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Polarization Dependence of the Spin-Density-Wave Excitations in Single-Domain Chromium

Description: A polarised neutron scattering experiment has been performed on a single-Q, single domain sample of Cr in a magnetic field of 4 T in the transverse spin-density-wave phase. It is confirmed that the longitudinal fluctuations are enhanced for energy transfers E {lt} 8 meV similarly as in the longitudinal spin-density-wave phase. The spin wave modes with deltaS parallel and perpendicular to Q are isotropic within the E-range investigated.
Date: December 31, 1997
Creator: Boeni, P.; Sternlieb, B.J.; Shirane, G.; Roessli, B.; Werner, S.A. & Lorenzo, J.E.
Partner: UNT Libraries Government Documents Department

Spin--wave spectrum of an amorphous ferromagnet

Description: The spin-wave spectruin of an amorphous Heisenberg ferromagnet is calculated by a diagrammatic expansion making use of a transformation due to Taylor and Wu Phys. Rev., B2: 1752 (1970). The upper limit of the spectrum is found to occur at frequencies below that of the corresponding crystalline system, while the low-frequency part of the spectrum is enhanced. Internal van Hove singularities are absent in the spin-wave spectrum of the amorphous ferromagnet. (auth)
Date: January 1, 1973
Creator: Gubernatis, J.E. & Taylor, P.L.
Partner: UNT Libraries Government Documents Department

Helical spin-density wave in Fe/Cr trilayers with perfect interfaces

Description: Despite the presence of only collinear, commensurate (C) and incommensurate (I) spin-density waves (SDW`s) in bulk Cr, the interfacial steps in Fe/Cr multilayers are now believed to stabilize a helical (H) SDW within the Cr spacer. Yet H SDW`s were first predicted in an Fe/Cr trilayer with perfect interfaces when the orientation of the Fe moments does not favor C ordering: if the number of Cr monolayers is even (odd) and the Fe moments are pointing in the same (opposite) direction, then a C SDW does not gain any coupling energy. Under these circumstances, a simple model verifies that H ordering is indeed favored over 1 ordering provided that the Fermi surface mismatch is sufficiently small or the temperature sufficiently high.
Date: July 1, 1998
Creator: Fishman, R.S.
Partner: UNT Libraries Government Documents Department

Excitations of the transversely polarized spin density waves in chromium

Description: Inelastic neutron scattering measurements across the TSDW satellites of chromium were performed at {Delta}E=5,20meV, both under zero field and 5T magnetic field. Analysis concerning the transverse magnetic excitations (T{sub 1},T{sub 2}) and the longitudinal magnetic excitations (L) indicates that T{sub 1} and L are equally intense while T{sub 2} dominates and becomes more so at higher energies.
Date: July 15, 1997
Creator: Lee, W.T.; Werner, S.A.; Fernandez-Baca, J.A. & Fishman, R.S.
Partner: UNT Libraries Government Documents Department

Spin waves in CsVBr{sub 3}

Description: Inelastic neutron scattering has been used to measure spin wave excitations in the quasi-one dimensional S = 3/2 magnetic material CsVBr{sub 3}. Dispersion relations were determined using standard triple-axis methods. Fits to linear spin wave theory yield model Hamiltonian parameters describing magnetic interactions in the system.
Date: July 10, 1997
Creator: Nagler, S.E.; Mandrus, D.G. & Tennant, D.A.
Partner: UNT Libraries Government Documents Department

Magnetization rotation or generation of incoherent spin waves? Suggestions for a spin-transfer effect experiment.

Description: ''Spin-transfer'' torque is created when electric current is passed through metallic ferromagnets and may have interesting applications in spintronics. So far it was experimentally studied in ''collinear'' geometries, where it is difficult to predict whether magnetization will coherently rotate or spin-waves will be generated. Here we propose an easy modification of existing experiment in which the spin-polarization of incoming current will no longer be collinear with magnetization and recalculate the switching behavior of the device. We expect that a better agreement with the magnetization rotation theory will be achieved. That can be an important step in reconciling alternative points of view on the effect of spin-transfer torque.
Date: June 20, 2002
Creator: Bazaliy, Y. B. & Jones, B. A.
Partner: UNT Libraries Government Documents Department

Finite temperature spin-dynamics and phase transitions in spin-orbital models

Description: We study finite temperature properties of a generic spin-orbital model relevant to transition metal compounds, having coupled quantum Heisenberg-spin and Ising-orbital degrees of freedom. The model system undergoes a phase transition, consistent with that of a 2D Ising model, to an orbitally ordered state at a temperature set by short-range magnetic order. At low temperatures the orbital degrees of freedom freeze-out and the model maps onto a quantum Heisenberg model. The onset of orbital excitations causes a rapid scrambling of the spin spectral weight away from coherent spin-waves, which leads to a sharp increase in uniform magnetic susceptibility just below the phase transition, reminiscent of the observed behavior in the Fe-pnictide materials.
Date: April 29, 2010
Creator: Chen, C.-C.
Partner: UNT Libraries Government Documents Department

Spiral order in Ba{sub 2}CuGe{sub 2}O{sub 7}

Description: The quasi 2-D square-lattice antiferromagnet Ba{sub 2}CuGe{sub 2}O{sub 7} was studied by neutron scattering and bulk magnetic techniques. An incommensurate magnetic spiral structure with the propagation vector (1+{xi}, 1+{xi}, 0)({xi}=0.027) was observed below T{sub N}=3.26 K. Spin dynamics can be adequately described by conventional spin-wave theory with two exchange constants: nearest- neighbor in-plant antiferromagnetic coupling J{sub 1}{approx}0.48 meV and interplane ferromagnetic interaction J{sub 3}{approx}0.013 meV. This set of exchange parameters apparently fails to explain the spiral order. The non-centrosymmetric crystal structure suggests that the incommensurate phase may be the result of a Dzyaloshinskii-Moriya instability of the Neel ground state.
Date: August 1, 1996
Creator: Zheludev, A.; Shirane, G.; Sasago, Y.; Koide, N. & Uchinokura, K.
Partner: UNT Libraries Government Documents Department

Low energy magnetic fluctuations in the TSDW phase of chromium

Description: A polarized neutron study of chromium carried out in a field of 6T applied to a single-domain single-Q crystal indicates that the inelastic intensity observed close to the transverse spin density wave (TSDW) satellite positions (1 {+-} {delta}, 0,0) does not behave as expected for spin-wave scattering. In particular, the signal corresponds to magnetization fluctuations of almost equal magnitude both parallel and perpendicular to the ordered moments in the TSDW phase.
Date: December 1, 1997
Creator: Azuah, R.T.; Kulda, J.; Pynn, R. & Stirling, W.G.
Partner: UNT Libraries Government Documents Department

Low energy spin excitations in chromium metal

Description: Neutron scattering experiments with full polarization analysis have been performed with a single crystal of chromium to study the low-energy spin fluctuations in the transverse spin density wave (TSDW) state. A number of remarkable results have been found. Inelastic scattering observed close to the TSDW satellite positions at (1 {+-} {delta},0,0) does not behave as expected for magnon scattering. In particular, the scattering corresponds to almost equally strong magnetization fluctuations both parallel and perpendicular to the ordered moments of the TSDW phase. As the Neel temperature is approached from below, scattering at the commensurate wavevector (1,0,0) increases in intensity as a result of critical scattering at silent satellites (1,0, {+-} {delta}) being included within the spectrometer resolution function. This effect, first observed by Sternlieb et al, does not account for all of the inelastic scattering around the (1,0,0) position, however, Rather, there are further collective excitations, apparently emanating from the TSDW satellites, which correspond to magnetic fluctuations parallel to the ordered TSDW moments. These branches have a group velocity that is close to that of (1,0,0) longitudinal acoustic (LA) phonons, but assigning their origin to magneto-elastic scattering raises other unanswered questions.
Date: December 31, 1997
Creator: Pynn, R.; Azuah, R.T.; Stirling, W.G. & Kulda, J.
Partner: UNT Libraries Government Documents Department

Spin excitations in [La{sub 1-x}Ca{sub x}MnO{sub 3}] in the mixed-phase region.

Description: The magnetic excitations and the ferromagnetic order parameter have been studied by neutron scattering in a series of the manganese-based CMR perovskites [La{sub 1-x}Ca{sub x}MnO{sub 3}] (x=0.46, 0.48, 0.50) near the metallic ferromagnetic to insulating antiferromagnetic phase. Well-defined ferromagnetic spin waves were detected for the x=0.46 and x=0.48 compositions. From the measurements of the order parameter, only the x=0.48 sample showed conclusive evidence of a coexistence of an antiferromagnetic phase with the ferromagnetic phase. For this composition, hysteresis was observed in the spin wave intensity but not in the spin stiffness parameter. This effect indicates that the ferromagnetic exchange is not perturbed by the antiferromagnetic ordering. No measurable ferromagnetic magnetization was found in the x=0.50 sample; thus no spin waves could be detected. The results indicate that the onset of the antiferromagnetism upon hole doping for the series occurs in a narrow region of x below the x=0.50 phase boundary.
Date: December 18, 2001
Creator: Stumpe, L.; Kirby, B.; Kaiser, H.; Rhyne, J. J. & Mitchell, J. F.
Partner: UNT Libraries Government Documents Department

Low temperature spin wave dynamics in classical Heisenberg chains

Description: A detailed and quantitative study of the low-temperature spin-wave dynamics was made for the classical Heisenberg-coupled chain using computer simulation. Results for the spin-wave damping rates and the renormalization of the spin-wave frequencies are presented and compared with existing predictions.
Date: November 1, 1977
Creator: Heller, P. & Blume, M.
Partner: UNT Libraries Government Documents Department

Magnetic Contribution to Heat Capacity and Entropy of Nicke Ferrite (NiFe2O4)

Description: The heat capacity of nickel ferrite was measured as a function of temperature over the range from 50 to 1200 C using a differential scanning calorimeter. A thermal anomaly was observed at 584.9 C, the expected Curie temperature, T{sub c}. The observed behavior was interpreted by recognizing the sum of three contributions: (1) lattice (vibrational), (2) a spin wave (magnetic) component and (3) a {lambda}-transition (antiferromagnetic-paramagnetic transition) at the Curie temperature. The first was modeled using vibrational frequencies derived from an experimentally-based ir absorption spectrum, while the second was modeled using a spin wave analysis that provided a T{sup 3/2} dependency in the low temperature limit, but incorporated an exchange interaction between cation spins in the octahedral and tetrahedral sites at elevated temperatures, as first suggested by Grimes [15]. The {lambda}-transition was fitted to an Inden-type model which consisted of two truncated power law series in dimensionless temperature (T/T{sub c}). Exponential equality was observed below and above T{sub c}, indicating symmetry about the Curie temperature. Application of the methodology to existing heat capacity data for other transition metal ferrites (AFe{sub 2}O{sub 4}, A = Fe, Co) revealed the same exponential equality, i.e., m = n = 5.
Date: December 15, 2005
Creator: S Ziemniak, L Anovitz, R Castelli
Partner: UNT Libraries Government Documents Department

Fermi Liquid Instabilities in the Spin Channel

Description: We study the Fermi surface instabilities of the Pomeranchuk type in the spin triplet channel with high orbital partial waves (F{sub l}{sup a} (l > 0)). The ordered phases are classified into two classes, dubbed the {alpha} and {beta}-phases by analogy to the superfluid {sup 3}He-A and B-phases. The Fermi surfaces in the {alpha}-phases exhibit spontaneous anisotropic distortions, while those in the {beta}-phases remain circular or spherical with topologically non-trivial spin configurations in momentum space. In the {alpha}-phase, the Goldstone modes in the density channel exhibit anisotropic overdamping. The Goldstone modes in the spin channel have nearly isotropic underdamped dispersion relation at small propagating wavevectors. Due to the coupling to the Goldstone modes, the spin wave spectrum develops resonance peaks in both the {alpha} and {beta}-phases, which can be detected in inelastic neutron scattering experiments. In the p-wave channel {beta}-phase, a chiral ground state inhomogeneity is spontaneously generated due to a Lifshitz-like instability in the originally nonchiral systems. Possible experiments to detect these phases are discussed.
Date: March 16, 2010
Creator: Wu, Congjun; /Santa Barbara, KITP; Sun, Kai; Fradkin, Eduardo; /Illinois U., Urbana; Zhang, Shou-Cheng et al.
Partner: UNT Libraries Government Documents Department

Random walk approach to spin dynamics in a two-dimensional electron gas with spin-orbit coupling

Description: We introduce and solve a semiclassical random walk (RW) model that describes the dynamics of spin polarization waves in zinc-blende semiconductor quantum wells. We derive the dispersion relations for these waves, including the Rashba, linear and cubic Dresselhaus spin-orbit interactions, as well as the effects of an electric field applied parallel to the spin polarization wave vector. In agreement with calculations based on quantum kinetic theory [P. Kleinert and V. V. Bryksin, Phys. Rev. B 76, 205326 (2007)], the RW approach predicts that spin waves acquire a phase velocity in the presence of the field that crosses zero at a nonzero wave vector, q{sub 0}. In addition, we show that the spin-wave decay rate is independent of field at q{sub 0} but increases as (q-q{sub 0}){sup 2} for q {ne} q{sub 0}. These predictions can be tested experimentally by suitable transient spin grating experiments.
Date: September 27, 2010
Creator: Yang, Luyi; Orenstein, J. & Lee, Dung-Hai
Partner: UNT Libraries Government Documents Department

Final Report: X-ray Studies of Materials Dynamics at MHATT-CAT Sector 7, Advanced Photon Source

Description: This Final Report describes the scientific accomplishments that have been achieved with support from grant DE-FG02-03ER46023 during the period 12/01/02 ? 11/30/05. The funding supported a vigorous scientific program allowing the PI to achieve leadership in a number of important areas. In particular, research carried out during this period has opened way to ultrafast dynamics studies of materials by combining the capabilities of synchrotron radiation with those of ultrafast lasers. This enables the initiation of laser-induced excitations and studies of their subsequent dynamics using laser-pump/x-ray probe techniques. Examples of such excitations include phonons, shock waves, excitons, spin-waves, and polaritons. The breadth of phenomena that can now be studied in the time-domain is very broad, revealing new phenomena and mechanisms that are critical to many applications of materials.
Date: April 25, 2006
Creator: Clarke, Roy
Partner: UNT Libraries Government Documents Department

Spin and charge dynamics of chromium alloys

Description: Both the spin- and charge-density waves of Cr alloys are produced by the Coulomb attraction between electrons and holes on nearly nested Fermi surfaces. Driven by quasi-particle transitions, transverse spin- wave and longitudinal phason modes are associated with rotational and translational symmetries of pure Cr and its dilute alloys. At low frequencies, both spin and charge phasons have a nearly linear dispersion with a mode velocity which approaches the spin-wave velocity as T approaches T{sub N} or as the mismatch between the Fermi surfaces increases.
Date: July 1, 1996
Creator: Fishman, R.S.; Viswanath, V.S. & Liu, S.H.
Partner: UNT Libraries Government Documents Department

Absolute measurements of the high-frequency magnetic dynamics in high-{Tc} superconductors

Description: The authors review recent measurements of the high-frequency dynamic magnetic susceptibility in the high-T{sub c} superconducting systems La{sub 2{minus}x}Sr{sub x}CuO{sub 4} and YBa{sub 2}Cu{sub 3}O{sub 6+x}. Experiments were performed using the chopper spectrometers HET and MARI at the ISIS spallation source. The authors have placed their measurements on an absolute intensity scale, this allows systematic trends to be seen and comparisons with theory to be made. They find that the insulating S = 1/2 antiferromagnetic parent compounds show a dramatic renormalization in the spin wave intensity. The effect of doping on the response is to cause broadenings in wave vector and large redistributions of spectral weight in frequency.
Date: August 7, 1997
Creator: Hayden, S.M.; Aeppli, G.; Dai, P.; Mook, H.A.; Perring, T.G.; Cheong, S.W. et al.
Partner: UNT Libraries Government Documents Department

Neutron study of fracton excitations in percolating antiferromagnets

Description: The authors report the results of an inelastic neutron scattering experiment on nearly-percolating Heisenberg antiferromagnets (RbMn{sub c}Mg{sub 1{minus}o}F{sub 3}), in which the Mn concentrations (C = 0.31, 0.34 and 0.39) are very close to the percolation threshold (c{sub p} = 0.312). A broad peak superimposed on Ising-cluster excitations was observed throughout the Brillouin zone. The intensity of a broad peak increased on approaching the percolation threshold. The origin of this broad peak is attributed to the excitation of fractons in a percolating network.
Date: June 27, 1997
Creator: Ikeda, H.; Takahashi, M.; Fernandez-Baca, J.A. & Nicklow, R.M.
Partner: UNT Libraries Government Documents Department

Spin Dynamics of the Reentrant Spin Glass Fe{sub 0.7}A1{sub 0.3}

Description: Inelastic neutron scattering experiments are reported to study magnetic excitations in a single crystal of the reentrant spin glass Fe(0.7)Al(0.3) near the (111) Bragg peak. In the ferromagnetic phase, the magnetic excitation spectrum is separated in two areas: at low q, spin waves are propagating and at higher q, the response is quasielastic-like. In the spin glass phase, the response is always quasielastic.
Date: December 31, 1997
Creator: Raymond, S.; Bao, W.; Shapiro, S.M. & Motoya, K.
Partner: UNT Libraries Government Documents Department

Low energy spin-wave excitations in the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}.

Description: Inelastic neutron scattering experiments were performed on a single crystal of the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}. Low energy spin-wave excitations were observed along the c direction with a maximum energy of {approx} 0.5 meV at the zone boundary. The dispersion of these acoustic spin wave modes is modeled by a nearest-neighbor Heisenberg model with an inter-bilayer exchange interaction between neighboring spins in different bilayers of 0.048(1) meV and an anisotropy gap of {Delta} = 0.077(3) meV. These results confirm the two-dimensional nature of the spin-correlations in the bilayer manganites, with a ratio of the in-plane to inter-bilayer interaction of {approx}200. The temperature dependence of the energies and intensities of the spin wave excitations are in agreement with our earlier conclusion that the ferromagnetic transition is second-order.
Date: September 21, 1999
Creator: Rosenkranz, S.; Osborn, R.; Mitchell, J. F.; Vasiliu-Doloc, L.; Lynn, J. W. & Sinha, S. K.
Partner: UNT Libraries Government Documents Department

Spin Polarized Electron Probes and Magnetic Nanostructures

Description: OAK B188 This report summarizes progress to date in our theoretical research program, for the period from July 1, 2002 to November 1, 2003. In addition, our research priorities for the coming year are set forth. The reporting period has been a most exciting and significant one. For the past several years, one of our principal thrust areas has been development of the theory of spin dynamics in magnetic nanostructures with emphasis on the use of spin polarized electrons as probes of short wavelength spin dynamics in such entities. Our program stimulated the first experiment which detected large wave vector spin waves in ultrathin films in 1999 through spin polarized electron loss spectroscopy (SPEELS); the publication which announced this discovery was a joint publication between a group in Halle (Germany) with our theory effort. The continued collaboration has led to the design and implementation of the new SPEELS spectrometer and we now have in hand the first detailed measurements of spin wave dispersion in an ultrathin film. A second such spectrometer is now operational in the laboratory of Prof. H. Hopster, at UC Irvine. We are thus entering a most exciting new era in the spectroscopy of spin excitations in magnetic nanostructures. During the reporting period, we have completed very important new analyses which predict key aspects of the spectra which will be uncovered by these new instruments, and the calculations continue to be developed and to expand our understanding. In addition, we have initiated a new series of theoretical studies directed toward spin dynamics of single magnetic adatoms on metal surfaces, with STM based studies of this area n mind. In the near future, these studies will continue, and we will expand our effort into new areas of spin dynamics in magnetic nanostructures.
Date: October 15, 2003
Creator: Mills, D. L.
Partner: UNT Libraries Government Documents Department