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Physics of Magnetism Magnetic Materials 2011 Part 5

Tham khảo tài liệu 'physics of magnetism magnetic materials 2011 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 66 CHAPTER 7. ITINERANT-ELECTRON MAGNETISM This quantity describes the magnetic susceptibility in metallic systems in which there is no interaction between the band electrons. This means that i eff 0 and thus that the Stoner enhancement factor 1 UeS AI Ef - 1 reduces to unity. By contrast the enhancement factor can reach fairly high values when there is a strong interaction between the electrons and or when the density of electron states at Ef is high in fact the Stoner enhancement factor can become extremely high for metallic systems close to magnetic instability that is when the Stoner criterion is fulfilled. We mentioned already that such a situation occurs for Pd metal. Experimentally one finds that the susceptibility of Pd metal is roughly one order of magnitude larger than for example of Zr metal. 7.3. STRONG AND WEAK FERROMAGNETISM In order to explain the principles of 3d-electron magnetism up to now we have used a simplified model with rectangular 3d bands Fig. 7.1.1 . In a more realistic treatment one has to take account of the actual shape of the 3d band in the energy range of interest Friedel 1969 . This means that the density of states is no longer a constant over the whole energy range considered but may vary strongly as a function of the energy when moving from the bottom of the 3d band to the top. Generally one finds that the density of states of the 3d band first increases when moving from the bottom of the 3d band in upward direction. After reaching a region where the density of states is high one passes into a region where the density of states is fairly low. Moving further to the top of the band one encounters again a region where the density of states is high. When for a given degree of 3d-band filling the Fermi energy happens to be located in a region where the density of states is relatively low the Stoner criterion may not be met and a spontaneous moment may form only if higher and lower lying states are included where the density of states .