Metals band magnetism

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 ... 

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. 

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. 

With photons in the infra-red frequency range only the electrons very near the Fermi level may be excited. This method was used to detect the modification of the energy bands of rare earth metals by magnetic ordering. Measurements of the entire conduction band structure by either optical reflection or photoemission are possible with photons in the ultraviolet range. With X-ray photons one can use the photoelectric effect to study not only the band states but also the location of the core states. This gives a direct answer to the question that was raised in the last section on band structure calculations, namely where the 4f-levels are relative to the band levels. 

GdFei2 does not exist as a stable compoimd but electronic structure calculations using the LMTO-ASA method by Trygg et al. (1992) have been performed. As was observed in the experiment on the RMn systems there is a pronoimced influence of localized 4f magnetism on the conduction-band magnetism (transition-metal sublattice) which gives noticeable changes in the local moment of the iron (transition element). The presence of the 4f spin moment is found to induce a redistribution of the spin moment between the rare-earth and iron sites, while the total conduction-electron moment remains constant. It seems that these conclusions have also some importance for the ternary materials. 

The R-M systems, where M is a 3d transition metal, form an outstanding tool for the study of 3d band magnetism and in particular the interactions, instabilities and anisotropies of such magnetism. In the majority of cases, for a given M element, a series of compounds with different rare earths crystallize in the same crystallographic structure and thus have practically the same band structure. It is then possible to study the 3d magnetism under... 

Just as in the molecular case, and discussed in detail in Chapter 8, there is always a choice to be made between filling all the lowest levels with electron pairs and the alternative of allowing some of the higher energy levels to be occupied by electrons with parallel spins. 13.5 and 13.6 showed two extreme cases where all the electrons in a band were either all spin unpaired or all spin paired. An intermediate situation shown in 13.70 is also of importance, where not all of the spins are unpaired. An example of this type, which, in addition to being metallic is magnetic, is found in the... 

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