Bulk magnetic structure

Alternatives to XRD include transmission electron microscopy (TEM) and diffraction, Low-Energy and Reflection High-Energy Electron Diffraction (LEED and RHEED), extended X-ray Absorption Fine Structure (EXAFS), and neutron diffraction. LEED and RHEED are limited to surfaces and do not probe the bulk of thin films. The elemental sensitivity in neutron diffraction is quite different from XRD, but neutron sources are much weaker than X-ray sources. Neutrons are, however, sensitive to magnetic moments. If adequately large specimens are available, neutron diffraction is a good alternative for low-Z materials and for materials where the magnetic structure is of interest. 

These limitations, most urgently felt in solid state theory, have stimulated the search for alternative approaches to the many-body problem of an interacting electron system as found in solids, surfaces, interfaces, and molecular systems. Today, local density functional (LDF) theory (3-4) and its generalization to spin polarized systems (5-6) are known to provide accurate descriptions of the electronic and magnetic structures as well as other ground state properties such as bond distances and force constants in bulk solids and surfaces. 

This formula shows a smooth decrease of p toward the bulk magnetic moment with increasing N. However, the experimental results graphed in Figure 1 indicate that the variation of p with N has a more complex, oscillatory behavior. Its explanation requires a detailed consideration of the geometry of the cluster and a better treatment of its electronic structure. 

As reported by Blanco et al. (1999), neutron diffraction patterns of powder and bulk polycrystalline samples of GdCu were obtained for both structures in the cubic CsCl type of structure, which orders antiferromagnetically at 7n 150 K, a propagation vector of ( j 0) has been found with the moments probably parallel to the c-axis (note that other noncollinear magnetic structures might give rise to the same neutron-diffraction pattern). In the orthorhombic low temperature phase (7n 45 K) the available diffraction patterns... 

NMR is a widely used and important technique for molecular structure determination as applied to bulk materials, where it competes, often advantageously, with vibrational spectroscopy. However, a lack of sensitivity has limited its application to the study of adsorption on high-area finely divided surfaces. Also, certain metals with bulk magnetic properties—e.g., Fe, Co, and Ni (but not the other group Vlll transition metals)—cannot be studied by the technique as their magnetism causes very broad and weak resonances from adsorbed species. 

In the next few subsections we will describe in detail the lattice and electronic structure, and describe the results of the most important experiments, which determine the bulk magnetic properties of TDAE-C60. 

The pros and cons of these suggestions have been discussed in depth [357, 360, 368], Experimental evidence for high-spin states usually comes from the EPR fine structure and from magnetic susceptibility measurements. EPR is most often used for well defined radicals also in dilute solution. In the solid state the spin-spin interaction is usually referred to as zero-field splitting and represented by a traceless tensor D. The susceptibility data, on the other hand, are predictive for the bulk magnetization of the whole sample and typically measured for the powdered or crystalline material. 

Nanoparticles are discrete nanometer (10 m)-scale assemblies of atoms. Thus, they have dimensions between those characteristic of ions (lO m) and those of macroscopic materials. They are interesting because the number of atoms in the particles is small enough, and a large enough fraction of them are at, or near surfaces, to significantly modify the particle s atomic, electronic, and magnetic structures, physical and chemical properties, and reactivity relative to the bulk material. Nanoparticle surfaces themselves may be distinctive. Particles may be terminated by atomic planes or clusters that are not common, or not found, at surfaces of the bulk mineral. These, and other size-related effects will lead to modified phase stability and changes in reaction kinetics. 

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