Mössbauer effect and neutron diffraction measurements have been made as a function of composition for YBa2(Cu1−xFex)3O7+δ. A chemically driven disordering of the sublattices is observed with increasing Fe concentration which results in a transition from the orthorhombic to a tetragonal phase. However, as the occupancy of the oxygen site 0(1) in the chain is depleted and the site of the ordered vacancy 0(5) between the chains becomes occupied, the total oxygen content is not reduced but remains near seven in contrast to behavior observed in the quenched tetragonal phases. Mössbauer spectra for x =0.05 at temperatures below 15K show magnetic hyperfine splitting which is characteristic of a spin glass; i.e., a distribution of magnetic fields. It is inferred from Mössbauer results that Fe preferentially occupies the Cu(1) site.

The hydrogen rich ternary hydride phases of RFe2Hx (R = Y, Dy, Er) with x ≥ 4 have been investigated using 57Fe, 161Dy and 166Er Mössbauer spectroscopy and bulk magnetization. In all cases, the presence of hydrogen results in a dramatic reduction in the moment on Fe as well as the magnetic transition temperature. The data also imply a simultaneous weakening of the R-Fe magnetic exchange.

Research on polycrystalline RRh4B4 samples has shown that crystalline electric field (CEF) effects play an important role in these compounds. The successful synthesis of single crystal samples of RRh4B4 with R = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu has provided an opportunity to further investigate CEF effects in these materials. Magnetization and magnetic susceptibility measurements on the RRh4B4 single crystals revealed strong magnetic anisotropy, and the experimental results could be described well by CEF calculations based on the parameters derived from an analysis of experimental data for ErRh4B4 single crystals. The easy directions of magnetization of these compounds are consistent with the signs of the Stevens factor alpha J of the CEF Hamiltonian. A strong influence of magnetic anisotropy on superconductivity was also observed.

Experience over the last 15 years has shown that pulsed neutron spectrometers are able to make a unique contribution to the field of magnetic inelastic scattering. Pulsed neutron spectrometers are characterized by high resolution and wide dynamic range both of which are necessary in order to characterize the magnetic response of the complex systems of current interest, ranging from rare earth-transition metal permanent magnets to quantum critical scatterers. However, all these studies have been constrained by current flux limitations. The development of more powerful spallation neutron sources, such as the JHP, is likely to transform these investigations from interesting demonstrations of the potential of pulsed neutron scattering into routine tools for the study of magnetic correlations. .

Systematics of hyperfine parameters from 237Np Mössbauer resonance data of compounds with Np in a high formal charge state (Np6+,Np7+) are discussed with respect to electronic structure properties. In neptunyl(VI) species, we find a linear correlation between the isomer shift and the strength of quadrupole interaction. Both scale linearly with the actinide–oxygen bond length, stressing the central role of this parameter. Some compounds show paramagnetic relaxation spectra which makes their analysis difficult. The hyperfine interactions are often not rotational symmetric indicating a deviation from the simple linear O–Np–O configuration. Mössbauer spectra of NpO3·2H2O reveal that this compound should be described as a neptunyl. A comparison of hyperfine parameter systematics indicates that the Np valence electron properties in Np(VII) species are basically similar to those in Np(VI) neptunyls.

Heat capacity, resistivity, and magnetic susceptibility data have been obtained for the compounds REBa2Cu,0,_,, where RE = Dy, Ho or Er. Neutron diffraction data on the Ho compound show a structure identical to that of YBa,Cu,O,_,. Magnetic transitions are observed at T, = 0.95, 0.17 and 0.59 K for Dy, Ho and Er compounds, respectively. It is argued that these are due predominantly to dipolar interactions. Resistivity data show that the magnetic state is coexistent with superconductivity in all cases. From the heat capacity data, the degeneracies of the crystal field ground states are determined, and estimates are given for the magnetic moment in the ground state and the energy separation to the first excited crystal field state.

Two samples with the same nominal composition (La0.83Sr0.17Mn0.98Fe0.02O3)-Fe-57 were prepared in air and argon. Thermograviametric, structural, resistive, and magnetic measurements show that the air-made sample has slightly higher Mn4+/Mn3+ ratio; i.e., it is slightly more highly doped by similar to -0.15% cation vacancies. Substitution of Fe for Mn depresses the ferromagnetic transition temperature T-c but has very small effect on the rhombohedral/orthorhombic structural phase boundary. Dramatic differences are found in transport properties for small differences in Mn4+ concentration near a structural and magnetic phase boundary. The dynamics of charge hopping and exchange via the effect of relaxation of the magnetic hyperfine field distribution and its average value are monitored by temperature-dependent Mossbauer effect measurements. Ferromagnetic clustering is observed above T-c at Fe-57 sites. The compound undergoes complex temperature dependent magnetic and phase behaviors that are monitored locally by the Fe-57 Mossbauer effect. The iron probe has a stable 3+ valance configuration whose hyperfine interaction reflects the structural phase changes, the Jahn-Teller distortion and the time-dependent Mn3+-Mn4+ charge fluctuations. It is shown that regions of magnetic order exist over similar to 100 K range beginning well above T-c. Moreover, the low-temperature structural phase. reported to be orthorhombic at this composition, shows dynamic magnetic fluctuation behavior that motionally narrows at low temperature. Any changes in hyperfine interaction due to the structural transition are masked by the fluctuating magnetic behavior, but clearly the two structures coexist in the temperature region near T-c.

Two samples with the same nominal composition La{sub 0.83}Sr{sub 0.17}Mn{sub 0.98}{sup 57}Fe{sub 0.02}O{sub 3} were prepared in air and argon. Thermograviametric, structural, resistive, and magnetic measurements show that the air-made sample has slightly higher Mn{sup 4+}/Mn{sup 3+} ratio; i.e., it is slightly more highly doped by {approximately}0.15{percent} cation vacancies. Substitution of Fe for Mn depresses the ferromagnetic transition temperature T{sub c} but has very small effect on the rhombohedral/orthorhombic structural phase boundary. Dramatic differences are found in transport properties for small differences in Mn{sup 4+} concentration near a structural and magnetic phase boundary. The dynamics of charge hopping and exchange via the effect of relaxation of the magnetic hyperfine field distribution and its average value are monitored by temperature-dependent Mossbauer effect measurements. Ferromagnetic clustering is observed above T{sub c} at {sup 57}Fe sites. The compound undergoes complex temperature dependent magnetic and phase behaviors that are monitored locally by the {sup 57}Fe Mossbauer effect. The iron probe has a stable 3+ valance configuration whose hyperfine interaction reflects the structural phase changes, the Jahn-Teller distortion and the time-dependent Mn{sup 3+}-Mn{sup 4+} charge fluctuations. It is shown that regions of magnetic order exist over {approximately}100 K range beginning well above T{sub c}. Moreover,more » the low-temperature structural phase, reported to be orthorhombic at this composition, shows dynamic magnetic fluctuation behavior that motionally narrows at low temperature. Any changes in hyperfine interaction due to the structural transition are masked by the fluctuating magnetic behavior, but clearly the two structures coexist in the temperature region near T{sub c}. {copyright} {ital 1998} {ital The American Physical Society}« less

The electric quadrupole and magnetic hyperfine interactions rneasured from the 161Dy Mössbauer resonance in crystalline Dy(OH)3 and from the 166Er resonance in crystalline Ho(OH)3 and Er(OH)3 are interpreted using the crystal field and molecular exchange field model . The crystal field parameters established from previous optical spectroscopy results account weIl for these hyperfine parameters. The crystal-field and magnetic properties of these ferromagnetic insulators are described weIl within the model.

Mössbauer studies on 57Fe-doped superconducting REBa2Cu3O7+δ (RE=Er, Dy) were made as a function of temperature for x=0.15 and 0.30. The magnetic behavior of the 3d dopants, which mainly occupy Cu(1) sites, undergoes antiferromagnetic ordering which is coexistent with superconductivity at low temperature. The dimensionality of the magnetic interaction changes from 2D to 3D when the rare earth changes from Er to Dy. the line-widths of the Mössbauer subspectra are characteristic of magnetic fluctuation behavior in the vicinity of a phase transition. Combining these results with those of Fe-doped Y-123 (pseudo 1D) and Gd (3D), the magnitude of the rare earth moments appears to be strongly correlated with the dimensionality of the magnetic interaction of Fe dopants in these compounds. However, the Mössbauer spectrum for 155Gd in GdBa2Cu2.85Fe0.15O7+δ (TN(Fe) ∼ 14 K) shows no magnetic order at 4.9 K.

A Mössbauer study has been made on57Fe ions substituted into the Cu(1) site of REBa2Cu3−x Fe x O7+δ (RE=Y, Er, Dy, Gd;x=0.15, 0.30). At low temperature, the iron atoms antiferromagnetically order with a transition temperature which is dependent on the Fe concentration. The temperature dependence of the magnetic subspectra representing Fe ions with various local oxygen environments in YBa2Cu3−x Fe x O7+δ and ErBa2Cu3−x Fe x O7+δ fit a 2D-Ising model with a ratio of the anisotropic exchange between the two directions on the order of 0.5–1.0(10−3) for the Y-compounds and on the order of 1 for the Er-compounds. The magnitude of the local dopant magnetization is related to a short-range chemical order which determines the magnetic chain size and defines the correlation lengths. For the Y-compound, the order is quasi-1D with strong intrachain but very weak interchain coupling. For the Er-compounds, the magnetic coupling is Ising 2D. The strong fluctuation behavior expected in low dimensional systems above and belowT N is observed via characteristic relaxation in the Mössbauer linewidth nearT N. For both the Dy- and Gd-compounds, the magnetic order is 3D. The magnitude of the rare-earth magnetic moments appears to affect the character of the magnetic interaction in the Cu(1)-site. However, a Mössbauer effect measurement at155Gd nuclei in GdBa2Cu2.85Fe0.15O7+δ (T N(Fe)∼14 K) shows paramagnetic behavior at 4.9 K.

Single phase samples of (Ba1-xKx) (Bi1-ySby)O3 have been synthesized over a limited compositional range using a two-step procedure: high-temperature firing in a low oxygen partial pressure followed by low-temperature oxygen annealing. For y > 0 samples are superconducting only in a simple cubic structure with T(c) gradually decreasing as y increases for fixed x. As in the case of potassium only (y = 0) substituted material, the highest T(c) is found in the compositional range adjacent to the structural phase transition into the lower symmetry nonsuperconducting phase. Mossbauer effect measurements indicate that Sb is close to +5 valent state with small mixed valent behavior which increases with Sb content.

Neutron and Mössbauer effect measurements have been made as a function of Fe concentration in orthorhombic ( x = 0.01, 0.02) and tetragonal ( x ⩾ 0.05), YBa 2 (Cu 1 − x Fe x ) 3 O 7 + δ . A systematic Rietveld analysis of the neutron data for all concentrations shows that Fe principally occupies a site slightly displaced ( y , y , 0) from the Cul (0, 0, 0) site in order to approach tetrahedral coordination. Neutron data for the x = 0.05 compound at 10 K show no significant difference from those obtained at 297 K. Local ordering of oxygen about an Fe substituent is conducive to placement of a second Fe atom in an adjacent site leading to local aggregation into chains of various lengths. This clustering is reflected in the character of low temperature magnetic Mössbauer spectra and in specific heat measurements. The Mössbauer spectra yield the relative changes in oxygen configurations about Fe atoms as the Fe concentration changes.