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. In this view, with increasing temperature the local magnetization would thus decrease homogeneously, as single delocalized electrons are moved from the up- to the down-channel. On the other hand, in the local-moment case the electronic states are localized to specific atoms, giving atomic spins, which interact only over a short range and typically are analyzed with the Heisenberg model. Here, finite temperatures lead to a deviation of the atomic spins' orientations from the ideal configuration, thus for a ferromagnet also decreasing the macroscopic magnetization.
Techniques to study them
Such ordering can be studied by observing the magnetic susceptibility as a function of temperature and/or the size of the applied magnetic field, but a truly three-dimensional picture of the arrangement of the spins is best obtained by means of neutron diffraction. Neutrons are primarily scattered by the nuclei of the atoms in the structure. At a temperature above the ordering point of the magnetic moments, where the material behaves as a paramagnetic one, neutron diffraction will therefore give a picture of the crystallographic structure only. Below the ordering point, e.g. the Néel temperature of an antiferromagnet or the Curie-point of a ferromagnet the neutrons will also experience scattering from the magnetic moments because they themselves possess spin. The intensities of the Bragg reflections will therefore change. In fact in some cases entirely new Bragg-reflections will occur if the unit cell of the ordering is larger than that of the crystallographic structure. This is a form of superstructure formation. Thus the symmetry of the total structure may well differ from the crystallographic substructure. It needs to be described by one of the 1651 magnetic (Shubnikov) groups rather than one of the non-magnetic space groups.
Although ordinary X-ray diffraction is 'blind' to the arrangement of the spins, it has become possible to use a special form of X-ray diffraction to study magnetic structure. If a wavelength is selected that is close to an absorption edge of one of elements contained in the materials the scattering becomes anomalous and this component to the scattering is (somewhat) sensitive to the non-spherical shape of the outer electrons of an atom with an unpaired spin. This means that this type of anomalous X-ray diffraction does contain information of the desired type.
Magnetic structure of the chemical elements
Only three elements are ferromagnetic at room temperature and pressure: iron, cobalt, and nickel. This is because their Curie temperature, Tc, is higher than room temperature (Tc > 298K). There are also elements which are paramagnetic at room temperature, but become ferromagnetic at low temperatures, including Dysprosium, Erbium, Holmium, Terbium, and Thulium. There is also antiferromagnetic ordering, which becomes disordered above the Néel temperature. Those elements which become superconductors exhibit superdiamagnetism below a critical temperature.
|No.||Name||Superconducting Tc||Curie temperature||Néel temperature|
|29||Copper||6 * 10-8 K|
|47||Silver||5.6 * 10-10 K|
|65||Terbium||221 K||230 K|
|66||Dysprosium||92.1 K||180.2 K|
|67||Holmium||20 K||132.2 K|
|68||Erbium||18.74 K||85.7 K|
|69||Thulium||32 K||56 K|
- Neutron diffraction of magnetic materials / Yu. A. Izyumov, V.E. Naish, and R.P. Ozerov ; translated from Russian by Joachim Büchner. New York : Consultants Bureau, c1991.ISBN 030611030X
- A demonstration by Brian Toby
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- G.W. Webb, F. Marsiglio, J.E. Hirsch. "Superconductivity in the elements, alloys and simple compounds" (PDF).CS1 maint: Multiple names: authors list (link)
- "Elements handbook: Neel point". Retrieved 27 Sep 2018.
- "Elements handbook: Curie point". Retrieved 27 Sep 2018.