Polarized neutron diffraction: a way of investigating the electronic properties of magnetic ions
(F. Givord, J.X. Boucherle)
The interaction of the neutron spin with the periodic magnetic fields that are present in a magnetically ordered crystal allows performing diffraction experiments. Magnetic structure factors, which are the Fourier components of the magnetization density, are measured. With polarized neutron diffraction, one can reach high precision, especially in measuring weak magnetic moments.
Two types of analysis are commonly used:
Expansion of the magnetic densities of 4f, 5d and 6s type in Rare Eath alloys
In the direct space
The magnetization density map can be calculated by Fourier transform. This technique has been widely boosted by the use of the Maximum Entropy method, which allows to fully use all the meaning of the measurements (values and errors) and to be free from the missing data which cannot be measured.
The magnetization density is characteristic of the type of electrons that are involved in the magnetic properties. In the case of experiments performed on Rare Earth alloys, the expansions of these electrons are quite different : the 4f density is very concentrated, the 5d one is spread out and the 6s one is constant.
Magnetic form factors of 4f, 5d and 6s type electrons in Rare earth alloys
In the reciprocal space
It is possible to get the form factor for each magnetic atom.
The characteristics of the magnetization density can be found in the form factor but in a reversed way. The expansions of the form factors are very different : the 4f form factor is widespread, the 5d one is tightened and the 6s one peaked at the origin, which allows to study them independently.
For each compound, measurements are characteristic of the surrounding of magnetic atoms (electric or magnetic field). Atomic models based on the local site symmetry, yield the parameters required for the understanding of these compound properties.
Last update : 02/28 2006 (332)