I. Halevy

High-resolution67Zn-Mössbauer study of oxide spinels

We report on first experiments to investigate the electronic structure in the normal spinels ZnAl2O4 and ZnFe2O4 and in the inverse spinels Zn2SnO4 and Zn2TiO4 using the high-resolution67Zn-Mössbauer spectroscopy. The electric field gradient for67Zn at the B (octahedral) site in ZnAl2O4 is negative, whereas the A (tetrahedral) site remains essentially cubic, however, with a more positive center shift. ZnFe2O4 orders antiferromagnetically at ≈10K. Due to superexchange a magnetic field is observed at67Zn. In the inverse spinels short range order leads to only few (Zn,Sn) and (Zn,Ti) configurations at the octahedral sites. The s-electron densities at the67Zn sites are distinct and cover a surprisingly broad range for Zn2TiO4. This strongly suggests that d-electrons of Ti play an essential role in the chemical bond of this compound.

Electronic structure of Zn in oxide spinels

Using67Zn Mössbauer absorption and emission spectroscopy, we have investigated the electronic structure at the A and B sites in the normal spinels (Zn)[Al2]O4, (Zn)[Fe2]O4 and (Zn)[Ga2]O4. Within each system, the center shift Sc at the A site is more positive. In all systems investigated, the electric field gradientVu at the B site is negative. The values for SC andVU scale with oxygen nearest-neighbour distance to Zn. In the Fe spinel, a transferred magnetic hyperfine field is observed at the Zn site below the antiferromagnetic ordering temperatureTN=10 K. For a more detailed discussion of the chemical bond, we have performed ab initio Hartree-Fock cluster calculations for the Al and Fe spinels. Our experimental and theoretical results show that all hyperfine parameters are essentially determined by covalency effects. Our data on the Ga spinel raise the question of a partially inverse structure.

Dynamic short-range order observed in the (Zn)[Fe2]O4 spinel by neutron diffraction, μSR, and Mössbauer experiments

Using neutron diffraction (ND), muon-spin rotation/relaxation (μSR), and57Fe-Mössbauer spectroscopy (MS) we have investigated magnetic properties of the normal spinel (Zn)[Fe2]O4. In compounds which are slowly cooled from 1200°C to room temperature inversion is below detection limits. AtTN = 10.5 K the spinel exhibits long-range antiferromagnetic order (LRO). The transition as seen in thermal-scan spectra by MS is very sharp. However, ND andμSR experiments show that already at temperatures of ∼ 10TN a short-range antiferromagnetic ordering (SRO) develops which extends through ∼70% of the sample volume just aboveTN. BelowTN SRO and LRO coexist. At 4.2 K still ∼25% of the sample is short-range ordered. The regions over which the SRO extends have a size of ∼ 3 nm. Their fluctuation rates are in the GHz range. Modern ab initio cluster calculations successfully describe the magnetic hyperfine fields as well as the electric field gradient (EFG) tensor at the Fe sites. Covalency of the Fe-O and Zn-O bonds is important. The physical origin of the regions exhibiting SRO, however, remains unresolved at this point.

Magnetic behavior of YFexAl12−x

Abstract Previous μSR studies on the RFe6Al6 (R=Tb, Ho, Er) ferrimagnets (T N ≈340 K ) showed effects of frustration due to competing exchange between the R and Fe sublattices. This puts the compounds on the borderline between spin glass and long-range order. In continuation of this work, μSR and 57 Fe Mossbauer spectroscopy were carried out on YFe6Al6 and YFe7Al5. In these compounds, no moments exist on the normally strongly magnetic R sublattice. Neutron diffraction was unable to detect magnetic Bragg peaks, but the μSR and Mossbauer spectra clearly reveal a ferrimagnetic transition near 340 K in both materials. Not all of the Fe ions order at this temperature. The Fe ions on the Fe sublattice (8f) order only below ∼70 K . Since competing exchange is absent, the Fe sublattices possess inherent frustration reflected in distributions of moment size and orientation, short correlation lengths and strong spin fluctuations.

The Magnetism of Frustrated RFe6Al6 Compounds Studied by μSR and Mössbauer Spectroscopy

AbstractμSR studies on the RFe6Al6 (R = Tb, Ho, Er) ferrimagnets (TN≈340 K) show that the presence of frustration by competing exchange causes a slow approach of the ordered spins towards their static limit. This explains the non-Brillouin-like behavior of the magnetic Bragg peak intensities seen in neutron diffraction. Furthermore, considerable local spin disorder is found, despite the existence of long-range order. In YFe6Al6, having no moments on the normally strongly-magnetic R sublattice, neutron diffraction was unable to detect any magnetic Bragg peaks. The μSR and Mössbauer spectra however, clearly reveal a magnetic transition near 340 K, but not all of the Fe ions order at this point. The Fe ions on the mixed Fe-Al sublattice (8j) order first, while the pure Fe sublattice (8f) orders only below ∼70 K. The Fe ions on this site must be inherently frustrated which is also reflected in distributions of moment size and orientation, short correlation lengths and strong spin fluctuations.

Mössbauer effect and magnetization studies of mixed valence solid solution NpOH·NpH2−x

Abstract By dissolving Np metal in a mixed solution of NaCl, MgCl2 and Na2SO4 in water, a new compound NpOH precipitates together with NpH2−x forming a solid solution (double salt) NpOH·NpH2−x. 237Np Mossbauer spectroscopy, d.c. magnetization and X-ray studies of the precipitate are reported. NpOH crystallizes in the CaF2 (Fm3m) structure with a lattice parameter of 5.375 A. The Mossbauer absorption spectrum of NpOH is very well resolved from that of NpH2−x (single absorption line) owing to their large difference in Mossbauer isomer shift (IS; 28.1 mm s−1). The Mossbauer studies indicate that the Np ion in NpOH is tetravalent whereas in NpH2−x it is trivalent. Both behave paramagnetically down to 2.5 K; however, NpOH exhibits paramagnetic spin relaxation. The Mossbauer spectra can be fitted within a relaxation model assuming an isolated Kramers doublet and, even better, assuming a quartet ground state (T8(2) of 4 I 9 2 in cubic symmetry). The saturation hyperfine field derived is 350(50) T. NpH2−x in the solid solution exhibits a positive IS relative to NpH2.0 as a result of volume contraction. The IS of the neptunium dihydride system depends on the hydrogen concentration. From this result we derive for NpH2−x stabilized in the solid solution the value x≈0.4.