Showing papers on "Magnetic structure published in 1988"

Spin-density-wave antiferromagnetism in chromium

Abstract: A comprehensive account is given of the macroscopic and microscopic physical properties of chromium (and where appropriate those of its dilute alloys) that relate to its antiferromagnetism. Neutron scattering is treated in great detail, first in the historical introduction, then as an experimental probe of both the magnetic structure and the excitations of the incommensurate spin-density-wave state and (with the assistance of x rays) of the concomitant charge-density wave and strain wave. Neutron scattering is considered as a tool to explore not only the disappearance of long-range order with increasing temperature through the growth of excitations as the weak first-order N\'eel transition is approached, but also the persistence of these spin fluctuations well into the paramagnetic state---processes that are still little understood. The article surveys, without mathematical details, model systems designed to reproduce the magnetic and thermodynamic properties of Cr. The energy-band structure calculations are given a more comprehensive review. Special attention is paid to calculations of the wave-vector-dependent susceptibility that reproduce the observed wave vector of the spin-density wave, and to a recent finite-temperature calculation that gives almost the right N\'eel temperature. The review of Fermi-surface studies emphasizes those designed to relate the spin-density wave vector (and its pressure dependence) to the nesting vector of the Fermi surface. An account is given of the spectroscopic determination of the energy gap(s), whose theoretical analysis is still unclear, and of experiments aimed at determining physical properties that throw light on the origin of the weak first-order N\'eel transition. The article describes the use of magnetic anomalies in the elastic moduli to determine the volume dependence of the exchange interaction responsible for antiferromagnetism in Cr. The experimental features of the spin-flip transition are reviewed, although a theory of this phenomenon is wanting. The experimental study of microscopic structure by the use of hyperfine-interaction properties is surveyed. An account is given of both experimental and theoretical studies of the surface of Cr and of Cr films and sandwiches. Finally, "technical antiferromagnetism" is discussed: the effect of severe internal strain in producing a commensurate antiferromagnetic state, wave-vector Q domains, polarization S domains (for which the experimental evidence is scanty), and ultrasonic attenuation as a tool to study them.

Neutron-diffraction determination of antiferromagnetic structure of Cu ions in YBa2Cu

Abstract: Neutron-diffraction experiments on ceramic powders of YBa/sub 2/Cu/sub 3/O/sub 6+//sub x/ (nonsuperconducting, with x = 0.0 and 0.15) have confirmed the existence of long-range, three-dimensional, antiferromagnetic order of the Cu spins. The structure determination was aided by the use of uniaxially oriented powders. The ordering wave vector within a CuO/sub 2/ plane is (1/21/2) and the planes are coupled antiferromagnetically along the c axis. The Neel tempreature is 400 +- 10 K for the x = 0.15 +- 0.05 sample and approx. >500 K for x = 0.0.

Dynamics and Structure of Quiescent Solar Prominences

Abstract: 1 Introduction to Quiescent Solar Prominences (E R Priest).- 1.1 Basic Description.- 1.1.1 Different Types.- 1.1.2 Properties.- 1.1.3 Development.- 1.1.4 Structure.- 1.1.5 Eruption.- 1.2 Basic Equations of MHD.- 1.2.1 Magnetohydrostatics.- 1.2.2 Waves.- 1.2.3 Instabilities.- 1.3 Prominence Puzzles.- 2 Overall Properties and Steady Flows (B Schmieder).- 2.1 Basic Properties.- 2.1.1 Description and Classification.- 2.1.2 Fine Structure in H?.- 2.1.3 Evolution of Filaments During the Solar Cycle.- 2.2 Physical Characteristics: Density and Temperature.- 2.2.1 Density and Ionization Degree.- 2.2.2 Non LTE Models.- 2.2.3 Turbulent Velocity and Electron Temperature.- 2.3 Velocity Field and Mass Flux.- 2.3.1 Instrumentation.- 2.3.2 H? Profile Analysis.- 2.3.3 Vertical Motions.- 2.3.4 Horizontal Motions.- 2.3.5 Oscillations.- 2.4 Instability.- 2.4.1 Disparition Brusque of Filaments.- 2.4.2 Model Support.- 2.4.3 Post-Flare Loops and Loop Prominences.- 2.5 Conclusion.- 3 Prominence Environment (O Engvold).- 3.1 Introduction.- 3.2 Helmet Streamers.- 3.2.1 Eclipse Photography.- 3.2.2 Morphology.- 3.2.3 Location of Current Sheet.- 3.2.4 Brightness.- 3.3 Coronal Cavities.- 3.3.1 Brightness and Structure.- 3.3.2 Temperature and Density.- 3.4 Filament Channels.- 3.4.1 Association with Neutral Lines.- 3.4.2 Poleward Migration of Filament Channels.- 3.4.3 Presence of Prominences.- 3.4.4 Temperature and Electron Pressure.- 3.4.5 Cool Matter in the Filament Channels.- 3.5 Prominence-Corona Transition Region.- 3.5.1 Line Emission.- 3.5.2 Empirical Modelling.- 3.5.3 A Fragmented and Dynamic Transition Region.- 3.6 Prominences and Environment.- 3.6.1 Magnetic Fields and Chromospheric Structure.- 3.6.2 Association with Supergranulation Network.- 3.6.3 Dynamics.- 3.6.4 The Mass of Coronal Cavity and Prominence.- 3.6.5 Coronal Voids - a Source of Prominence Mass?.- 3.7 Modelling of the Helmet Streamer/Prominence Complex.- 3.7.1 Helmet Streamer and Cavity.- 3.7.2 Magnetic Field Topology.- 3.7.3 Siphon-Type Models.- 3.8 Conclusions.- 4 Observation of Prominence Magnetic Fields (J L Leroy).- 4.1 Historical Steps.- 4 2 Investigations Based on the Polarimetry of Spectral Lines.- 4.2.1 Zeeman Effect.- 4.2.2 HanleEffect.- 4.2.3 180 1 Introduction to Quiescent Solar Prominences (E R Priest).- 1.1 Basic Description.- 1.1.1 Different Types.- 1.1.2 Properties.- 1.1.3 Development.- 1.1.4 Structure.- 1.1.5 Eruption.- 1.2 Basic Equations of MHD.- 1.2.1 Magnetohydrostatics.- 1.2.2 Waves.- 1.2.3 Instabilities.- 1.3 Prominence Puzzles.- 2 Overall Properties and Steady Flows (B Schmieder).- 2.1 Basic Properties.- 2.1.1 Description and Classification.- 2.1.2 Fine Structure in H?.- 2.1.3 Evolution of Filaments During the Solar Cycle.- 2.2 Physical Characteristics: Density and Temperature.- 2.2.1 Density and Ionization Degree.- 2.2.2 Non LTE Models.- 2.2.3 Turbulent Velocity and Electron Temperature.- 2.3 Velocity Field and Mass Flux.- 2.3.1 Instrumentation.- 2.3.2 H? Profile Analysis.- 2.3.3 Vertical Motions.- 2.3.4 Horizontal Motions.- 2.3.5 Oscillations.- 2.4 Instability.- 2.4.1 Disparition Brusque of Filaments.- 2.4.2 Model Support.- 2.4.3 Post-Flare Loops and Loop Prominences.- 2.5 Conclusion.- 3 Prominence Environment (O Engvold).- 3.1 Introduction.- 3.2 Helmet Streamers.- 3.2.1 Eclipse Photography.- 3.2.2 Morphology.- 3.2.3 Location of Current Sheet.- 3.2.4 Brightness.- 3.3 Coronal Cavities.- 3.3.1 Brightness and Structure.- 3.3.2 Temperature and Density.- 3.4 Filament Channels.- 3.4.1 Association with Neutral Lines.- 3.4.2 Poleward Migration of Filament Channels.- 3.4.3 Presence of Prominences.- 3.4.4 Temperature and Electron Pressure.- 3.4.5 Cool Matter in the Filament Channels.- 3.5 Prominence-Corona Transition Region.- 3.5.1 Line Emission.- 3.5.2 Empirical Modelling.- 3.5.3 A Fragmented and Dynamic Transition Region.- 3.6 Prominences and Environment.- 3.6.1 Magnetic Fields and Chromospheric Structure.- 3.6.2 Association with Supergranulation Network.- 3.6.3 Dynamics.- 3.6.4 The Mass of Coronal Cavity and Prominence.- 3.6.5 Coronal Voids - a Source of Prominence Mass?.- 3.7 Modelling of the Helmet Streamer/Prominence Complex.- 3.7.1 Helmet Streamer and Cavity.- 3.7.2 Magnetic Field Topology.- 3.7.3 Siphon-Type Models.- 3.8 Conclusions.- 4 Observation of Prominence Magnetic Fields (J L Leroy).- 4.1 Historical Steps.- 4 2 Investigations Based on the Polarimetry of Spectral Lines.- 4.2.1 Zeeman Effect.- 4.2.2 HanleEffect.- 4.2.3 180 Ambiguity.- 4.2.4 Instrumental Achievements.- 4.3 Indirect Magnetic Field Determinations.- 4.4 Magnetic Field at the Photospheric Level.- 4.5 Main Features of the Magnetic Field in Quiescent Prominences.- 4.5.1 Field Strength.- 4.5.2 Angle with Horizontal.- 4.5.3 Angle with Prominence Axis.- 4.5.4 Magnetic Structure with Normal or Inverse Polarity.- 4.5.5 Homogeneity of the Field.- 4.6 Some Important Problems.- 4.6.1 Magnetic Field in Sub Arc Second Structures.- 4.6.2 Paradox of Fine Vertical Structures.- 4.6.3 Determination of Currents.- 4.6.4 Evolution of Prominence Magnetic Structure.- 5 The Formation of Solar Prominences (J M Malherbe).- 5.1 Introduction.- 5.2 Overview of Observations.- 5.3 Main MHD Instabilities Involved in Prominence Formation.- 5.3.1 Radiative Thermal Instability.- 5.3.2 Resistive Instabilities.- 5.4 Steady Reconnection in Current Sheets.- 5.4.1 Incompressible and Compressible Theories.- 5.4.2 Unification of Different Regimes.- 5.5 Static Models.- 5.5.1 Condensation in a Loop.- 5.5.2 Condensation in an Arcade.- 5.5.3 Condensation in a Sheared Magnetic Field.- 5.5.4 Condensation in a Current Sheet.- 5.6 Dynamic Models: Injection from the Chromosphere into Closed Loops.- 5.6.1 Surge-Like Models.- 5.6.2 Evaporation Models.- 5.7 Dynamic Models: Condensation in Coronal Current Sheets.- 5.7.1 Numerical Simulations.- 5.7.2 Role of Shock Waves in Condensation Process.- 5.8 Unsolved Problems.- 5.9 Conclusion.- 6 Structure and Equilibrium of Prominences (U Anzer).- 6.1 Introduction.- 6.2 Prominence Models.- 6.2.1 Global Structure.- 6.2.1.1 Two-Dimensional Equilibria.- 6.2.1.1.1 Models with Normal Magnetic Polarity.- 6.2.1.1.2 Models with Inverse Magnetic Polarity.- 6.2.1.1.3 Force-Free Fields.- 6.2.1.2 Quasi-Three-Dimensional Models.- 6.2.1.3 Support by Alfven Waves.- 6.2.2 Internal Structure and Thermal Equilibrium.- 6.2.2.1 Hydrostatic Equilibrium.- 6.2.2.2 Thermal Equilibrium.- 6.3 Concluding Remarks.- 7 Stability and Eruption of Prominences (A W Hood).- 7.1 Introduction.- 7.2 Description of MHD Instabilities.- 7.3 Methods of Solution.- 7.3.1 Normal Modes.- 7.3.2 Energy Method.- 7.3.3 Non Equilibrium.- 7.4 Effect of the Dense Photosphere.- 7.4.1 Physical Arguments.- 7.4.2 Ballooning Modes.- 7.5 Coronal Arcades.- 7.5.1 Distributed Current Models - Eruptive Instability.- 7.5.2 Localised Modes - Small Scale Structure.- 7.5.3 Arcades Containing a Current Sheet.- 7.6 Thermal Stability.- 7.7 Resistive Instabilities - Tearing Modes.- 7.7.1 Introduction.- 7.7.2 Estimate of Tearing Mode Growth Rate.- 7.7.3 Effect of Line Tying.- 7.8 Simple Model of Prominence Eruption and a Coronal Mass Ejection.- 7.9 Conclusions and Future Work.- References.

Noncollinear magnetic structure of CoFe2O4 small particles

Abstract: The magnetic structure of small CoFe2O4 particles has been investigated as a function of the particle size. Samples (in the 10–100 nm size range and up) were prepared by chemical precipitation followed by a heat treatment at relatively low temperatures. Mossbauer spectra of the 57Fe nuclei, obtained with a longitudinal magnetic applied field, unambiguously establish that a noncollinear structure exists that is most pronounced for the smallest particles. The analysis indicates that a surface effect of the crystallites that make up a particle is the origin of this phenomenon. A model is proposed in which the CoFe2O4 crystallites that make up a particle consist of a core with the usual spin arrangement and a boundary surface layer with atomic moments inclined to the direction of the net magnetization. The temperature dependency of this structure is also examined.

The magnetic structure of holmium. I

Abstract: The magnetic structure of the incommensurate spiral phase of holmium has been studied using elastic neutron scattering for temperatures between 30 and 18K, the temperature region just above the transition to a ferromagnetic cone phase with a wavevector of 1/6 c*. The results show that the structure consists of commensurate regions separated by spin discommensurations or spin slips. In the commensurate regions, pairs of spins (one on each sublattice) deviate by about 10 degrees from each of the easy directions within the plane, and there is evidence for a c axis antiferromagnetic component which changes domain type from one commensurate region to another. The spin slips, which occur on alternate sublattices, have one plane of spins aligned with the planar easy directions, and are found to disturb the commensurate regions for about three planes on either side of each spin slip.

Note on the crystallographic and magnetic structure of YFe10V2

Abstract: Etude de l'occupation des sites par le vanadium. Variation du moment du fer aux trois differents sites du fer

Low-temperature magnetic structure of MnO: A high-resolution neutron-diffraction study.

Abstract: The antiferromagnetic structure of MnO was studied using high-resolution neutron diffraction from a polycrystalline sample at 8 K. The ''allowed-reflection rule'' which is obeyed by the observed reflections was deduced. By use of this rule, and without recourse to calculation of intensities, it is shown unambiguously that the magnetic structure is collinear, and the magnetic configuration is deduced. By using the (115)-to-(333) magnetic intensity ratio (whose splitting is observable only with high resolution), we found that within our experimental resolution, the spin axis is perpendicular to the unique (111) direction.

Neutron-powder-diffraction study of nuclear and magnetic structure in YBa2Cu3-xCoxO7+y with x=0.84 and y=0.32.

Abstract: Neutron powder diffraction has been used to study the crystal structure and magnetic ordering of YBa/sub 2/Cu/sub 3-//sub x/Co/sub x/O/sub 7+//sub y/ with x = 0.84 and y = 0.32. Rietveld refinement of the crystal structure was carried out at 295 K (space group P4/mmm,a = 3.888 A,c = 11.636 A) and showed in particular that Co substitutes in the Cu(1) position of YBa/sub 2/Cu/sub 3/O/sub 7-//sub y/. Long-range, three-dimensional antiferromagnetism was observed up to a Neel temperature of about 405 K. All five detected magnetic peaks could be indexed on a wave vector ((1/2 1) / 2( 1/2) with h,k,l all odd, indicating a body-centered tetragonal magnetic unit cell with a/sub mag/ = a ..sqrt..2, c/sub mag/ = 2c. At 9 K the proposed magnetic structure has moments parallel to c antiferromagnetic ordering within both Cu and Co layers. Adjacent moments in neighboring Cu layers are antiparallel whereas the small moments found in the Co layers are parallel to their adjacent moments in the neighboring Cu layers. The ordered moment was 0.87..mu../sub B/ for the Cu layers and 0.16..mu../sub B/ in the Co layers, respectively. With increased temperature the ordered moments in the Co layers decrease faster than thosemore » in the Cu layer. At room temperature the ordered moment was (0.65(7))..mu../sub B/ for the Cu layers with no detectable ordered moment in the Co layers.« less

Neutron scattering study of the antiferromagnetic ordering in YBa2Cu3O6+x powder and single crystal samples

Abstract: Neutron diffraction experiments have been performed on powder and single crystal (2×5×2×1.5 mm 3 ) samples of YBa 2 Cu 3 O 6+x compounds. The reduced compound YBa 2 Cu 3 O 6 is found to order antiferromagnetically belowT N = 420 ± 10K. The magnetic moment is about 0.6 ± 0.05 μ B on the Cu 2+ sites and aligned perpendicular to the tetragonal c-axis. The magnetic structure is characterized by an in-plane wave vector|1/2 1/2 0| and an antiferromagnetic coupling between the two CuO 2 planes of the unit cell. The addition of oxygen induces only a decrease of T N and the moment value (290 ± 10 K, 0.45 ± 0.05 μ B and 245 ± 10 K, 0.28± 0.05 μ B , for x = 0.25 ± 0.03 and 0.38 ± 0.03, respectively). The long range order disappears abruptly aroundx = 0.40.

Magnetism of metastable phases: band theory and epitaxy (invited)

Abstract: Total‐energy band calculations are used to analyze the magnetic phases of metallic elements as functions of volume. The calculations utilize a fixed‐spin‐moment procedure, which is described and justified as a natural generalization of density‐functional theory. This procedure finds the ground‐state energies of electronic systems under two constraints, and hence determines the system energy as a function of two variables—volume and magnetic moment. The energy function is used to find the ferromagnetic phases and their ground‐state properties, including bulk moduli and magnetic susceptibilities. The systems studied are fcc Fe, fcc Co, bcc Ni, fcc Pd, and bcc Mn, each of which undergoes a phase transition for small changes of the lattice constant from equilibrium (zero‐pressure) values.

Magnetic structures under pressure in cubic heavy-fermion compounds

Abstract: Neutron diffraction experiments under pressure on the cubic heavy-fermion compounds CeIn3 and CePb3 are reported. Resistivity measurements were performed on CeIn3 in order to study the approach to the magnetic-nonmagnetic transition. Comparisons are made with previous results obtained on CeAl2. In CePb3 and CeAl2, the pressure-induced change in the magnetic structure from modulated to simple-antiferromagnetic may correspond to a reduction of the magnetic anisotropy. The strong pressure decrease of the Neel temperature of CeIn3 for P>15 kbar indicates the proximity of the transition from a magnetic to a nonmagnetic ground state. In cubic heavy-fermion compounds, the appearance of a nonmagnetic ground state seems to be directly connected to the recovery of the full multiplet degeneracy of the rare-earth ion.

Magnetic rare-earth superlattices (invited)

Abstract: The magnetic structures of several single‐crystal, magnetic rare‐earth superlattice systems grown by molecular‐beam epitaxy are reviewed. In particular, the results of recent neutron diffraction investigations of long‐range magnetic order in Gd‐Y, Dy‐Y, Gd‐Dy, and Ho‐Y periodic superlattices are presented. In the Gd‐Y system, an antiphase domain structure develops for certain Y layer spacings, whereas modified helical moment configurations are found to occur in the other systems, some of which are commensurate with the chemical superlattice wavelength. References are made to theoretical interaction mechanisms recently proposed to account for the magnetic states of these novel materials.

Structure of magnetic fields on the quiet sun

Abstract: To obtain quantitative temporal and spatial information on the network magnetic fields, auto- and cross-correlation techniques are applied to the Big Bear videomagnetogram data. The average size of the network magnetic elements derived from the auto-correlation curve is about 5700 km. The distance between the primary and secondary peak in the auto-correlation curve is about 17,000 km, which is half of the size of the supergranule as determined from the velocity map. The canceling features and the emergence of ephemeral regions are the major sources for the loss and replenishment of magnetic flux on the quiet sun.

Anomalous lattice contraction and magnetism of γ-Fe precipitates in Cu

Abstract: The lattice parameter of FCC ( gamma -)Fe precipitates in a Cu matrix is investigated as a function of the size of the precipitates. With prolonged aging, noncoherent gamma -Fe precipitates with about 0.2% smaller lattice spacings than the coherent ones are found. The structural phase transition, the magnetic structure and the Neel temperature of this new state are studied using X-ray and neutron diffraction methods. Antiferromagnetic ordering takes place accompanying a structural phase transition as in the coherent state. The observed Neel temperature is slightly higher than the previously reported value TN=67 K. Various magnetic properties of gamma -Fe precipitates in Cu are discussed on the basis of the present experimental data.

Ferromagnetism in metastable 304 stainless steel with bcc structure

Abstract: We have studied the consequence of structure on the magnetic properties of 304 stainless steel in two distinct crystalline states. Ordinary 304 stainless steel has an fcc structure and is nonmagnetic at room temperature. By using a vapor quenching method, we have fabricated single‐phase metastable bcc 304 stainless steel which is strongly ferromagnetic. Films a few μm thick have been made by high‐rate sputter deposition onto substrates at room temperature or liquid‐nitrogen temperature. Vibrating sample magnetometry and 57Fe Mossbauer spectroscopy reveal that the bcc phase has a magnetization of 130 emu/g, due largely to the Fe moment. The Curie temperature is found to be excess of 550 °C. Upon subsequent annealing above 550 °C, the metastable bcc state transforms back into the usual nonmagnetic fcc phase. The changes in the magnetic properties and the structure of these films during the transformation are examined.

Control of the tokamak edge plasma by static and rotating helical magnetic limiters

Abstract: Rotation of a helical magnetic limiter in the poloidal and toroidal directions is shown to be a good method of avoiding the non-uniform wall loading that is obtained when a static local helical field is applied. Mechanisms of the static and dynamic modification of the plasma potential at the tokamak edge are discussed in terms of magnetic field structure, ambipolarity, electron transport along the stochastic magnetic fields and cross-field drift motion of plasma particles. The magnetic structure is studied by Poincare mapping of field lines traced numerically, using a code that accounts for the exact configuration of local helical coils.

Neutron Scattering Investigation of Itinerant Electron System Fe2P

Abstract: Neutron elastic and inelastic scattering experiments have been performed on Fe 2 P single crystals in the temperature range from 8 K to 800 K. Fe 2 P is of the hexagonal C-22 type structure and exhibits c -axis ferromagnetism below 130 K. In the intermediate range between 130 K and 217 K, the canted ferromagnetism with a uniform component along the c -axis M 0 and a staggered component in the c -plane M Q appears, and the first order transition takes place at T c =217 K from the canted ferromagnetism to paramagnetism. The most interesting behaviour is the existence of giant short range order up to T =3 T c . It originates in the small stiffness constants D and leads to the strong deviation of susceptibility from the Curie-Weiss law. The amplitude of the spin fluctuation observed In Fe 2 P In the pararnagnetic phase is understood by the simple model with parameters deduced from the spin wave dispersion at low temperature, the magnetization and uniform susceptibility.

Mn magnetism and magnetic structures in RMn2

Abstract: Because of the tetrahedral arrangement of the Mn atoms in the Laves phases, the magnetic structures of the RMn2 are frustrated. To reduce this frustration, very unusual magnetic structures are stabilized with large crystallographic distortion.

Crystallographic and magnetic structure of TbFe10V2 and ErFe10V2

Abstract: The compounds TbFe10V2 and ErFe10V2 crystallize in the tetragonal ThMn12 structure. The vanadium atoms exhibit a strong preference for occupying the 8i site. At room temperature both compounds have an easy magnetization direction parallel to the c axis. At 4.2 K both compounds behave differently. In TbFe10V2 the easy magnetization direction deviates from the c axis by 50°. In ErFe10V2 the easy magnetization direction is perpendicular to the c axis.

Antiferromagnetism in YBa2Cu3O6

Abstract: We report on the first single-crystal determination of the magnetic order in YBa 2 Cu 3 O 6 by neutron scattering. The magnetic structure is found to consist of antiferromagnetic square-lattice layers of Cu(2) ions which are antiferromagnetically stacked along the c-axis. Magnetic moments of Cu(2) lie in the (ab) plane and have an amplitude of 0.6 μ B whereas Cu(1) show no ordered moment.

Long-range antiferromagnetic order of the Cu in oxygen-deficient RBa

Abstract: PHYSICAL REVIEW B VOLUME 37, NUMBER 16 Long-range antiferromagnetic order of the Cn in oxygen-deficient W-H. Li and J. W. JUNE 1988 RBa2Cu30t;+ Lynn Department of Physics; University of Maryland, College Park, Maryland 20742 and lvational Bureau of Standards, Gaithersburg, Maryland 20899 H. A. Mook and B. C. Sales Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Z. Fisk Los Alamos National Laboratory, MS-K764, Los Alamos, 1Ve~ Mexico 87545 (Received 18 January 1988) We have employed polarized and unpolarized neutron-diffraction techniques on both powders and single crystals to establish the nature of the magnetic order of the Cu ions in oxygen-deficient (nonsuperconducting) Magnetic RBa2Cu306+, (R Y,Nd). Bragg peaks of the type ( lt/2, k/2, l) are observed that yield a magnetic structure in which the Cu spins are coupled anti- both in the Cu-0 planes as well as along the tetragonal c axis, with the spin ferromagnetically direction in the tetragonal plane. The Neel temperature is very sensitive to the oxygen content, with T~~„o~ 500 K, while the spin configuration is independent of x. The relation between magnetism and superconductivity has attracted considerable interest for many years. In the ternary magnetic superconductors such as ErRh4B4 and HoMosSs, the magnetic and superconducting electrons re- side on separate sublattices, and hence there is only a these two between weak electromagnetic coupling cooperative phenomena which nevertheless gives rise to some interesting competitive behavior at very low temper- atures. ' An analogous situation exists for the present ox- ide superconductors, where the heavy rare-earth moments s also order at very low temperatures. However, a much more interesting and direct competition has been observed in the La2Cu04 — „system, where the oxygen-deficient materials are found to order anti- (nonsuperconducting) and with ferromagnetically at quite high temperatures, magnetic fluctuation energies which are much larger than Similar behavior might be expected for the k Ttv. oxygen-deficient RBa2Cu30s+, (R rare earth), ' and initial powder neutron data indicate magnetic ordering of the Cu spins. This correlation between the magnetic and superconducting properties is interesting not only from the magnetic-superconductor point of view, but also because it may hold the key to the origin of the supercon- ductivity in these oxides. '2 '5 Here we report our neutron scattering results on YBa2Cu30s+„powders and —, ). We observe NdBa2Cu30s+ single crystals (for x long-range antiferromagnetic order of the Cu spins, with transition temperatures as high as 500 K. The neutron measurements were carried out at the Na- tional Bureau of Standards Research Reactor, and con- sisted of three separate types of experiments. Initially un- polarized powder diffraction measurements were made with an incident energy of 13. 5 meV and a pyrolytic graphite PG(002) monochromator at the BT-9 triple-axis spectrometer to characterize the sample and search for possible additional Bragg peaks which might reveal a Polarized neutron measurements phase transformation. were then taken to establish that the phase transition which was found was magnetic in origin. These data were obtained at the BT-2 triple-axis polarized beam spectrom- eter with a Heusler alloy monochromator, also with an in- cident energy of 13. 5 meV. A supermirror was employed to determine the polarization of the scattered beam. Fi- nally, measurements were taken on a small single crystal utilizing an incident energy of 14. 8 meV, along with a PG(002) analyzer to improve the signal-to-noise ratio. Angular collimations before and after the monochromator and analyzer (when used) were 40' (full width at half maximum) in all cases, and a PG filter was employed to suppress higher-order wavelengths. Powder diffraction patterns over a wide angular range were taken with unpolarized neutrons at several tempera- tures at and below room temperature to characterize the system. The sample of YBa2Cu30&+„weighed g and was prepared by the usual solid-state reaction technique discussed in the literature. This fully oxygenated sample was then warmed in fiowing helium gas at the rate of 10'C/min up to 855'C, and then rapidly cooled to room Thermogravimetric analysis revealed an temperature. average oxygen concentration of 6. 13 0.05. The sample was subsequently sealed in a cylindrical sample holder with an atmosphere of helium gas for the neutron mea- surements. The basic diffraction pattern was consistent lat- with room-temperature with tetragonal symmetry' tice parameters of a 3.894(4) A. and c 11.784(2) A. However, several additional weak Bragg peaks were also observed, and an example of one such peak is shown in For min/point. Fig. 1(a). The counting time was comparison, the intensity of this peak is about 0. 79% of the (001) nuclear peak at this temperature. The intensity is also quite sensitive to T, and hence it is likely to origi- nate from either a structural distortion which increases the size of the unit cell or an antiferromagnetic transition. Although we do not have detailed measurements at tem- I 1988 The American Physical Society

UPdIn?A New Magnetic Heavy-Fermion Compound

Abstract: Magnetization (susceptibility), specific-heat and electrical-resistivity studies on the ternary equiatomic compound UPdIn are presented. At 20.4 K the material undergoes a phase transition to an antiferromagnetic state and below 7 K remanent magnetization is observed. The results can be interpreted by the antiferromagnetic structure becoming canted below 7 K resulting in a ferromagnetic component of the magnetization. The low-temperature specific heat shows that UPdIn is another heavy-fermion compound.

Unusual magnetic and lattice transformation in UNiSn, a possible half‐metallic ferromagnetic system

Abstract: UNiSn, as well as certain other ternary actinide compounds, displays very anomalous behavior in the temperature dependence of the electrical resistivity, ρ(T). In UNiSn this unusual ρ(T) dependence has been attributed to half‐metallic behavior with a semiconducting gap in the minority spin band and metallic behavior in the majority spin band with a ferromagnetic transition in the vicinity of 50 K. We have studied UNiSn using 119Sn Mossbauer spectroscopy and observed that annealing procedures influence the microscopic properties of the sample in a profound way. As‐cast samples are nonmagnetic. Annealing in the neighbor of 800 °C results in the formation of a magnetic component. The conversion to a magnetic phase is incomplete even after annealing for 210 days at 800 °C. The magnetic component goes through two transformations: (1) a region below approximately 60 K where there is a coexistence and gradual transformation of paramagnetic to magnetically ordered material and (2) a region below 43 K where the ma...