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in subjects: "Antiferromagnetic Materials"

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Giant magnetic effects and oscillations in antiferromagnetic Josephson weak links

Josephson junctions with an antiferromagnetic metal as a link are described. The junction can be switched off by a relatively small magnetic field. The amplitude of the current oscillates as a function of the field.

Date: April 1, 2001

Creator: Gorkov, L. & Kresin, Vladimir

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Studies of the magnetic structure at the ferromagnet - antiferromagnet interface

Antiferromagnetic layers are a scientifically challenging component in magneto-electronic devices such as magnetic sensors in hard disk heads, or magnetic RAM elements. In this paper we show that photo-electron emission microscopy (PEEM) is capable of determining the magnetic structure at the interface of ferromagnets and antiferromagnets with high spatial resolution (down to 20 nm). Dichroism effects at the L edges of the magnetic 3d transition metals, using circularly or linearly polarized so… more

Date: January 2, 2001

Creator: Scholl, A.; Nolting, F.; Stohr, J.; Luning, J.; Seo, J.W.; Locquet, J.-P. et al.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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MAGNETIC EXCITATIONS AND DOUBLE GAP IN THE S=1/2 LINEAR CHAIN QUANTUM ANTIFERROMAGNET BaCu{sub 2}Si{sub 2}O{sub 7}

The magnetic excitation spectrum of the ideal S=1/2 1-dimensional quantum antiferromagnet (AF), contains no single-mode contributions, and, in fact, can be described as a 2-particle continuum of spinons. Chain-MF theory predicts that a weakly coupled chains system becomes ordered at low temperatures. In the ordered state a sharp magnon excitation is present. The continuum persists, but is separated from the magnon branch by an appreciable energy gap. On the other hand, if weakly coupled chains … more

Date: March 16, 2000

Creator: Raymond, S.; Zheludev, A.; Masuda, T.; Kakurai, K.; Ressouche, E. & Wildes, A.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Direct imaging of the first order spin flop transition in the layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7}.

The spin-flop transition in the antiferromagnetic layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} was studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polarized domains at crystal defect sites and not through the expansion of polarized domains due to domain… more

Date: August 31, 1999

Creator: Berger, A.; Gray, K. E.; Miller, D. J.; Mitchell, J. F.; Vlasko-Vlasov, V. K. & Welp, U.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Theoretical studies of magnetic systems. Final report, August 1, 1994--November 30, 1997

During the grant period the authors have studied five areas of research: (1) low dimensional ferrimagnets; (2) lattice effects in the mixed valence problem; (3) spin compensation in the one dimensional Kondo lattice; (4) the interaction of quasi particles in short coherence length superconductors; and (5) novel effects in angle resolved photoemission spectra from nearly antiferromagnetic materials. Progress in each area is summarized.

Date: December 31, 1997

Creator: Gor`kov, L. P.; Novotny, M. A. & Schrieffer, J. R.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Phase diagram of the random-field Ising system Fe{sub 0.60}Zn{sub 0.40}F{sub 2} at intense fields

The critical and irreversibility phase boundaries of the d = 3 diluted uniaxial antiferromagnet Fe{sub 0.60}Zn{sub 0.40}F{sub 2} have been determined under strong external magnetic fields by means of magnetization measurements. The data reveal that the random-field-induced glassy phase, previously observed in the upper part of the (H,T) phase diagram for highly diluted samples (x {approx_equal} 0.3), is extended to higher values of x.

Date: October 1, 1997

Creator: Montenegro, F. C.; Lima, K. A.; Torikachvili, M. S. & Lacerda, A. H.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Neutron study of fracton excitations in percolating antiferromagnets

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 … more

Date: June 27, 1997

Creator: Ikeda, H.; Takahashi, M.; Fernandez-Baca, J. A. & Nicklow, R. M.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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

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.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Phase diagram of imperfect ferromagnetic/antiferromagnetic-bilayers

The phase diagram for ferromagnetic/antiferromagnetic bilayers with imperfect interfaces is calculated, using a Ising spin-1/2 model which is solved numerically in the mean field approximation for finite temperatures. We identify 3 stable phases: domains in the ferromagnet, domains in the antiferromagnet, and domain walls near the interfaces with homogeneous order within the layers. Phase transitions between these phases occur as a function of temperature, relative film thicknesses, and step de… more

Date: October 1, 1996

Creator: Berger, A. & Fullerton, E.E.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Reduced size of ordered moments of a quasi 1d antiferromagnet Sr{sub 2}CuO{sub 3}

The quasi one-dimensional antiferromagnet Sr{sub 2}CuO{sub 3} exhibits Neel order of Cu{sup 2+} moments (S = 1/2) at T{sub N} = 5.41(1) K, as demonstrated by (1) almost resolution-limited magnetic Bragg reflections in neutron scattering measurements, and (2) spontaneous muon spin precession in zero-field muon spin relaxation ({mu}SR) measurements. The temperature dependence of the order parameters are consistent between the two experimental techniques. From the neutron data, the authors obtaine… more

Date: September 1, 1996

Creator: Kojima, K.; Larkin, M. & Luke, G.M.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Magnetic properties of the S=1/2 quasi-one-dimensional antiferromagnet: Sr{sub 14-x}Y{sub x}Cu{sub 24}O{sub 41}

Magnetic susceptibility and inelastic neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system Sr{sub 14-x}Y{sub x}Cu{sub 24}O{sub 41}, which has both simple chains and two-leg ladder chains of copper ions. The experimental results show that the simple chain in Sr{sub 14}Cu{sub 24}O{sub 41} has a spin gap in the excitation spectrum, which originates from a dimerized state. We have also studied the effect of Y{sup 3+} substitution for Sr{sup 2+} site on the d… more

Date: July 1, 1996

Creator: Matsuda, M.; Katsumata, K.; Shapiro, S.M. & Shirane, G.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Charge inhomogeneity and high temperature superconductivity

When the condensation of a gas of fermions into a (self-bound) liquid state is frustrated by the long-range Coulomb interaction, the consequence is a large local fluctuation of the charge density, together with pairing on a high energy scale. The competition between these two effects at long length scales determines the nature of the ordered state at low temperatures. Evidence for the central role of this competition in determining the physical properties of the high temperature superconductors… more

Date: May 1, 1996

Creator: Emery, V.J. & Kivelson, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Heavy fermions in high magnetic fields

This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The National High Magnetic Field Laboratory (NHMFL) has established major new facilities at LANL. This project sought to explore some exciting new problems in condensed matter physics that could be studied using these facilities. We studied the behavior of heavy-fermion compounds in high-magnetic fields. The unusual properties of these materials are… more

Date: May 1, 1996

Creator: Trugman, S.; Bedell, K.; Bonca, J.; Gulacsi, M. & Yu, C.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Superconductivity in doped insulators

It is shown that many synthetic metals, including high temperature superconductors are ``bad metals``, with such a poor conductivity that the usual meanfield theory of superconductivity breaks down because of anomalously large classical and quantum fluctuations of the phase of the superconducting order parameter. It is argued that the supression of a first order phase transition (phase separation) by the long-range Coulomb interaction leads to high temperature superconductivity accompanied by s… more

Date: December 31, 1995

Creator: Emery, V.J. & Kivelson, S.A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Chebyshev recursion methods: Kernel polynomials and maximum entropy

The authors describe two Chebyshev recursion methods for calculations with very large sparse Hamiltonians, the kernel polynomial method (KPM) and the maximum entropy method (MEM). They are especially applicable to physical properties involving large numbers of eigenstates, which include densities of states, spectral functions, thermodynamics, total energies, as well as forces for molecular dynamics and Monte Carlo simulations. The authors apply Chebyshev methods to the electronic structure of S… more

Date: October 1, 1995

Creator: Silver, R. N.; Roeder, H.; Voter, A. F. & Kress, J. D.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Crystallographic and magnetic structure of UCu{sub 1.5}Sn{sub 2}

We report on the crystallographic and magnetic structures of the antiferromagnet UCu{sub 1.5}Sn{sub 2}, as determined by x-ray and neutron powder diffraction. It forms in the tetragonal CaBe{sub 2}Ge{sub 2} structure type, with space group P/4nmm, and we find no site disorder between two different Sn 2c sites, in contrast with a previous report. UCu{sub 1.5}Sn{sub 2} orders antiferromagnetically with a Neel temperature of about 110 K. This is unusually high amongst uranium intermetallics. The u… more

Date: September 1995

Creator: Purwanto, A.; Robinson, R. A.; Nakotte, H.; Swainson, I. & Torikachvili, M.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Selected studies of magnetism at high pressure

Most previous studies of magnetism in various compounds under extreme conditions have been conducted over a wide pressure range at room temperature or over a wide range of cryogenic temperatures at pressures below 20 GPa (200 kbar). We present some of the most recent studies of magnetism over an extended range of temperatures and pressures far beyond 20 GPa, i.e., in regions of pressure-temperature (P-T) where magnetism has been largely unexplored. Recent techniques have permitted investigation… more

Date: September 1, 1995

Creator: Hearne, G. R.; Pasternak, M. P. & Taylor, R. D.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Growth induced magnetic anisotropy in amorphous thin films. Annual progress report year 1, November 4, 1994--October 31, 1995

The work in the past year has primarily involved three areas of magnetic thin films: amorphous rare earth-transition metal alloys, epitaxial COPt3 thin films, and exchange coupled antiferromagnetic insulators. In the amorphous alloys, the authors have focused on understanding the cause and the effect of the growth-surface-induced perpendicular magnetic anisotropy. Using the results of previous work, they are able to control this anisotropy quite precisely. This anisotropy is predicted to have d… more

Date: July 1, 1995

Creator: Hellman, F.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Magnetoresistance behavior of UNiGe

The authors have measured the temperature dependences of the magnetoresistance of single-crystalline UNiGe for both parallel (i//B//c-axis) and perpendicular configurations (i//a-axis, B//c-axis) in magnetic fields up to 18 T. Both configurations yield similar magnetoresistance behavior, which emphasizes the strong magnetic contribution to the resistivity in all directions. Crossing magnetic-phase boundaries causes anomalies in the magnetoresistance, which allowed a completion of the previously… more

Date: May 1, 1995

Creator: Nakotte, H.; Lacerda, A.; Purwanto, A.; Havela, L.; Sechovsky, V.; Prokes, K. et al.

Item Type: Report

Partner: UNT Libraries Government Documents Department

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Magnetic excitations in a random exchange system Fe{sub x}Mn{sub 1-x}TiO{sub 3}

In order to examine the influence of exchange frustration on spin wave excitations, inelastic neutron scattering experiments have been performed on a random exchange system Fe{sub x}Mn{sub l-x}TiO{sub 3} with x = 0.00, 0.10, 0.25 and 0.33. For all three mixed systems, the magnetic excitation spectra consist of two components: a well-defined spin wave component and a broad damped component. The latter can be well fitted to a spectral weight function for a damped harmonic oscillator.

Date: August 24, 1994

Creator: Kawano, H.; Yoshizawa, H.; Nicklow, R. M. & Ito, A.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Neutron diffraction study of the magnetic ordering of BaCuO{sub 2+x}

Neutron diffraction measurements have revealed that BaCuO{sub 2+x} orders antiferromagnetically below T{sub N} = (15.0 {+-} 0.5) with a magnetic propagating vector {kappa} = [1 1 1]. The Cu atoms in the Cu{sub 6} ring clusters, located at the (1/4, 1/4, 1/4) positions, order ferromagnetically within these clusters while the clusters themselves order antiferromagnetically. The ordered magnetic moment of each of these Cu atoms is (0.89 {+-} 0.05){mu}{sub B} at T=4.2K. No evidence of long-range ma… more

Date: August 8, 1994

Creator: Wang, X. L.; Fernandez-Baca, J. A.; Wang, Z. R.; Vaknin, D. & Johnston, D. C.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Spin correlations in percolating networks with fractal geometry

Using neutron scattering techniques, the authors investigated the magnetic correlations in diluted antiferromagnets close to the percolation threshold in which the magnetic connectivity takes a fractal form. Recent experimental results concerning the self-similarity of the magnetic order, and magnetic excitations in two-dimensional Ising and three-dimensional Heisenberg antiferromagnets are presented.

Date: July 28, 1994

Creator: Ikeda, H.; Iwasa, K.; Fernandez-Baca, J. A. & Nicklow, R. M.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Electronic phase separation and high temperature superconductors

The authors review the extensive evidence from model calculations that neutral holes in an antiferromagnet separate into hole-rich and hole-poor phases. All known solvable limits of models of holes in a Heisenberg antiferromagnet exhibit this behavior. The authors show that when the phase separation is frustrated by the introduction of long-range Coulomb interactions, the typical consequence is either a modulated (charge density wave) state or a superconducting phase. The authors then review so… more

Date: January 11, 1994

Creator: Kivelson, S. A. & Emery, V. J.

Item Type: Article

Partner: UNT Libraries Government Documents Department

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Spin-hole doping in the Kondo insulator Ce{sub 3}Bi{sub 4}Pt{sub 3} studied by neutron scattering

To investigate the effect of non-magnetic impurities on gap formation and stability in Kondo insulators, we have performed inelastic neutron scattering experiments on (Ce{sub 1-x}La{sub x}){sub 3}Bi{sub 4}Pt{sub 3}, with x = 0.0, 0 1, 0.25 and 1.0. Data were collected at the Rutherford spallation source using incident neutron energies of 30 and 100 MeV and with the sample temperature fixed at 10K, well below {Delta}g/k{sub B}. The gap edge, defined from the rapid onset of scattering intensity, … more

Date: September 1, 1993

Creator: Severing, A.; Perring, T.; Thompson, J. D.; Fisk, Z. & Canfield, P. C.

Item Type: Article

Partner: UNT Libraries Government Documents Department