bluuebunny/crossref_metadata_2025_split huggingface dataset:
Neutron Magnetic Scattering from Rare-Earth Ions
Authors: Blume M. , Freeman A. J. , Watson R. E. 
Published: April 2005

The theory of neutron scattering by paramagnetic rare-earth ions has been given by Trammell and more recently by Odiot and Saint-James. Unlike the case for the iron series transition metals, for which the orbital angular momentum of the 3d electrons is almost completely quenched, the magnetic scattering of neutrons from the rare earths (except for gadolinium and europium) arises from both the spin and orbital contributions to the magnetization. Using recently determined Hartree-Fock wave functions for some of the trivalent rare-earth ions we have determined the functions 〈jn〉 and 〈gn〉 (for n=0, 2, 4, and 6) which are necessary for the theoretical evaluation of the spin and orbital contributions respectively to the magnetic form factor. A comparison of these theoretical results with the experiments of Koehler and Wollan for Nd3+ and Er3+ and of Koehler, Wollan and Wilkinson for Ho3+ is presented. Differences are found between theory and experiment and some possible reasons for this are discussed.
A paper on these matters is being submitted to the Physical Review.
Far Infrared Spectra of Magnetic Materials
Authors: Tinkham M. 
Published: April 2005

The far infrared spectral region (∼10−100 cm−1) corresponds to kT for T=15−150°K or βH for H=105−106 oe. Materials with characteristic temperatures or fields of these orders of magnitude may have interesting far infrared spectra. We have studied far infrared resonance spectra in antiferromagnetic and in ferromagnetic rare-earth iron garnets.
Antiferromagnets have a resonance frequency which depends on the exchange and anisotropy fields, HE and HA. If HE is found from χ⊥, HA can be found from ω0, and compared with theory. We have done this with FeF2, MnO, and NiO, which have resonances for T≈0 at frequencies of 52.7 cm−1, 27.5 cm−1, and 36.6 cm−1, respectively. As the temperature is raised, these frequencies fall, reaching zero at TN. When a magnetic field is applied along the easy axis of FeF2, there is a first-order Zeeman splitting, from which a g value of 2.25 was determined. In MnO and NiO, however, there is an easy (111) plane. In this case the resonant mode is nondegenerate and hence the Zeeman effect is second-order and unobservably small.
The exchange coupling between the rare earth and iron ions in garnets is typically of order 20 cm−1 (30°K), while the iron ions are coupled strongly together, corresponding to their Curie temperature of about 550°K. Because anisotropy breaks down selection rules, transitions in which a single rare-earth ion spin flips in the exchange field of the iron may be observed at far infrared frequencies. We also observe Kaplan-Kittel exchange resonances, in which the entire sublattice of rare-earth ions precess together and induce a corresponding precession of the iron sublattice. Since the frequency of such a resonance depends on the rare earth sublattice magnetization, it is quite temperature dependent, in contrast to the single ion absorptions. To obtain quantitative agreement with experiment, the Kaplan-Kittel theory must be generalized to take account of anisotropy energy. In YbIG at 2°K, with the magnetization along the [111] easy direction, we find single ion resonances at 23.4 and 26.4 cm−1 and an exchange resonance at 14.1 cm−1. The latter rises in frequency as the temperature is raised, whereas the former frequencies are nearly constant up to 60°K, where the intensity becomes too low for observation. The corresponding frequencies at low temperatures in ErIG are 18.2, 21.6, and 10.0 cm−1, whereas in SmIG the exchange resonance occurs at 33.5 cm−1 and decreases with increasing temperature. The small temperature dependence of the single ion exchange splittings suggests a rare earth-rare earth coupling (perhaps via spin waves in the iron sublattice) of magnitude ∼4% of the iron-rare earth coupling.
Magnetodynamic Mode Ferrite Amplifier
Authors: Roberts Roy W. , Auld Bert A. , Schell Robert R. 
Published: April 2005

A longitudinally pumped ferrite amplifier has been operated using two magnetodynamic modes for the signal and idler resonant circuits. This is in contrast to the more usual electromagnetic cavity-type resonance or the ferrite magnetostatic modes. These magnetodynamic modes result from the coupling of a cavity-type resonance with a magnetostatic resonance. The coupling or mixing is strongest when the two unperturbed resonant frequencies are the same.
For the amplifier described in this paper a single crystal yttrium-iron garnet sphere was used. The magnetic bias field applied was 17 kMc/γ oe. The resulting magnetodynamic modes were resonant at 21 and 14 kMc, respectively, for the two branches of the mode tuning curve.
Advantages of these modes for ferrite amplification are ease of coupling and lack of interference from other resonant modes.
Millimeter Wave Parametric Amplification with Antiferromagnetic Materials
Authors: Moore R. A. 
Published: April 2005

The use of material with natural magnetic resonances in the millimeter and submillimeter wave bands makes possible electromagnetic interaction without an extremely large applied magnetic field. Antiferromagnetic materials most suitable for microwave application Cr2O3 and MnFe2 display resonances at approximately 160 and 250 kMc.
In the presence of a magnetic field applied parallel to the sublattice magnetizations, the uniform precessional mode resonates at different frequencies for oppositely directed circularly polarized excitation. A perturbation analysis has been conducted to determine the parametric response of the oppositely directed uniform precessional modes to longitudinal pumping. For materials with a sufficiently narrow linewidth, very close coupling between the uniform precessional mode on an external coaxial, stripline, or waveguide signal circuit is possible. Calculations have been conducted on the response of an antiferromagnetic medium in a silver cavity resonant to the pump frequency. The pump power absorbed by the cavity at the threshold of amplification for Cr2O3 would be 2(10)4 w. Narrower linewidth materials are needed for efficient antiferromagnetic millimeter and submillimeter amplification.
A more efficient utilization of the pump power and a simplified structure for millimeter wave application results when the pump fields are supported by a dielectric rod mode resonance. If the rod is placed across the narrow dimention of rectangular waveguide, the broad metallic wall forms the conducting end plates of the H11 mode. The signal TE01 waveguide mode couples to the appropriately directed circularly polarized uniform precessional resonance of the antiferromagnetic material on the rod.
On the Possibility of Obtaining Large Amplitude Resonance in Very Thin Ferrimagnetic Disks
Authors: Morgenthaler F. R. 
Published: April 2005

A theoretical prediction is made that the uniform resonance and the spinwaves in a very thin ferrimagnetic disk can be made nondegenerate. If this condition can be achieved experimentally, the usual second order spinwave instabilities will be forbidden and cannot limit the component of the magnetization occurring at the resonance frequency (ω0). The precession cone angle (θ) should then increase, in response to the usual rf driving field, until limited by third order instabilities involving spinwaves with frequencies equal to 32 ω0.
The state of nondegeneracy is to be achieved in the following manner: A thin disk, magnetized in a direction perpendicular to the plane, is excited by two transverse positive-circularly-polarized pulsed rf magnetic fields of specified amplitude. One of these fields has a frequency equal to ω0, the other a frequency detuned from ω0 by an amount comparable to the width of resonance. As θ increases from zero during the ensuing transient, there will occur a point at which the familar second order coupled spinwaves become unstable (the state of nondegeneracy has not yet been reached). Since there is a time lag before these modes can effect the uniform precession, the cone angle will continue to increase. When some larger value of θ has been attained the degeneracy between the spin waves and the resonance is removed; the previously unstable spin waves are then decoupled from the uniform resonance.
The degeneracy is removed because, as the author has already shown theoretically, the resonance frequency of the thin disk should lie below the main spinwave band (volume band) provided θ&gt; (2Nt)½, where Nt (the transverse demagnetizing factor) is by assumption very small. This level of excitation would create the desired state of nondegeneracy, if it were not for the fact that a range of degenerate spinwaves (separate from the main band) still exists. These degenerate modes may be destroyed, however, by introducing the additional transverse rf field.
Large amplitude resonance would clearly be desirable for a variety of reasons. Optical modulation experiments, to give only one example, would be of great interest. The necessity for using thin disks or films is, in this case, a fortuitous circumstance. It also follows that any nonlinear device involving harmonic generation, frequency mixing, etc. would become more efficient.
Saturation Magnetization of Swaged Mn–Al
Authors: Bohlmann M. A. 
Published: April 2005

Two years ago remarkable properties were reported for a new alloy based on manganese and aluminum. The value for saturation magnetization in Mn=Al had been extrapolated to infinite magnetizing field and was given as 6200 gauss. Our measurements indicate that the extrapolated figure is too low. Magnetization values of over 7100 gauss have been measured at 13 000 oe. Extrapolations of these values to infinite fields are not made because of anomalies in the magnetization curves at 7500 oe and 12 000 oe. The material having the high magnetization had an energy product of 4.0×106 gauss oe.
To reach the high values, it was necessary to adhere to processes and procedures having narrow limits. Saturation magnetization depended directly on composition, homogenizing, quenching rate, annealing, and deformation. The latter was most critical because a preferred axis of orientation is developed by this process. Mn–Al is very brittle, but the brittleness problem was circumvented by swaging the material in suitable jackets.
Rockwell tests indicate that hardness is directly related to 4πM.
Possibility of Domain Wall Nucleation by Thermal Agitation
Authors: Aharoni Amikam 
Published: April 2005

Very fine particles reverse their magnetization spontaneously, passing the energy barrier by means of thermal activation. In the same way one could argue that small portions of larger particles could be spontaneously reversed, thereby serving as nucleii for reversed domains. These can then grow, because it is known that the energy favors subdivision into domains. In treating such a possibility, however, one should take into account the additional barrier for forming a wall around the ``nucleus,'' which is usually left out in standard domain theory calculations when the wall energy is used. This energy is rather small but the barrier for formation of the wall is large.
The model studied is a small sphere, centrally located inside a spherical particle, the magnetization in which starts to rotate gradually from the z direction in which the rest of the particle is magnetized. The angle between magnetization and the z axis is assumed to vary linearily in the radial coordinate, from zero on the surface of the inner sphere, to its maximum value in the center. This value eventually reaches π, thus completing a wall around a reversed domain of radius zero, which should then start to grow by further supply of energy.
Studying the energy barrier for such a process, it is seen that the main contribution is from the exchange energy, which practically forbids the process. The probability for a domain wall to nucleate at room temperature is thus shown to be negligibly small, which justifies the neglection of temperature affects in Brown's equations.
Kossel Line Studies of Irradiated Nickel Crystals
Authors: Hanneman R. E. , Ogilvie R. E. , Modrzejewski A. 
Published: April 2005

Irradiation damage of nickel single crystals bombarded with 3 Mev electrons has been studied by Kossel line techniques. Changes of lattice parameter and x-ray line breadth were determined for crystals bombarded with integrated dosages of 1016, 1017, and 1018 electrons/cm2 at liquid nitrogen temperature. It is concluded that the formation of Frenkel defects in the lattice due to the electron bombardment is consistent with the data obtained. Subsequent annealing studies support this conclusion.
Using an electron microbeam probe to obtain the Kossel patterns, the absolute precision of the lattice parameter obtained is better than 1 part per 100 000 or ±0.00003 A. The limit of relative precision of change in lattice parameter of a given crystal is 3 parts per 1 000 000 or 0.00001 A. Basic principles of the formation of Kossel lines, experimental arrangements, various errors, and related models are presented.
Observation of Fission Fragment Damage in Thin Films of Metals
Authors: Kelsch J. J. , Kammerer O. F. , Goland A. N. , Buhl P. A. 
Published: April 2005

Direct observation of the effects of fission events has been made in thin films of Al and Pt by transmission electron microscopy.
A very thin layer (10 to 20 A thick) of UO2 was incorporated between evaporated metal films. The samples were exposed to a total thermal neutron flux of 2.8×1015 n/cm2. The fission fragments produced tracks in the matrix, the characteristic appearance of which is strongly influenced by the thickness and the atomic number of the matrix.
The mechanism of track formation is discussed from the viewpoint of dissipation of energy by the thermal spike mechanism.
Na(Nb1−xVx)O3 System and ``Ferrielectricity''
Authors: Miller Robert C. , Wood Elizabeth A. , Remeika Joseph P. , Savage Albert 
Published: April 2005

Following a report of ferroelectricity at room temperature in Na(Nb1−xVx)O3, crystals of pure NaNbO3 and Na(Nb1−xVx)O3 have been investigated electrically, optically, and by x-ray diffraction. Attempts to introduce amounts of vanadium larger than x≈0.005 into the crystals were unsuccessful. The room temperature phase of pure NaNbO3 has been reported to be antiferroelectric, that of NaVO3 nonferroelectric but perhaps also antiferroelectric. The vanadium-doped crystals appear to be antiferroelectric in the room temperature as grown state. A transition to a ferroelectric phase can be induced in both the vanadium-doped and the pure crystals by a sufficiently high field (of the order of 50 kv/cm), called the transition field, applied normal to the orthorhombic b axis. In one type of crystal plate, when the field-induced transition into the ferroelectric phase occurs, a crystallographic reorientation is also observed. Some crystals revert to the nonferroelectric phase when the transition field is removed.
All of the crystals investigated while in the ferroelectric phase were found to have the length of the b axis twice the length of the simple perovskite cube, whereas in the antiferroelectric phase, the length of the b axis is four times that of the simple perovskite cube.
In so far as the phenomena studied in the present investigation are concerned, it can be stated that the vanadium-doped crystals behave in most respects like pure NaNbO3 crystals. In particular, no evidence was found for an unbalanced antidipole configuration, i.e., ``ferrielectric'' structure, proposed by another investigator in an earlier paper. Further differences between the content of that paper and the present results are pointed out.
Growth of Cubic ZnS Single Crystals by a Chemical Transport Process
Authors: Samelson Harold 
Published: April 2005

A chemical transport process for the growth of zinc sulfide single crystals is described. The reaction employed is ZnCl2(g)+H2S(g)=2HCl(g)+ZnS(s).
The rate of transport of ZnS is discussed, and details of the thermodynamics of the system and the conditions for the growth of single crystals with a pure cubic structure are given. Crystals having this structure and as high a chemical purity have been difficult to obtain heretofore. The chemical analysis and some of the luminescent properties of these crystals are described briefly.
Magnetic Films: Nucleation, Wall Motion, and Domain Morphology
Authors: Schuler Frederic 
Published: April 2005

Nucleation fields for the reversal of magnetization in thin iron-nickel films were experimentally measured. The reversal was observed by the Kerr magneto-optical effect. Different regions or types of imperfections were observed at which lower nucleation fields are required than for the more perfect regions. Small regions of study in the ``interior'' of the film were localized by using an inhomogeneous field a millimeter or less in extension. Nuclei formed reproducibly about a center, with a statistical probability of formation which depended on the pulse length and field strength. A mode value of nucleation can be associated with a given center. The average of the mode values for different centers increased somewhat with film thickness, and showed a marked dependence upon composition, with a minimum value for films containing 80–85% nickel.
Repeated pulsing with fields smaller than those required for nucleation increased the size of nuclei, while maintaining the same lenticular shape, to limiting dimensions which increased with film thickness. At higher fields, wall motion was observed in detail for two types of motion: (1) the tip of the domain was displaced along the easy axis with an axial or ``endwise'' extension; (2) the ``180° wall'' was displaced along the hard axis with a normal or ``sidewise'' extension. Endwise extension takes place at lower fields than sidewise extension. Evidence was obtained to indicate that an energy barrier exists for sidewise motion, e.g., repeated 1.5-μsec pulses caused no motion, whereas a single 20-μsec pulse did. With very narrow drive wires or with thick films, additional nucleation took place instead of ``sidewise'' motion of the wall; otherwise, domains could be enlarged at will. Some films were produced which required larger fields to ``erase'' or contract the domain lengthwise than for tip extension.
The instabilities and general morphology of the domains as they depend on composition, film thickness, distance of extension of the inhomogeneous field along the easy axis, and the magnitude of the drive field were studied. Certain configurations led to a morphology of many small nuclei, which eventually were dense enough to join into a single domain; other configurations showed domain growth by wall motion and a large domain formed from very few nuclei. Very small nuclei were generally unstable, disappearing in a short time after formation. The smallest stable nucleus observed was about 0.25 mm×0.04 mm. The transition in behavior between 1.0- and 0.5-μsec pulses stresses the importance of multiple-nucleation processes in the nonuniform rotation region. The finescale observations on fields for nucleation and wall motion make possible comparisons with the processes taking place when the complete specimen is switched in a B-H loop tester.
Effect of Impurities on the Flow Stress of LiF Crystals
Authors: Johnston W. G. 
Published: April 2005

The mechanical properties of LiF single crystals with relatively low impurity content are compared with those containing ∼75 ppm of magnesium. These crystals represent the extremes of material that have been obtained from the Harshaw Chemical Company during the past six years.
The recently produced, ``pure'' crystals are very soft (80–100 g/mm2 critical resolved shear stress), and insensitive to heat treatments; and their flow stress varies slowly with test temperature. The impure crystals are much harder as received (∼1500 g/mm2) and respond to heat treatment in a manner that suggests precipitation hardening. The temperature dependence of the flow stress is much different in slowly cooled and rapidly cooled impure crystals. Whereas the slowly cooled crystals are much harder at room temperature, air-cooled crystals in which the magnesium is present in vacancy-impurity pairs are harder at low temperatures.
Two questions that must be answered are: (1) How does such a small amount of impurity present in aggregates produce a large hardening at room temperature? and (2), how do isolated vacancy-impurity pairs produce the large, temperature-sensitive hardening at low temperatures? The effects of impurities must be understood in terms of how they resist the motion of fresh dislocations.
One possibility that should be further explored is that impurities in these crystals affect the resistance to dislocation motion by altering the steady-state number of jogs on the screw dislocations. The existence of many jogs and the fact that they contribute to the flow stress have been previously established.
Parametric Effects in a Two-Level Electric Dipole System
Authors: Fontana J. R. , Pantell R. H. , Smith R. G. 
Published: April 2005

In a two-level system with induced electrical dipole properties, the relationship between an externally applied electric field and the expectation value of the dipole moment is expressed by a differential equation, in which one coefficient is proportional to the instantaneous system energy. The nonlinear properties arising from this relationship allow parametric amplification of signals by pumping at another frequency, in the presence of a material containing active systems of this type. No population inversion is necessary.
For optimum operation, both the pump and the signal should be within a few per cent of the natural frequency of the system. The electromagnetic Q required for parametric oscillations in a device of this sort is inversely proportional to the on-frequency absorption coefficient for the material. A numerical example shows that, at 1 mm wavelength, parametric oscillations will occur with a Q of 12 000 and a pump field of 10 kv/cm in a gas with an absorption coefficient of 0.1 cm−1.
Attenuation of Fast Electrons in Crystals and Anomalous Transmission
Authors: Heidenreich R. D. 
Published: April 2005

The two-beam dynamical theory of electron diffraction (Laue case) with attenuation is treated using imaginary potentials and a damping vector α in the Schrödinger equation. Explicit formulas are obtained for centrosymmetric crystals when the Bragg condition is realized relating the damping vectors for the two wave fields with the mean imaginary inner potential P0 and the imaginary Fourier coefficient Pg.
Using a simplified model, the imaginary Fourier coefficients are expressed in terms of the imaginary potential Pf for nearly free electrons (plasma) and Pc for bound atom cores. It is shown that the imaginary potentials can be directly related to the total inelastic collision cross sections and absolute numerical values so calculated. The values computed in this way are sufficiently near experimental results reported in the literature to warrant confidence in the approach. For 105-V electrons, the damping vectors α are around 10−3 Å−1, and the imaginary potential Pf≅0.6 V.
The question of anomalous transmission or an electron ``Borrmann effect'' is considered on the basis of the results obtained. It is concluded at this stage that the effect will not be large for metals but will increase with atomic number and may become significant for Z&gt;30. Uncertainties in both plasma and core electron cross sections make such predictions unreliable except for a few elements.
A calculation of the contribution of those electrons that have lost energy to E1&lt;E indicates that the E1 electrons also can produce an ``anomalous transmission.'' A velocity filter will be necessary to distinguish this E1 effect from a true anomalous transmission.