Free hyperfine field

Again, although the purpose of a recent study by Nowik et al. (15) was to study the free ion hyperfine fields in intermetallic compounds of... 

Measurements of the hyperfine field at low temperatures Bhf(7 0) make it possible in favourable cases to determine the nature on the ground state. This offers the possibility to obtain information of the second term in eq. (15). In the particular case that the ground state is a pure 17Z = —7) state, the first term in eq. (15) reduces to the free-ion value QS4f, so that QSlat can be obtained as the difference QS(T - 0) — QS4f. Experimental values of QSlat are sometimes very useful because they can provide information on the crystal-field parameters. For rare earth ions... 

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. 

Spectrum is typical of the Dy " free ion in a /z = state and a typical spectrum measured at 77 K is shown in Fig. 17.17 [96]. The observed temperature dependence of the hyperfine field shown in reduced form in... 

Plots of the hyperfine field at nuclei against the ordered magnetic moments in uranium intermetallics. Open circles are the results reported by Ref. 2. Closed circles were the results calculated in free ions of uranium atoms. Closed squares were results measured in this work. 

At sufficiently low temperatures, usually below 10 20 K for transition metal complexes with 5 = and below 50 K for free radicals, transverse relaxation of electron spins is dominated by fluctuation of dipolar hyperfine fields from distant protons. These fluctuations are driven by coupling among the protons,... 

Magnetic hyperfine field parameters for the lanthanide ions based on the results of Freeman and Watson (1962). The experimental free ion values of the total hyperfine field are derived from electron spin resonance data on dilute lanthanide impurities in insulators [Bleaney (1972)]. A positive sign for Htu means that the held has the same direction as the electronic moment. 

As with the NMR measurements, the a and P values at low temperatures agree within a few percent with the free ion values. The NMR and NGR results are considered together in sections 2.1.1.1 and 2.1.1.2 where the separate contributions to the magnetic hyperfine field and quadrupole coupling are discussed. 

Nowik et al. (1966) measured the hyperfine fields in a variety of such dysprosium intermetallics at 4.2 K, finding only small differences from the free-ion values in compounds with noUTmagnetic metals. However, in compounds with Fe and Co substantial departures from the free-ion values arise. 

Calculated free-ion hyperfine fields for the elements under discussion are summarized in table 2. Hund s rule configuration is assumed. This gives the maximum hyperfine field possible for each charge state since it sets ntj = J. This is appropriate in ordered materials if the free-ion configuration is present and if exchange interaction dominates all other energies. Usually this situation prevails in the heavy lanthanides. Examples will be given later on. Crystal field effects can reduce and this aspect will also be discussed. 

Table 2 also lists the free-ion valence electric field gradient (for T -> 0). The free-ion field gradient should be observed in a cubic ordered magnet if the free-ion hyperfine field is present. If B j is reduced from the free-ion value one may use the observed... 

Calculated free-ion hyperfine fields and ionic electric field gradients (without Sternheimer shielding) in the lanthanides and actinides of interest. 

Zinn 1976). The Eu ion is in the divalent state and remains so at least up to lOGPa. At higher pressures the valence of Eu in EuO is a matter of controversy to which we shall return further below. The 87 2 configuration of the free Eu ion leads to pure spin magnetism. In terms of the hyperfine field at the Eu nucleus this means that Bojb which dominates in other lanthanide ions (except Gd which also has 87/2) is absent here. We may write... 

This lanthanide Laves phase becomes ferromagnetic at Tc = 61 K under ambient conditions. The saturation hyperfine field and magnetically induced quadrupolar coupling correspond within the experimental errors to the free-ion values (see also fig. 37). Similarly, the isomer shift lies well inside the typieal range of metallic trivalent... 

QJ01a. The dashed line is the Np free-ion field. The arrow shows the change in hyperfine parameters of NpAs under 8.3 GPa applied pressure. [Taken from Kalvius et al. (1992).]... 

Looking at the behavior of the isomer shift, the same general trends are manifested but no break in the linear correlation of shift with volume is observed between NpAs and NpSb. In contrast to the behavior of the hyperfine field, the isomer shift in none of the materials reach the theoretical free-ion Np value, stressing the lingering influence of ligand electrons within the whole series. The systematic difference in isomer shift between the pnictides and the chalcogenides corresponds to a higher covalency of the latter in accordance with the behavior of the hyperfine field. 

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