Showing papers on "Ferromagnetism published in 1990"

The application of the specular reflection of neutrons to the study of surfaces and interfaces

Abstract: The application of the specular reflection of neutrons to the study of surface and interfaces is described. The theoretical and experimental background to the technique is presented. A range of recent experimental results in surface chemistry, solid films and surface magnetism is discussed. In surface chemistry the results include adsorption of surfactants at the air-solution interface, insoluble monolayers and polymers at the air-liquid interface, soap films and adsorption at the liquid-solid and liquid-liquid interfaces. In solid films results on Langmuir-Blodgett films, hard carbon films, polymer films and some semiconductor layers are discussed. In surface magnetism experimental data which illustrate the nature of magnetism in magnetic multilayers and ferromagnetic films, and which describe flux penetration in superconductors, are presented.

Magnetic properties of layered transition metal compounds

Abstract: to Low-Dimensional Magnetic Systems.- 1. Experimental realizations of 2-d magnetic systems.- 2. Magnetic model Hamiltonians.- 3. Survey of the predicted magnetic behaviour.- 4. Lattice- and spin-dimensionality crossovers in quasi 2-d magnetic systems.- 5. Magnetic and nonmagnetic impurity doping in quasi 2-d magnets.- References.- Theory of Two-Dimensional Magnets.- 1. Introduction.- 2. Ising magnets.- 2.1. Ising model. Excitations and phase transitions.- 2.2. Onsager solution.- 2.3. Critical exponents and scaling.- 2.4. Dual transformation. Order and disorder.- 3. Planar magnets.- 3.1. XY model.- 3.2. Excitations.- 3.3. Scaling and correlations.- 3.4. Phase transition.- 3.5. Magnetic vortices as a Coulomb gas.- 3.6. Relationships with other models.- 3.7. Planar antiferromagnets.- 4. Heisenberg magnets.- 4.1. Heisenberg model and real magnets.- 4.2. Renormailzation of the temperature.- 4.3. Heisenberg ferromagnets in an external magnetic field.- 4.4. Excitations of the 2-d Heisenberg model.- 4.5. Dipolar interactions.- 5. Experimental layered magnets.- 5.1. Ising layered magnets. ANNNI model: application to CeSb and CeBi.- 5.2. Layered planar magnets.- 5.3. Layered Heisenberg magnets.- 6. Dynamics of 2-d magnets.- 6.1. Equations of motion.- 6.2. Spin-wave dynamics.- 6.3. Spin-diffusion dynamics.- 6.4. Dynamics of localized excitations.- 6.5. Resonant paramagnetic cxcitation of vortex pairs.- 6.6. Summary.- Acknowledgement.- References.- Application of High- and Low-Temperature Series Expansions to Two-Dimensional Magnetic Systems.- 1. Introduction.- 1.1. Series expansions.- 1.2. Methods applied in series analysis.- 1.2.1. Ratio methods.- 1.2.2. Pade approximant methods.- 1.2.3. Other methods of series analysis.- 2. Series expansions and predictions for the 2-d Ising model.- 2.1. Spin 1/2 model with nearest neighbours only (simple 2-d lattices).- 2.1.1. High-temperature series.- 2.1.2. Low-temperature series.- 2.1.3. Properties in nonzero parallel field.- 2.1.4. Properties in nonzero perpendicular field.- 2.2. Ising model with general S.- 2.3. Other series for I (1/2).- 2.3.1. Restricted dimensionality systems.- 2.3.2. Further-neighbour interactions.- 2.3.3. Crossover from 2-d to 3-d behaviour.- 3. Series expansions and predictions for the Heisenberg model.- 3.1. Series for S = 1/2, arbitrary S and S = ?.- 3.1.1. Properties at nonzero field.- 3.2. Other series for the Heisenberg model.- 3.2.1. Restricted dimensionality.- 3.2.2. Further-neighbour interactions.- 3.2.3. Crossover from 2-d to 3-d behaviour.- 4. Series expansion in the X Y and Ising-to Low-Dimensional Magnetic Systems.- 1. Experimental realizations of 2-d magnetic systems.- 2. Magnetic model Hamiltonians.- 3. Survey of the predicted magnetic behaviour.- 4. Lattice- and spin-dimensionality crossovers in quasi 2-d magnetic systems.- 5. Magnetic and nonmagnetic impurity doping in quasi 2-d magnets.- References.- Theory of Two-Dimensional Magnets.- 1. Introduction.- 2. Ising magnets.- 2.1. Ising model. Excitations and phase transitions.- 2.2. Onsager solution.- 2.3. Critical exponents and scaling.- 2.4. Dual transformation. Order and disorder.- 3. Planar magnets.- 3.1. XY model.- 3.2. Excitations.- 3.3. Scaling and correlations.- 3.4. Phase transition.- 3.5. Magnetic vortices as a Coulomb gas.- 3.6. Relationships with other models.- 3.7. Planar antiferromagnets.- 4. Heisenberg magnets.- 4.1. Heisenberg model and real magnets.- 4.2. Renormailzation of the temperature.- 4.3. Heisenberg ferromagnets in an external magnetic field.- 4.4. Excitations of the 2-d Heisenberg model.- 4.5. Dipolar interactions.- 5. Experimental layered magnets.- 5.1. Ising layered magnets. ANNNI model: application to CeSb and CeBi.- 5.2. Layered planar magnets.- 5.3. Layered Heisenberg magnets.- 6. Dynamics of 2-d magnets.- 6.1. Equations of motion.- 6.2. Spin-wave dynamics.- 6.3. Spin-diffusion dynamics.- 6.4. Dynamics of localized excitations.- 6.5. Resonant paramagnetic cxcitation of vortex pairs.- 6.6. Summary.- Acknowledgement.- References.- Application of High- and Low-Temperature Series Expansions to Two-Dimensional Magnetic Systems.- 1. Introduction.- 1.1. Series expansions.- 1.2. Methods applied in series analysis.- 1.2.1. Ratio methods.- 1.2.2. Pade approximant methods.- 1.2.3. Other methods of series analysis.- 2. Series expansions and predictions for the 2-d Ising model.- 2.1. Spin 1/2 model with nearest neighbours only (simple 2-d lattices).- 2.1.1. High-temperature series.- 2.1.2. Low-temperature series.- 2.1.3. Properties in nonzero parallel field.- 2.1.4. Properties in nonzero perpendicular field.- 2.2. Ising model with general S.- 2.3. Other series for I (1/2).- 2.3.1. Restricted dimensionality systems.- 2.3.2. Further-neighbour interactions.- 2.3.3. Crossover from 2-d to 3-d behaviour.- 3. Series expansions and predictions for the Heisenberg model.- 3.1. Series for S = 1/2, arbitrary S and S = ?.- 3.1.1. Properties at nonzero field.- 3.2. Other series for the Heisenberg model.- 3.2.1. Restricted dimensionality.- 3.2.2. Further-neighbour interactions.- 3.2.3. Crossover from 2-d to 3-d behaviour.- 4. Series expansion in the X Y and Ising-Heisenberg models.- 4.1. Series for the 2-d XY model.- 4.2. Series for the 2-d Ising-Heisenberg model.- 5. Applications to magnetic systems.- 5.1. Ising model.- 5.2. Heisenberg model.- 5.2.1. Spin 1/2.- 5.2.2. Spin 1.- 5.2.3. Spin 3/2 and spin 2.- 5.2.4. Spin 5/2.- 5.2.5. Restricted dimensionality.- 5.3. XY and Ising-Heisenberg models.- Acknowledgements.- References.- Spin Waves in Two-Dimensional Magnetic Systems: Theory and Applications.- 1. Introduction.- 2. Magnetic structures and spin Hamiltonians.- 3. Spin wave theory of model systems.- 4. Dispersion relation.- 5. Thermodynamic properties.- 6. Impurities in antiferromagnets.- References.- Neutron Scattering Experiments on Two-Dimensional Heisenberg and Ising Magnets.- 1. Introduction.- 2. 2-d systems with Ising and Heisenberg interactions.- 2.1. K2CoF4: a 2-d Ising system.- 2.2. K2FeF4: a 2-d planar antiferromagnet.- 2.3. K2MnF4 and K2NiF4: weakly anisotropic Heisenberg magnets.- 2.4. Rb2CrCl4: a planar Heisenberg ferromagnet with small anisotropy.- 2.5. K2CuF4: a planar Heisenberg ferromagnet.- 3. 2-d random magnetic systems.- 3.1. Phase transitions and critical phenomena.- 3.2. Excitations.- 3.3. Random field effects.- 3.4. Relaxation front 2-d to 3-d order.- 3.5. Competing anisotropics and interactions.- 4. Triangular lattice antiferromagnet (TALAF).- 4.1. Fluctuations.- 4.2. An additional degree of freedom.- 4.3. Perturbation.- 4.4. Quantum effect RbFeCl3 and CsFeCl3 VX2 (X = Cl, Br, I) AMX2 (A = Li, Na, K M = 3d metal ion X = O, S, Se).- References.- Phase Transitions in Quasi Two-Dimensional Planar Magnets.- 1. Introduction.- 2. Phase transition and excitations in the 2-d XY model.- 3. Crystallographic properties of BaM2(X)4)2 compounds.- 4. Magnetic properties of BaNi2(PO4)2.- 4.1. Static properties.- 4.2. Dynamic properties.- 4.3. Critical properties.- 5. Magnetic properties of BaCo2(AsO4)2.- 5.1. Static properties.- 5.2. Magnetic phase diagrams.- 5.3. Dynamic properties.- 6. Magnetic properties of BaNi2(AsO4)2.- 6.1. Static properties.- 6.2. Dynamic properties.- 7. Magnetic properties of BaCo2(PO4)2.- 8. Other experimental realizations of the 2-d planar model.- 8.1. K2CuF4.- 8.2. NiCl2 and CoCL2 graphite intercalated compounds NiCl2-GIC CoCl2-GIC.- 9. Concluding remarks.- Acknowledgement.- References.- Spin Dynamics in the Paramagnetic Regime: NMR and EPR in Two-Dimensional Magnets.- 1. Introduction.- 1.1. Dynamics of the 2-spin correlation functions.- 1.2. Nuclear magnetic resonance (NMR).- 1.3. Electron paramagnetic resonance (EPR).- 2. General formalism.- 2.1. Diffusion and dimensionality.- 2.2. Cut-off and EPR linewidth.- 3. EPR spectrum.- 3.1. Diffusion of 4-spin correlation functions.- 3.2. Secular contribution D0.- 3.3. Nonsecular contributions.- 3.4. Satellite line.- 4. Experiments on quasi 2-d Heisenberg magnets.- 4.1. NMR experiments.- 4.2. EPR experiments.- 4.2.1. Angular dependence of linewidth.- 4.2.2. Frequency dependence of magic angle linewidth.- 4.2.3. Dynamic shift.- 4.2.4. Lineshape of the main line.- 4.2.5. Satellite lines at half resonance field.- 5. Critical dynamcis.- 5.1. Critical behaviour of the NMR line.- 5.1.1. Isotropic regime.- 5.1.2. Anisotropic regime.- 5.1.3. Experiments.- 5.2. Critical behaviour of the EPR linewidth.- 5.2.1. Ferromagnets.- 5.2.2. Antiferromagnets.- 5.3. AC susceptibility.- 6. Conclusions.- References.- Field-Induced Phenomena in Two-Dimensional Weakly Anisotropic Heisenberg Antiferromagnets.- 1. Introduction.- 2. Effective, field-dependent anisotropies.- 3. The phase diagram.- 4. Random fields and domain walls (solitons).- 5. The spin flop transition.- 6. The bicritical point.- 7. Concluding remarks.- Acknowledgements.- References.- Index of Names.- Index of Chemical Compounds.- Index of Subjects.

Safety considerations in MR imaging.

Abstract: The authors identify eight areas of potential safety concern during clinical magnetic resonance (MR) imaging. These include (a) biologic effects of the static magnetic field; (b) ferromagnetic attractive "projectile" effects of the static magnetic field; (c) potential effects of the relatively slowly time-varying magnetic field gradients; (d) effects of the rapidly varying radio-frequency (RF) magnetic fields, including RF power deposition concerns; (e) auditory considerations from noise caused by the rapidly pulsed magnetic field gradients; (f) safety considerations concerning superconductive systems, including quenches, use of cryogens, and cryogen storage and handling; (g) psychological effects, such as claustrophobia and anxiety induced because of the examination; and (h) possible effects of the intravenous use of the MR contrast agent gadopentetate dimeglumine. The concerns in each of these categories are elaborated upon, and the available data are presented to clarify their status.

Magnetization reversal in ultrathin ferromagnetic films with perpendicular anistropy: Domain observations.

Abstract: We report on the first observation of magnetic-domain structure during spin reversal under field in ultrathin metallic films with perpendicular anisotropy. The magnetization reversal is either dominated by the nucleation process or by domain-wall motion. The related magnetic aftereffect is analyzed starting from the Fatuzzo theory.

Magnetic x-ray dichroism in core-level photoemission from ferromagnets.

Abstract: We report on the novel effect of magnetic x-ray dichroism in core-level photoemission from ferromagnets with circularly polarized radiation. Depending on the relative orientation of photon spin and sample magnetization (parallel or antiparallel), a single emission line may be resolved into two lines, due to exchange splitting of the core level. The mechanism is explained in terms of spin-selective dipole transitions in the presence of spin-orbit coupling. Possible applications of magnetic x-ray dichroism in photoemission from magnetic samples are pointed out.

Spin-wave calculations for multilayered structures.

Abstract: Results are presented on the calculations of spin-wave frequencies in ferromagnetic layers, double layers, and multilayered structures for small, nonzero wave vectors such as can be investigated by, e.g., Brillouin light scattering. The underlying continuum-type magnetostatic theory includes both dipolar and exchange contributions and fully takes into account magnetic surface and interface anisotropies as well as interlayer exchange coupling. For single magnetic layers the detailed influence of surface anisotropies on both film surfaces is studied. For magnetic double layers the interlayer exchange coupling mechanism is investigated. In the case of multilayers consisting of alternating magnetic and nonmagnetic layers, the crossing regime between dipolar and exchange modes shows a strong dependence of the gap width on the amount of interface anisotropy. For small layer thicknesses the interlayer exchange coupling shifts the spin-wave frequencies of all but the highest-frequency dipolar modes into the exchange-mode regime. In the case of all-magnetic multilayered structures, a new type of collective spin-wave excitations arising from coupled exchange modes is predicted.

Magnetic anisotropy in low-dimensional ferromagnetic systems: Fe monolayers on Ag(001), Au(001), and Pd(001) substrates.

Abstract: A second-variation full-potential linear augmented-plane-wave total-energy method for thin-film ferromagnetic systems is used to study the spin-orbit-interaction contribution to the magnetic anisotropy. For a free-standing Fe monolayer, the spin magnetization is determined to lie in the plane. Results for Fe monolayers on Au(001), Ag(001), and Pd(001) substrates indicate a preference for the spin direction to be perpendicular to the plane of the film. Computational details for this magnetic anisotropy are also discussed.

Hysteresis properties of ultrathin ferromagnetic films

Abstract: We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.

A Molecular Ferromagnet with a Curie Temperature of 6.2 Kelvin: [Mn(C5(CH3)5)2]+[TCNQ]-.

Abstract: The study of magnetic phase transitions in insulating molecular solids provides new insights into mechanisms of magnetic coupling in the solid state and into critical phenomena associated with these transitions. Only a few such materials are known to display cooperative magnetic properties. The use of high-spin molecular components would enhance intermolecular spin-spin interactions and thus a series of chargetransfer (CT) salts have been synthesized that utilize the spin S = 1 molecular cation, [Mn(C5(CH3)5)2]+ (decamethylmanganocenium). The structure and cooperative magnetic behavior of [Mn(C5(CH3)5)2]+[TCNQ- (decamethylmanganocenium 7,7,8,8-tetracyano-p-quinodimethanide) are reported. This salt is a bulk molecular ferromagnet with the highest critical (Curie) temperature (Tc = 6.2 K) and coercive field (3.6 x 103 gauss), yet reported for such a material.

Mössbauer effect in small particles

Abstract: Mossbauer spectroscopy is very sensitive to the special magnetic properties of ultrafine particles. Studies of particles in the superparamagnetic state allow determination of the particle size and the magnetic anisotropy energy constant A strong magnetic interaction between the particles may result in ordering of the magnetic moments of particles which would be superparamagnetic if they were non-interacting. This so-called superferromagnetic state can also be characterized by Mossbauer spectroscopy measurements. Furthermore, because a large fraction of the atoms in very small particles are in the surface layer, Mossbauer spectroscopy can be applied for studies of surface magnetism in small particles.

Domain-wall dynamics and Barkhausen effect in metallic ferromagnetic materials. II, Experiments

Abstract: Barkhausen effect (BE) phenomenology in iron‐based ferromagnetic alloys is investigated by a proper experimental method, in which BE experiments are restricted to the central part of the hysteresis loop, and the amplitude probability distribution, P0(Φ), and power spectrum, F(ω), of the B flux rate Φ are measured under controlled values of the magnetization rate I and differential permeability μ. It is found that all of the experimental data are approximately consistent with the law P0(Φ)∝Φc−1 exp(−cΦ/〈Φ〉), where all dependencies on I and μ are described by the single dimensionless parameter c>0. Also the parameters describing the shape of F(ω) are found to obey remarkably simple and general laws of dependence on I and μ. The experimental results are interpreted by means of the Langevin theory of domain‐wall dynamics proposed in a companion paper. The theory is in good agreement with experiments, and permits one to reduce the basic aspects of BE phenomenology to the behavior of two parameters ...

Epitaxial ferromagnetic τ-MnAl films on GaAs

Abstract: We report the growth of epitaxial τ‐MnAl ferromagnetic films on GaAs substrates by molecular beam epitaxy (MBE). Reflection high‐energy electron diffraction and x‐ray diffraction show that the τ‐phase films grow with the c axis of the tetragonal unit cell normal to the {100}GaAs substrate surface. In the bulk, τ‐MnAl is a metastable ferromagnetic phase with uniaxial magnetocrystalline anisotropy. The large hysteresis observed in the Hall resistance versus applied magnetic field suggests that the easy magnetization direction is indeed parallel to the c axis in the MBE‐grown films. The growth of these ferromagnetic films with perpendicular magnetization on compound semiconductor substrates creates the possibility of novel devices that combine magnetic memory and magneto‐optic functions with semiconductor electronics and photonics.

Chapter 3 Invar: Moment-volume instabilities in transition metals and alloys

Abstract: Publisher Summary The term “invar” stands for alloys showing minimum thermal expansion coefficients (maximum spontaneous volume magnetostriction) in certain ranges of composition and temperatures. Invar effect originates from investigations by Ch.E. Guillaume who found that ferromagnetic fcc FeNi alloys at concentrations around Fe 65 Ni 35 show almost constant thermal expansion as a function of temperature in a wide range around room temperature. According to his results, the linear thermal expansion coefficient α = (1/ l )/(d l /d T ) of Fe 65 Ni 35 Invar at 300 K is about 1.2 × 10 -6 K -1 , thus an order of magnitude smaller than in the pure components Fe and Ni and even smaller than in a Pt–10% Ir alloy, the material used for the prototype meter. The chapter summarizes the magnetic phase diagrams of the most important binary and ternary alloy systems showing moment–volume instabilities in general and Invar or Elinvar behavior in special composition ranges. Besides the magnetic-transition temperatures, Curie temperatures T C for ferromagnetic (FM) transitions, Neel temperatures T N for antiferromagnetic (AF) transitions, and T f for spin glass (SG) or re-entrant spin glass (RSG) like transitions, the diagrams contain information about the structural phases occurring in the systems.

Growth and magnetic studies of lattice expanded Pd in ultrathin Fe(001)/Pd(001)/Fe(001) structures.

Abstract: Ultrathin Fe/Pd and Pd/Fe bilayers and Fe/Pd/Fe trilayers were studied using ferromagnetic resonance and Brillouin scattering. Pd interlayers always couple the Fe layers ferromagnetically. For thicknesses up to four monolayers the Pd is ferromagnetic, in agreement with theory. One additional atomic layer of Pd destroys the ferromagnetism, but the Pd still exhibits a fluctuating magnetic moment partly polarized by the exchange field from adjacent Fe layers. The magnetic moments in the interfaces were determined from the ferromagnetic-resonance absorption intensities

Dipolar magnets and glasses: Neutron-scattering, dynamical, and calorimetric studies of randomly distributed Ising spins.

Abstract: We have measured the magnetic correlations, susceptibility, specific heat, and thermal relaxation in the dipolar-coupled Ising system LiHo_xY_(1-x)F_4. The material is ferromagnetic for spin concentrations at least as low as x=0.46, with a Curie temperature obeying mean-field scaling relative to that of pure LiHoF_4. In contrast, an x=0.167 sample behaves as a spin glass above its transition temperature, while an x=0.045 crystal shows very different glassy properties characterized by decreasing barriers to relaxation and nonexponential thermal relaxation as T→0. We find the properties of the x=0.045 system to be consistent with a single low-degeneracy ground state with a large gap for excitations. The x=0.167 sample, however, supports a complex ground state with no appreciable gap, in accordance with prevailing theories of spin glasses. The underlying causes of such disparate behavior are discussed in terms of random clusters as probed by neutron studies of the x=0.167 sample.

Ordering due to disorder in dipolar magnets on two-dimensional lattices

Abstract: We calculate the effects of thermal fluctuations, dilution, and a uniform external field on the continuously degenerate ground state of dipolar magnets on the square and honeycomb lattices. In all cases, these perturbations make terms in the free energy that favor particular states, reducing the continuous degeneracy to a discrete symmetry. Dilution and thermal selection compete; i.e., they select different states. For the cases of thermal fluctuations and dilution, the discrete symmetry is fourfold on the square lattice and sixfold on the honeycomb lattice; for a uniform field it is twofold and threefold, respectively. In addition, dilution generates effective random axes, which disorder all of the selected phases in each system, except for the dilution-selected phase of the square lattice. We present a proposed phase diagram including speculations on the finite-temperature transition.

The dipole-exchange spin wave spectrum for anisotropic ferromagnetic films with mixed exchange boundary conditions

Abstract: A theory is developed for dispersion characteristics of spin waves in ferromagnetic films taking into account both dipole-dipole and exchange interactions, crystallographic anisotropy and mixed exchange boundary conditions on the film surfaces. An arbitrary orientation of the external bias magnetic field with respect to the orientation of crystallographic axes and the film normal is assumed. The influence of crystallographic anisotropy on the spin wave spectrum of a ferromagnetic film is discussed. The theoretical results obtained are compared with the results of experiments performed in yttrium-iron garnet (YIG) and Mn-ferrospinel films.

Magnetisation processes, hysteresis and finite-size effects in model multilayer systems of cubic or uniaxial anisotropy with antiferromagnetic coupling between adjacent ferromagnetic layers

Abstract: Magnetisation processes have been investigated for model multilayer systems where antiferromagnetic interactions couple adjacent ferromagnetic layers. In this first study, only coherent rotations of the magnetisation of each layer are taken into account. Depending on the direction of the applied magnetic field, the initial moment configuration and the magnetocrystalline anisotropy, various first- or second-order magnetic transitions may be observed. The cases of cubic and uniaxial anisotropy bilayer systems are treated in detail. Spin-flop and spin-flip transitions are calculated to occur for both symmetries when starting from antiferromagnetic configurations that are parallel to the field axis. In the cubic case, various other transitions have been found. In particular, first-order transitions between symmetric and nonsymmetric states have been calculated, involving asymmetric behaviour of the magnetisation vectors of adjacent layers. Such transitions give rise to a transverse magnetisation. All the critical transition fields have been calculated as a function of the anisotropy and are reported in the various phase diagrams. Hysteresis loops have also been calculated. They generally consist of an upper and a lower part shifted symmetrically about the origin as in the case of bulk antiferromagnets. The influence of the number of layers has been investigated in some particular cases. It is shown that in most instances when the number of layers n becomes very large (i.e. when boundary effects disappear), the multilayer behaves like a bilayer but with different transition-field values. For small numbers of layers, whether or not the magnetism is compensated (n even or odd) has a very strong effect on the magnetisation processes.

The magnetic phases of pseudobinary Ce(Fe1-xMx)2 intermetallic compounds; M=Al, Co, Ru

Abstract: Powder neutron diffraction measurements of the magnetic and structural properties of a range of pseudobinary Ce(Fe1-xMx)2 intermetallic compounds, with M=Al, Co, Ru, are reported. In each of these pseudobinaries addition of a small amount of the metallic impurity (M) to CeFe2 reduces the ferromagnetic ordering temperature and enhances the small antiferromagnetic component which appears at low temperature. At higher impurity concentrations antiferromagnetism is achieved. This antiferromagnetism is accompanied by a cubic to rhombohedral distortion of the crystalline lattice. The antiferromagnetic component of the ordered moment in these compounds is oriented at approximately=18.5 degrees to the antiferromagnetic propagation vector ( tau =(111)). An ordered cerium moment is seen in the ferromagnetic phase of CeFe2 and Ce(Fe1-xRux)2. This moment is coupled ferrimagnetically to the iron-sublattice moment and mu Ce/ mu Fe approximately=-0.3.

Theory of ferromagnetism in CeCo5.

Abstract: Electronic-structure calculations for ${\mathrm{CeCo}}_{5}$ in the hexagonal ${\mathrm{CaCu}}_{5}$ structure and, for comparison, for the isostructural compounds ${\mathrm{YCo}}_{5}$ and ${\mathrm{LaCo}}_{5}$ are reported. It is shown that the anomalous properties of ${\mathrm{CeCo}}_{5}$ can be understood as due to itinerant 4f states, in particular its reduced magnetic moment relative to ${\mathrm{YCo}}_{5}$ and ${\mathrm{LaCo}}_{5}$. An important concept in the electronic structure of ${\mathrm{CeCo}}_{5}$ is the hybridization between the 3d states of cobalt and the 4f states of cerium. With spin-orbit coupling included in the calculation, good agreement with experiment for the total magnetic moment is achieved. A non-negligible 4f magnetic moment composed of both spin and orbital contributions is found on the cerium site. This manifests the existence of itinerant 4f magnetism in this compound. Common to all three compounds are the quite large orbital moments on the cobalt sites.

Two‐dimensional magnetic phase transition of ultrathin iron films on Pd(100)

Abstract: Epitaxial Fe films grown on Pd(100) are used to study monolayer magnetism, critical behavior, and surface magnetic anisotropy, by means of in situ surface magneto‐optical Kerr‐effect measurements. Auxiliary LEED‐Auger observations in 10−11 Torr vacuum are used to characterize the (1×1) epitaxy and the layer‐by‐layer film‐growth mode. Ferromagnetic hysteresis loops were detected for all Fe thicknesses from 0.6–4 monolayers (ML) with the TC monotonically increasing with thickness, independent of the easy‐axis orientation. The easy axis is perpendicular to the film plane below a critical thickness of 2.5 ML for 100‐K film growth, and reorients in‐plane above this thickness, and for all thicknesses for films grown at 300 K. The temperature dependence of the magnetization was obtained from the height of the Kerr loops in the remanent state and used to extract an effective magnetization exponent β for different film thicknesses and spin orientations. A value of β=0.127±0.004 is reported for a 1.2‐ML Fe film wit...

Magnetic susceptibility effects and their application in the development of new ferromagnetic catheters for magnetic resonance imaging.

Abstract: Newly developed ferromagnetic catheters (Fe-Caths) are more conspicuous than conventional radiographic catheters (Rad-Caths) on magnetic resonance (MR) images because they produce recognizable ferromagnetic signal patterns (FSPs). To determine how MRI parameters influence these patterns, the imaging characteristics of nine Fe-Caths (ferromagnetic concentration 0.01 to 1.0 weight/weight %) were studied systematically and compared with three Rad-Caths. All catheters were studied in stationary and moving phantoms at mid-field (0.38 T) and high-field (1.5 T) strength using spin-echo and gradient-echo pulse sequences. Rad-Caths always produced a signal void. Fe-Caths produced FSPs, the size of which depended on the orientation of the catheter with respect to the main magnetic field, the concentration of ferromagnetic agent in the catheter, and the direction and strength of the frequency encoding gradient. When Fe-Caths were positioned perpendicular to the main magnetic field, they produced FSPs; however, when they were parallel to the main magnetic field, Fe-Caths produced no FSP, thus having a similar appearance to the Rad-Caths. Ferromagnetic catheters produce conspicuous patterns on MR images that depend on catheter orientation in the main magnetic field and vary predictably with the MRI parameters.