In a magnetizable material, the elementary magnet is the unit that has a magnetic dipole of fixed size but variable direction. If all directions are equally frequent, the magnetic fields of all elementary magnets cancel each other out and the body appears non-magnetic. If, on the other hand, the elementary magnets are preferably aligned along one direction, the sum of their magnetic fields forms an externally measurable magnetic field and the body is magnetized.
- The cause of the magnetization can be an existing external magnetic field, to which the previously disordered elementary magnets are preferably positioned parallel and as a result strengthen the external field ( paramagnetism ).
- If the elementary magnets are preferably set parallel to one another even without an external field, then in small areas of the body ( domains or Weiss areas ) a complete alignment spontaneously results with a correspondingly stronger reaction to an external magnetic field; such a body is called a ferromagnet . With ferromagnets, the magnetization is partially retained as remanent magnetization after the external field has been switched off. This can e.g. B. by heating above the Curie temperature or by mechanical stress (shock) are lost. The orientation of the elementary magnets in ferromagnetic materials can have many reasons, mostly the exchange interaction between neighboring atoms plays a decisive role.
- Not by elementary magnets is diamagnetism caused, which occurs upon application of an external magnetic field in all materials and the external field weakens . This effect is so minor that it can usually be ignored if elementary magnets are present.
With paramagnetic and diamagnetic materials, the alignment is proportional to the external magnetic field and disappears as soon as it is switched off. The strength of the magnetization generated by the alignment is described by the magnetic susceptibility .
The strongest elementary magnets are formed by atomic shells that have a certain angular momentum other than zero and thus a magnetic moment . The atomic nuclei appear about 1000 times weaker when they have a nuclear spin . In metallic solids , because of the electron spin , the conduction electrons are also to be regarded as elementary magnets, but they can only be aligned very weakly because of the Pauli principle . In many solids there are no noticeable elementary magnets.
Understanding the behavior of elementary magnets is particularly important for data storage : hard drives store data in the form of differently oriented ferromagnetic domains. In order to further increase the storage capacity with advancing miniaturization , it is necessary to reduce the size of these domains to fewer and fewer elementary magnets.