicist Louis-Eugène-Félix Néel, who proposed this explanation of the magnetic behaviour of such materials in 1936. Values of the Néel temperature for some typical antiferromagnetic substances are given in the Table.
| Néel temperature of antiferromagnetic substances | |
|---|---|
| chromium | 311 K |
| manganese fluoride | 67 K |
| nickel fluoride | 73 K |
| manganese oxide | 116 K |
| ferrous oxide | 198 K |
The ordered antiferromagnetic state is naturally more complicated than the ordered ferromagnetic state, since there must be at least two sets of dipoles pointing in opposite directions. With an equal number of dipoles of the same size on each set, there is no net spontaneous magnetization on a macroscopic scale. In most insulating chemical compounds, the exchange forces between the magnetic ions are of an antiferromagnetic nature.
Ferrimagnetism
Lodestone, or magnetite (Fe3O4), belongs to a class of substances known as ferrites. Ferrites and some other classes of magnetic substances discovered more recently possess many of the properties of ferromagnetic materials, including spontaneous magnetization and remanence. Unlike the ferromagnetic metals, they have low electric conductivity, however. In alternating magnetic fields, this greatly reduces the energy loss resulting from eddy currents. Since these losses rise with the frequency of the alternating field, such substances are of much importance in the electronics industry.
A notable property of ferrites and associated materials is that the bulk spontaneous magnetization, even at complete magnetic saturation, does not correspond to the value expected if all the atomic dipoles are aligned parallel to each other. The explanation was put forward in 1948 by Néel, who suggested that the exchange forces responsible for the spontaneous magnetization were basically antiferromagnetic in nature and that in the ordered state they contained two (or more) sublattices spontaneously magnetized in opposite directions. In contrast to the simple antiferromagnetic substances considered above, however, the sizes of the magnetization on the two sublattices are unequal, giving a resultant net magnetization parallel to that of the sublattice with the larger moment. For this phenomenon Néel coined the name ferrimagnetism, and substances that exhibit it are called ferrimagnetic materials.
Brebis Bleaney Edwin Kashy Sharon Bertsch McGrayne