Orbital hyperfine field

Each of the eight hyperfine resonances is an unresolved quadmpole doublet, due to the quadmpole interaction of Os in the hexagonal Os metal source. The authors have interpreted the hyperfine fields in terms of core polarization, orbital and spin-dipolar contributions. 

Although attempts have been made to correlate the hyperfine field of a mixed-valence compound with the weighted mean of the Fermi contact-interaction field H, = 220 < Sz > kOe , the orbital and spin-moment contributions Hl and Hd, which may be comparable to H, at Fe(II) ions, make any such correlations dubious ... 

Figure 11. The best fit has been found for 5 = 0.7 0.01 nuns, Aitg =—3.25 0.01 nuns, " =(2.11, 2.19, 2.00), "T/ nMn = (-45, 10, 19) T, tj = 0.74 0.1, D = 7.2 0.5 cm, and EID = 0.16 0.02. The zero-field splitting parameter D = 7.2 0.5 cm is comparable to that found for rabredoxin from Clostridium pasteurianum (D = 7.6cm Surprisingly, the rhombicity parameter E/D = 0.16 0.02 differs somewhat from that of rubre-doxin from C. pasteurianum (E/D = 0.28). The hyperfine coupling tensor has been determined to be A = (-14.5, -9.2, -27.5) T. The anisotropy of the hyperfine conpling tensor is cansed by spin-orbit confribntions to the internal magnetic hyperfine field.

Low-spin iron(III) ions have an electron hole in the t2g orbitals. Therefore, these centers have S = 1/2 and spin-orbit interaction contribntes considerably to the magnetic hyperfine field. Low-spin iron(III) componnds in solution always show a rather complicated magnetic Mossbauer pattern at temperatures around 4.2 K and low external fields, which means that the relaxation rate of these centers is lower than the nnclear precession rate of 10 s. Sometimes a magnetic sphtting is observed even at 77 K. Therefore, in order to pin down 8 and A g, it is advisory to measure between 100 and... 

Within a nonrelativistic calculation of the hyperfine fields in cubic solids, one gets only contributions from s electrons via the Fermi contact interaction. Accounting for the spin-orbit coupling, however, leads to contributions from non-s elections as well. On the basis of the results for the orbital magnetic moments we may expect that these are primarily due to the orbital hyperfine interaction. Nevertheless, there might be a contribution via the spin-dipolar interaction as well. A most detailed investigation of this issue is achieved by using the proper relativistic expressions for the Fermi-contact (F), spin-dipolar (dip) and orbital (oib) hyperfine interaction operators (Battocletti... 

This correlation exists despite any a priori reason why Np should always behave as a rare-earth element where the large orbital contribution unambiguously determines the hyperfine field Hhf. Such a type of relation is less reliable in transition metals due to variations of the localization of the d states, where the spatial extent of the spin density can differ markedly according to bonding conditions. 

The NpT2X2 compounds are usually treated as materials with localized 5f states, and crystalline electric-field effects are held responsible for the depression of the magnetic moments (2.4/tB is expected for the free-ion moment of Np). However, the heavy-fermion behaviour (found in NpCu2Si2) is able to introduce an instability of the localized 5f states. In this sense the reduction in hyperfine field at the Np nucleus could be understood as being due to a partial loss of the orbital moments resulting from 5f-electron delocalization. 

The orbital term //l and spin moment are more difficult to estimate. For an Fe + ion in cubic symmetry, both are zero, and even in distorted environments they are small. Calculations of these contributions in a-Fe203 have been made [32]. The limiting value of ferric hyperfine fields is therefore about 550 kG, and it is difficult to distinguish between two cation sites in the same sample unless they show significant differences in chemical isomer shift or quadrupole splitting. By contrast, for ferrous iron, the magnitudes of //l and can be of the order of and the resultant field H can be anywhere... 

A contribution from the nuclear dipolar field AN( i) = (fio/4n)rN](jlN x rNi) yields the spin-other orbit hyperfine correction... 

E/Z) = 0.16 0.02 differs somewhat from that of rubredoxin from C. pasteurianum ElD = 0.28). The hyperfine coupling tensor has been determined to be A = (—14.5, —9.2, —27.5)T. The anisotropy of the hyperfine coupling tensor is caused by spin-orbit contributions to the internal magnetic hyperfine field. 

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