Electron spin correlation

Many ferromagnets are metals or metallic alloys with delocalized bands and require specialized models that explain the spontaneous magnetization below Tc or the paramagnetic susceptibility for T > Tc. The Stoner-Wohlfarth model,6 for example, explains these observed magnetic parameters of d metals as by a formation of excess spin density as a function of energy reduction due to electron spin correlation and dependent on the density of states at the Fermi level. However, a unified model that combines explanations for both electron spin correlations and electron transport properties as predicted by band theory is still lacking today. 

Chemical stability indicates that in the cubic, metallic perovskites the interstitial C and N are probably neutral. They represent, therefore, an M atom with half filled p or s-p orbitals, and in the cubic structure the metal-M-metal interaction is defined by Figure 86. [This is to be contrasted with low-temperature CrN, which has considerable ionic character and an ordering of its covalent character along a given axis.] In contrast to the M atoms of the Heusler alloys, the p electrons of C and N correlate with, and therefore spin pair, the near-neighbor eg electrons. The metal- -metal interactions are determined by the Ug electron-spin correlations since Hu 2.76 A < Rc. 

Just as for polyelectronic atoms, the electronic wavefunction for a molecule must be antisymmetric the Pauli principle. Thus, electron spin correlation is accommodated by the definition of the wavefunction as a Slater determinant whose elements are the occupied molecular orbitals. 

The sign of the contact shift usually alternates along a chain if the spin-delocalization is mainly via n orbitals a delocalization gives like signs, and sharper attenuation with distance from the spin source. Simple considerations of electron spin correlation, such as the Pauli principle and Hund s rule, give overall guidance, but the reality is often complex (e.g.. Ref. 176). 

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