Mossbauer magnetic hyperfine splitting

Magnetic hyperfine splitting in Ru Mossbauer spectra was observed in the 1960s by Kistner [110, 143] for an absorber of 2.3 at% ruthenium dissolved in metallic iron. The spectra obtained with an unpolarized absorber (a) and with polarized absorbers, i.e., magnetization parallel to incident y-rays (b), and magnetization perpendicular to incident y-rays (c) are shown in Fig. 7.35. The stick spectra on top... 

In order to examine in which cases the different Mossbauer parameters can be observed, we will in this section make use of parameters R (which will be defined below) where i = 1,2, and 3 refers to the isomer shift, quad-rupole splitting, and magnetic hyperfine splitting, respectively. 

Iron-supported-on-MgO catalysts behave in some ways differently from the above catalyst systems. That is, while the catalytic activity of these metallic-iron particles for the atmospheric-pressure ammonia synthesis depends markedly on particle size in the range 1.5-10 nm (206), the Mossbauer parameters (isomer shift, quadrupole splitting, and magnetic hyperfine splitting) are independent of iron particle size in this range (97). This thus rules out an electronic effect in the interpretation of the effect of particle... 

Beff interacts with the magnetic moment of the nucleus, /< = —g P BI, yielding the magnetic hyperfine splitting of the nuclear ground and excited states that we infer from the Mossbauer spectrum by measuring the transition energies. 

Fig. 3. Mossbauer spectrum showing magnetic hyperfine splitting.

For completeness the reader should refer to two review articles, by Lang (9) and Miinck (10), which are in preparation. Both articles deal mainly with the interpretation of magnetically hyperfine split Mossbauer spectra of ferric heme proteins and other biological molecules. 

Mossbauer spectra obtained at very low temperatures (18-23.5°K) on samples B, C, and D are shown in Figure 4.33. At these low temperatures the samples become ferromagnetic, and the spectra exhibit magnetic hyperfine splitting (3,41). As a result, the spectra consist of six lines instead of two. While the six lines in the spectra of Figure 4.33 are not very pronounced, they are nonetheless real. The positions of the lines, as determined from a computer analysis of the data, are shown above the spectra in Figure 4.33. 

The magnetic hyperfine splitting (MHS) depends on the nuclear spin quantum numbers and /g of the excited and ground state of the Mossbauer nucleus and on the effective magnetic field at the Mossbauer nucleus, which includes contributions from the local electronic spin, from the orbital momentum, from dipole terms, and from external fields. 

This anisotropy was also studied by Mossbauer spectroscopy at temperatures down to 32 mK. The absence of magnetic hyperfine splitting in the Eu 6d derived signal was attributed to incoherent tuneUing of Eu between the split sites in the 24 cage [36]. The characteristic tunneling frequency of about 450 MHz was also confirmed by microwave absorption measurements [36]. 

Magnetic hyperfine splitting is observed in the Mossbauer spectra of NpCr Al below 50K (Gal et al. 1987b). The temperature variation of is consistent with a second-order transition into antiferromagnetism around this temperature. 

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