Perturbations, magnetic interactions

The low-temperature Mossbauer spectra of the spinel type oxides, NiCr204 [14,18] (Fig. 7.6b) and NiFe204 [3, 18], have been found to exhibit combined magnetic dipole and electric quadrupole interaction (Fig. 7.7). For the evaluation of these spectra, the authors have assumed a small quadrupolar perturbation and a large magnetic interaction, as depicted in Fig. 7.3 and represented by the Hamiltonian [3]... 

A more accurate description is obtained by including other additional terms in the Hamiltonian. The first group of these additional terms represents the mutual magnetic interactions which are provided by the Breit equation. The second group of additional terms are known as effective interactions and represent, to second order perturbation treatment, interaction with distant configurations . These weak interactions will not be considered here. 

Crystal-Field Potential. Before we can discuss the magnetic interactions, we must consider the effect of the lattice on the ion, since this is by far the largest perturbation on the wave function of the ion. In crystal-field theory the electrons and nuclei of the surrounding atoms are replaced by a potential field of the same symmetry, and the effects of this crystal field on the metal ion are evaluated. Thus we take for V in Eq. (5) the expression... 

The chemical perturbations of the nuclear levels can be divided into two classes. One effect is the electrostatic interaction between the nuclear charge and the surrounding electric charges, and the other effect is the magnetic interaction between the nuclear magnetic moment and the surrounding electron spin density. The former of these interactions will be discussed next. 

Figure 2 Schematic description of some 57 Fe Mossbauer spectra showing original nuclear energy levels (A) slightly perturbed through interaction with S electron density (B) giving rise to a chemical shift (8) resulting from quadrupole interactions (C) and magnetic splitting of the ground and excited states (D). Arrows indicate transitions responsible for one,...

Figure 1 Nuclear energy levels responsible for one-, two-, and six-line iron Mossbauer spectra I, original energy levels II, small perturbation via interaction with s-electron density III, quadrupole interaction splitting IV, magnetic splitting of the ground and excited states...

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