The mixed valence active center of the diiron-oxo protein uteroferrin (Ufr), a porcine purple acid phosphatase, has been analyzed by Mössbauer spectroscopy. Low and high magnetic field spectra are consistent with an antiferromagnetically spin-coupled center. Theoretical spectra that simulate experimental data have been calculated by diagonalizing the spin Hamiltonian H )Here the subscripts 1 and 2 refer to Fe 3+ (S 1 ) 5/2) and Fe 2+ (S 2 ) 2), respectively. For T e 4.2 K, both irons exhibit resolved magnetic hyperfine splittings which depend on the direction and magnitude of the external field, and the static limit of H applies. For T g 100 K, the spin fluctuations are fast and pure quadrupole doublets are observed unless strong fields are applied. In Ufr, the dominant isotropic exchange is strongly perturbed by the zero field splitting (ZFS). We have also calculated the EPR g eff tensor by diagonalizing the electronic terms of H. Our work confirms the claim (Sage, J. T.; et al. J. Am. Chem. Soc. 1989, 111, 7239) that the anisotropy of g eff arises from the admixture of higher spin manifolds to the S eff ) 1 / 2 ground state by the ZFS. In order to find a solution to the Hamiltonian, we searched in its large parameter space with a genetic algorithm. This highly effective searching procedure allowed us to find an optimal parameter set that simultaneously reproduces Mössbauer spectra and EPR g values. The strategies employed for the search in the parameter space of H are discussed. We have determined the exchange constant J ) 34.7 cm -1 , ZFS parameters D 1 ) -0.10 cm -1 , E 1 ≈ 0, D 2 ) +10.81 cm -1 , and E 2 ) +3.17 cm -1 , and hyperfine tensors ã 1 /g n n ) -(21.4,21.2,17.8) T and ã 2 /g n n ) -(15.2,12.2,14.1) T. We have also interpreted the 4.2 K spectra with an effective S eff ) 1 / 2 Hamiltonian for the ground state and determined effective hyperfine tensors à 1 eff /g n n ) -(45.6,64.1,31.3) T and à 2 eff /g n n ) +(11.4,24.9,16.7) T. For Fe 2+ , the ZFS and the electric and magnetic hyperfine interactions were consistent with the presence of axial and rhombic distortions of the dominant octahedral electrostatic potential. The principal axes of the orthorhombic field, which are rotated by 45°about the z axis of the octahedral field, defined an orbital ground state for Fe 2+ of |x 2 -y 2 〉 symmetry with some |z 2 〉 admixture, consistent with the sign of the electric field gradient. Knowledge of the orbital ground state has allowed us to estimate the intrinsic Fermi contact, orbital, and dipolar hyperfine tensors for Fe 2+ .

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The magnetic anisotropy in amorphous Fes28j2Si6 (METGLAS~2605S) has been studied by the determination of both surface and bulk properties using Mossbauer, x-ray, and density measurements.Upon annealing above 650 K, a substantial anisotropy with an easy direction perpendicular to the rib/on plane develops. It is proposed that the origin of this out-of-plane anisotropy, also observed for other amorphous ribbons, is a compressive stress from the formation of higher-density crystalline surface layers. The compressive stress produces the perpendiculaf anisotfopy ln the flbbon bulk thfough positive magnetostfictlon.

Mossbauer spectra of ferrimagn«ic Fe7be, have been taken at various temperatures ranging from liquid-nitrogen temperature to the specimen's Neel point. Three sets of six-line hyperfine patterns were obtained and assigned to three magnetically nonequivalent sites of a superstructure of the crystal. Isomer shifts indicate nearly Fe'+ for all three sites. Change of sign of quadrupole splittings above and below 130'K were observed and may be interpreted to be due to the spin-rotation from c axis to c plane with increasing temperature. The Neel temperature was determined to be (447+ 1) 'K.

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