Hyperfine temperature dependence

Contact shifts give information on the electronic structure of the iron atoms, particularly on the valence distribution and on the magnetic coupling within polymetallic systems. The magnetic coupling scheme, which is considered later, fully accounts for the variety of observed hyperfine shifts and the temperature dependence. Thus, through the analysis of the hyperfine shifts, NMR provides detailed information on the metal site(s) of iron-sulfur proteins, and, thanks to the progress in NMR spectroscopy, also the solution structure 23, 24 ). 

Fig. 4. Top Theoretical temperature dependence of the hyperfine shift of the H/3 protons of reduced spinach [Fe2S2] ferredoxin 151). The solid line corresponds to the situation where only one species exists in solution, whereas the dashed line corresponds to a situation where there is fast equilibrium between two species (in a 20/80 ratio) differing for the location of the extra electron 151). Bottom.-. Experimental temperature dependence of the H NMR shifts. The signals are labeled as in Fig. 2B.

The temperature dependence of the magnetic hyperfine splitting in spectra of interacting nanoparticles may be described by a mean field model [75-77]. In this model it is assumed that the magnetic energy of a particle, p, with volume V and magnetic anisotropy constant K, and which interacts with its neighbor particles, q, can be written... 

The interactions also suppress the collective magnetic excitations at low temperatures. Figure 6.17 shows the temperature dependence of the magnetic hyperfine... 

Combines sensitivity of EPR and high resolution of NMR to probe ligand super-hyperfine interactions For paramagnetic proteins enhanced chemical shift resolution, contact and dipolar shifts, spin delocalization, magnetic coupling from temperature dependence of shifts Identification of ligands coordinated to a metal centre... 

The Mossbauer effect involves the resonance fluorescence of nuclear gamma radiation and can be observed during recoilless emission and absorption of radiation in solids. It can be exploited as a spectroscopic method by observing chemically dependent hyperfine interactions. The recent determination of the nuclear radius term in the isomer shift equation for shows that the isomer shift becomes more positive with increasing s electron density at the nucleus. Detailed studies of the temperature dependence of the recoil-free fraction in and labeled Sn/ show that the characteristic Mossbauer temperatures Om, are different for the two atoms. These results are typical of the kind of chemical information which can be obtained from Mossbauer spectra. 

The superparamagnetic properties of -Fe203 have also been studied via the Mossbauer effect. Nakamura et al, (32) have investigated the temperature dependence of the internal field in -Fe203 particles of approximately 50 A. diameter. At 120 °K. they obtain a spectrum which is almost identical to the bulk material, except that no Morin transition has occurred and the spins still lie in the basal plane. At room temperature the magnetic hyperfine spectrum collapses (even though the bulk... 

To determine static properties of the SeO radical in KDP and DKDP, the temperature dependence of the hyperfine interaction between unpaired electron and Se (I = 1/2) nucleus was measured [53]. The hyperfine tensor component A, where the direction is along the c-axis, shows an isotope effect, because its value is higher in DKDP than in KDP. Furthermore, its value shows a jump at Tc for DKDP and a considerable temperature dependence in the PE phase of both crystals, approximated by the relation A (T) = A (0) - B coth(ro/T), where To 570 K for both crystals. It is interesting to note that A, similarly to the As NQR frequency and P isotropic chemical shift, should be constant in the PE phase if the two-state order-disorder mechanism of the corresponding tetrahedron holds. However, while the temperature dependencies of the As NQR frequency and P isotropic chemical shift in the PE phase were explained as originating from a six-state order-disorder mechanism [42] and additional displacive mechanism [46], respectively, here it was assumed that excitation of some extra lattice vibration mode with frequency Tq affects the hyperfine tensor components and causes the temperature dependence of A. ... 

Fig. 26. Temperature dependence of the hyperfine magnetic field and of the longitudinal relaxation parameter in NpSns. The J = 1/2 Brillouin curve for the temperature dependence of is shown. (Gal et al, )...

Fig. 34. Temperature dependence of the hyperfine field components along the tetragonal b and c axes, (W, r)b and (Whf)c, of a GdNi2B2C sample, reflecting the temperature dependence of the corresponding components of the Gd magnetic moment. The lines leading to the ordering temperature Tn = 20 K and the spin reorientation temperature Tr = 14 K are guides for the eye (after Tomala et al. 1998).

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