Mossbauer Zeeman spectra

Fig. 4.9 Magnetic dipole splitting (nuclear Zeeman effect) in pe and resultant Mossbauer spectrum (schematic). The mean energy of the nuclear states is shifted by the electric monopole interaction which gives rise to the isomer shift 5. Afi. g = Sg/tN and A M,e = refer to the...

The Mossbauer effect is sensitive to the oxidation and spin state of iron and the environment around the iron nucleus therefore different chemical species yield different Mossbauer spectra. Furthermore all spin states and oxidation states of iron are accessible to the technique. There are three main components of a Mossbauer spectrum. The isomer shift arises from the electron density at the 57Fe nucleus the quadrupole splitting results from the electric field gradient produced by electrons and ions around the 57Fe nucleus, and the nuclear Zeeman splitting is sensitive to the spin state and magnetic coupling of the iron. 

The interpretation of a complex Mossbauer spectrum will obviously be simplified if the relative intensities of the various components are known. Once the energy levels of the Zeeman/quadrupole Hamiltonian have been calculated, and the spin quantum numbers for each state assigned (or appropriate linear combinations if the states are mixed), it is possible to calculate the intensities from the theory of the coupling of two angular momentum states [32, 33]. 

NpFc2 and NpNi2 are established FM (with Tq =492 and 32 K). The magnetic ground state of NpCo2, in contrast, has been under some discussion. Usually it is listed as an AFM with 7n = 13 K, based on susceptibility and mainly on Mossbauer data (Aldred et al. 1975, Sanchez et al. 1992). There a Zeeman pattern appears (in the absence of an applied field) below 13 K. The spectrum shows, however, rather wide resonance lines and a clear distinction between static (distribution of internal fields) and dynamic broadening cannot... 

Fig. 4.7. Six allowed transitions in the Mossbauer spectrum of the 14.4 keV transition in Fe. The internal/external magnetic field results in Zeeman splitting of the nuclear levels while an EFG interacts with the excited state level producing further shifts. The insets show the schematic of resultant spectra in both cases.

To analyze the recorded spectra, the spectrometer needs to be calibrated. The three main calibration parameters are the velocity scale, the center point of the spectrum and the nonlinearity of the velocity/time profile of the oscillation compared to a standard reference. The calibration is performed using a spectrum recorded from an a-iron foil at room temperature using the well defined line positions of the sextet from a-iron, which occur at 5.312mms , 3.076mms , and 0.840mms The center of this a-iron spectrum at room temperature is taken as the reference point (0.0 nun s ) for isomer shift values of sample spectra. The typical Mossbauer spectrum of the 14.4 keV transition of Fe in natural iron (Fig. 4.10) represents a simple example of pure nuclear Zeeman effect. Because of the cubic symmetry of the iron lattice, there is no quadrupole shift of the nuclear energy levels. The relative intensities of the six magnetic dipole transitions are... 

In a powder with no preferred orientation, /3 is equally distributed between 0 and ir (/3 = 54.T). The lines of a Zeeman sextet are in the intensity ratio 3 2 1 1 2 3, and those of a quadrupole doublet have equal areas. These patterns along with the singlet represent the three basic types of Pe Mossbauer spectrum. 

---