Showing papers on "Magnetic structure published in 1986"

Magnetic structure of erbium.

Abstract: We present a synchrotron x-ray scattering study of the magnetic phases of erbium. In addition to the magnetic scattering located at the fundamental wave vector tau/sub m/ we also observe scattering from magnetoelastically induced charge modulations at the fundamental wave vector, at twice the fundamental, and at positions split symmetrically about the fundamental. As the temperature is lowered below 52 K the charge and magnetic scattering display a sequence of lock-in transitions to rational wave vectors. A spin-slip description of the magnetic structure is presented which explains the wave vectors of the additional charge scattering.

Microscopic Methods in Metals

Abstract: 1. Concerning Methods.- 1.1 Descriptive Methods.- 1.2 Abbreviated Methods.- 1.3 Name-Tag Methods.- 2. Scanning Acoustic Microscopy.- 2.1 Principle of Scanning Acoustic Microscopy (SAM).- 2.2 The Image Contrast of Solids in the Reflection Scanning Acoustic Microscope V(z)-Curves.- 2.3 Examples of Practical Applications of Reflection Scanning Acoustic Microscopy.- 2.3.1 Grain Structure.- 2.3.2 Diffusion Zones.- 2.3.3 Materials Defects.- 2.4 Outlook.- References27.- 3. High-Resolution Electron Microscopy.- 3.1 Background.- 3.1.1 Historical Development.- 3.1.2 Conventional vs. High-Resolution Electron Microscopy.- 3.2 Basic Principles of High-Resolution Electron Microscopy.- 3.2.1 Formation of Lattice Fringe Images.- 3.2.2 Formation of Many-Beam Lattice Images.- 3.2.3 Image Simulation by the Multislice Method.- 3.3 Applications.- 3.3.1 Defect and Defect Analysis.- 3.3.2 Amorphous Metals and Alloys.- 3.3.3 Ordered Alloys and Intermetallic Compounds.- 3.3.4 Phase Transformation.- 3.3.5 Surface, Grain Boundary and Interface.- 3.4 Outlook.- References.- 4. Field Ion Microscopy.- 4.1 Principles and Techniques.- 4.1.1 Magnification, Resolution, and Image Formation.- 4.1.2 Field Evaporation and Desorption.- 4.1.3 Specimen Preparation.- 4.1.4 Image Detection.- 4.1.5 Variants of the Field Ion Microscope.- 4.1.6 Field Emission Field Ion Microscopy.- 4.1.7 The Atom-Probe.- 4.2 Illustrative FIM Studies.- 4.2.1 Atomic Events on Solids.- 4.2.2 Field Evaporation and Desorption Measurements.- References.- 5. X-Ray and Neutron Diffraction.- 5.1 Diffraction of Neutrons and X-Rays by Poly- and Non-Crystalline Alloys.- 5.1.1 Neutron and X-Ray Scattering.- 5.1.2 General Scattering Theory for Solid and Liquid Solutions.- 5.1.3 Binary Alloys.- 5.1.4 Chemical Short-Range Order in Binary Alloys.- 5.1.5 Topological Order in Crystalline Solid Solutions.- 5.1.6 Integrated Intensity.- 5.2 Experimental Techniques.- 5.2.1 X-Ray and Neutron Sources.- 5.2.2 Instrumentation.- 5.3 Applications.- 5.3.1 Structure of Metallic Glasses and Liquids.- 5.3.2 Phase Analysis of Poly-Crystalline Mixtures.- 5.3.3 Small-Angle Scattering.- 5.3.4 Line Profile Analysis of Powder Pattern Peaks.- 5.3.5 Residual Stress Measurements.- 5.3.6 Grazing Incidence X-Ray Scattering.- References.- 6. Extended X-Ray Absorption Fine Structure.- 6.1 Theory.- 6.1.1 Overview.- 6.1.2 The Standard EXAFS Formula.- 6.1.3 Validity of the Theory.- 6.2 Experimental Techniques.- 6.3 Analysis.- 6.3.1 Basic Manipulations.- 6.3.2 Determination of Structural Parameters.- 6.3.3 Guidelines for Using EXAFS Spectroscopy.- 6.4 Experimental Applications.- 6.4.1 Local Environment Surrounding Solute Atoms.- 6.4.2 Comparison to Theoretical Models.- 6.4.3 Debye-Waller Factors - Mean-Square Displacements.- 6.4.4 Structure of Amorphous Metals.- References.- 7. X-Ray Photoelectron Spectroscopy.- 7.1 Historical.- 7.2 Basic Principles.- 7.2.1 Photoemission.- 7.2.2 The Core-Electron Binding Energy in a Metal.- 7.2.3 Core-Electron Satellites.- 7.2.4 Plasmons, Electron Mean-Free Path, and Surface Aspects of XPS.- 7.2.5 Measurement of Core-Electron Binding Energy by XPS.- 7.3 Related Methods.- 7.3.1 Angle-Resolved Photoemission Spectroscopy (ARPES).- 7.3.2 Inverse Photoemission Spectroscopy (IPES).- 7.3.3 X-Ray Absorption Edge Spectroscopy (XAS).- 7.3.4 X-Ray Emission Spectroscopy (XES).- 7.3.5 Auger Electron Spectroscopy (AES).- 7.3.6 Electron Energy Loss Spectroscopy (EELS).- 7.4 Applications.- 7.4.1 Chemical Analysis.- 7.4.2 Binding Energy Shifts.- 7.4.3 Valence Electron Density of States.- 7.4.4 Conduction-Electron Screening.- 7.5 Recent Developments.- 7.5.1 Surface Atoms.- 7.5.2 Metal Clusters.- References.- 8. Auger Electron Spectroscopy.- 8.1 History.- 8.2 Principles.- 8.2.1 The Auger Energies.- 8.2.2 The Auger Electron Emission Depth.- 8.2.3 Quantitative Analysis by AES.- 8.2.4 Composition Depth Profiling.- 8.2.5 Spatial Resolution in Auger Microscopy.- 8.3 The Instrument.- 8.4 Related Methods.- 8.5 Applications.- 8.5.1 Grain Boundary Segregation Studies.- 8.5.2 Surface Segregation.- 8.5.3 Grain Boundary Diffusion.- 8.5.4 Defect-Enhanced Diffusion.- 8.5.5 Other Studies.- 8.6 Future Developments.- References.- 9. Positron Annihilation.- 9.1 Background.- 9.2 Basic Principles.- 9.2.1 Positron Thermalization.- 9.2.2 Annihilation Process.- 9.3 Experimental Methods.- 9.3.1 Positron Sources.- 9.3.2 Angular Correlation of Annihilation Photons.- 9.3.3 Doppler Broadening of Annihilation Radiation.- 9.3.4 Lifetime Measurements of Positrons.- 9.3.5 Monoenergetic Positron Beams.- 9.4 Applications.- 9.4.1 Fermi Surfaces in Metals and Alloys.- 9.4.2 Metals at Various Temperatures.- 9.4.3 Radiation Induced Defects.- 9.4.4 Amorphous Alloys.- 9.4.5 Surfaces.- 9.5 Conclusions and Outlook.- References.- 10. Muon Spectroscopy.- 10.1 Basic Principles of the Experimental Techniques.- 10.2 The Depolarization Functions.- 10.2.1 Slow Dipole Fluctuations.- 10.2.2 Dipole Fluctuations and Correlation Functions.- 10.3 Diffusion Studies by ?+ SR.- 10.3.1 Standard Theory of the Diffusion of a Light Interstitial in a Metal.- 10.3.2 Effects of Impurities and Defects on the ?+ Damping Rate.- 10.3.3 Quantum ?+ Diffusion in Metals.- 10.3.4 Classical ?+ Diffusion in Metals.- 10.3.5 ?+ Diffusion in Hydrides.- 10.4 Magnetic Studies by ?+ SR.- 10.4.1 Static Properties.- 10.4.2 Dynamic Properties.- 10.5 Conclusions.- References.- 11. Perturbed Angular Correlation.- 11.1 Background.- 11.2 Principles.- 11.2.1 Spin Alignment.- 11.2.2 Spin Precession.- 11.3 Detection of Hyperfine Fields.- 11.3.1 Magnetic Dipole Interaction.- 11.3.2 Electric Quadrupole Interaction.- 11.4 Radioactive Probes, Preparation and Techniques.- 11.4.1 Probe Atoms and Sample Preparation.- 11.4.2 Data Recording and Analysis.- 11.4.3 PAC and Mossbauer Spectroscopy.- 11.5 Applications.- 11.5.1 Hyperfine Fields at Impurities.- 11.5.2 Surface Studies.- 11.5.3 Diffusion of Light Gases in Tantalum.- 11.5.4 Defects and Impurities.- 11.6 Future Developments and Conclusions.- References.- 12. Nuclear Magnetic Resonance.- 12.1 Introductory Comments.- 12.2 Physical Background of an NMR Experiment - Hyperfine Interactions.- 12.2.1 Nuclear Paramagnetism.- 12.2.2 Thermal Equilibrium and Dynamic Properties of the Spin System.- 12.2.3 Electric Interaction - Nuclear Quadrupole Moment.- 12.2.4 Summary.- 12.3 Basic NMR Experiment - Principles and Setup.- 12.3.1 Spin Movement in a Magnetic Field.- 12.3.2 Free Induction Decay - Transverse Relaxation Time.- 12.3.3 Spin Echo - Homogeneous and Inhomogeneous Broadenings.- 12.3.4 Spin-Lattice Relaxation Measurement.- 12.3.5 Spectrum Measurement.- 12.3.6 NMR Techniques and Instruments.- 12.3.7 Phase Coherent Pulsed NMR Spectrometer.- 12.3.8 Feasibility of an NMR Observation.- 12.4 NMR Outputs - Microscopic Origin.- 12.4.1 Hyperfine Fields.- 12.4.2 Frequency Shifts.- 12.4.3 Relaxation Times.- 12.4.4 Electric Field Gradient.- 12.4.5 Summary.- 12.5 Applications - Structural Investigations.- 12.5.1 Phase Analysis.- 12.5.2 Chemical Short-Range Order.- 12.5.3 Structure of Amorphous Metals.- 12.5.4 Atomic Motion in Metals.- 12.6 Applications - Electronic and Magnetic Properties.- 12.6.1 Local Magnetic Susceptibilities and Moments.- 12.6.2 Impurities in Metals.- 12.6.3 Magnetic Impurities - Occurrence of Magnetism.- 12.6.4 Concentrated Alloys - Local Environment Effects.- 12.6.5 Magnetic Structure and Phase Transition.- 12.6.6 Spin Fluctuations in Rare Earth Based Compounds.- 12.6.7 Electronic Phase Transitions.- 12.7 Conclusion and Outlook.- References.- 13. Mossbauer Spectroscopy.- 13.1 History.- 13.2 Principles.- 13.2.1 Line Width.- 13.2.2 Line Shape.- 13.2.3 Line Intensity (Recoil-Free Fraction).- 13.3 Mossbauer Isotopes.- 13.3.1 Sources.- 13.3.2 Absorbers.- 13.4 Methodology.- 13.4.1 Classical Setup.- 13.4.2 Scattering Geometry.- 13.5 Hyperfine Interactions.- 13.5.1 Isomer Shift.- 13.5.2 Magnetic Hyperfine Interaction.- 13.5.3 Electric Quadrupole Interaction.- 13.5.4 Mixed Interactions.- 13.5.5 Polarimetry.- 13.6 Relativistic Effects.- 13.7 Time-Dependent Effects.- 13.8 Applications.- 13.8.1 Iron.- 13.8.2 Phase Analysis.- 13.8.3 Texture.- 13.8.4 Defects.- 13.8.5 Amorphous.- 13.8.6 Relaxation Phenomena.- 13.9 Outlook.- References.- Additional References with Titles.

Spin slips and lattice modulations in holmium: A magnetic x-ray scattering study

Abstract: The analysis of data obtained in magnetic X-ray scattering experiments on rare earth metals, particularly holmium, has led to a phenomenological model for one-dimensional spatially propagating magnetic structures. Based on the concept of spin slips or discommensurations, this model explains the observed lock-in transitions in the magnetic spirals of the rare earths in terms of simple commensurate structures. Further, the anomalous intensities of previously observed higher harmonic magnetic satellites, as well as the qualitative behavior of the magnetic wave vector in the presence of a magnetic field, are understood directly within the spin slip description. We discuss how the presence of spin slips in the magnetic structure can lead to modulations of the crystal lattice as demonstrated by a recent magnetic X-ray study of holmium.

Magnetic structure and dynamics in the alpha and beta phases of solid oxygen.

Abstract: We describe a comprehensive neutron scattering study of ..cap alpha..- and ..beta..-O/sub 2/. In ..cap alpha..-O/sub 2/, an inelastic feature at 10 MeV is identified with the b(/2 zone boundary, enabling the first measurement of the intrasublattice exchange in this system. We find the intrasublattice exchange constant to be -1.2 +- 0.1 MeV. Taken in conjunction with the intersublattice exchange, -2.4 MeV determined from magnetic susceptibility, we find that the system is very close to a magnetic instability. The ..cap alpha..-phase staggered magnetization drops with increasing temperature, extrapolating to a purely magnetic transition at 31 K, which is 7 K above the ..cap alpha..-..beta.. transition. This is only 40% of the mean-field ordering temperature, implying that the magnetic couplings are predominantly two-dimensional (2D). ..beta..-O/sub 2/ has 2D short-range magnetic correlations. These results are discussed in light of a model of magnetoelastic coupling in a frustrated triangular antiferromagnet.

Magnetic structure of the heavy-fermion compound U2Zn17.

Abstract: PHYSICAL REVIE%' VOLUME 33, NUMBER 5 Magnetic structure of the heavy-fermion compound MARCH 1986 U2Znt7 D. E. Cox, G. Shirane, and S. M. Shapiro Physics Department, A Tcf T Brookhaven National Laboratory, Bell Laboratories, Upton, New York G. Aeppli 600 Mountain Avenue, Murray Hill, New Jersey 07974 Z. Fisk and J. L. Smith Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 l. Kjems Risd National Laboratory, Roskilde, Denmark H. R. Ott Laboratorium I'ur I estkorperphysik, Eidgenossische Technische Hochschule-Honggerberg, 8093 Zurich, Switzerland (Received 26 September 1985) The phase transition of U2zni7 at 9. 7 K has been investigated by neutron powder diffraction. The transi- tion corresponds to the onset of antiferromagnetic order where the U moments are oriented antiparallel to their nearest neighbors within the basal planes and the near neighbor along the c axis of this rhombohedral compound. At 5 K, the ordered moments lie within the basal planes and are of magnitude (0. 8+0. 1) p, &, which is substantially below the paramagnetic moment of 2.25', &/U atom given by high-temperature sus- ceptibility data. The list of heavy-fermion systems, which display ex- traordinarily large ( & 400m, ) effective conduction electron masses as deduced from the low-temperature specific heat, and metals remaining now includes both superconductors Recent magnetic paramagnetic to the lowest temperatures. susceptibility and specific-heat measurements suggest that there are also heavy-fermion systems which undergo transi- tions to magnetically ordered ground states. ' In this paper, we present the first neutron-scattering determination of magnetic order in a heavy-fermion system, U2Zni7. The essential results are as follows. First, the ordering, which 10 K, is exceedingly simple, with the mag- sets in at T~ netic unit cell identical to the nuclear unit cell, and the mo- ments associated with the two U atoms in the primitive unit cell oriented in opposite directions. Second, the ordered moment is (0. 8+0. 1)p, s/U atom which is well below the moment of 2. 25@, s /U atom deduced from the high- temperature (bulk) susceptibility. U2Zni7 was made by heating the appropriate amounts of U and Zn to 1050'C in an evacuated BeO crucible. Ap- proximately 20 g of material were crushed into a coarse pounder and loaded into a cylindrical aluminum sample hold- er in an atmosphere of helium. Data were collected at the Brookhaven High Flux Beam Reactor with 2.353-A neu- trons from a pyrolytic-graphite monochromator in the (002) filter suppressed higher-order setting. A pyrolytic-graphite wavelengths. For the structure determination at 15 K ( T~), a pyrolytic-graphite analyzer in the (004) setting was used in order to optimize the resolution at higher scattering angles. The collirnations were 20'-open-40'-20' for in-pile, monochromator-sample, sample-analyzer, and analyzer-detector, respectively. The resulting diffraction pattern showed a number of weak impurity peaks in addi- tion to those characteristic of U2Zni7. The major impurity was identified as Zn, estimated to be about 5'/0 by weight. The remaining peaks were an order of magnitude weaker «1% of the strong U2Zn~7 peaks) and (with intensities could not be identified. The d spacings are listed in the cap- tion to Fig. 1, and do not correspond to those of UZni2 or e-U. At room temperature, U2Zn&7 (Ref. 3) has the Th2Zn~7- type structure, which is one of a series of ordered structures [e. g. , Pu3Zn22, UZnt2 (Ref. 5)] which can be derived from that of CaCu5. In U2Zni7, Zn occupies the Cu sites and U is ordered on two-thirds of the Ca sites, the remaining one- third being replaced by pairs of Zn atoms about 2. 6 A apart. The ordered structure has rhombohedral symmetry, space group R3m, with hexagonal lattice constants v 3a and 3c with respect to those of the parent CaCu5-type cell. Be- cause the structural parameters for U2Zni7 near its 10-K transition were unknown, we carried out a Rietveld analysis of the data sho~n in Fig. 1. Regions around the impurity peaks and Al reflections from the sample holder were ex- cluded from the refinement, and background contributions were estimated by interpolation between values obtained by averaging over regions where no Bragg peaks were present. Table I displays the lattice parameters and atomic coordi- nates given by the profile refinement. In the upper frame of Fig. 1, the solid line represents the calculated profile which best fits the data, while in the lo~er frame, the difference between the calculated and observed profiles is shown. The results are in exce11ent agreement with a room-temperature x-ray scattering determination of the structure. Belo~ 10 K some very weak additional scattering at some of the low-angle nuclear peak positions was observed. To gain more intensity tics to be obtained and thus allow adequate counting statis- in a reasonable period of time, the

The static, paramagnetic, spin susceptibility of metals at finite temperatures

Abstract: The authors present a theory for the static, paramagnetic, spin susceptibility for metals at finite temperatures. It is based on a 'first principles' mean-field theory of itinerant magnetism and provides an expression which resembles that of a classical Heisenberg model added to an itinerant component. The quantities which occur in this expression all depend on the underlying electronic structure of the disordered local moment (DLM) paramagnetic state and thus demonstrate the subtle combination of the localised and itinerant aspects of this problem. The theory is applied to BCC iron and FCC nickel. A Curie temperature of 1280K and effective Curie constant moment of 1.97 mu B is obtained for iron in reasonable agreement with experiment. From the calculations of the wavevector dependent susceptibility, which are compared with quasi-elastic neutron scattering data, equal time spatial, magnetic correlation functions are inferred which are consistent with the magnetic structure of the initially imposed (DLM) paramagnetic state. The calculations for nickel provide a very different picture. At temperatures at which the DLM paramagnetic state differs from the conventional 'Stoner' state, it is shown that this state is unstable via the sensitivity of the moments to their orientational environment and that the theory for the paramagnetic state is equivalent to that of the Stoner-Wohlfarth picture. It is concluded that an improved theory for nickel must incorporate a mechanism for 'local' moment formation on several sites.

Magnetic structure and magnetic properties of the spinel solid solutions ZnCr2xAl2-2xS4 (0.85?x?1). I. Neutron diffraction study

Abstract: The spinel solid solutions ZnCr2xAl2-2xS4 have been studied by means of neutron diffraction in the concentration range 0.85

Polarived neutron diffraction study of UNi2

Abstract: A polarived neutron diffraction study of the hevagonal UNi2 was performed. The magnetivation of this weak ferromagnet (with Curie temperature Tc = 21 K and saturation moment μs ≈ 0.08 μB per formula unit) is essentially due to the 5f electrons of the uranium. The 5f form factor is most unusual for uranium, with a pronounced mavimum at about (sin gv)λ ≈ 0.2 A−1 indicating the presence of a very large orbital moment. This is the first evperimental evidence of a large orbital magnetivation density in an itinerant electron ferromagnet.

Intrinsic magnetic properties of compounds with the Nd2Fe14B structure

Abstract: Results on magnetization, Curie temperature, magnetocrystalline anisotropy, magnetic structure, spin-reorientation transitions and magnetostriction are reviewed for the R2M14B series of compounds, with M ≡ Fe or Co. Some Fe-Co solid solutions and hydrides are also discussed. The results are analysed in terms of exchange and crystal-field interactions, and some unsolved problems are identified.

Magnetic properties of ferrihydrite

Abstract: The magnetic properties of a sample of synthetic ferrihydrite have been investigated by use of Mossbauer spectroscopy. The area ratios of the Mossbauer lines are not significantly changed by application of magnetic fields at 5 K, indicating an amorphous magnetic structure. An average magnetic moment per particle of 3.1×10−21 JT−1 was estimated from the dependence of the hyperfine splitting of the Mossbauer spectra on external magnetic fields at 80 K.

Magnetic Structure Change in Ba2Mg2Fe12O22

Abstract: Neutron diffraction study was carried out on a single crystal of Ba 2 Mg 2 Fe 12 O 22 . The magnetic structure of Ba 2 Mg 2 Fe 12 O 22 changes at 195 K, collinear ferrimagnet above 195 K and proper helix below 195 K. The helix is propagated along the c -axis by rotations of the large and small spin-bunches. The turn angle of the helix is 59.6° at 77 K and rapidly decreases as the temperature approaches 195 K. Results are discussed from a point of view of the exchange interactions.

Reorientation phase transition in NdfeO3

Abstract: Crystal and magnetic structure of polycrystalline NdFeO3 was determined by high resolution neutron diffraction in the spin reorientation region. The magnetic moment direction of the Fe3+ ion changes from Gz via GxGz to Gx between 105 and 200 K. Moreover, a change of the Fe-Nd distance within the reorientation region is observed.

Magnetic X-ray scattering with synchrotron radiation

Abstract: With the availability of high-brilliance synchrotron radiation from multiple wigglers, magnetic X-ray scattering has become a powerful new probe of magnetic structure and phase transitions. Similar to the well-established magnetic neutron scattering technique, magnetic X-ray scattering methods have many complementary advantages. A brief review is presented of the history of magnetic X-ray scattering as well as recent results obtained in studies of the rare-earth magnet holmium with emphasis on instrumentational aspects. In particular, the development of a simple polarization analyzer to distinguish charge and magnetic scattering is described.

Cation distribution and random spin canting in LaZnFe11O19

Abstract: A new M-type hexagonal ferrite with composition LaZnFe11O19 has been synthesised and its cation distribution and magnetic properties have been investigated. The location of Zn ions among the five sublattices of the M structure has been deduced from temperature dependent Mossbauer spectra. It is found that less than 10% of Zn ions may occupy the pseudo-tetrahedral 4e sublattice, the remaining Zn ions being located at the tetrahedral 4fIV sublattice. From both, the low value of the saturation magnetisation, M0=19.0 mu BFU-1, and the observation of a high field differential susceptibility it is concluded that LaZnFe11O19 has a non-collinear magnetic structure. Based on the hyperfine interactions and cation distribution obtained from the Mossbauer spectra the authors propose the existence of a random spin canting in the octahedral 12k and 2a sublattices. A theoretical analysis in the scope of the random spin canting model is performed in order to justify the observed noncollinear disordered spin structure.

Spin density and bonding in Mn(H2O)2+6in ammonium manganese Tutton salt

Abstract: The structure of (NH 4 ) 2 Mn(SO 4 ) 2 6H 2 O has been determined at 4.2 K by neutron diffraction. It confirms the main features of an earlier X-ray structure and gives details of the water molecules and the hydrogen bonding. The flipping ratios associated with the more intense Bragg reflections were measured by polarized neutron diffraction. By using the neutron structure factors, 495 unique magnetic structure factors were obtained and the spin distribution in the Mn(H 2 O) 2+ 6 ion was modelled. There appears to be no reduction in spin population in the Mn 2+ d-orbitals (t 3.1 2g e 2.0 g ), but a small negative diffuse population, 4s 0.2 -, is present, which together with a total ‘overlap’ population of –0.1 spins balances the 0.2 net positive spin found on the water molecules. Unexpectedly, the spin on the water molecules seems to be concentrated on the side of the oxygen atoms far from the manganese atom, and to reside more on the hydrogen atoms than the oxygen atoms. Evidence for the expected negative spin component in the Mn—O bonds arising from antibonding overlap is found.