bluuebunny/crossref_metadata_2025 huggingface dataset:
Ferrimagnetic Resonance Relaxation in Rare-Earth Iron Garnets
Authors: Seiden P. E.
Published: April 2005

A quantitative analysis has been made of the ferrimagnetic resonance relaxation in rare-earth iron garnets. The data considered consist of measurements of the ferrimagnetic resonance linewidth of yttrium iron garnet substituted with neodymium, samarium, dysprosium, holmium, erbium, and ytterbium taken from 4.2°K to just below the Curie point, as well as data of other investigators for terbium and thulium. The data are conveniently divided into the two groups of odd and even electron ions. The ions in each group behave in a similar manner, the only exceptions being thulium and ytterbium. It is assumed that all the relaxation occurs through the strongly relaxing rare-earth ions but we find that both the ``fast relaxation'' theory of de Gennes, Kittel, and Portis and the ``slow relaxation'' theory of Clogston, Van Vleck, and Orbach fail to provide agreement with experiment. We have, however, formulated an empirical relation containing a relaxation term of the ``slow relaxation'' type that provides an excellent fit to the data when one regards the relaxation of the rare earth as being the familiar one-ion spin-lattice relaxation due to the direct (one phonon) and Orbach (two phonon) processes. The observed temperature dependence of the linewidth is satisfactorily accounted for if one assumes the direct process for the odd electron ions and the Orbach process for the even electron ions. In the case of thulium, one finds it necessary to assume the direct process, and for ytterbium one finds evidence for both processes, the direct process being dominant at low temperatures and the Orbach process dominant at high temperatures. The analysis allows evaluation of the iron-rare-earth exchange frequency and the rare-earth relaxation parameters. The exchange frequencies are found to be in good agreement with the values obtained by other investigators. In the one case where independent relaxation time measurements exist, that of samarium iron garnet, the behavior we find from the linewidth measurements agrees with that found by Caspari et al. by γ-γ correlation experiments. The frequency dependence predicted by the empirical relation is also in accord with experiment and a quantitative calculation of the frequency dependence of the peak value of the linewidth and the temperature of the linewidth peak yields excellent agreement with the data of Dillon for terbium.
Microwave Resonance and Relaxation of Manganese Iron Ferrites
Authors: Clarke B. H. , Teale R. W.
Published: April 2005

Microwave resonance measurements are reported at 16.85 Gc/sec in the temperature range 1.5° to 300°K for single crystals of composition MnxFe3−xO4 with x = 1.318, 1.007, and 0.990. For x = 1.318, the linewidth and the g factor, if this is calculated in the usual way, both pass through maxima as a function of temperature. These effects are compared to the predictions of a slow relaxation model, and indicate a splitting of a low-lying pair of energy levels of about 7.5 cm−1, and relaxation times across these levels of about 10−11 sec. The specimens close to manganese ferrite also show these relaxation effects but on a much reduced scale. They give splittings and relaxation times similar to those above. Sharp peaks in the linewidth as a function of orientation were found at [110] directions at 4.2°K and below, and were similarly dependent on composition. It is suggested that the slow relaxation is associated with manganese ions of some sort.
Ferromagnetic Resonance Relaxation, Wide Spin-Wave Coverage by Ellipsoids
Authors: Risley Allan S. , Bussey Howard E.
Published: April 2005

The role of degenerate spin-wave coupling in ferromagnetic resonance (FMR) losses has been studied as a function of sample shape. Measurements of linewidth ΔH and peak absorption intensity s″ both show that FMR loss above the 90°, k→0 limit of the spin-wave spectrum is much less than within the spectrum. An investigation of unsymmetrically magnetized spheroids, i.e., with the external field H0 not along the symmetry axis revealed that there are regions of the spin-wave spectrum which can be reached by more than one sample shape and that in these regions of overlap the different shapes yield different values of ΔH and s″. A similar result can be calculated from the usual equation of motion using either Landau-Lifshitz or Bloch-Bloembergen damping. (The asymmetric case has not yet been treated by the spin-wave model.) Despite the strong variations of ΔH and s″, the gyromagnetic ratio changed less than 1% under all variations of shape and orientation. The variable shape technique gives greater coverage of the spectrum than previous variable frequency work. Another advantage is that the unknown effects of changing frequency on other possible loss mechanisms are avoided.
Spin-Wave Anisotropy Effects in Parallel Pumping in YIG
Authors: Sethares J. C. , Seavey M. H. , Purnhagen T. G. , Stiglitz M. R.
Published: April 2005

A second ``corner'' has been observed on the ``butterfly curve'' of the parallel pumping experiment on YIG (yttrium iron garnet) at x-band frequency. The extra corner occurs in all crystallographic directions in a (110) plane except the [100] and [111]. The separation between the two corners closely equals the anisotropy-produced azimuthal angle splitting of the top of the spin-wave band. This splitting occurs in cubic YIG in all directions except the [100] and [111]. The shape of the butterfly curve in the region between the corners is partially explained by the variation in spin-wave ellipticity with azimuthal angle. A variation in spin-wave linewidth for k→0 with azimuthal angle may be responsible for the remaining discrepancy.
Ferromagnetic Resonance of Single-Crystal Gadolinium
Authors: Moore T. W. , Rodbell D. S.
Published: April 2005

Gadolinium metal is a system of relative simplicity as far as exchange interactions and ionic states are concerned. The 8S7/2 state of Gd3+ is consistent with the observed 7μb/atom and the moment contributing f electrons are most likely exchange coupled via conduction electrons. The present work concerns the measurement of the ferromagnetic resonance behavior of a single crystal at frequencies of 21 and 35 Gc/sec and at temperatures from above the Curie point (20°C) to 4.2°K. The resonance absorptions are well defined at both frequencies and at all temperatures for most crystal orientations. The minimum linewidth observed is about 400 Oe but the line shape and width degrade when the applied field departs from the easy axis, particularly at the lower frequency and at low temperatures. Strong, well-defined domain wall resonances are also frequently observed.

The deduced magnetocrystalline anisotropy energy is accurately described by the standard uniaxial expansion EA = K1 sin2θ+K2 sin4θ+⋯, about the hexagonal axis. The anisotropy constants derived from the resonance data agree reasonably well with those measured by Graham1 using static torque methods. The samples used in these experiments are from the same source.2

The demagnetization fields at the surface of the coin-shaped sample are measured directly from the shift in the resonance field of a small chip of the free radical DPPH in intimate contact with the sample surface.3 The corresponding demagnetization factors lie between that calculated by Schlömann4 for a uniformly magnetized right cylinder (an accurate description of our sample shape) and that calculated for an oblate spheroid of the same axial ratio, tending toward the latter at low temperatures.

Using the magnetization measurements of Graham (as a function of temperature and applied field) we find that the spectroscopic splitting factor, g, is 2.00±0.02 below 0°C, changing smoothly to 1.94±0.02 at 28°C in the paramagnetic region. This latter values agrees well with those reported by Kip5 and by Popplewell and Tebble.6 The g value at low temperatures, also deduced from the resonance data, gives better (but still not exact) agreement between the directly measured saturation magnetization and that implied by the 8S7/2 ionic state.
Observations of the Ordering Process in a Mg–Fe Ferrite
Authors: Heck Carl , Vaccari Gianantonio
Published: April 2005

Slowly cooled magnesium-iron ferrites of the approximate composition MgO2FeO·4Fe2O3 have constricted hysteresis loops in a wide range of temperature. Applying a magnetothermic treatment one can induce an anisotropy which is accompanied by a square hysteresis loop. The speed of the ordering process can be varied in a certain range by variation of the temperature. The treatment of samples was performed at about 100°C allowing an exact study of the process.

During the conversion of the shape of the hysteresis loop the remanence ratio Br/Bm increases. An activation energy of 0.37 eV can be derived from the temperature dependence of this change. This is in good agreement with results of Sixtus.

More exact investigations revealed an unexpected effect; the peak induction increases with time discontinuously in two main steps while the field is kept constant. The time at which the steps occur varies with temperature. An activation energy of 1.4 eV can be derived.

Nearly the same value was calculated from the temperature dependence of an observed minimum of the magnetization energy occurring during loop transformation. The depth of this minimum can be taken as a measure of the induced anisotropy.
Coexistence of Magnetic and Electric Ordering in Crystals
Authors: Smolenskii G. A. , Bokov V. A.
Published: April 2005

Magnetic and electric properties of some single crystals of complex composition of perovskite-type structure were investigated and coexistence of ferroelectric and antiferromagnetic properties was shown in them. It was found that single crystals YMnO3 and YbMnO3 of the new class of ferroelectrics, discovered by Bertaut and others, are also antiferromagnetics. The results of the investigation of solid solutions based on BiFeO3, which has a perovskite-type structure, are discussed. The opinion is expressed that BiFeO3 is probably not a ferroelectric. The main results concerning the thermodynamic theory of substances which are ferroelectric and ferromagnetic simultaneously are given in this report.
Flux Reversal in Rare-Earth-Substituted Yttrium Iron Garnets
Authors: Palmer Wilfred
Published: April 2005

The switching properties of polycrystalline toroids of pure YIG (yttrium iron garnet) and YIG containing 1% Dy, 3% Dy, and 4% Sm have been measured over the temperature range 4.2° to 300°K. In each of the rare-earth-substituted materials, the field required to switch 50% of the flux in a given time shows a peak at a temperature which is approximately 0.4 times the temperature at which a peak is observed in the ferrimagnetic resonance linewidth. It is assumed that at low switching speeds flux reversal occurs by domain wall motion, and an explanation of the temperature dependence of the switching field is sought in a consideration of the effect of the loss mechanisms which determine the linewidth on the domain wall mobility. The field above threshold H − H0 required for complete dissipation of magnetostatic energy in the moving wall at a given domain wall velocity V, and thus a given switching speed, should be proportional to the linewidth at the effective precession frequency applicable to domain wall motion. For 180° domain walls this frequency is approximately Vπ/d, where d is the domain wall thickness. Measurements of the frequency dependence of the linewidth of rare-earth-substituted YIG have indicated that the loss mechanisms in these materials are ``slow'' relaxation processes. For slow relaxation processes the linewidth is given by the expression ΔH (T, ωτ) = F (T)ωτ/1+ω2τ2, where ω is the experimental frequency and τ is the relaxation time. H − H0 (T, Vπτ/d) should depend in a similar way on Vπτ/d. This requires that Vπτ/d be a double-valued function of H − H0, the higher value of which is inaccessible. The increase of ωτ beyond unity contributes to the decrease of the linewidth at low temperature, but the decrease in the switching field at low temperature cannot be explained in this way. If the switching time chosen for reference were sufficiently short that at low temperatures Vπτ/d increases beyond unity, the required field would not be reduced but a discontinuous increase in switching speed, which is not observed, would occur at the field H − H0 (T, Vπτ/d=1). However, the behavior of the switching field is understandable if the explicit temperature dependence of H − H0 (T, Vπτ/d) is such as to produce the observed decrease at low temperature.

A report of this work and related topics has been submitted for publication.
Magnetization Reversal in Weak Ferromagnets
Authors: Joenk R. J.
Published: April 2005

Reversal of the net ferromagnetic moment in an antiferromagnet exhibiting weak ferromagnetism would appear qualitatively to require only small angle rotation of the sublattice moments. However, consideration of the canting mechanism responsible for the weak ferromagnetism shows that it is asymmetric in a way which requires full 180° reversal of the sublattice moments, i.e., M1×M2 retains the same direction in space. This is true for both the Dzialoshinsky-Moriya and single-ion anisotropy canting mechanisms. The critical values of the applied field for reversal of the magnetization and the corresponding initial modes of deviation from equilibrium have been calculated for single-domain particles of antiferromagnetic material having either type of weak ferromagnetic moment. Of the four modes found, two have extremely high critical fields since they involve rotation in opposition of the sublattice moments at the expense of exchange energy. The two reversal modes of interest are (initially) rotations of the Mi about axes parallel and perpendicular to the particle axis, principally at the expense of anisotropy energy. If the initial character of the motion is assumed to persist, the mode Ω⊥ would be a 180° rotation of m≡M1+M2 and 1≡M1−M2 in the plane of the Mi and the particle axis. The mode Ω∥ would be a 180° rotation of 1 in the plane perpendicular to the particle axis while simultaneously m decreases to zero and then grows to equilibrium value in the opposite direction along the particle axis such that 12+m2=constant. The critical fields for these two modes are given by the zero-frequency limits of the dynamic resonance equations as pointed out by Brown. In general, the smallest critical fields will be found for the Ω⊥ mode in Dzialoshinsky-Moriya type weak ferromagnets with small, but finite, basal plane anisotropy. The lowest |Hc| calculated is 40 Oe for αFe2O3.
Domain Structure of hcp and fcc Cobalt above Room Temperature
Authors: Grundy P. J. , Tebble R. S.
Published: April 2005

Lorentz transmission electron microscopy has been used to observe the magnetic domain structures in thin cobalt foils above room temperature. The changes in domain pattern occurring with increase in temperature in a crystal of the hcp phase are related to magnetocrystalline anisotropy measurements, and to the rotation of the easy direction of magnetization from the [0001] c axis to a direction in the basal plane at about 275°C. The domain pattern, in a crystal of the fcc phase, at elevated temperatures, is also in agreement with anisotropy measurements. In a region of mixed phase there is rotation of the magnetization direction from a [111] direction to a [110] direction in the fcc areas, in disagreement with anisotropy measurements.
Interactions between Ni and NiO
Authors: Berkowitz A. E. , Greiner J. H.
Published: April 2005

Polycrystalline Ni films, evaporated onto single-crystal NiO substrates, have been studied by torque measurements from 77° to 473°K in fields up to 22 kOe. The NiO films had (111) and (001) orientations, representing planes of parallel and compensated spins, respectively. After short vacuum anneals above TN, the Néel temperature, the major torque symmetry of the films on (001) NiO substrates was sin 4θ with an amplitude that decreased with increasing field. This was due to the rhombohedral distortion of NiO below TN; a (001) crystal consists of twinned, T-domain regions. This oriented strain field in the NiO substrate propagates into the Ni film and produces two orthogonal uniaxial anisotropies by a strain-magnetostriction mechanism.

The (111) NiO substrates were also twinned. After annealing above TN, the Ni films on these substrates showed a peak in the rotational hysteresis at ∼1 kOe, and a torque component of sixfold symmetry in the moderate to high-field range. One sample also showed rotational hysteresis that was increasing rapidly at 22 kOe. This behavior is consistent with a model involving exchange anisotropy, the motion of T-domain walls, and a strain-magnetostriction mechanism.
Magnetic Transition Layer Spin Configuration
Authors: Soohoo R. F.
Published: April 2005

The spin configuration in the transition layer between an antiferromagnet and a ferromagnet is investigated. The calculation is carried out by minimizing the free energy of the system with exchange, anisotropy, magnetostatic, and Zeeman terms. The results of our calculation indicate that a domain wall is formed between the antiferromagnetic and ferromagnetic media. The configuration of the walls, although similar to that in ferromagnets, is more complex and sensitively dependent upon the relative values of the exchange constant and anisotropy for the two media as well as upon the strength and nature of the interaction across the interface. A specific example involving an oxidized ferromagnetic film is given to illustrate the utility of our general result. It is noted that the formation of an antiferromagnetic-ferromagnetic domain wall may influence the magnetization of and surface spin pinning in thin films.
Pressure Dependence of the Magnetization of Cobalt
Authors: Kouvel J. S. , Hartelius C. C.
Published: April 2005

Single-crystal and polycrystalline specimens of cobalt were subjected at room temperature to hydrostatic pressures up to about 5 kbars, and the effects on their magnetizations were measured in fields up to 14 kOe. For the single crystal magnetized along its easy axis, complete saturation was achieved at about 3 kOe, and above this field the pressure dependence of the magnetization (per g) remained constant at a saturation value, σs−1 (∂σs/∂p), of −2.18×10−4 kbars−1. This value is shown to be identifiable with the pressure dependence of the atomic moment at 0°K and has the same sign and even roughly the same magnitude as the recent low-temperature pressure results for iron and nickel. For the cobalt polycrystal, the change in magnetization with pressure varied widely (though smoothly) with field before attaining the above saturation value at about 13 kOe. The lack of saturation at lower fields was attributed entirely to magneto-crystalline anisotropy, and a graphical integration method applied to the data gave the result: K−1(∂K/∂p) = −23.2×10−4 kbars−1, where K is the total anisotropy energy (per g) and is linearly related to the anisotropy coefficients, K1 and K2. The similarity in sign and the order of magnitude difference between the K−1(∂K/∂p) and σs−1 (∂σs/∂p) values for cobalt are closely analogous to the situation previously found in iron and an iron-silicon alloy.
Antiferromagnetic Transition in γ-Phase Mn Alloys
Authors: Zimmerman J. E. , Arrott A. , Sato H. , Shinozaki S.
Published: April 2005

Specific heat measurements on Mn-rich Mn-Cu alloys reveal first-order antiferromagnetic phase transitions at around room temperature and above. Magnetic entropy increases with Mn content but in all alloys it is smaller than would be expected from localized spins. The same is true of an equiatomic FeMn alloy. However the specific heat exhibits a well-defined cusp at 475°K, indicating a transition of second or higher order. By extrapolating the Mn-Cu results to pure Mn, the thermodynamic properties of γ-Mn in the region of the transition are derived.
Recent Developments in Permanent Magnetism
Authors: McCaig Malcolm
Published: April 2005

The properties of permanent magnets together with their theoretical background are reviewed. The study of micromagnetics has shown that because of processes such as fanning, buckling, and curling, the high energies once predicted for elongated single domain particle magnets are unlikely to be obtained by shape anisotropy alone. There may be a better chance of obtaining high energies from materials with crystal anisotropy or if displaced loops due to exchange anisotropy could be produced in a practical material at room temperature. The properties of barium ferrite are due to crystal anisotropy, but its magnetization is low. Other materials with high crystal anisotropy at present require expensive materials or manufacturing processes. The properties of Alnico V and related alloys can be explained in terms of shape anisotropy due to two-phase structure. Progress has been made in a group of high coercivity alloys containing more than 4% Ti and 29% Co. A single crystal of one of these alloys has given the highest (BH)max so far obtained, but it is only recently that it has been discovered that small additions of S or Se enable these alloys to be made columnar by simple methods.
Magnetic Exchange Anisotropy in Mixtures of Barium, Strontium, or Lead Ferrite with Potassium Ferrite
Authors: Roth W. L. , Luborsky F. E.
Published: April 2005

This paper describes some new systems F1−xAx, showing exchange anisotropy at room temperature and above. F is ferrimagnetic lead (P), barium (M), or strontium (S) ferrite, all with the magnetoplumbite structure, PbFe12O19, Tc=455°C. A is antiferromagnetic potassium ferrite (KFe11O17, TN=530°C) which crystallizes in the closely related β-alumina structure. Mixtures F1−xAx were prepared with 0≤x≤1. Specimens were cooled from above 600°C to room temperature in a field of 4 kOe and measurements made of the magnetic saturation σs, intrinsic coercive force Hci, torque L, and rotational hysteresis Wr, from −197° to 480°C. The observation of nonvanishing rotational hysteresis, sinθ torque curves, displaced hysteresis loops and changes in σs are interpreted in terms of exchange anisotropy in the M A and P A systems; compositions in the S A system behaved as simple mixtures of magnetic and nonmagnetic phases. X-ray diffraction indicated the formation of a new phase A′, possibly related to the development of exchange anisotropy. The identity of A′ has not been established; it may be a potassium-rich superstructure of A with lattice parameters A0′≈3A0 and C0′≈C0.
Rotational Hysteresis in Tungsten Steel and in Hysteresis Motors
Authors: Lommel J. M.
Published: April 2005

The starting torque for hysteresis motors is discussed in terms of the rotational hysteresis torque of the permanent magnet rotors as measured with a dc torque magnetometer. The rotational hysteresis torque was found to agree within experimental error with the starting torque of a rotor suspended in a magnetic field rotating at 60 cps. The rotational hysteresis in tungsten steel was measured for as-quenched and tempered samples. The rotational hysteresis had a peak value of 2.1×105 ergs/cm3 and decreased with tempering. The applied field at which the peak rotational hysteresis was found was not changed by tempering. The rotational hysteresis integral, a measure of the magnetization reversal mechanism, varied from 2.2 to 3.2, increasing as the coercive force decreased. The maximum anisotropy field in the material was found to be 6700 Oe.
Static Magnetization Reversal in Thin Films of Co–Ni–P
Authors: Bate G. , Speliotis D. E. , Morrison J. R.
Published: April 2005

Electrodeposited films of Co–Ni–P containing 14% nickel and 1% phosphorus by weight and having thickness 0.05 to 0.2 μ showed the following magnetic properties—high coercivity (300–550 Oe), high ratio of remanent-to-saturation moment (∼0.90), steep-sided hysteresis loops and isotropy in the film plane.

The process of magnetization reversal was studied by using a sensitive Hall probe to measure and map the horizontal component of the flux above the surface of the film after increasing reverse fields has been applied. This technique was used since neither the Bitter pattern method nor the Kerr effect revealed the natural pattern of magnetization reversal. However, it was possible to introduce, deliberately, a small region of reversed magnetization by means of the field from a magnetic recording head and examine this region by all three methods. This provided a method of checking the interpretations of the Hall effect data.

It was found that these films employ a unique mode of magnetization reversal. Initially, small regions of reversed magnetization are established throughout the film, and these regions grow together to form elongated switched areas whose long dimensions are always perpendicular to the direction of applied field. The reversal pattern is highly reproducible in a given sample for a given field direction and reverses when the field direction reverses.

This switching behavior is dependent on the direction of the field with respect to the sample, the long direction of the reversed regions always being perpendicular to the applied field. The sign of the vertical component of the flux from the sample was compared above and below the sample to determine whether magnetization perpendicular to the film plane existed. No such regions were found with a Hall probe of sensitive area down to 25×25 μ.
Ferromagnetism and Incipient Superconductivity in a CeRu2+7.3% GdRu2 Alloy
Authors: Drautman J. J. , Anderson C. J. , Del Grosso R.
Published: April 2005

The susceptibility, magnetic moment, and resistivity of an alloy of CeRu2+7.3% GdRu2 prepared by arc melting have been measured in the range 1.3° to 20°K. This alloy has been previously reported to be both ferromagnetic and superconducting. The sample is weakly ferromagnetic, and undergoes a superconducting transition which reduces the positive susceptibility. The resistance becomes smaller at the transition temperature, but does not become zero. The transition temperature in zero field is in good agreement with that previously reported.
Magnetization of Superconducting Niobium—Zirconium Alloys
Authors: McEvoy J. P. , Decell R. F.
Published: April 2005

The magnetization of hollow cylinders of niobium-25 at.% zirconium has been measured at 4.2°K. The experimental results can be described by a model in which the magnetization is derived from an induced supercurrent density given by Jc=α/B0+H, where H is the transverse magnetic field. This relationship has been verified for Nb-25%Zr with a microstructure varied by mechanical deformation. The values of α and B0 for several samples cold worked from 25% to 65% have been determined. Extrapolation of the functional dependence of these parameters to negligible cold work (annealed samples) and maximum cold work (heavily strained wire) are shown to be in agreement with previously reported results on cylinders and wires. The current density in the absence of the Lorentz force, (H=0) is found to be somewhat independent of the cold working, implying that the constant J(o)=α/B0 may be related to bulk electronic parameters.
Spin Waves in EuS
Authors: Charap S. H.
Published: April 2005

The ferromagnetic compounds of Eu2+ have recently been studied as nearly ideal examples of the Heisenberg model with considerable success. Measurements of the zero-field specific heat and magnetization of powdered EuS in the liquid-helium temperature region have recently been reported. The data were separately analyzed in terms of simple spin-wave theory to give rather indefinite results, especially for the second-neighbor exchange. But together these data provide an unprecedented opportunity to test the Heisenberg model. It is necessary to analyze the data using the complete spin-wave theory as derived by Holstein and Primakoff, including the effect of magnetic dipolar coupling in producing a variation with propagation vector of the spin-wave energy and, hence, spin-wave moment. It is also essential that the finite size of the Brillouin zone be taken into account. By numerical calculation values of the exchange integrals between first neighbors (J1) and second neighbors (J2) as well as an average field H acting in the domains (presumably due to anisotropy and dipolar fields) were sought which would simultaneously produce good fits to both the specific heat and magnetization. It was found that J1/kB=0.20°±0.01°K, J2/kB=−0.08°∓0.02°K, and H=4±2 kOe. By combining the Rushbrooke and Wood value for the transition temperature Tc when J2=0 with the molecular field result for the dependence of Tc on J2 we may calculate from the above values Tc=15.8°±0.6°K. The paramagnetic Curie point is calculated to be 20.1°±0.8°K. These are in good agreement with measured values. The origin of the large field H is not known, but it is consistent with the fields required to saturate powdered EuS.
EPR of Exchange-Coupled Pairs of Eu2+ in CaO and SrO
Authors: Calhoun B. A. , Overmeyer J.
Published: April 2005

The EPR spectra of isolated Eu2+ ions have been studied in crystals of CaO and SrO. In SrO, there is no crystal field splitting of the 8S7/2 level of the Eu2+. The g values and hyperfine constants are the same in both crystals. Crystals of CaO and SrO containing 1%–2% Eu exhibit in addition a large number of weak lines at both lower and higher fields than the single ion spectra observed in more lightly doped crystals. These resonances arise from coupled pairs of Eu2+ ions, and their analysis provides direct information about the exchange interactions between the Eu2+ ions. The temperature dependence of these intensities shows that the nearest neighbor exchange interaction Jnn is ferromagnetic and its magnitude is approximately 1°. In the [100] direction, some of the resonances due to next-nearest neighbor pairs have been identified. This exchange interaction Jnnn is very small, less than 0.1°. Its sign cannot be determined from the temperature dependence of the intensities of the resonances. These values for the exchange interactions are in agreement with those inferred from the magnetic properties of the europium chalcogenides.
Mass Spectrometric Study of Neutral Particles Sputtered from Cu by 0- to 100-eV Ar Ions
Authors: Woodyard James R. , Cooper C. Burleigh
Published: April 2005

A low-pressure magnetically confined argon arc discharge was used in a mass spectrometer ion source to study low-energy (0–100 eV) sputtering of polycrystalline copper. Target bombarding ion current densities ranged from 60–200 μA/cm2. Neutral particles were studied. Cu atoms and Cu2 molecules were detected. The mass ratio of analyzed Cu2 molecules to Cu atoms increased with bombarding ion energies to about 5½% at ion energies of 100 eV. Target voltages for appearance of Cu atoms and Cu2 molecules were − 19 and − 50 V, respectively. No Cu3 molecules were detected; if they were present, it was estimated that the ratio 189Cu3 to 63Cu is less than 0.09%. The method has been found to be promising for the study of neutral particles in low-energy sputtering. Yield curves agree well with results of other observers; sensitivities of 7×10−4 atoms/ion were attained, and this figure can be improved.
Methods of Determining Centroid X-Ray Wavelengths: CuKα and FeKα
Authors: Mack M. , Parrish W. , Taylor J.
Published: April 2005

For an exact solution of the Bragg equation it is necessary that there be a one-to-one correspondence (equivalence) between the angular measure θ and the wavelength measure λ. Because the aberrations inherent in all x-ray diffraction profile measurements distort the profile, it is necessary to correct any given measure (peak, centroid, etc.) for all aberrations. The only angular measure of powder diffractometer profiles that can be corrected to a high degree of accuracy is the centroid. To realize the greater accuracy inherent in the centroid method, accurate centroid spectral wavelengths are required. Centroids of CuKα and FeKα two-crystal spectrometer profiles prepared by Bearden have been calculated and corrected for axial divergence, distortions arising from variation in absorption across the spectral distributions, crystal asymmetry, and other factors. To determine the centroid wavelength it is necessary to truncate the profile; equivalence of λ and θ is maintained by truncating the powder and spectral profiles in the same manner. Four methods for systematically truncating the profiles have been published; the methods are evaluated and the superiority of one of the methods is demonstrated. It is shown that the centroid wavelength is a function of integration (wavelength) range, primarily as a result of inclusion or exclusion of the Kα satellite group within the truncated profile. The two-crystal spectrometer profiles are compared with models based on a Cauchy distribution. The necessity for additional spectral data before reference centroid wavelengths can be established is discussed.
Mechanism of Afterglow in Neon Flashtubes
Authors: Johnson P. D. , Rautenberg T. H. , Harris B.
Published: April 2005

The afterglow following pulsed dc excitation of neon can be explained by the requirement that the electron temperature must decrease before electron-ion recombination followed by radiative transitions can take place. This conclusion is supported by the suppression of afterglow by simultaneously applied rf power. The absence of negative absorption rules out energy level population inversion and stimulated emission.
Photon Avalanches from a Population Inversion
Authors: Tonks Lewi
Published: April 2005

High-power lasers encounter increasing losses of stored energy from stimulated emission by once-through avalanches of photons as higher powers are sought. This occurs even without resonance buildup. Even in the simple geometry of a thin rod, a satisfactory analysis would require elaborate machine calculation. A very crude model has, however, produced rough results from which some guidance is obtainable.

The model exhibits the expected linear increase of outgoing photon flux near the end of the rod and the huge depletion of excited atom population away from the rod equator. A criterion has been developed for the maximum useful excitation relative to emission parameters, rod length and rod cross section. It is concluded that this excitation level is fairly insensitive to aspect ratio (radius/length) but that internal reflection from the rod cylindrical surface can markedly lower this level by increasing the effective aspect ratio enormously.
Shaped Electroluminescent GaAs Diodes
Authors: Franklin A. R. , Newman R.
Published: April 2005

Electroluminescent GaAs diodes have been shaped as truncated cones with the p-n junction at the apex to direct a large fraction of the radiation toward the base of the cone within the critical angle for transmission. Devices having an efficiency at 77°K of 8% have been fabricated. Efficiency is defined here as the radiant power out the base per unit electrical power input. It is calculated that improved devices of this type will have efficiencies of about 18%.
Optical Observation of Elastic Resonances in NH4H2PO4
Authors: Gainon Denis J. A. , Jaffe Hans , Sliker T. R.
Published: April 2005

Elastic resonances in NH4H2PO4 (ADP) have been studied by an optical method which depends on the elasto-optic effect. Displacement models of the overtone resonance modes have been inferred from the optical patterns. For a (zxt) 45° plate, the transmission patterns for the stronger resonances are consistent with the picture of two longitudinal standing waves traveling at right angles; the reduced frequency constants are independent of the mode number m, and the sample resonates as m2 Lamé plates. For a (zxt) 0° plate, the greater part of the transmission patterns can be explained in terms of two shear standing waves traveling at right angles; the reduced frequency constants depend on the mode number n, but they tend to become constant as n increases. For both types of plate, the configuration of the electrodes is important in determining which modes are excited and to what extent.
Electronic Conduction in Single Crystals of Uranium Dioxide
Authors: Nagels P. , Devreese J. , Denayer M.
Published: April 2005

Electrical conductivity measurements between 90° and 800°K have been performed on single crystals of uranium dioxide with compositions in the range UO2.000–UO2.007. The activation energy calculated from the lnσT vs 1/T curves decreases from 0.34 to 0.19 eV with increasing excess oxygen content. Hall effect measurements by dc and ac techniques yielded no measurable response, and indicate that the Hall mobility is smaller than 0.015 cm2V−1 sec−1 at room temperature. This result and a consideration of the fundamental parameters determining the transport mechanism suggest an interpretation based on small polaron theory.
Paramagnetic Susceptibility of Eu2O3 from 300° to 1300°K
Authors: Arajs Sigurds , Colvin R. V.
Published: April 2005

The magnetic susceptibility of Eu2O3 has been studied as a function of temperature between 300° and 1300°K. The results can be satisfactorily explained by the Van Vleck theory of paramagnetism with Judd's energy levels of free Eu3+ ions.
Some Structure-Property Relationships in Thin Magnetic Films
Authors: Ahn K. Y. , Ansell G. S. , Barton C. J. , Beam W. R.
Published: April 2005

When Permalloy films are annealed, the Hc and Hk of the as-deposited films are observed to decrease and α, the dispersion angle, is seen to increase. Electron diffraction studies show the development of long-range order in these films with annealing, as evidenced by the appearance of superlattice lines. The changes in Hk and α can be partly explained by this transition from short- to long-range order, and a model is proposed.
Pressure-Induced Transformations to Crystal Structures of Lower Packing Density
Authors: Weddeling Franz K.
Published: April 2005

An explanation of pressure-induced polymorphic transitions from close-packed structures to nonclose-packed arrangements of atoms is attempted and compared with recent experimental data for ytterbium.
Modulation Effects in Magnetic Resonance: Widths and Amplitudes for Lorentzian and Gaussian Lines
Authors: Smith George W.
Published: April 2005

The effect of modulation amplitude on magnetic resonance linewidths δH (between slope extrema) and derivative signal intensity is examined. Experimental results are compared with theory for Lorentzian and Gaussian lines. Modulation amplitudes, hm, of 0.15δH [or 2hm (peak to peak) = 0.30δH] produce about 2% error in linewidths for Lorentzian signals and less than 1% error for Gaussian signals. Effects of modulation on linewidth and signal amplitude are presented in graphic and tabular form.
Some Aspects of Polarization Reversal in Ferroelectric TGS
Authors: Wieder H. H.
Published: April 2005

The time of maximum switching current tm of triglycine sulfate (TGS) crystals has been measured as a function of field E and crystal thickness d in the ranges 102<E<103 V/cm and 0.02<d<0.98 cm. The results may be expressed as tm=t∞ exp[α0d−⅔/E] with t∞≃(1.3±0.3)×10−4 sec and α0=(4±1)×102 V/cm. The symmetry of the switching transients about tm is a function of d and increases slightly with E. The symmetry factor m varies from m=0.7 to m=0.4 for 0.5<d<1.0 cm in agreement with a model for polarization reversal in which the virtual sidewise displacement of domain walls is the dominant mechanism. For thin crystals, d<0.15 cm and m<0.1, a model based on dominant nucleation and growth of antiparallel domains is consistent with experimental observations.
Miscibility of III-V Semiconductors Studied by Flash Evaporation
Authors: Müller Egon K. , Richards John L.
Published: April 2005

A study has been made of the mutual solid solubility of the phosphides, arsenides, and antimonides of gallium and indium, all of which crystallize in the zinc-blende structure. The films were prepared by flash evaporation of mixed powders. X-ray diffraction analysis of the films showed that all of the phosphides and arsenides form a complete range of solid solutions, while the phosphides and antimonides are largely immiscible. In the arsenide-antimonide systems, only those containing a common group III element are completely miscible (e.g., InAs–InSb, GaAs–GaSb). All quasibinary systems with a common group V element form a complete range of solid solutions. The results have been extrapolated to give a schematic representation of the limits of solubility in the multicomponent system In–Ga–P–As–Sb. The solubility has been correlated with the size of the component atoms involved in each system. Where solid solution occurred, the lattice constants were found to obey Vegard's law within the limits of accuracy. Epitaxy was achieved by depositing the films onto single-crystal germanium. Optical absorption has been measured on films covering a wide range of compositions.
Modulation of Infrared by Free Carrier Absorption
Authors: McQuistan R. B. , Schultz J. W.
Published: April 2005

The phenomenon of free carrier absorption in germanium has been utilized to modulate infrared radiation. Criteria are established which provide bases for judging and comparing modulator performance. By solving the small-signal continuity equation for the spatial and temporal distribution of excess holes, expressions are obtained which relate the modulator performance to the fundamental electro-optical properties of the modulator material. Modulators have been fabricated having modulation bandwidths in the range 105 to 106 cps. Experimentally determined modulator characteristics are compared with theoretical predictions.
Bombardment of Tungsten with 20-keV Helium Atoms in a Field Ion Microscope
Authors: Sinha M. K. , Müller E. W.
Published: April 2005

Fast He atoms are shot into one side of the tungsten emitter tip of a field ion microscope, and the resulting damage to the surface, consisting of vacancies, interstitials, and their clusters, is observed in atomic resolution. About one-half of the number of atoms calculated to have received a displacement energy of 50 eV in the interior appear as surface defects. The surface integrates the damage by focusing collisions coming from as deep as 1000 Å. Stress enhanced mobility of subsurface interstitials is detectable at 21°K, and rapid thermal diffusion from the depth occurs at 90°K.
Selected Area X-Ray Diffraction Studies of Surface Damage in Sliding on Single-Crystal Copper
Authors: Roshon David D.
Published: April 2005

Selected area x-ray diffraction was employed to study the localized structural changes resulting from sliding of a hemispherical copper stylus on the (001) face of a copper single crystal. Although a relatively large (0.25 mm) x-ray beam was used, significant structural deformations were resolved. Features observed included deformation of boundary regions by rotation from slip, crystal bending in areas of pileup, the micro-crystalline Beilby layer, a preferentially oriented transition region, and transferred crystalline particles. The observed deformations exhibited a dependence on sliding direction.
Spherulitic Crystallization from the Melt. I. Fractionation and Impurity Segregation and Their Influence on Crystalline Morphology
Authors: Keith H. D. , Padden F. J.
Published: April 2005

A systematic study of impurity segregation in spherulitic crystallization is described. The experiments deal principally with high polymers, in which the role of ``impurities'' is fulfilled by stereoirregular molecules or by molecules of low molecular weight. It is shown that these species are rejected preferentially by growing crystals and that their diffusion plays a vital part in governing over-all morphology. In particular, openness of texture is related to the concentration of impurity present; and coarseness of texture, which is a measure of the ``diameters'' of crystalline fibers between which impurities become concentrated during crystallization, is determined by δ=D/G, where D is the diffusion coefficient in the melt and G is the radial growth rate of the spherulite. Results provide substantial support for a theory of spherulitic crystallization proposed by the authors.
Influence of Temperature, Strain Rate, Surface Condition, and Composition on the Plasticity of Transition-Metal Carbide Crystals
Authors: Williams Wendell S.
Published: April 2005

A fundamental study of the ductile-brittle behavior of the transition-metal monocarbides has been initiated through an investigation of the plastic deformation of single crystals of TiC, ZrC, and NbC. A vacuum furnace was used with an Instron testing machine for compression measurements in the temperature range 800° to 1600°C. The specimens were Linde single-crystal boules, cleaved into bars approximately 1×1×4 mm. The compressive stress was applied longitudinally in a 〈100〉 direction in the NaCl structure. The three carbides studied behaved similarly, but NbC was several times harder than TiC and ZrC. The critical resolved shear stress τ decreases exponentially with increasing temperature, increases approximately linearly with increasing carbon content, is little affected by surface condition, and increases as the logarithm of the strain rate. No yield point was observed. The effective stress exponent depends on the strain rate, ranging from 1 to 10. These results are discussed in relation to recent theories of the stress-strain relation.
Model for the Electrical Behavior of a Microplasma
Authors: Haitz Roland H.
Published: April 2005

The complex current fluctuations observed in connection with microplasma breakdown can be explained by a simple model containing two constants: extrapolated breakdown voltage Vb and series resistance Rs; and two continuous probability functions: turnoff probability per unit time p10(I) as a function of pulse current I and turn-on probability per unit time p01. Experimental methods allowing an accurate measurement of these four quantities are described. The new concept of an extrapolated breakdown voltage Vb is discussed based on two independent measurements: one of secondary multiplication and the other of instantaneous current, both as a function of voltage. Within the experimental accuracy of 20 mV both methods extrapolated to one and the same breakdown voltage. The turnoff probability p10(I) is determined by a new combination of experimental techniques to cover the current range from 5 to 70 μA with a variation of 11 decades for p10(I). The observation of a narrow turnoff interval is explained quantitatively.
Statistics of Time in Photomultiplication
Authors: Euling Raymond
Published: April 2005

Photomultiplication is considered as a branching process. The output current of the photomultiplier is studied as a function of the statistical distributions of photon emission, electron flight times, and secondary emission yields. Expressions are derived for the timing accuracy of photomultipliers and scintillation counters.
Determination of Particle Structure from Light Scattering
Authors: Hart R. W. , Gray E. P.
Published: April 2005

Light scattering is well known as a useful tool in the investigation of the structure of particles. One of the disadvantages is that, whereas the scattering often may be calculated for a given structure, it has not been possible to carry out a Fourier inversion of the scattered intensity function to determine an unknown structure.

There are two major difficulties which are immediately recognizable. (1) The phase as well as the amplitude of the scattered radiation must be measured, and (2) the usual technique of Fourier inversion is not applicable because it requires knowledge of the scattered wavefunction for experimentally unattainable values of parameters. Both of these problems are discussed, and it is noted that they may be overcome in certain cases of interest. A method of phase determination suited to optical wavelengths is suggested for scattering by a single particle. Further, for ``soft'' scatterers, a method of inverting the scattering data is developed that does not suffer from the difficulty associated with the Fourier method. Thus, the inversion may be carried through and the desired structure of a scatterer may be obtained.