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[en] In the last few years, considerable advances have been made in our understanding of the properties of the light actinides. Although these are 5f transition elements formally equivalent to the lanthanide (4f) elements, these materials show a much more varied behavior due to the larger spatial extent and ionizability of the 5f electrons. A review is given of some areas of current interest, especially where hyperfine measurements have played an active role. These include studies of a variety of magnetic phenomena, systematics of isomer shift measurements, and studies of paramagnetic relaxation

[en] Theoretical development in the line shape calculation of Mossbauer resonance spectra involving spin relaxation effects is introduced. The ''eigenvalue'' procedure of calculating line shapes saves computational times by two orders of magnitude compared to traditional methods. This was illustrated by calculating the relaxation spectra of hemoglobin cyanide and a GAMMA₈ quartet electronic state. The ''white noise approximation'' involved in all the relaxation theory is discussed, and its limitations are pointed out. The theory without such an approximation is used to analyze the data for Cs₂NaYbCl₆

[en] Using the Mori formalism, a general theory is developed for the calculation of paramagnetic relaxation effects in a general crystal field doublet. For systems with extreme magnetic anisotropy, the relaxation rates show a strong angular dependence, being severely peaked in a direction perpendicular to the magnetic easy axis. Application to the RRh₄B₄ (R = rare-earth) compounds is discussed. 5 refs

[en] Until recently it has been commonly accepted that small impurities of magnetic atoms were severely detrimental to superconductivity, and that superconductivity and long-range magnetic ordering could not occur in the same materials. In known binary and pseudo-binary compounds, this is still the case. However, many recent experiments on ternary superconductors have shown that the effects of magnetism are considerably more complex. In some cases, the addition of magnetic atoms has been found to enhance superconducting properties by increasing the superconducting critical field, without significantly lowering the transition temperature. In many cases, compounds will show both superconducting and long range magnetic ordering transitions. The destruction of superconductivity by ferromagnetic ordering and the coexistence of superconductivity with antiferromagnetic ordering is now well established. Hyperfine interaction measurements have played a significant role in the investigations of these materials, including measurement of the magnitude of the exchange interaction between rare-earth spin and conduction electron spin, elucidation of the mechanism for critical field enhancement, specification of crystalline field ground states, and studies of the nature of magnetic ordering

[en] A superconductor comprised of a polycrystalline metal oxide such as YBa₂Cu₃O_7-X (where 0 < X < 0.5) exhibits superconducting properties and is capable of conducting very large current densities. By aligning the two-dimensional Cu-O layers which carry the current in the superconducting state in the a- and b-directions, i.e., within the basal plane, a high degree of crystalline axes alignment is provided between adjacent grains permitting the conduction of high current densities. The highly anisotropic diamagnetic susceptibility of the polycrystalline metal oxide material permits the use of an applied magnetic field to orient the individual crystals when in the superconducting state to substantially increase current transport between adjacent grains. In another embodiment, the anisotropic paramagnetic susceptibility of rare-earth ions substituted into the oxide material is made use of as an applied magnetic field orients the particles in a preferential direction. This latter operation can be performed with the material in the normal (non-superconducting) state. 4 figs

[en] This paper reviews recent high-field (up to 80 kOe) magnetization, nuclear-gamma-ray resonance, and neutron diffraction measurements on a number of actinide ferromagnets with the C-15 Laves phase crystal structure. NpAl₂ and NpOs₂ are an interesting contrast; NpAl₂ behaves as a localized 5f system whereas NpOs₂ exhibits properties usually associated with itinerant ferromagnets. The second part of the paper reports similar measurements on the series AnFe₂, where An = U, Np, Pu, and Am. The properties of these compounds suggest an increasing localization of 5f electrons as one proceeds from uranium to americium. The Pu ion in PuFe₂ is definitely trivalent 5f⁵, but the observation of a small negative moment on the Am site in AmFe₂ implies a partial occupancy of the Am²⁺ 5f⁷ state in this compound. This is the first indication of a mixed valence configuration in an actinide system

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[en] The electric quadrupole and magnetic hyperfine interactions measured from the ¹⁶¹Dy Moessbauer 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 well for these hyperfine parameters. The crystal-field and magnetic properties of these ferromagnetic insulators are described well within the model