Chapter 2
Combining Mössbauer Spectroscopy and Magnetometry
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Κ. E. Kauffmann and E. Münck Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213
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Combining Fe Mössbauer spectroscopy with magnetometry can provide significant insight into the physical and electronic structure of iron-containing compounds. Such multi-instrumental studies are particularly useful for analyzing the magnetic behavior of synthetic complexes, available as polycrystalline powders. In addition to difficulties due to frequently present metal contaminants, the study of powder samples poses problems associated with alignment of individual crystallites by an applied magnetic field and/or texture resulting from packing. Moreover, interactions between neighboring molecules in solid samples complicate the analysis of the magnetic behavior for individual sites. By combining Fe Mössbauer spectroscopy with magnetic susceptibility measurements, such problems can be observed and overcome. We address these issues by discussing a simple S = 5/2 system and an exchange-coupled diferric complex. 57
Previous published articles have explored and reviewed the correlations between Mossbauer spectroscopy and Electron Paramagnetic Resonance (EPR) for ironcontaining systems with half-integral (7-3) and integral electronic spin (4). These relations provide researchers with strong arguments for establishing the presence of novel structures, or they allow one to correlate EPR signals with specific iron environments in multi-metal systems such as the nitrogenases, hydrogenases and carbon monoxide dehydrogenases, to mention examples from biologically oriented research. Similarly, when Fe Mossbauer spectroscopy is used in conjunction with magnetic susceptibility measurements, Mossbauer spectroscopy becomes an extremely powerful technique in characterizing the magnetic behavior of iron 57
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©1998 American Chemical Society In Spectroscopic Methods in Bioinorganic Chemistry; Solomon, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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complexes. Recently in our laboratory, we have studied model compounds using both Mossbauer spectroscopy and SQUID magnetometry (5). These studies show that by using magnetic susceptibility to limit the range of values in the electronic parameters of a given spin system it is possible to obtain consistent fits to both Mossbauer and susceptibility data. In the following, we describe some aspects of the methodology of our combined studies. We will assume that the reader is familiar with the relevant aspects of Mossbauer spectroscopy and magnetometry, in particular with the spin Hamiltonian formalism. Suitable introductions into the Mossbauer technique, for instance, are given in (7-5); magnetometry studies have been described by Day (6), Butzlaff et al. (7), and Trautwein et al. (8). Theoretical Considerations This section briefly reviews the theoretical basis of the connections between magnetization studies and certain aspects of Mossbauer spectroscopy. Let us consider a mononuclear iron compound with spin S, and assume that the energy levels of interest are well described in the spin Hamiltonian approximation. In the presence of an applied magnetic field, the electronic part of the spin Hamiltonian can be written as
H. =d\s] -^2>
§^ -«φ/S.g.B.
+
(1)
where D and Ε are the axial and rhombic zero-field splitting parameters, respectively, and g is the g-tensor. For simplicity we assume that the g-tensor is collinear with the zero-field splitting tensor. This assumption frequently does not hold for exchangecoupled clusters; however, because the g-values are generally close to g = 2, violation of this assumption does not seriously affect the data analysis. For a given orientation of the appliedfield,the magnetic moment, , of the η eigenstate due to H can be written as η
Λ
e
n
-
r)F _ ,
(2)
where ê is the unit vector along Β and Β is the magnitude of B. The net magnetic moment, M , observed for Ν paramagnetic sites at temperature Τ is the thermal average of , η
η
Σ*Γ M =
£ %
ψι all η
In Spectroscopic Methods in Bioinorganic Chemistry; Solomon, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
(3)
18 This expression is fundamental to the study of molecular magnetism (9). From eqs. 1 and 2, it follows that »g
(4)
where is the expectation value of the electronic spin taken for level n. Using eq. 4 we can write eq. 3 as _£
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