Polarized neutron technique

In both cases, sample size was found to be a crucial parameter. As we have seen in Section XII. A, growing very large crystals of organic conductors is not easy. To make the experiment feasible one can think of a bunch of carefully aligned smaller crystals and/or take advantage of improvements in polarized neutron techniques since the 1980s. 

This work is to be followed by an analysis of an experimental set of intensity data obtained with both the X-ray and polarized neutron techniques. This will allow application of the formalisms described above, and a direct comparison of experimental charge density parameters with theoretical results. 

Such an experiment is not easy to perform and is very time consuming because of the very small magnetic structure factors for the forbidden reflections. The investigation of MnF2 was important however, both in demonstrating the application of polarized neutron techniques to antiferromagnetic materials and in revealing directly the covalent spin density transferred to the fluorines [the existence of this spin density was already known of course, from NMR measure-... 

The joint use of X-ray and neutron diffraction data is particularly expedient. Firstly, the interaction between the magnetic moments of neutrons and electrons is the basis for polarized-neutron diffraction, from which the unpaired spin density in a system can be derived. The diffraction of spin-polarized neutrons is an important technique, beyond the scope of this volume. Secondly, the interaction between neutrons and the atomic nuclei, which is the basis for structure determination by neutron diffraction, leads directly to information on the positions and mean-square vibrations of the nuclei. 

PuP amongst compounds of metallic character, plutonium pnictides PuP, PuAs, PuSb are ferromagnetic. PuP has been studied not only by the magnetization technique but also by neutron diffraction specific heat and Powder polarized neutron... 

The X-ray and neutron diffraction data mentioned previously have been used in conjunction with the technique of polarized neutron diffraction (at 4.2 K) to deduce spin-density distributions in [MnPc].519,520 In further investigations514 it proved possible to determine individual 3d and 4s orbital populations on the manganese ion together with an estimate of 24% for the d-orbital electron density delocalized in the macrocyclic ring. From these studies it appears that the charge on the manganese is approximately +1 this charge appears to be achieved primarily by the loss of a 3d electron rather than a 4s electron. 

For a long time, sample size has limited the use of some investigation techniques. For example, polarized neutron studies failed in part for sample-size problems (see Section XII.C). Similarly, to perform neutron inelastic scattering experiments on TTF-TCNQ, Shirane et al. [73] had to use an assembly of 17 aligned crystals to reach a total volume of about 0.015 cm3. Of course, there is a gradual improvement in both measurement techniques and crystal growth, but almost every time a new type of material is found, problems recur. In this section we first recall the needs of the main crys-... 

Polarized neutron diffraction makes it possible to measure magnetic structure factors and thus to obtain the distribution of unpaired spins in magnetic materials (see, e.g., Ref. 201). Let us discuss here the use of this technique for the study of low-dimensional organic conductors. 

The techniques of single-crystal diffractometry have been discussed by Arndt and WUlis (51). We should note that extinction is a very serious problem in the determination of accurate magnetic intensity data from single crystals. Although extinction must always be accounted for in conventional crystallographic studies, it is particularly important to make proper correction in polarized neutron experiments where the ratio of magnetic to nuclear structure factors is determined. 

Coherent and isotopic-incoherent scattering involve no spin-flip, whereas spin incoherent scattering (i.e., for hydrogenated molecules) inverts the neutron spin with a probability of 2/3. Since spin-polarized neutrons are used in the neutron spin-echo technique, the polarization of the neutron beam, after spin-incoherent scattering woifld be reversed and three times less intense. 

The determination of Xj as a function of T in US using polarized neutrons (sect. 4.7) represents a new type of study, with special application to the actinides, although the technique as presently described requires single crystals. 

In this article the greatest emphasis is given to wide angle neutron diffraction from isotropic samples. However, a range of other neutron diffraction techniques is also available and it is only possible to describe some of them briefly here. (One of the major advantages of neutrons is that polarized beams can be used and these are especially useful for the study of magnetic structures. However, a discussion of polarized neutron diffraction instrumentation is beyond the scope of this article.)... 

F. Mezei. Neutron spin echo A new concept in polarized thermal neutron techniques. Zeitschrift Physik, 255 (1972), 146-160. 

The highest energy resolution is obtained by using the neutron spin echo technique. In this type of instrument, the velocities of neutrons incident on, and scattered from, a sample are coded as the number of Larmor precessions that the neutron spins undergo in a well-defined applied magnetic field. The method requires a beam of polarized neutrons with a distribution of velocities. As indicated in Figure 7, these neutrons enter the precession-field region with their spins perpendicular to the field. As a neutron... 

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