Hyperfine interaction single-crystal spectra

Fig. 4.13 Combined magnetic hyperfine interaction for Fe with strong electric quadrupole interaction. Top left, electric quadrupole splitting of the ground (g) and excited state (e). Top right first-order perturbation by magnetic dipole interaction arising from a weak field along the main component > 0 of the EFG fq = 0). Bottom the resultant Mossbauer spectrum is shown for a single-crystal type measurement with B fixed perpendicular to the y-rays and B oriented along...

Ms level (selection rule AMs = 1). EPR is applicable to solid (powder, single crystal, and frozen solutions) as well as liquid samples, and to electronic ground-state systems as well as to photoexcited and carrier injection-induced systems (i.e., device structures). The information contained in an EPR spectrum also reflects hyperfine interactions and magnetic anisotropy (zero-field) effects. 

While the effective g value is expressed in terms of three principal values directed along three axes or directions in a single crystal, only the principal values of g can be extracted from the powder spectrum rather than the principal directions of the tensor with respect to the molecular axes. (Therefore it is more correct to label the observed g values as gi, g2, g3 rather than g gyy, in a powder sample.) In the simplest case, an isotropic g tensor can be observed, such that all three principal axes of the paramagnetic center are identical (x = y = z and therefore gi= gi = g-i). In this case, only a single EPR line would be observed (in the absence of any hyperfine interaction). With the exception of certain point defects in oxides and the presence of signals from conduction electrons, such high symmetry cases are rarely encountered in studies of oxides and surfaces. 

Altaiskii et al [1] were the first to report on the ESR spectrum of nitrogen donors in Lely grown 3C single crystals. In that study the three-line spectrum is partially resolved. Later work by Carlos et al [2] showed that a well resolved three-line spectrum could be obtained for the CVD films as shown in FIGURE 1. The g-tensor and hyperfine interaction are isotropic with g = 2.0050 and A = 0.11 mT. Both of these parameters are in good agreement with expectations from simple effective-mass-approximation calculations for this polytype [5]. 

The temperature dependence of the magnetic field and quadrupole interaction in erbium metal have been followed between 4-2 and 40 K and analysed [142]. An estimate of —1-9(4) bam was suggested for the excited-state quadra-pole moment. In a more detailed study, the line intensities of the hyperfine spectrum in a single crystal of Er metal have been correlated with the magnetic structure previously determined by neutron diffraction methods [143]. 

Magnetic structure determinations In certain cases details of the magnetic structure can be elucidated from hyperfine interaction measurements. For example, in Mbssbauer effect experiments employing single crystal absorbers, the relative intensities of the component lines of the spectrum depend on the angle between the gamma ray quantum and the hyperfine field at the nucleus. The latter is in turn parallel to the ionic moment. By such an experiment, Reese and... 

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