ALBERT

Notes: The result of an elastic neutron-scattering study of the phase transition in the random magnet with competing anisotropies Fe0.75Co0.25Br2 is reported. It was found that the ordering of the spin component parallel to the crystalline c axis at the transition from the paramagnetic to antiferromagnetic phase induces an ordering of the spin component perpendicular to it. Thus, there is no boundary separating the higher-temperature antiferromagnetic and lower-temperature mixed-ordering phase. This type of spin ordering is interpreted as arising from a coupling between the orthogonal spin components.

Notes: We have performed inelastic neutron scattering study of the cerium γ↔α valence transition (T0(approximately-equal-to)150 K) in polycrystalline Ce0.74Th0.26 at the Intense Pulsed Neutron Source (IPNS) of Argonne National Laboratory. An incident neutron energy of 300 meV was used to measure the excitation energy spectra of Ce0.74Th0.26 at 100, 140, 155, and 200 K by chopper spectrometers. By comparing the neutron total scattering of Ce0.74Th0.26 with that of La0.74Th0.26, an isostructural nonmagnetic alloy measured under idential experimental conditions, the magnetic scattering function of Ce0.74Th0.26 averaged over all q in the Brillouin zone, Savemag (Q,E), is derived. We find that the magnetic response of Ce0.74Th0.26 at all temperatures is predominately due to moments of 4f character. The obtained magnetic scattering function in this temperature region consists of a broad quasielastic peak, which is well fitted by a spin relaxational spectral function, namely, a Lorentzian centered at 0 energy. The peak shifts to higher energies and broadens as the temperature is lowered. As the temperature decreases across the transition temperature, the magnetic intensity drops sharply, accompanied by an abrupt broadening of the linewidth corresponding to a spin fluctuation energy much higher than the thermal energy. Γ, the Lorentzian HWHM's, were found to be 16, 25, 63, and 110 meV at T=200, 155, 140, and 100 K, respectively. Within experimental precision, we find no evidence of additional inelastic peaks due to crystal-field excitations. These results are in good agreement with those from an earlier neutron experiment1 using thermal-energy neutrons in which the measured spectra were limited to about 70 meV. The static single-site susceptibility obtained by a Kramers–Kronig analysis agrees well with the bulk susceptibility.1 The 4f occupation per Ce atom, deduced by summing theneutron data from −100 to 230 meV, is about 0.6 and 0.4 for the γ and α phase, respectively. These values are considerably smaller than those obtained from photoemission2 and other measurements.1 This indicates that the neutron experiment did not extend to high enough energies to account for all the intensity. In the high-temperature γ phase the relaxational model represents a reasonable approximation to the spin dynamics. By integrating the Lorentzian scattering function obtained from the fits of the neutron data over an energy interval of −0.1 to 2 eV, we obtained an f occupancy close to unity in the γ phase. For the α phase there is currently no analytical expression of the magnetic scattering function from first-principle calculations. Model calculations3,4 of the ground state (T=0) for a single f impurity in the metal, on the other hand, predicts a magnetic response function having a thresholdlike rise at a finite energy, followed by a long tail extending to high energies. In order to be able to compare with the theory, we have also undertaken the measurements of the magnetic scattering function of Ce0.74Th0.26 at 10 K with an incident neutron energy of 1.2 eV. We find that Savemag shows an inelastic peak at about 138 meV and a tail at higher energies. The line shape of the measured spectrum agrees qualitatively with the single-impurity theory3,5,6 and the result of a recent polarized neutron study7 of α-Ce. By summing the measured magnetic intenstiy up to 500 meV, we estimated a 4f occupancy of 0.76 per Ce atom. The static susceptibility at 10 K and the electronic specific heat coefficient obtained from the neutron data and the theory3,8 agree well with the values obtained from bulk measurements.1,9,10 The above results are also in fair agreement with the electronic spectroscopy data.2 Since the neutron measurements were made using polycrystalline samples, we are unable to detect, if any, coherence effects6,11 due to interaction of the f electrons in the lattice. A detailed report on the inelastic neutron scattering investigations is presented elsewhere.12

Notes: Neutron-spin-echo (NSE) experiments were performed on polycrystalline samples of EuxSr1−xS for x=0.4 and 0.54 in the temperature range of 1.2〈T〈10 K and for 0.036〈Q〈0.18 A(ring)−1. The x=0.4 sample exhibits a paramagnetic (PM) to spin-glass (SG) transition near Tg ∼2 K. In the x=0.54 sample, large ferromagnetic correlations develop below 5 K and a SG state appears at lower temperatures. In the NSE experiment, the spin-spin correlation function S(Q,t) is measured directly for times between 0.03〈t〈5 nsec. At low temperatures, both materials exhibit a weak Q dependence in the dynamics and the spins are essentially frozen over the time range explored. On heating the x=0.4 sample, the spins start to fluctuate more rapidly, but no dramatic change occurs around Tg. On heating the x=0.54 sample, S(Q,t) decreases rapidly with time. Near 5 K, S(Q,t) is exponential (e−Γt) with Γ being strongly Q dependent. Measurements of the depolarization of the scattered beam confirms the absence of true long-range ferromagnetic order below Tc.

Notes: We report an inelastic neutron scattering study of the uniaxial dilute antiferromagnets Fe1−xMgxCl2, for x=0.3, 0.45, and 0.6. The first two samples have long-range antiferromagnetic (AF) order. Their spin-wave peaks are broadened by the exchange fluctuations. The x=0.6 sample orders like an Ising spin glass with a very short AF correlation length (ξ≈10 A(ring)). For wavelength λ(approximately-less-than)ξ, spin wave peaks are similar to the dilute antiferromagnets, but for λ(approximately-greater-than)2ξ, the peak sharpens up and becomes resolution limited at the zone center. We give a simple explanation for this behavior.

Notes: Neutron scattering experiments have been performed on a spin-glass CuMn (5 at. %, Tg =27.6 K). Polarized neutron measurements with coarse energy resolution show no change of instantaneous spatial spin correlation below T=60 K. Unpolarized neutron studies with fine energy resolution (ΔE=260 μeV) demonstrate that the slowing down of Mn spin fluctuations occur in the same way for all the wave vectors between 0.2 and 4.0 A(ring)−1. These results indicate that the spatial and dynamic spin correlations are completely decoupled in the spin freezing process. We also study the short-range spatial correlation of the "frozen'' spins at T=5 K using a simple model, and demonstrate the importance of the ferromagnetic coupling between the third nearest-neighbor Mn moments.

Notes: Fe1−xMgxCl2 is a diluted Ising antiferromagnet with competing first- and second-neighbor exchange interactions. For x≈45, the system undergoes a Néel transition at TN ≈7.4 K and a reentrant spin-glass transition at Tsg ≈3.0 K. From neutron scattering experiments we find that (i) the antiferromagnetic Bragg peak persists down to 1.2 K, well below Tsg; and (ii) the diffuse scattering becomes temperature independent below about 6.0 K, with the correlation length frozen at about 2.8 lattice spacings. These results suggest that long-range antiferromagnetic order and spin-glass-like short-range order coexist. Such a behavior is predicted by the infinite-range model for spin glasses with strong uniaxial anisotropy.

Notes: Abstract Stage-2 CoCl2-GIC approximates a two-dimensional easy-plane (XY) ferromagnet on a triangular lattice. It has been found in prior work to order in two steps, with the intermediate phase showing long-ranged ferromagnetic correlations within the intercalate plane, but no correlations between neighboring planes. We have probed the wave vector and temperature dependence of the static and dynamic spin correlations in detail, including measurements of the critical scattering, the quasielastic scattering from vortex diffusion and the spin wave excitations with and without an external magnetic field. Some of the predictions for a Kosterlitz-Thouless type transition are met in this compound, at least qualitatively, including an apparent jump in the spin stiffness at the critical point and the existence of a diffusive central peak in the scattering function possibly originating from vortex autocorrelations. However, there are some inconsistencies between our observations and recent analytical studies as well as Monte Carlo-molecular dynamics simulations of the vortex dynamics that prevent unambigous assignment of the upper critical temperature as a vortex-binding transition.

Notes: Abstract Short-range order effects in amorphous polycondensates, including the technologically important bisphenol-A-polycarbonate, have been investigated by elastic diffuse neutron scattering with spin polarization analysis. Selectively deuterated samples of each polycondensate have been used in order to vary the scattering contrast and thereby emphasize different pair correlations. The technique of spin polarization analysis allowed a reliable separation of the coherent scattering and an intensity calibration on the basis of the incoherent scattering as an internal standard. Thus, (dσ/dΩ)coh has been measured directly by this method. The experimental results are compared to calculated cross-sections from computer-generated structures. Simulations have been performed with the “amorphous cell” method which models the static structure of the amorphous polymer in full chemical detail on the basis of a “random coil” conformation. The results of the simulations yield a fertily ground for the discussion of the measured cross-sections, though a direct comparison with the experiment is not always satisfactory. The observed discrepancies indicate a still insufficient structural relaxation of the simulated structures.

Notes: Abstract Inelastic neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system Sr14Cu24O41, which has both simple chains and two-leg ladders of copper ions. We have observed that both the chain and the ladder exhibit a spin gap, which originate from a dimerized state.