Modification of magnetic anisotropy in metallic glasses using high-energy ion beam irradiation

Heavy ion irradiation in the electronic stopping power region induces macroscopic dimensional change in metallic glasses and introduces magnetic anisotropy in some magnetic materials. The present work is on the irradiation study of ferromagnetic metallic glasses, where both dimensional change and modification of magnetic anisotropy are expected. Magnetic anisotropy was measured using Mössbauer spectroscopy of virgin and irradiated Fe₄₀Ni₄₀B₂₀ and Fe₄₀Ni₃₈Mo₄B₁₈ metallic glass ribbons. 90 MeV ¹²⁷I beam was used for the irradiations. Irradiation doses were 5×10¹³ and 7.5×10¹³ ions/cm². The relative intensity ratios D ₂₃ of the second and third lines of the Mössbauer spectra were measured to determine the magnetic anisotropy. The virgin samples of both the materials display in-plane magnetic anisotropy, i.e., the spins are oriented parallel to the ribbon plane. Irradiation is found to cause reduction in magnetic anisotropy. Near-complete randomization of magnetic moments is observed at high irradiation doses. Correlation is found between the residual stresses introduced by ion irradiation and the change in magnetic anisotropy.     

Room-temperature laser action at 4.34.4 mum in CaGa(2)S(4):Dy(3+)

We report room-temperature mid-IR laser operation in a new low-phonon-frequency nonhygroscopic host crystal, calcium thiogallate (CaGa(2)S(4)) . Laser action at 4.314.38 mum on the Dy(3+)H(11/2)(6)?H(13/2)(6) transition occurred with a maximum slope efficiency of 1.6%.     

PHOSPHORESCENCE QUENCHING OF POLY rA IN GLUCOSE-SODIUM ACETATE SOLUTION AT 77 K*

Abstract. The phosphorescence of poly rA at 77 K in an aqueous medium containing glucose is half-quenched by bound ethidium bromide at a fractional concentration, r_1/2, of 0.005, and by bound proflavine at r_1/2= 0.002: r_1/2= 0.024 for Co²⁺ and r_1/2= 0.039 for Mn²⁺. The decay of the dye-quenched poly rA phosphorescence is markedly nonexponential and decays more rapidly with increasing dye concentration, while the decay of the metal-ion quenched poly rA phosphorescence parallels that of the unquenched poly rA, independent of metal-ion concentration. Förster overlap integrals and critical distances for transfer of both poly rA singlet and triplet excitation to dye singlet states are calculated, and used to explain a consistent interpretation of the experimental results in terms of one-step direct excitation transfer from base to dye or metal ion in a highly folded polymer conformation.     

Synthetic magnetic opals

We present studies of novel nanocomposites of BiNi impregnated into the structure of opals as well as inverse opals. Atomic force microscopy and high resolution elemental analyses show a highly ordered structure and uniform distribution of the BiNi filler in the matrix. These BiNi-based nanocomposites are found to exhibit distinct ferromagnetic-like ordering with transition temperature of about 675 K. As far as we know there exists no report in literature on any BiNi compound which is magnetic.     

Magnetization curves for general cylindrical samples in a transverse field

Using the recent results for the surface current density on cylindrical surfaces of arbitrary cross-section producing uniform interior magnetic field we propose a method for obtaining solutions of Bean’s critical state model for general cylindrical samples. The method uses the technique of conformal mapping to express the sample surface and the flux-fronts in terms of a set of coefficients that depend on a parameter. The flux-fronts are to be determined by solving a system of nonlinear ordinary differential equations for the coefficients. Retaining only a certain finite number of leading coefficients we get an approximate solution. The procedure is illustrated by considering two cyclindrical samples — one with an elliptical cross-section and the other with a non-elliptical cross-section. The virgin curve and small and large magnetization hysteresis loops for the two samples are obtained.     

Low-field vortex dynamics in various high-T c thin films

We present a novel ac susceptibility technique for the study of vortex creep in superconducting thin films. With this technique we study the dynamics of dilute vortices in c-axis oriented Y-123, Hg-1212, and Tl-1212 thin films, as well as a axis oriented Hg-1212 thin films. Results on the Hg-1212 and Tl-1212 thin films indicate that dislocation-mediated plastic flux creep of single vortices dominates at low temperatures and fields. As the temperature (or the field) is increased, the increasing vortex-vortex interactions promote a collective behavior, which can be characterized by elastic creep with a non-zero μ exponent. Also, in some of these samples effects of thermally assisted quantum creep are visible up to 45 K in some of these samples. In Y-123 thin films, creep is found to be collective down to the lowest temperatures and fields investigated, while the quantum creep persists only up to 10–11 K.     

Molecular Mechanics Investigation of an Adenine-Adenine Non-Canonical Pair Conformational Change

Conformational changes are important in RNA for binding and catalysis and understanding these changes is important for understanding how RNA functions. Computational techniques using all-atom molecular models can be used to characterize conformational changes in RNA. These techniques are applied to an RNA conformational change involving a single base pair within a nine base pair RNA duplex. The Adenine-Adenine (AA) non-canonical pair in the sequence 5'GGUGAAGGCU3' paired with 3'PCCGAAGCCG5', where P is Purine, undergoes conformational exchange between two conformations on the timescale of tens of microseconds, as demonstrated in a previous NMR solution structure [Chen, G., et al., Biochemistry, 2006. 45: 6889-903]. The more populated, major, conformation was estimated to be 0.5 to 1.3 kcal/mol more stable at 30 °C than the less populated, minor, conformation. Both conformations are trans-Hoogsteen/sugar edge pairs, where the interacting edges on the adenines change with the conformational change. Targeted Molecular Dynamics (TMD) and Nudged Elastic Band (NEB) were used to model the pathway between the major and minor conformations using the AMBER software package. The adenines were predicted to change conformation via intermediates in which they are stacked as opposed to hydrogen-bonded. The predicted pathways can be described by an improper dihedral angle reaction coordinate. Umbrella sampling along the reaction coordinate was performed to model the free energy profile for the conformational change using a total of 1800 ns of sampling. Although the barrier height between the major and minor conformations was reasonable, the free energy difference between the major and minor conformations was the opposite of that expected based on the NMR experiments. Variations in the force field applied did not improve the misrepresentation of the free energies of the major and minor conformations. As an alternative, the Molecular Mechanics Poisson-Boltzmann Surface Area (MMPBSA) approximation was applied to predict free energy differences between the two conformations using a total of 800 ns of sampling. MM-PBSA also incorrectly predicted the major conformation to be higher in free energy than the minor conformation.