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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 chemistry.
Additional recommended knowledge
Most solid materials are diamagnetic, paramagnetic or in the case of metals Pauli-paramagnetic. These materials either do not have electron spins or these spins are not ordered unless an external field induces some non-random orientation. Such materials are not considered to 'have' a magnetic structure. This is different for ferro-, ferri- and antiferomagnetic materials. They differ in the relative ordering of their spins within the lattice. In some ferromagnetic cases the structure may be relatively simple in that all spins point in the same direction, or at least that would be the case at very low temperatures. Towards higher temperatures there will be more and more 'rebellious' spins defying the order of the magnetic structure and at a certain temperature the order will break down and the spins will point in random directions. In some materials the pattern in which the spins order is much more complicated. In antiferromagnetic materials spins point in opposite directions so that the overall magnetic moment is zero. However, this is not necessarily achieved by a simple up and down pattern. Much more complicated structures can arise. At times one can recognize layers in which all spins point in one direction (as in a ferromagnet) but in the next layer they point in the opposite direction giving an overall antiferromagnetic arrangement. The possible number of arrangements is very large and can include spirals, clusters, tetrahedra etc.
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 of 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. 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Magnetic_structure". A list of authors is available in Wikipedia.