Band magnetism

SERRS Surface-enhanced RRS [214, 217] Same as SERS but using a wavelength corresponding to an absorption band Magnetic Spectroscopies Same as SERS... 

The problem could be stated from another point of view. In an isostructural series the uranium and neptunium compounds tend to be itinerant electron magnets or band magnets (like iron) and their orbital contribution is at least partially quenched. For much heavier actinides we know that the compounds will make local moment magnets with orbital contributions. It is quite possible that in between these two clear cut forms of magnetism that the intermediate case could be dominated by fluctuations, and no recognizable form of magnetism would occur. To state that the... 

In the Mott-Hubbard theory on the other hand, it is shown that there exists an instability in the narrow-band electronic structure (Peierls instabihty ) and if the bandwidth decreases below a critical value, a sudden transition (Mott transition) takes place toward a complete localized situation. In this approach, it is assumed, in fact, that band magnetism does not exist and one has to deal only with 2 classes of materials... 

The Stoner product of UN (see Chaps. A and D) is greater than one, in agreement with the antiferromagnetic behaviour of this solid. The antiferromagnetism was attributed to itinerant band magnetism (as in some d-metals and compounds but unlike light actinide metals). In fact, cohesive properties of this solid have been well explained in a pure spin-polarised picture and Fournier et al. have shown that the magnetic uranium sublattice moment and the Neel temperature have a similar pressure dependence. Discrepancies existed, however, between calculations and experiments ... 

Fig. 23. Schematic energy diagram of the Heisenberg exchange energy J vs. bandwidth W at the crossover from localized-electron to band magnetism, after Zhou and Goodenough (2002a).

Choosing a ligand field description of the electronic structure, we will deal only with spectral and magnetic properties which refer to the mainly metal d orbitals. Therefore we will review electronic spectra (d-d bands), magnetic susceptibility measurements, ESR spectra and MCD spectra. We will also briefly comment on NMR spectra which are determined by the nature of the ground state in a paramagnetic complex15. We will not go much into the theory of these techniques, since they are well known and adequately treated elsewhere, but will briefly summarize their fundamental basis. 

In terms of both abundance in the Earth s crust (and on planetary surfaces) and numbers of species, most minerals are ionic and covalent in character, rather than metallic. Their magnetism, therefore, is appropriately described in terms of the magnetism of their cations and anions, rather than in terms of band magnetism of conduction electrons. Prominent exceptions are the Fe-Ni phases found in meteorites and that are relevant to planetary cores. Even in metallic minerals, however, the magnetism of the closed shell cation cores can, to a good first approximation, be treated independently from the conduction electron magnetism. 

The effects of an external pressure on the magnetic moment at 4.2 K and on Tc were found to be large and negative (Franse 1983) and to become larger with decreasing Fe content (table 2.2). The results are in accordance with the predictions of the SEW theory (Wohlfarth 1968,1981) which suggests that these compounds can be treated within the framework of band magnetism. 

The following table contains values for the molar i(sceptibilitv of Xm, s iecific us ceptibihty x and volumetric susceptibility I The cgs (laiissiaii sistem of units is employed In the Oaussian units the relation between magnetic induction Band magnetic field strength H is... 

A more complex magnetic behaviour is expected for RI compounds in which the second component is a 3d transition metal such as Mn, Fe, or Co. The magnetic behaviour of the transition metal component is now based on the magnetic polarization of the electronic d-bands. Consequently, in this section we summarize the theory of itinerant or band magnetism and its application to transport properties. We begin with the Stoner-Wohlfarth model and include a summary of recent works. 

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