14 Matching Results

Search Results

Advanced search parameters have been applied.

Neutron scattering studies of amorphous Invar alloys

Description: This paper reviews recent inelastic neutron scattering experiments performed to study the spin dynamics of two amorphous Invar systems: Fe/sub 100-x/B/sub x/ and Fe/sub 90-x/Ni/sub x/Zr/sub 10/. As in crystalline Invar Fe/sub 65/Ni/sub 35/ and Fe/sub 3/Pt, the excitation of conventional long-wavelength spin waves in these amorphous systems cannot account for the relatively rapid change of their magnetization with temperature. These results are discussed in terms of additional low-lying excitations which apparently have a density of states similar to the spin waves.
Date: January 1, 1989
Creator: Fernandez-Baca, J.A.
Partner: UNT Libraries Government Documents Department

Neutron scattering at the high flux isotope reactor at Oak Ridge National Laboratory

Description: Since its beginnings in Oak Ridge and Argonne in the late 1940`s, neutron scattering has been established as the premier tool to study matter in its various states. Since the thermal neutron wavelength is of the same order of magnitude as typical atomic spacings and because they have comparable energies to those of atomic excitations in solids, both structure and dynamics of matter can be studied via neutron scattering. The High Flux Isotope Reactor (HFIR) provides an intense source of neutrons with which to carry out these measurements. This paper summarizes the available neutron scattering facilities at the HFIR.
Date: March 1, 1995
Creator: Yethiraj, M. & Fernandez-Baca, J.A.
Partner: UNT Libraries Government Documents Department

Random-field critical scattering at high magnetic concentration in the Ising antiferromagnet Fe{sub 0.93}Zn{sub 0.07}F{sub 2}

Description: The high magnetic concentration ising antiferromagnet Fe{sub 0.93}Zn{sub 0.07}F{sub 2} does not exhibit the severe critical scattering hysteresis at low temperatures observed in all lower concentration samples studied. The system therefore provides equilibrium neutron scattering line shapes suitable for determining random-field Ising model critical behavior.
Date: March 24, 1997
Creator: Slanic, Z.; Belanger, D. P. & Fernandez-Baca, J. A.
Partner: UNT Libraries Government Documents Department

Neutron diffraction study of the magnetic structures of CeMn{sub 2}Ge{sub 2} and CeMn{sub 2}Si{sub 2}

Description: The magnetic properties of the layered compounds of the form RMn{sub 2}X{sub 2} (R = Rare Earth, X = Si, Ge) have been thought to be sensitive to the intralayer Mn-Mn distance. Thus it has been reported that the Mn moments in CeMn{sub 2}Si{sub 2} are aligned antiferromagnetically (AF) below T{sub N} = 380K, while the Mn moments in CeMn{sub 2}Ge{sub 2} are ferromagnetic (FM) below T{sub C} = 316K. Recently, however, there has been some debate about the actual magnetic structures of this family of compounds, and for this reason the authors have performed high-resolution neutron powder diffraction measurements on these compounds for temperatures between 12K and 550K. The measurements indicate that at high temperatures both compounds are paramagnetic. Below T{sub N} = 380K CeMn{sub 2}Si{sub 2} becomes a collinear AF, with a structure similar to that reported by Siek et al. in which the magnetic propagation vector is {tau} = (0 0 1). CeMn{sub 2}Ge{sub 2} on the other hand, exhibits two different magnetic transitions. At T{sub N} {approx} 415K there is a transition to a collinear AF phase characterized by the commensurate propagation wavevector {tau} = (1 0 1). At T{sub C} = 318K there is a transition to a conical structure with a ferromagnetic component along the c-axis and a helical component in the ab plane. The helical component is characterized by the incommensurate propagation vector {tau} = (1 0 1-q{sub z}), where q{sub z} is temperature dependent. These findings are consistent with the recent results of Welter et al.
Date: December 31, 1995
Creator: Fernandez-Baca, J.A.; Chakoumakos, B.C.; Hill, P. & Ali, N.
Partner: UNT Libraries Government Documents Department

(A neutron scattering experiment to study the high-energy spin dynamics of the itinerant antiferromagnet Mn sub 90 Cu sub 10 )

Description: The traveler performed a neutron scattering experiment to study the high-energy spin dynamics of the itinerant antiferromagnet. This experiment was conducted at a unique instrument located at the hot-neutron source at the ILL. The traveler also held various scientific discussions with ILL research staff members and visiting scientists.
Date: October 26, 1990
Creator: Fernandez-Baca, J.A.
Partner: UNT Libraries Government Documents Department

Neutron scattering study of the magnetic correlations of iron rich Fe-Zr glasses

Description: Small angle neutron scattering (SANS) and polarized-beam spin rotation measurements show that amorphous Fe/sub 80/Zr/sub 10/ does not exhibit conventional long range ferromagnetic order below T/sub C/. The SANS measurements show the existence of two characteristic length scales; one provides evidence of relatively large spin clusters, while the other is characteristic of the spin dynamics. 4 refs., 3 figs.
Date: January 1, 1988
Creator: Fernandez-Baca, J.A.; Rhyne, J.J.; Erwin, R.W. & Fish, G.E.
Partner: UNT Libraries Government Documents Department

Two-Dimensional Anisotropy in a Layered Metallic Antiferromagnet RE{sub 1-x}Sr{sub x}MnO{sub 3} with x approx.1/2

Description: Some RE{sub 1-x}Sr{sub x}MnO{sub 3} (RE=Nd,Pr) systems with x {approximately} 1/2 exhibit an A-type anti-ferromagnetic (AFM) structure instead of the expectged CE-type magnetic structure. This can be understood as being a consequence of the increase of the band width (W). These A-type magnetic systems also show salient features in their crystal structures. We proposed a two dimensional anisotropy of the transport as well as the magnetic properties for the A-type AFM structure and we show that we have, indeed, observed anisotropic spin wave dispersion relations by inelastic neutron measurements.
Date: July 1, 1997
Creator: Kawano, H.; Yoshizawa, H.; Fernandez-Baca, J.A.; Tomioka, Y. & Kuwahara, H.
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

Effect of pressure on the magnetic phase diagram of the antiferromagnetic spin-density-wave alloy Cr-1.6% Si

Description: A neutron diffraction experiment has been performed to study the effects of high pressure on the magnetic phase diagram of a single crystal specimen of Cr-1.6%Si. At applied pressures up to 0.49 GPa there are two clear transitions. At T{sub NC} there is a transition from a paramagnetic (P) to a commensurate spin density wave (CSDW), while at lower temperatures there is a transition to a mixed state where the CSDW seems to coexist with an incommensurate SDW (ISDW) state. T{sub NC} decreases with the application of pressure while the transition temperature T{sub CI} to the mixed C-I state seems to be insensitive to pressure. At 0.69 GPa the ISDW state disappears while the P-C transition changes drastically.
Date: December 31, 1996
Creator: Fernandez-Baca, J.A.; Fawcett, E.; Alberts, H.L.; Galkin, V.Yu. & Endoh, Y.
Partner: UNT Libraries Government Documents Department

Magnetism in BaCoS{sub 2}

Description: BaCoS{sub 2} is a layered Mott-Hubbard insulator that orders antiferromagnetically near 300 K. We report magnetic susceptibility measurements on polycrystalline BaCoS{sub 2} from 77 K to 800 K, and in- and out-of-plane measurements on single crystals from 2 K to 350 K. We also report a powder neutron refinement of the magnetic structure of BaCoS{sub 2}. The neutron measurements indicate that the moments lie in the plane, and that the magnetic unit cell is the same as the (orthorhombic) chemical cell. The ordered moment obtained from the neutron refinement is close to 3 {mu}{sub B} indicating that the Co ions are in the high-spin (S = 3/2) configuration. The effective moment inferred from the high temperature susceptibility, p = 5.26 {mu}{sub B} is also consistent with high-spin Co{sup +2}.
Date: August 1996
Creator: Mandrus, D.; Chakoumakos, B. C.; Fernandez-Baca, J. A.; Nagler, S. E.; Sales, B. C. & Sarrao, J. L.
Partner: UNT Libraries Government Documents Department

Magnetic Excitation of CuGeO{sub 3} under Applied Pressure

Description: Magnetic excitations of the spin-Peierls compound CuGeO{sub 3} under applied pressure of 2 GPa have been studied. The dispersion along the chain direction up to zone boundary has been obtained. The spin-Peierls gap energy increases to 4.2 meV and the zone boundary energy decreases to 14.1 meV. The pressure dependence of dispersion relation can be interpreted by the increase of the next-nearest-neighbor intra-chain interaction under applied pressure causing the increase of both the spin-Peierls gap energy and transition temperature.
Date: July 31, 1997
Creator: Nishi, M.; Kakurai, K.; Fujii, Y.; Yethiraj, M.; Tennant, D. A.; Nagler, S. E. 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

Calculation of thermal diffuse scattering

Description: The authors developed a computer program to calculate the thermal diffuse scattering (TDS) intensity distribution for single crystal specimens in a diffractometer with no energy analysis. They assumed that the phonon frequencies are approximated by those of elastic waves and that the elastic constants, density and lattice parameters of the system under study are known. The results of the calculations were compared to experimental data obtained for single crystals of Si, diamond and NiAl at the Wide Angle neutron Diffractometer at the HFIR at Oak Ridge National Laboratory. Excellent agreement was found between the calculations and the experimental observations.
Date: July 14, 1997
Creator: Wakabayashi, N.; Nicklow, R.M.; Child, H.R.; Smith, H.G.; Fernandez-Baca, J.A.; Katano, S. et al.
Partner: UNT Libraries Government Documents Department

Magnetic excitations in thulium metal

Description: We have performed inelastic neutron scattering measurements on a single crystal specimen of Tm at wavevectors {rvec {kappa}} = (1,1, {zeta}) and (0,0,2 + {zeta}) ({zeta} = 0, {hor ellipsis}, 1). Most of the measurements have been made at T = 5K, where Tm exhibits a seven layer ferrimagnetic-antiphase-domain structure (four moments up, parallel to the c-axis, followed by three moments down). At this temperature the excitation spectra consist of three peaks. The two lower energy excitations have been identified as originating from magneto-vibrational scattering from the TA phonon, while the higher energy excitation is magnetic and exhibits only a weak dispersion (between 8.3 and 9.6 meV). At T = 50K, a temperature at which the system exhibits a c-axis sinusoidally modulated structure, the magnetic mode shows significant softening and broadening. The magneto-vibrational scattering vanishes above the Neel temperature (T{sub N} = 58.5K) while the magnetic mode persists at least up to T = 70K. These results suggest that the Hamiltonian in this system is dominated by the crystal-field-anistropy energy, and that the exchange interaction is relatively weak. 9 refs., 2 figs.
Date: January 1, 1989
Creator: Fernandez-Baca, J.A.; Nicklow, R.M.; Rhyne, J.J. (Oak Ridge National Lab., TN (USA) & National Inst. of Standards and Technology (NCTL), Gaithersburg, MD (USA))
Partner: UNT Libraries Government Documents Department