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  • 1
    Electronic Resource
    Electronic Resource
    Search for the breakdown of the spin-wave approximation in EuO (abstract) : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 7023-7023 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The spin-wave approximation in a Heisenberg ferromagnet is valid at low temperatures where the spin-wave interactions are negligible. At higher temperatures these interactions become highly nonlinear and the spin-wave approximation should break down. In a recent paper Fishman and Vignale1 predicted that this breakdown should occur near the temperature T¯=0.4zJs, where z is the number of nearest neighbors, J is the ferromagnetic coupling constant, and s is the spin of the ferromagnet. In another related paper Fishman and Liu2 have calculated that above this temperature T¯ a second pole appears in the correlation function at an energy close to 2zJs. The origin of this new mode is the strong coupling of transverse and longitudinal fluctuations that forces the spins to precess about their local mean field. When the "spin-wave'' energies are close to the precessional mode energy 2zJs a splitting of the "spin-wave'' branch is predicted. In order to test this prediction we have performed a high-resolution inelastic neutron scattering experiment to measure the spin waves in EuO, a prototypical Heisenberg ferromagnet, in the vicinity of the q-E region where this splitting should occur. Although our experiment was performed at T=20 K, which is above T¯≈12 K, our measurements failed to observe the predicted splitting of the "spin-wave'' branch. Only a single resolution-limited spin-wave branch was observed, in agreement with earlier measurements by Passell et al.3
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354050
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  • 2
    Electronic Resource
    Electronic Resource
    Recent neutron studies of high-Tc materials (invited) (abstract) : 35th annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 5391-5391 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Neutron scattering results on high-temperature superconductors have included measurements of the magnetic ordering of the Cu and rare earth ions and studies of fluctuations of the magnetic moments. The fluxoid lattice of YBa2Cu3O7−δ has also been observed using neutron diffraction techniques. The most recent studies of these topics will be reviewed with particular emphasis placed on measurements of the magnetic fluctuations for the (La1−xSrx)2CuO4 system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348042
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  • 3
    Electronic Resource
    Electronic Resource
    Neutron measurements of the vortex lattice in YBa2Cu3O7 (invited) : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 5855-5859 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Neutron diffraction has been used to measure the vortex lattice scattering for YBa2Cu3O7. A square pattern is found when the field is applied along the c axis, while a triangular pattern is found when the field is applied well away from the c axis. High-resolution measurements for the square pattern show that the vortex lattice has long-range orientational order but only short-range positional order. The temperature dependence of the penetration depth is not that expected for a superconductor with a conventional s-wave BCS-type gap. Preliminary measurements for temperatures near the irreversibility line are consistent with the occurrence of melting of the vortex lattice or glass phase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353551
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  • 4
    Electronic Resource
    Electronic Resource
    Neutron scattering studies on lanthanum cuprates (invited) (abstract) : 35th annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 4519-4519 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The results of recent inelastic neutron scattering investigations of the magnetic excitations in La2CuO4 are reviewed. Experiments were performed using a wide range of incident neutron energies. Our study of the paramagnetic phase is consistent with a picture for the magnetic dynamics which shows conventional spin waves at higher energies and an overdamped response due to spin-wave interactions at lower energies. The evolution of the magnetic dynamics with doping in the La2−x(Ba,Sr)xCuO4 system is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348344
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  • 5
    Electronic Resource
    Electronic Resource
    High-energy spin waves in bcc iron : 35th Annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 6219-6221 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dispersion relation of the spin-wave excitations in bcc iron has been studied by neutron inelastic scattering at the spallation neutron source ISIS. Magnetic intensity was followed up to 550 meV along the [100] direction. The general form of the dispersion curve is in qualitative agreement with that calculated from a spin-polarized band model, and in particular the prediction of propagating modes extending above 300 meV has been confirmed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348814
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  • 6
    Electronic Resource
    Electronic Resource
    Small-angle neutron scattering study of the flux-line lattice in a single crystal of Bi2.15Sr1.95CaCu2O8+x (invited) : The 6th joint magnetism and magnetic materials (MMM)-intermag conference (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 6784-6787 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A flux-line lattice (FLL) was observed in a single crystal of Bi2.15Sr1.95CaCu2O8+x (BSCCO) using small-angle neutron scattering methods. The sample has a superconducting transition at 85 K. The flux-line lattice is observed to melt, evidenced by the rapid disappearance of diffracted intensity as the temperature is increased above a field-dependent melting temperature. Diffracted intensity due to the vortex lattice also falls off as the applied field is increased. It is believed that this is a manifestation of the transition of the three-dimensional flux lines into two-dimensional pancake vortices. The Bragg intensity of the FLL peak is inversely proportional to the fourth power of the London penetration depth (λL). Hence, the temperature (T) dependence of the order parameter can be measured quite accurately from the intensity of the Bragg spots at different temperatures. In BSCCO with an applied field of 50 mT, the measured T dependence appears linear. The low-T behavior is of great interest for an understanding of the underlying mechanism for superconductivity in these materials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358128
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  • 7
    Electronic Resource
    Electronic Resource
    Transmission optical device to produce intense polarized neutron beams (1988)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 53 (1988), S. 648-650 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown how a device can be constructed using a stack of magnetic supermirrors to produce high-intensity polarized neutron beams with a large area. The device makes use of the fact that single-crystal Si is transparent to neutrons. A prototypical device has been constructed which provides 64% neutron efficiency with 20 min collimation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.99840
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  • 8
    Electronic Resource
    Electronic Resource
    Measurements of the magnetic excitations above Tc in iron and nickel (invited) (1985)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 57 (1985), S. 3006-3011 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High-resolution neutron scattering measurements, utilizing full polarization analysis techniques, have been carried out to explore the nature of the magnetic response above Tc in these itinerant ferromagnets. Large isotopically enriched single crystals were employed in order to obtain reliable determinations of the scattering function S(q,E). The scattering in Ni is found to evolve in a continuous fashion from the well-known spin diffusion response at small wave vectors q to a response with peaks centered at ±E at larger q, in agreement with our original work. The ratio of the observed widths of these spin waves to the spin-wave energies, as determined directly from measurements at constant q, is found to decrease with increasing ||q|| as expected. We also show that the recent measurements by Shirane and collaborators were taken with resolution which was too coarse to observe these spin-wave excitations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.335196
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  • 9
    Electronic Resource
    Electronic Resource
    Small-angle neutron scattering of (Er0.8Ho0.2)Rh4B4 (1985)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 57 (1985), S. 3110-3112 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The (Er1−xHox)Rh4B4 pseudoternary alloy system has a minimum in the phase boundary between the superconducting and ferromagnetic phases near x=0.3. This minimum has been identified as due to the competing magnetic anisotropies of Er and Ho. It has also been suggested that there could be a Lifschitz point near the minimum. Using the 30-m SANS camera at the National Center for Small-Angle Scattering Research at ORNL, we have observed a peak in the SANS pattern for (Er0.8Ho0.2)Rh4B4 at Q=0.065 A(ring)−1. This peak appears for temperatures between Tc2, measured upon cooling, and Tm, and corresponds to a modulation of the magnetic moment with a wavelength of about 100 A(ring), demonstrating that the modulated moment phase exists away from the ErRh4B4 end of the phase diagram. The wavelength of the modulation is the same as was previously observed in ErRh4B4. The fact that the wavelength of the modulation remains finite near x=0.3 appears to rule out the possibility of Lifschitz behavior near this point.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.335174
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  • 10
    Electronic Resource
    Electronic Resource
    Discrete thin-film multilayer design for X-ray and neutron supermirrors (1989)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 22 (1989), S. 35-41 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Multilayer structures, analogous to broadband optical filters, may be used to reflect X-rays or neutrons at angles larger than the total external reflection angle intrinsic to the bulk mirror material. These so-called supermirrors have generally been designed using continuum theories, which predict that the thickness of the jth layer in the multilayer should vary smoothly as j−1/4. A new approach is proposed, based on considering the contribution of each bilayer to the extinction in a given stack of bilayers, and the discrete equations governing the choice of layer thicknesses are derived and solved. In the limit of zero layer thickness, the continuum result is recovered. The design produces essentially perfect reflectivity over the entire supermirror range. An optimal technique for trading reflectivity to gain angular range also emerges naturally from the physics of the design.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889888010003
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