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Concept
Magnetic structure
Résumé
The term magnetic structure of a material pertains to the ordered arrangement of magnetic spins, typically within an ordered crystallographic lattice. Its study is a branch of solid-state physics.
Most solid materials are non-magnetic, that is, they do not display a magnetic structure. Due to the Pauli exclusion principle, each state is occupied by electrons of opposing spins, so that the charge density is compensated everywhere and the spin degree of freedom is trivial. Still, such materials typically do show a weak magnetic behaviour, e.g. due to Pauli paramagnetism or Langevin or Landau diamagnetism.
The more interesting case is when the material's electron spontaneously break above-mentioned symmetry. For ferromagnetism in the ground state, there is a common spin quantization axis and a global excess of electrons of a given spin quantum number, there are more electrons pointing in one direction than in the other, giving a macroscopic magnetization (typically, the majority electrons are chosen to point up). In the most simple (collinear) cases of antiferromagnetism, there is still a common quantization axis, but the electronic spins are pointing alternatingly up and down, leading again to cancellation of the macroscopic magnetization. However, specifically in the case of frustration of the interactions, the resulting structures can become much more complicated, with inherently three-dimensional orientations of the local spins. Finally, ferrimagnetism as prototypically displayed by magnetite is in some sense an intermediate case: here the magnetization is globally uncompensated as in ferromagnetism, but the local magnetization points in different directions.
The above discussion pertains to the ground state structure. Of course, finite temperatures lead to excitations of the spin configuration. Here two extreme points of view can be contrasted: in the Stoner picture of magnetism (also called itinerant magnetism), the electronic states are delocalized, and their mean-field interaction leads to the symmetry breaking.
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