Mössbauer and magnetic studies of nanosize (Fe,Co) x C1 − x alloys

We report on preliminary results from a systematic study of the hyperfine (HF) structure of antiprotonic helium. This precise measurement which was commenced in 2006, has now been completed. Our initial analysis shows no apparent density or power dependence and therefore the results can be averaged. The statistical error of the individual lines is a factor of 60 smaller than that of three body quantum electrodynamic (QED) calculations, while the difference has been resolved to a precision comparable to theory (a factor of 10 better than our first measurement). Agreement between theory and experiment would lead to an increased precision of the measurement for the antiproton magnetic moment and provide a test of CPT invariance.     

Mössbauer spectroscopic studies of the disintegration of hexavalent iron compounds (BaFeO4 and K2FeO4)

Mössbauer spectroscopic studies of BaFeO₄ and K₂FeO₄ as prepared, then either sealed, or exposed to air, or exposed to moist air for a period up to more than one year, were performed at room temperature as a function of time. Some of the samples were studied as a function of temperature down to 4.2 K. K₂FeO₄ and BaFeO₄ after preparation, exhibit a pure Fe⁶⁺ spectrum. K₂FeO₄ shows low stability. After a period of 14 months in a sealed sample holder, the spectrum exhibits 83% noncrystalline Fe³⁺, as Fe₂O₃ nanoparticles, and only 17% of the original Fe⁶⁺. BaFeO₄ sealed, or exposed to dry air disintegrates slowly, exhibiting a spectrum composed of three subspectra. In addition to the original Fe⁶⁺ and final Fe³⁺ subspectra, a subspectrum, of an intermediate stage of a crystalline Fe⁴⁺ system, is present. In the first month the increase of the Fe³⁺ subspectrum is 15%, and that of the Fe⁴⁺ is 8%. BaFeO₄ exposed to moist air, disintegrates at a very fast rate. The Fe³⁺ subspectrum, due to Fe₂O₃ nanoparticles, increases in the first days at the rapid rate of ∼13%/day, and there is no evidence for Fe⁴⁺ in the spectrum. The Fe⁶⁺ in BaFeO₄, Fe³⁺ and Fe⁴⁺ in the disintegrated systems are all magnetically ordered at 4.2 K. Above 90 K the Fe³⁺ subspectra exhibit a superposition of a paramagnetic doublet and a diffuse magnetic sextet, with relative intensities changing with temperature, and changing from sample to sample according to their blocking temperatures, which are determined by the distribution in size of the nanoparticles.     

Mean-Square Amplitudes of Vibration and Anisotropic Motion of Metal Atoms in Iron Organometallics

Hyperfine Interactions - Inelastic nuclear resonant scattering spectra of 57Fe atoms were measured on Pt3 57Fe and Pd3 57Fe with the L12 crystal structure, and with the same crystallographic sites...     

Exploring metal atom dynamics in organometallics: methodology and results for C60 ferrocenes

Two new bucky-ferrocene compounds, incorporating two η⁵-CpFe ligands in the C₆₀ framework, have been examined by temperature-dependent ⁵⁷Fe Mössbauer spectroscopy over the temperature range 86<T<350 K. The root-mean-square-amplitude-of-vibration of the iron atom have been determined and compared to those observed for related mono-iron bucky ferrocenes, as well as for the parent ferrocene complex.     

Ferrimagnetism and hyperfine interactions in RFe5Al7(R = rare earth)

Magnetization and Mössbauer (⁵⁷Fe, ¹⁵⁵Gd) studies of RFe₅Al₇(R = Y, Sm to Lu, ThMn₁₂ crystal structure) in magnetic fields up to 50 kOe and temperatures 4.1 to 500 K have been performed. The Mössbauer studies yield the distribution of the iron ions among the various inequivalent crystallographic and magnetic sites, the hyperfine fields and their temperature dependence. The magnetization curves display a great variety of unusual magnetic phenomena. Among those; strong anisotropy, magnetic and thermal hysteresis (H_c = 24 kOe for DyFe₅Al₇ at 4.1 K), negative magnetization at low temperatures when the sample is cooled in a magnetic field (even in 1 Oe), compensation points, maxima points and time-dependent magnetization. Most of the phenomena observed can be explained in terms of a spin structure previously suggested for the RFe₆Al₆ compounds, composed of 4 magnetic sublattices. The rare earth moments lie antiparallel to the iron moments in the (j) site and to the ferromagnetic component of a canted antiferromagnetic structure of iron in the (f) site. Iron in the (i) site is nonmagnetic.     

Exchange and hyperfine fields acting on dilute Eu In SmCo5, Sm2(Co1-xFex)17 and Sm2CoxMy( rm M = Fe, Cu, Zn, Zr)

Mössbauer source experiments of dilute ¹⁵³Eu and ⁵⁷Fe in SmCo₅ and ¹⁵³Eu in Sm₂(Co_1-xFe_x)₁₇ and Sm₂ Co_x M_y at 4.1 K were performed. After the ${}^{153}\text{Sm}\text{→β?}{}^{153}$ decay the Eu ion is trivalent and exhibits extremely large hyperfine interactions due to strong exchange fields. Since the exchange interactions are comparable to the Eu³⁺ spin-orbit coupling, we calculated the expectation values of the Eu³⁺ spin, magnetic hyperfine field and electric field gradient as a function of exchange field and second order crystalline field, by diagonalization of the full Hamiltonian of spin orbit, exchange and crystalline field. For SmCo₅ and Sm₂(Co_1-xFe_x)₁₇ the exchange and crystalline fields are known and thus allow us to analyze our experimental results and obtain the polarized conduction electron contributions to the magnetic hyperfine field. The contribution due to magnetic neighbour polarization of conduction electrons is found to be linear in exchange field. The experimental results together with the theoretical analysis yield the Eu electric field gradient 4f Sternheimer shielding factor R_Q to be 0.26±04. It is shown that Mössbauer studies of two probes (¹⁵⁵Gd and Eu³⁺) in magnetic systems yield directly the second order crystalline field, the exchange field and the various contributions to the hyperfine field acting on the Eu nucleus. From the experimentally measured magnetic hyperfine fields alone, approximate values for the exchange fields in the mixed systems Sm₂Co_xM_y were determined.     

High-Temperature Spectroscopy of Organometallics: The Gol'danskii–Karyagin Effect in Ferrocene Complexes

The temperature dependence of the mean-square-amplitude of vibration and the motional anisotropy of the metal atom in a number of structurally related organometallics has been examined in detail and can be related to the molecular level architecture of these solids. In particular, it is noted that in octamethylferrocenylmethanol, there is a competition between the “gear-wheel effect” observed in related octamethylferrocenyl complexes and hydrogen bonding between neighboring molecules in the solid below the melting point. This revised version was published online in August 2006 with corrections to the Cover Date.     

Anisotropic transferred hyperfine fields acting on 119Sn in gadolinium metal and alloys

Mossbauer studies of 0.1% ¹¹⁹Sn in gadolinium alloys at 4.1 K were performed. The spectra were well reproduced by a superposition of hyperfine fields according to the RKKY model with k_F = 1.27 ± 0.04 A^-1. In Gd_0.95Tm_0.05 where the magnetization is along the c-axis, the transferred field per Gd ion is 4 ± 1% lower than in Gd_0.97Tb_0.03 where the magnetization is perpendicular to the caxis.     

Magnetostrictive effects at a spin reorientation transition in a cubic crystal

Mössbauer studies of spin reorientation transitions in the high magnetostrictive materials RFe₂(R = rareearth) were performed. The transitions studied were [100] → [110] (Ho_0.9Tb_0.1Fe₂ at 240 K), [100] → [111] (Dy_0.7Tb_0.3Fe₂ at 220 K), [110] → [100] (Ho_0.5Er_0.5Fe₂ at 135 K and Ho_0.3Tm_0.7Fe₂ at 110 K), [111] → [100] (Dy_0.2Er_0.8Fe₂ at 140 K) and [110] → [111] (Ho_0.7Tb_0.3Fe₂ at 120 K). The first four transitions were found to be of second order, continuous reorientation of magnetization. In Dy_0.2Er_0.8Fe₂ and Ho_0.7Tb_0.3Fe₂ a sharp discontinuous first order transition is observed. In all systems the Mössbauer recoil free fraction, its anisotropy, the isomer shift and hyperfine interaction parameters change sharply through the spin reorientation transition. All phenomena observed can be understood in terms of changes in static magnetostrictive distortions combined with critical phonon softening at the spin reorientation phase transitions. The system Ho_1-xTb_xFe₂ is unique in behaviour as Ho_0.9Tb_0.1Fe₂ shows the highest magnetostrictive effects, whereas Ho_0.7 Tb_0.3Fe₂ shows almost none.     

Self-assembled dendrimers with uniform structure

Calix[4]arenes substituted at their wide rim by four aryl urea residues (1) form hydrogen-bonded dimers in apolar solvents. Replacement of one urea residue by an acetamido moiety leads to calix[4]arene derivatives (5) which form hydrogen-bonded tetramers under the same conditions. Both self-assembly processes occur independently. Therefore, molecules have been prepared in which a tetra-urea calix[4]arene and a tri-urea mono acetamide derivative are covalently connected between their narrow rims by a long, mainly aliphatic chain [-O-(CH(2))(n)-C(O)-NH-(CH(2))(m)-O-] (7). In the presence of an equimolar amount of tetra-tosyl urea calix[4]arene () they form dendritic assemblies since the well known heterodimerization of tetra-tosyl and tetra-aryl urea calix[4]arenes prevents the formation of a cross-linked structure. Covalent connection of adjacent urea residues leads to tetra-loop derivatives (3) that cannot form homodimers, but instead form heterodimers with tetra-aryl or tetra-tosylureas. Therefore, similar dendrimers should be available using the selective dimerization observed for 3. The formation of a single, structurally uniform dendrimer from eight building blocks is confirmed by (1)H NMR spectra, showing only peaks that are also found for respective model assemblies. Translational diffusion coefficients of the assemblies have been determined using (1)H DOSY NMR.