Showing papers on "Magnetic structure published in 1992"

Neutron diffraction study of the heavy fermion superconductors UM2Al3 (M=Pd, Ni)

Abstract: An elastic neutron scattering study was performed on the new superconducting heavy fermion systems UPd2Al3 and UNi2Al3. The neutron diffraction patterns reveal unambiguously long range antiferromagnetic order in UPd2Al3 with an ordered magnetic momentμ U = (0.85±0.03)μ B , which coexists with the superconducting state. This is by far the largestμ U value observed for any heavy fermion superconductor. For UNi2Al3, no long-range magnetic order could be observed for temperaturesT≧1.5 K, yielding an upper limit of the ordered moment of 0.2μ B .

Calculated electronic and magnetic structure of the nitrides NiFe3N and PdFe3N.

Abstract: The electronic and magnetic structure of the antiperovskite-structure transition-metal nitrides ${\mathrm{NiFe}}_{3}$N and ${\mathrm{PdFe}}_{3}$N are calculated employing the augmented-spherical-wave method. From these calculations the binding character is found to be mainly covalent for the nitrogen-iron bonds and metallic between the iron and nickel atoms, as stated in the earlier literature. The magnetic structure exhibits itinerant moments for the iron atoms and more localized moments at the Ni (Pd) atoms. Effects of external pressure are studied by varying the lattice constant. Such calculations yield the pressure dependence of the magnetic moments and the hyperfine fields. The total energy is computed as a function of magnetic moment and volume, yielding total-energy surfaces, that provide the basis for a spin-fluctuation model at finite temperatures. The possibility of an Invar-like behavior is found.

Spin-liquid ground state of the half-filled Kondo lattice in one dimension.

Abstract: By finite-size analysis of exact diagonalization the existence of a finite spin gap is confirmed for both antiferromagnetic and ferromagnetic coupling in the half-filled Kondo lattice model. For all finite values of antiferromagnetic coupling the system belongs to a class of coherent Kondo spin singlet, whereas for any finite ferromagnetic coupling it scales to a spin S=1 antiferromagnetic chain with a Haldane gap.

Magnetic imaging at the atomic level

Abstract: Magnetic contrast at the atomic level has been observed for the first time in scanning tunneling microscopy experiments on a magnetite (Fe3O4(001)) surface using in-situ prepared ferromagnetic Fe tips. A periodic corrugation with a 12 A periodicity is clearly observed along the rows of FeB-sites which corresponds to the repeat period of Fe2+ and Fe3+ along these rows. This periodicity is not observed by using non-magneticW tips although the rows of FeB-sites can be resolved as well. The magnetic contrast observed with Fe tips is attributed to the different spin configurations of the magnetic ions Fe2+ and Fe3+ in Fe3O4.

Perpendicular anisotropy and antiferromagnetic coupling in Co/Ru strained superlattices.

Abstract: Despite the large lattice mismatch (\ensuremath{\simeq}8%), we demonstrate the growth of epitaxially ordered Co/Ru superlattices consisting of hcp (0001) Co and Ru sublayers. For small Ru interlayer thickness, a large antiferromagnetic exchange coupling between the Co sublayers is observed that influences strongly the magnetization process, as the magnetization within Co sublayers can no longer be considered to be fully locked ferromagnetically. For small Co and Ru thicknesses, the magnetization is oriented along the film normal, while the adjacent Co layers are strongly coupled antiferromagnetically. For this peculiar magnetic structure, a first-order spin-flop transition from an antiferromagnetic state to a ferromagnetic canted state is observed and clearly revealed by use of a ferromagnetic-resonance technique.

Three-dimensional structures of magnetostatic atmospheres. IV: Magnetic structures over a solar active region

Abstract: A static model is presented for the long-lived structures over a solar active region dominated by a pair of sunspots of opposite magnetic polarities. The magnetic field is approximately force-free high in the atmosphere but interacts strongly with the plasma and gravity in the lower region through cross-field electric currents. Basic atmospheric features long discussed in the literature can be reproduced in realistic geometry, such as the density depletion over a sunspot, magnetic shear, and levitated magnetic flux ropes interpretable as chromospheric filaments

Magnetic structure of CuO by neutron diffraction with polarization analysis

Abstract: Neutron diffraction with polarization analysis was performed on a single crystal of CuO, oriented with its alpha * and c* axes in the diffraction plane. It was verified that in the commensurate phase I(0-212.7 K; k=(1/2, 0, -1/2)), the magnetic moments were indeed along b*, but it was found that in the incommensurate phase II (212.7-232.5 K; k=(0.506, 0, -0.483)) there also exists a component of the magnetic moment along b* in disagreement with an earlier study by Forsyth et al. (1988). The data are in agreement with the very recent results of Brown et al. (1991). The authors propose a helimagnetic arrangement in the incommensurate phase II, with the magnetic moments rotating in the plane (b*; 0.506 alpha *+1.517c*). They present a short account of their experimental results, a study of the stability of the magnetic structure in phase II and the symmetry analysis of the possible magnetic space groups.

Neutron-scattering studies on CeM2Ge2(M=Ag, Au, and Ru).

Abstract: The results of elastic, quasielastic, and inelastic neutron-scattering studies on polycrystalline CeM 2 Ge 2 (M=Ag, Au, and Ru) are presented. All compounds reveal long-range magnetic order at low temperatures. Ferromagnetic (M=Ru), antiferromagnetic (M=Au), and incommensurate (M=Ag) structures were detected. Using time-of-flight (TOF) techniques, the crystalline electric-field splittings were determined. With high-resolution TOF experiments the temperature and wave-vector dependence of the magnetic relaxation rate was studied

Mn moment instability in the TbMn2 intermetallic compound

Abstract: Neutron diffraction experiments have been performed on a TbMn2 single crystal and on Tb(Mn0.96Fe0.04)2 powder samples. The magnetic structure of TbMn2 is metastable poised between two structures, S1 with propagation vector (2/3 2/3 0) and S2 with propagation vector (1/2 1/2 1/2). A transition from S1 to S2 can be induced either by an applied field of 4.5 T at 25 K or by chemical pressure induced by substitution of Mn by Fe. The S2 structure has been studied in Tb(Mn0.96Fe0.04)2. The transition to this structure is accompanied by a huge rhombohedral distortion and the structure itself is notable for the coexistence of magnetic and non-magnetic manganese atoms. This peculiar feature is attributed to instability of the Mn moment combined with frustration of the Mn itinerant antiferromagnetism.

Magnetization and Magnetoresistance of Fe/Gd Ferrimagnetic Multilayer Films

Abstract: Spin-flop and compensation phenomena of the magnetization in ferrimagnetic Fe/Gd multilayer films have extensively been investigated. It turns out that experimental results of the field and temperature dependence of magnetization agree fairly well with the calculated ones based on the molecular field model. In the calculation, we have obtained the magnetic structure, assuming that the composition is modulated stepwise, and that the exchange interactions in Fe and Gd layers are the same values as those in bulk Fe and Gd metals, respectively. Both longitudinal and transverse magnetoresistances show a crossover near the spin flop field due to the change in the magnetic structure. This crossover behavior is qualitatively well simulated by the calculation on the assumption of the parallel current flow in each atomic layer.

Magnetic ordering of Sm in Sm 2 CuO 4

Abstract: Long-range antiferromagnetic order of the Sm ions in ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ is observed via neutron diffraction at ${\mathit{T}}_{\mathit{N}}$=5.95 K. The magnetic structure consists of ferromagnetic sheets within the a-b planes, with the spins in alternate sheets aligned antiparallel. This spin structure and spin direction are completely different from those observed in any other copper-oxide superconductor system. The temperature dependence of the upper critical field ${\mathit{H}}_{\mathit{c}2}$ for the superconductor ${\mathrm{Sm}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ (${\mathit{T}}_{\mathit{c}}$=23.5 K) shows a sudden increase in ${\mathit{H}}_{\mathit{c}2}$ at T/${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}0.7, but this anomaly cannot be attributed to the Sm magnetic ordering. No anomaly in ${\mathit{H}}_{\mathit{c}2}$ is observed at the ${\mathit{T}}_{\mathit{N}}$ for Sm.

Magnetic structure of the Laves phase compound TiFe2

Abstract: The magnetic structure of TiFe2 which crystallizes with the C15 Laves phase structure has been determined from neutron diffraction measurements made on both single-crystal and polycrystalline samples. In the antiferromagnetic structure the Fe2 atoms in the (6h) sites with the same z coordinate are coupled ferromagnetically with the moments in successive layers alternately parallel and antiparallel to the c-axis. The Fe1 atoms which are in the (2a) sites are at an anti-centre of symmetry and carry no ordered moment. Magnetization measurements support this structure, giving a positive paramagnetic Curie temperature and showing the metamagnetic transition characteristic of ferromagnetically coupled sheets. Polarized neutron measurements indicate that in the antiferromagnetic phase the two types of Fe atom contribute almost equally to the susceptibility and suggest that the magnetic moment on the Fe1 atoms is only induced by the molecular field due to the aligned moment on the surrounding Fe2 atoms.

EPR and AFMR of Bi2CuO4 in Submillimeter Wave Region

Abstract: Paramagnetic and antiferromagnetic resonances have been observed for powder samples of Li 2 CuO 2 in submillimeter wave region using pulsed magnetic fields at temperatures from 1.9 K to 265 K. At the paramagnetic state, the g -values have been determined to be g // =2.30 and g ⊥ =2.01. The line-width estimated from the powder signal increases as the frequency increases. Antiferromagnetic resonance modes along the principal axes have been clearly observed even in the powder samples below the Neel temperature and explained by the conventional antiferromagnetic resonance theory. It has been found that the easy axis is parallel to the direction of g // and the anisotropy is almost uniaxial and fairly strong. This fact is quite different from other antiferromagnetic copper oxides. Obtained parameters by AFMR are A =90 mol/emu, A x ′ =0.061 A, A y ′ =-0.021 A, A z ′ =-0.040 A, Γ x ′ =-0.028 A, Γ y ′ =0.018 A and Γ z ′ = 0.010 A.

Magnetic structures and interactions in erbium

Abstract: The magnetic structures in a single crystal of isotopically enriched erbium have been studied using a high-resolution neutron diffraction technique at the HFIR reactor at Oak Ridge National Laboratory. In addition, these structures have been examined by mean-field calculations, in which the primary interactions were derived from magnetization and spin-wave measurements. The agreement between theory and experiment is generally very good. At intermediate temperatures, the structure is based on an elliptical cycloid in the a-c plane, and blocks of moments with alternating positive and negative components in the c-direction give rise to a series of commensurable structures in the manner proposed by Gibbs and co-workers (1986). These structures are, however, distorted by two-ion couplings of trigonal symmetry, which reflect the different orientation of the two HCP sublattices and have a magnitude which is a substantial fraction of the isotropic exchange interaction. The result is a wobbling cycloid, in which there is an oscillating moment in the b-direction whose period differs from that of the basic cycloidal structure. In the low-temperature cone phase, the moments bunch around alternating a-directions in a pattern with trigonal rather than hexagonal symmetry. Some further consequences of the trigonal couplings on the low-symmetry magnetic structures in erbium and holmium are discussed.

The magnetic structure of HoMn2

Abstract: The crystal and magnetic structure of the C15 Laves phase of HoMn2 was restudied using powder neutron diffraction. High-resolution spectra showed HoMn2 to remain cubic Fd3m below the magnetic transition temperature. The Ho spins assume a spin-canted ferromagnetic structure with 7.9 mu B per Ho atom. One out of four Mn sites carries a moment of 0.6 mu B induced by the strongly polarizing magnetic environment of ferromagnetically coupled near-neighbour (111) planes of rare-earth spins. A small thermal expansion anomaly accompanied by a spin reorientation is found at the Curie point of 25 K; the Neel point of the system lies at 31 K.

On the magnetic structure of iron

Abstract: Publisher Summary A ferromagnetic crystal in the non-magnetized state consists of separate regions of spontaneous magnetization, each of which is magnetized to saturation. The magnetic free energy of a ferromagnetic crystal is composed of the energy of magnetic anisotropy. The energy connected with magnetostriction deformation of the body should also be taken into account in cubic crystals. It should be determined from the minimum condition of the total energy for a given orientation of the layers, after which the most advantageous orientation of the layers themselves with respect to the coordinate planes has to be determined from the same condition. The width of the layers, however, is found to be inexpressible explicitly in the general case. In regard to the experimental data on the ferromagnetic structure of iron, quite a large number of investigations are available in which this structure was studied by the well-known method of ferromagnetic powders or colloids.

The low-temperature antiferromagnetic structure of Mn5Si3 revised in the light of neutron polarimetry

Abstract: The magnetic structures previously proposed for the low-temperature antiferromagnetic phase (AF1) of the intermetallic compound Mn5Si3 have been revised in the light of the results of neutron polarimetry in conjunction with unpolarized-neutron single-crystal integrated-intensity measurements. At 4.2 K the commensurate magnetic unit cell is orthorhombic with a=6.889, b=11.901 and c=4.805 AA and it is related to the hexagonal cell of the paramagnetic phase by b approximately= square root 3a. The hexagonal cell contains four Mn1, six Mn2 and six Si atoms. The non-collinear magnetic structure of the AF1 phase is stable below 66 K and has monoclinic symmetry. At 4.2 K the Mn1 atoms have moments of 1.20(5) mu B inclined parallel and antiparallel to the polar direction theta =116(1) degrees , phi =105(1) degrees , where theta is measured from (001) and phi from (010). One third of the Mn2 atoms carry no moment, one third have moments of 2.30(9) mu B at +or-( theta =70(1) degrees , phi =93(1) degrees ) and the remainder have 1.85(9) mu B at +or-( theta =21(1) degrees , phi =11(7) degrees ). The non-collinearity is attributed to topological frustration and the wide variation in the magnetic moments to the combined effects of Mn moment instability, frustration and single-ion anisotropy.

Infrared lines as probes of solar magnetic features. V - The magnetic structure of a simple sunspot and its canopy

Abstract: The strength and inclination of the magnetic field of a simple, relatively symmetric sunspot are determined using the extremely Zeeman sensitive Lande g=3 Fe I line at 15648 A. This spectral line allows a reliable determination of the field strength from the umbra to the outer boundary of the penumbra. The largest field strength observed in the sunspot is approximately 3100 G, the field strength at different points along the outer penumbral edge varies between 800 G and 1000 G. The angle of inclination at the outer penumbral boundary is close to 80°. Using λ 15648 A, the magnetic field of the sunspot can be followed well beyond the outer edge of the penumbra

Magnetic and electrical properties of GdNi1-xCux compounds

Abstract: The magnetic properties of the orthorhombic compounds GdNi1-xCux have been studied by means of magnetization, resistivity and neutron diffraction measurements. GdNi and GdNi0.7Cu0.3 show ferromagnetic structures while for GdNi0.4Cu0.6 the authors propose a helimagnetic structure. The link between the macroscopic magnetic properties in the ordered phase and the magnetic structures is also stressed. Comparison with other RNi1-xCux compounds with strong magnetocrystalline anisotropy allows one to clarify the role of the magnetic interactions as well as the importance of the magnetocrystalline anisotropy in all these pseudo-binary compounds.

Investigation of the crystal and magnetic structure of the perovskite system Sr2FeTiO6 –y by Mössbauer spectroscopy and neutron diffraction

Abstract: The crystal structure of the cubic perovskite Sr2FeTiO5.81 has been refined in space group Pmm using neutron powder diffraction data collected at 2 K. There is no evidence for long-range magnetic order. Mossbauer and magnetic susceptibility data show a phase transition in the vicinity of 20 K which has been interpreted in terms of spin-glass behaviour. The long-range structural disorder of the B-site cations, and the mixed oxidation states of the iron (shown to be Fe3+ and Fe4+) lead to incompatible superexchange interactions and frustration of the spin system in establishing long-range order.

Magnetic STM with a non-magnetic tip

Abstract: An apparatus and method for imaging the magnetic structure and the magnetic domains of a sample is described incorporating a scanning tunneling microscope (STM), a voltage generator for varying the voltage on the tip, an ammeter for measuring the current through the tip, circuitry to determine the tip voltage at zero current and a current source for passing current longitudinally through the sample. The invention may further include an applied magnetic field in the plane of the sample and orthogonal to current passing through the sample. A high density non-volatile memory is described incorporating the above apparatus except for the applied magnetic field and further incorporating a layer of ferromagnetic material having magnetic domains therein indicative of information.