bluuebunny/crossref_metadata_2025 huggingface dataset:
Switching Properties of a Single-Crystal Specimen of Nickel Ferrite
Authors: Miller J. C. , Barker R. C.
Published: April 2005

A single crystal of nickel ferrite was cut to form a toroidal core whose major face was a (111) plane and given a thermal and magnetic anneal to induce an effective easy axis with circumferential symmetry. This paper gives the results of an experimental study of the switching properties of the specimen. The crystal could be switched in a ``single wall mode'' which resulted in a rectangular sense signal when the drive was low, and for increased drive it exhibited the double peak, sense signal characteristic of multiple wall switching. Unlike polycrystalline materials, the sense signal did not show a smooth envelope, but was the superposition of rather reproducible gross features and a random fine structure. No two switching waveforms were alike in detail. Data are presented on the effect of drive and reset upon the sense waveform, the switching coefficient Sw, the differential sense signal observed on opposite sides of the core, and the irregular envelope of the elastic flux. The wall velocity was found to be linear with drive for the single wall mode. The data are interpreted in terms of a wall motion model (stressing the statistical nature of the growth process), and its relation to polycrystalline materials is discussed.
Disturb Thresholds in Cylindrical Film Memory Wire
Authors: Gianola U. F.
Published: April 2005

The magnetization of the circularly anisotropic magnetic wire developed for cylindrical film memories can be disturbed by a combination of two fields parallel to the hard and easy anisotropy axes, respectively. The measured threshold levels of the fields needed to disturb the wire are smaller than predicted by the rotational switching model. Under the specific conditions encountered in the memory, the reduction in the threshold levels is considerable and must be taken into account in the design of the memory. The threshold levels are very dependent on the way that the disturbing fields are applied. A pulsed field applied many times in a direction normal to the magnetization is considerably more effective in reducing the threshold levels than is a dc field of the same amplitude. The threshold relationships for different combinations of pulsed and dc fields in both easy and hard anisotropy directions are presented. The effect of reduced threshold levels on the design of a memory is discussed. They can result in inter-bit interaction. The amplitude of the readout signal may also change following particular writing sequences.
Spin-Wave Resonance in Thin Films at Oblique Angles
Authors: Rossing Thomas D.
Published: April 2005

Spin-wave spectra in thin Permalloy films have been observed as the angle between the applied field and the film is varied from 0 to 90 degrees. The applied field corresponding to the main resonance peak falls very close to its calculated value for all angles. At some critical angle the spin-wave spectrum collapses to a single peak. The observed critical angle is very near the angle at which the resonance frequency becomes independent of small changes in the magnetization, which supports a dynamic-pinning model. The separation between the main peak and the subsidiary peaks reaches a maximum at an angle slightly less than the critical angle and drops sharply as the critical angle is approached. At no angle does the wavelength of the spin waves appear to be a submultiple of the film thickness as would be expected if the surface spins were completely pinned.

As the angle between the applied field and the film increases from 0, the linewidth increases and reaches a maximum near 80°. Beyond this angle the linewidth decreases rapidly, and in many films the linewidth at 90° is less that at 0. Simple theory explains the increase in linewidth with angle, locates the angle of maximum linewidth, but fails to predict the narrowness at 90°.
Angular Dependence of Spin Pinning in Thin Ferromagnetic Films
Authors: Wigen P. E. , Kooi C. F. , Shanabarger M. R. , Cummings U. K. , Baldwin M. E.
Published: April 2005

A discussion of the origin of surface spin pinning in thin ferromagnetic films due to a surface layer possessing slightly different magnetic properties is presented. This model predicts that at a critical angle, measured between the normal to the film and the direction of the external magnetic field, the uniform precession condition will be the same in all regions of the film and the magnetization can be excited as a unit in the uniform precession mode. Spin-wave spectra were obtained at 9.6 and 14.6 kMc from thin films composed of Permalloy (80 Ni−20 Fe) and cobalt. The critical angle depends on the value of ω/4πγ M which varied from 0.167 to 0.667 and gave critical angles that varied from 2° to 15°. These results agree to within ±1° of the predicted values. A critical angle has occurred in every film independent of the substrate material, the cleaning procedure, or the substrate temperature during deposition. The angle dependence of the strength of the spin-pinning condition as predicted by this model at angles other than the critical angle is also compared with the experimental results.
Spin-Wave Instabilities in Magnetic Thin Films
Authors: Comly J. B. , Penney T. , Jones R. V.
Published: April 2005

High-power resonance effects have been observed in thin films of Permalloy. These effects are characterized by a saturation of the main resonance and in some cases the growth of a subsidiary absorption peak at magnetic fields below resonance. The hcrit associated with the saturation of the main resonance is found to be sensitive to the film thickness. The size dependence of nonlinear effects in films seems to be associated with a distortion in the spin-mode dispersion relation at low k numbers.
Theory of Ferro- and Antiferromagnetism by the Method of Green Functions
Authors: Oguchi Takehiko , Honma Akio
Published: April 2005

Theoretical studies on the magnetization of ferro- and antiferromagnets have been investigated by means of the equivalent operators, devised by one of the authors, introduced into the two-time and temperature-dependent Green function of Bogolyubov and Tjablikov. Our theory is applicable over the whole range of temperatures for arbitrary magnitudes of spin. At low temperatures, the dependence of magnetization on temperature is in good agreement with that of spin-wave theory except for an extra term which probably results from the approximation of decoupling the chain of equations for the Green functions. But this term is negligible when the magnitude of spin is more than one for the ferromagnetic case and32 for the antiferromagnetic case. It should be emphasized that our theory possesses spin-wave character as the collective modes in the low temperature region, and gradually changes to the individual spin-mode theory similar to the molecular field theory with increasing temperature.
Variable Delay Magnetic Strip Line
Authors: Kornreich P. , Pollack S. R.
Published: April 2005

A thin-film strip line, which uses the coherent rotation of the magnetization of a Permalloy film to increase delay, has been developed. The delay and frequency response for this line have been calculated assuming small angle coherent rotations. Results indicate that delays of 2.3 nsec/cm should be typical and that the delay should be a function of applied bias field, decreasing with increasing field. Structures were built with delay-to-risetime ratios of 2.5, the delay was found to be field dependent in approximate agreement with theoretical calculations.
Thick-Film Read-Only Memory Device
Authors: Matick R. E.
Published: April 2005

A ``Read-Only'' memory device has been developed which circumvents many of the usual difficulties associated with thin-film devices. The device consists of a relatively thick, uniaxially anisotropic, magnetic film electroplated on a suitable substrate and sandwiched between the conductors of a strip transmission line. A ``thick'' film here is defined as one for which the shape demagnetizing field exceeds the coercive force, thereby causing the film to split into a multi-domain structure. A current applied to the strip line magnetizes the film in the hard direction. Upon removal of the drive current, the shape demagnetizing field and anisotropy field reset the film to its initial multidomain structure. Since the switching and relaxation time of this device will determine the ``ultimate'' cycle time of an array, it is of interest to investigate the switching behavior of thick films. Experimental and theoretical results indicate that both the switching and relaxation of these films take place in time intervals as short as 3 nsec with an applied field of 2 nsec rise and fall time. Individual film elements have been operated with a pulse repetition rate of 20 Mc per sec and higher repetition rates appear possible.
Effect of Skew on the Performance of Magnetic-Film Storage Cells
Authors: Mossman P. , Williams M.
Published: April 2005

Results are given of write—disturb—read tests on elements of continuous film of 80Ni17Fe3Co on aluminum substrates. The curves relating output voltages to digit currents used for writing and disturbing are traced automatically. Measurements on a variety of films show that, irrespective of whether Hc/HK≶1 the following statements apply to 40-mil-square elements: the disturbing threshold current is about 1.5 times the value calculated; the writing threshold is usually about 3 times the value calculated; the output of one polarity may fall very rapidly as skew increases; creep occurs during about the first 100 disturbing pulses; even with optimum digit currents the disturbing pulses can reduce the output and may even just reverse it. However, where the reversed output is small a two-element storage cell can be made to give correct operation. The ultimate limit of skew occurs when the writing threshold approaches the disturbing threshold, so that films with higher Hc/HK tolerate more skew. Films with Hc/HK>1 give good performance, and can be made reliably, given suitable choice of substrate temperature and film thickness.
Magnetic Structure of MnSn2
Authors: Corliss L. M. , Hastings J. M.
Published: April 2005

An abrupt magnetic transition in MnSn2 was reported by Kouvel and Hartelius1 about one year ago. The salient features of the magnetic behavior are a smooth susceptibility maximum at 86°K and an almost discontinuous decrease in susceptibility at 73°K. At the suggestion of Kouvel, a neutron diffraction study was made to investigate possible changes in magnetic structure accompanying the transition.

MnSn2 has the body-centered tetragonal C16 structure, consisting of alternating layers of Mn and Sn perpendicular to the c axis. Each Mn atom is surrounded by 8 Sn atoms which connect it to its second and third nearest neighbors through angles of approximately 111° and 146°, respectively. Nearest-neighbor Mn atoms have no intervening Sn atoms and are separated by a distance of 2.72 Å in the c direction.

Neutron diffraction patterns show that above the abrupt transition at 73°K, MnSn2 is antiferromagnetic with a Néel temperature in the neighborhood of 330°K, in agreement with recent susceptibility and resistivity measurements of Kouvel2 on the same preparation, as well as with the earlier magnetic measurements of Yasukōchi et al.3 The structure consists of ferromagnetic (110) planes which alternate along the direction perpendicular to the sheets. Individual moments are oriented parallel to the direction of alternation. The superstructure lines, characteristic of the antiferromagnetic state, appear to saturate near nitrogen temperatures and show no anomaly in passing through the susceptibility maximum at 86°K. The value of the Mn moment at 77°K is 2.36 μB.

The abrupt transition at 73°K is marked by the disappearance of the high-temperature antiferromagnetic diffraction pattern and the appearance of a new pattern characteristic of a modulated spin structure. The low-temperature structure is a simple modification of the structure above the transition in which the magnitude of the moment is no longer constant but varies sinusoidally along the direction perpendicular to the ferromagnetically aligned sheets. Thus, the short-range structure and spin direction are essentially the same above and below the transition. The observed modulation has a wavelength of approximately 113 Å or twelve times the face diagonal of the chemical unit cell; the amplitude of the modulated moment is 3.11 μB at 4°K.
Atomic Moments and Magnetic Coupling in Cation Excess-Nickel Arsenides
Authors: Goodenough John B.
Published: April 2005

The concept of a critical cation-cation separation Rc, such that 3d electrons are collective if RRc, and the assumption that there is no discontinuity through Rc in the interatomic electron correlations, which are known for R>Rc from superexchange theory, permits interpretation of both the sign of the magnetic coupling and the magnitude of the observed magnetic moments in several cation-excess-nickel arsenides. It is necessary, however, to guess at the relative stabilities of a localized 3d level and the collective 3d states. This guess is qualitatively reasonable and consistent with the idea that half-filled shells should be more stable than partially filled shells. Since the same physical arguments were used to account for the magnitudes of the atomic moments and the sign of the magnetic coupling in cubic transition elements and binary alloys, this exercise suggests that these physical ideas, though qualitative, have power to give considerable insight into the essential features of quite complex problems.
Magnetic Moment Distributions in Dilute Nickel Alloys
Authors: Low G. G. E. , Collins M. F.
Published: April 2005

A neutron scattering technique has been developed which enables the spatial distribution of the magnetic moment disturbance around an impurity atom in a ferromagnet to be examined over a distance of several Angstroms. Measurements show that when vanadium or chromium is added dilutely to nickel the resulting magnetic disturbance occurs not only at the impurity atom sites themselves, but also on the neighboring nickel sites. In fact, most of the change in saturation magnetization observed on alloying is due to a decrease in the moments on nickel atoms in the vicinity of solute atoms rather than to changes in moment at the impurity atom positions. On the other hand, if iron or manganese is added dilutely to nickel the moment disturbance appears to be confined to the solute atoms. These results are discussed in terms of a theory suggested by Friedel.
Aspherical Spin Density in the Ferrimagnetic Compound Mn2Sb
Authors: Alperin H. A. , Brown P. J. , Nathans R.
Published: April 2005

The spatial distribution of the unpaired electrons about each manganese atom in the ferrimagnetic compound Mn2Sb (space group P4/nmm) has been measured by diffraction of polarized neutrons from single crystals at room temperature. It is found that this distribution is aspherical and highly compact. The moments for the manganese atoms at the 2a and 2c positions are measured to be −1.48±0.15 μB and 2.66±0.15 μB, respectively. Single-crystal x-ray determination of the manganese and antimony 2-c position parameters for this structure yield the values 0.2897±0.0006 and 0.7207±0.0002, respectively. An unusual feature of the spin density of the manganese atom at the 2-c position is the displacement of the center of the distribution from its nuclear position.
Measurement of Film Relaxation Time
Authors: Barrett W. A.
Published: April 2005

Magnetic film memories and other devices depend upon the relaxation from a state of hard direction magnetization to an easy direction state in the presence of a small easy direction ``tipping'' field HL. The relaxation times of five Permalloy films of thicknesses 0.1, 0.25, 0.4, 0.5, and 1.0 μ have been measured with an apparatus of switching field turn-off time <0.5 nsec and a detection risetime of 0.85 nsec. The relaxation time is defined as the time for a 10–90% easy direction flux change. It is found that the relaxation time varies roughly inversely with easy direction field and is to some extent independent of the initial hard direction field (8 Oe for the following remarks).

The tangent slope Sw to the 1/τs vs HL curve at a field HL=1 Oe fits the relation Sw=1+10d2 Oe nsec, where d is the film thickness in μ, within experimental error (∼20%). The Sw predicted from the Landau-Lifshitz equation for the rotational model with λ=108 cps is 1 Oe nsec. The rotational model also predicts that τs approaches 2 nsec for HL→0. The limiting τs for the real films varied from about 7 nsec for the 0.4-μ film to about 40 nsec for the 0.25-μ film with no thickness correlation. In addition, the 0.1-, 0.25-, and 0.5-μ films exhibited a distinct slow-switching component for |HL| <Hdisp (Hdisp is the field HL necessary for a 90% relaxation in one direction). Observation of the final state of the 0.1- and 0.25-μ films by the Bitter technique revealed a fine-scale domain structure for |HL| <Hdisp and a single domain state for |HL| ≳Hdisp.

The two thicker films were electrodeposited on a BeCu substrate, while the three thinner films were evaporated on a glass substrate. Since all five fit the same Sw vs d relation, it would appear that substrate-induced damping is not large.
Rotatable Anisotropy in Negative Magnetostriction Ni–Fe Films
Authors: Lehrer Sherwin S.
Published: April 2005

The phenomenon of rotatable anisotropy (RA) in Ni–Fe films refers to a reorientation of the easy axis along the direction of a previously saturating magnetic field in a time <1 sec at room temperature. The effect was first unambiguously observed by Lommel and Graham in Ni films and attributed by them to ferromagnetic-antiferromagnetic interaction between Ni and NiO. The present study shows that the RA effect occurs under the same conditions of film thickness and composition as a previous studied ``mottling'' effect reported by Huber and Smith. In both cases the effects only occur for compositions with negative magnetostriction λ and for thicknesses greater than a critical thickness tc which is nearly inversely proportional to λ. These thickness and compositional dependencies suggest that magnetostriction and strain are responsible for both effects; a detailed microdomain model is being developed.
Faraday Rotation and Hysteresis Properties of Ferrite Thin Films
Authors: Francombe M. H. , Rudisill J. E. , Coren R. L.
Published: April 2005

Faraday rotation measurements on ferrimagnetic oxides have so far been limited to single crystals which are rendered transparent by grinding. In the present studies films of NiFe2O4 and CoFe2O4, of thickness 1000–8000 Å, suitable for optical studies are prepared by oxidizing films of the alloys NiFe2 and CoFe2 sputtered on silica substrates. The Faraday rotation is determined with the film magnetized along axes both normal and parallel to its surface. Using the latter arrangement, optical hysteresis loops are recorded and checked against those obtained with a vibrating sample magnetometer. The dependence of loop shape and coercive force on film thickness and oxidation temperature is studied.
Chemically Deposited Thin Ferrite Films
Authors: Wade William , Collins Thomas , Malinofsky W. W. , Skudera William
Published: April 2005

A wide range of thin ferrite films has been prepared by a chemical-deposition process. Alcoholic solutions of ferric nitrate and other metal nitrates were combined in necessary proportions to yield stoichiometric ratios of the desired ferrites, which were then deposited on substrates. Firing the coated substrates between 900–1100°C in a controlled atmosphere resulted in spinel or garnet ferrite formation. Magnetic properties were evaluated at X band by cavity-resonance measurements. Ferrimagnetic linewidths obtained ranged between 125 and 1950 Oe. Crystal structure and composition were verified by x-ray diffraction analysis. Experimental data are given. A nickel film was measured in an experimental resonance isolator in the 35-Gc region resulting in a maximum reverse to forward loss ratio of 22.7 to 1. Data and results obtained suggest applications in millimeter and submillimeter ferrite devices.
Theory of the Magnetoelectric Effect in Cr2O3
Authors: Izuyama Takeo , Pratt G. W.
Published: April 2005

It is shown in detail how the magnetoelectric effect depends quadratically on the spin-orbit interaction. The effect is discussed in Cr2O3 using the spin Hamiltonian for the cases of an applied field parallel and perpendicular to the crystalline c axis.
Far-Infrared Magnetic Resonance in CoF2, NiF2, KNiF3, and YbIG
Authors: Richards P. L.
Published: April 2005

The far-infrared transmission spectra of a variety of magnetic insulators have been measured using the techniques of Fourier-transformation interferometry. Antiferromagnetic resonance has been observed for the first time in CoF2, NiF2, and KNiF3, and the ferrimagnetic resonance in YbIG has been found near 0°K. The temperature dependences of these resonances have been measured and, in the case of the iron-group fluorides, the results are compared with the Zener theory for the temperature dependence of the anisotropy energy. Resonances previously found by other investigators in YbIG, MnF2, MnO, and NiO have been observed with (in most cases) improved precision.
Low-Temperature Specific Heat Anomaly in Manganese Ferrites
Authors: Low Brooks
Published: April 2005

The specific heats of a series of ferrites Mnx FeyO4 with x+y≈3, have been measured in the range 1.5°K to 6°K. The specific heats are anomalously large in this temperature range, and the anomaly increases with increasing manganese concentration. If the relatively small estimated lattice contribution is subtracted out, the resulting Cv vs T curves are concave downward, suggesting a Schottky-type curve with a maximum in the range 7–10°K. Assuming that a valence transition, presumably involving manganese and iron atoms, is responsible for the specific heat anomaly, then the specific heat curves indicate that the temperature corresponding to the transition energy is in the range 18–25°K. An estimate of the amount of entropy involved indicates that the number of ion pairs involved in the transition is less than one-tenth the number of MnFe2O4 molecules. This suggests that transitions involving manganese ions on octahedral sites alone could account for the specific heat anomaly.
Influence of Grain Size on Square-Loop Properties of Lithium Ferrites
Authors: Schwabe E. A. , Campbell D. A.
Published: April 2005

Pure lithium ferrite can be obtained as a very good square-loop material if certain precautionary steps are taken in material preparation, firing, and cooling. Due to its simple composition and high Curie temperature, this material is of practical as well as of theoretical interest.

In a series of memory cores prepared under various conditions, grain size was found to be the determining factor for switching time and threshold field. The series covered a range of 2 to 30 microns in average grain size. Below 2 microns good squareness was not obtained even in samples with uniform grain size. Cores with 2.3-μ average grain size proved to have as little as 120-nsec switching times under coincident current test conditions. Results indicate that there is little magnetic interaction between radially adjacent grains. Therefore, the best guarantee to obtain a square hysteresis loop requires a relatively narrow distribution of grain sizes. Cores fired under conditions which resulted in a wide distribution of grain sizes exhibited poor squareness. The change of threshold field with temperature of lithium ferrite is only 0.15% per °C compared with 1.5% per °C in some extreme cases of commonly used MgO–MnO ferrites with high ZnO content. The procedures used for obtaining grains of uniform size are discussed. Adjusting the powder particle size to a certain fraction of the grain size desired in the finished core was found to be the best physical method of controlling grain growth without introducing complications through use of oxide additions acting as fluxes or grain growth inhibitors. Powder particle size was measured by use of x-ray diffraction line broadening techniques. As a very practical relative measure of the powder particle size in the submicron range, the intrinsic coercive force can be used.
Flux Growth of NiFe2O4 Crystals by the Czochralski Method
Authors: Kunnmann W. , Ferretti A. , Wold A.
Published: April 2005

Nickel ferrite crystals were grown from sodium ferrite flux by a modified Čzochralski method. The crystals were analyzed and corresponded to Ni1.01Fe2O4.01. The largest one was a little over 1 cm3.
Single-Crystal Ferroxdure, BaFe12−2x3+Irx4+Znx 2+O19, with Strong Planar Anisotropy
Authors: Tauber Arthur , Kohn J. A. , Savage R. O.
Published: April 2005

Ir4+, substituted in ferroxdure (BaFe12O19), has been observed to couple ferrimagnetically. Replacement of Fe3+ by Ir4+Zn2+, according to the formula BaFe12−2x3+Irx4+Znx 2+O19, reduces the intrinsic axial anisotropy. The room temperature anisotropy becomes planar at x=0.3–0.4 and increases to 22 000 Oe at x=0.6. Magnetization and Curie temperature are decreased by the substitution of Ir4+Zn2+; ferrimagnetic resonance linewidth is increased. The nonaxial anisotropy is believed to derive from spin-orbital coupling of Ir4+.

Single crystals up to ¼ in. across were grown from a Bi2O3 flux in platinum crucibles. Specimens are hexagonal tabular, black, lustrous, and well-developed. Single-crystal x-ray diffraction patterns showed that a0 increases with increasing Ir4+Zn2+ substitution while c0 remains unchanged.
Rare-Earth Ion Relaxation in Ferrimagnetic Resonance
Authors: Clarke B. H. , Pearson R. F. , Teale R. W. , Tweedale K.
Published: April 2005

The linewidths in ferrimagnetic resonance ΔH and the frequency dependence of the anisotropy field for iron garnets containing rare-earth ions are explained on the basis of a model for relaxation which was used by Clogston for nickel ferrous ferrite. It is assumed that the population of the rare-earth ion energy levels relaxes towards equilibrium with a finite relaxation time τ. Consequently for ωτ≳1, where ω is the measurement frequency, significant departures from equilibrium occur and give rise to the effects noted above.

If the static torque, linewidth, and the resonant field can be measured in a temperature range where only the lowest two rare-earth ion energy levels are appreciably populated, τ can be deduced for the magnetization along [100] and [110].

The consequences of the model are described; they differ quite extensively from those of the fast relaxing ion model. A quantitative comparison with the theory for iron garnets containing ytterbium supported the theory and yielded values for τ. In the case of erbium, however, the static torque was found to be strongly field dependent and hence only a qualitative comparison is possible. It is nevertheless concluded that the effects observed support the model. At 55°K τ for the ground doublet is about 10−11 sec.
Even and Odd Instabilities in Coupled Ferrite Spheroids
Authors: Auld B. A. , Holmes C. H.
Published: April 2005

A theoretical analysis supported by experimental evidence shows that coupling between two ferrimagnetic ellipsoids can lead to a type of instability analogous to the second-order, Suhl spin-wave instability in a single sample. Two ferrite samples coupled through their dipolar fields constitute a pair of coupled nonlinear resonators. If the samples are identical, the uniform precession of the system is describable in terms of an even mode in which the transverse components of magnetization in the two samples are parallel, and an odd mode in which the transverse components are antiparallel. The frequency separation of these modes is determined by the strength of the coupling. At sufficiently low power levels the even and odd modes are independent, and the response of the system to a driving field exhibits the usual double-peaked shape characteristic of coupled linear resonators. At high power levels, however, nonlinear terms, arising primarily from shape anisotropy, produce significant mode interaction. If, for example, the even mode is excited to a large amplitude at ωp by applying identical driving fields to the two samples, there results a torque at 2ωp that can lead to exponential growth of the odd mode. Conversely, excitation of the odd mode to a large amplitude can lead to parametric excitation of the even mode. The theoretical analysis predicts the conditions under which growth is attained. Experiments conducted on narrow linewidth disks of single-crystal YIG are in substantial agreement with the theory. The instability is observed as saturation of the directly driven mode and as radiation from the unstable mode.
Parallel Pumping in Planar Zn2Y
Authors: Green J. J. , Healy B. J.
Published: April 2005

Parallel pump absorption measurements have been made on a single-crystal sphere and a single-crystal thin flake of Zn2Y. The imaginary part of the susceptibility in each sample reaches a maximum value of about 2 for an rf magnetic field in the vicinity of 25 Oe. The dependence of the critical rf magnetic field upon dc magnetic field is compared to the theory of Bady and Schlömann and the agreement is found to be reasonably good. From the minimum rf critical field, spin-wave linewidths of 8.0 Oe for the sphere and 9.24 Oe for the thin flake have been calculated.
Unstable Growth of Phonons at High Microwave Power Levels in Magnetically Ordered Single Crystals
Authors: Morgenthaler Frederic R.
Published: April 2005

Conditions are given under which transverse and longitudinal microwave phonon standing waves present in a ferrimagnetic single crystal can be parametrically excited by the uniform precession of the magnetization. The instability thresholds for both first- and second-order processes are derived for crystals having cubic symmetry and are found to depend on higher-order magnetoelastic energy terms. The phonons need not be degenerate with any portion of the spin-wave band; in which case, the usual magnetoelastic coupling is absent. Competition from spin-wave processes is considered, and the conditions necessary for phonon domination are presented. The possibility of either maximizing or forbidding a particular phonon process is also discussed.
Spin-Wave Growth Under Parallel Pumping
Authors: Schlömann E. , Green J. J.
Published: April 2005

The transient growth of the spin-wave population immediately after the application of a strong microwave pulse has been calculated, assuming that the spin waves are in thermal equilibrium before the pulse is applied. The theory is compared with experiments performed at room temperature on a spherical single crystal of yttrium iron garnet at X band. Good agreement is found for a time interval of a few μsec starting several μsec after the pulse has been applied. Before this time interval, the absorption coefficient is too small to be measured. After this interval, the transient behavior is influenced by saturation effects which are not taken into account in the theory.
Effects of Electrical Conductivity on High-Power Linearity in Lithium-Titanium Ferrite
Authors: Hickernell Fred S. , West Russell G. , Sakiotis Nicholas G.
Published: April 2005

Nonlinear effects at high rf power levels were investigated in substituted polycrystalline lithium ferrite with respect to changes in the dc conductivity of the material. Selected samples of a lithium-titanium ferrite system were thermally annealed to produce material with dc resistivities in the range 1010 to 100 Ω-cm. The threshold field, spin-wave linewidth, and field-dependent characteristics of the subsidiary absorption curve were investigated by the perpendicular pump technique in a 5600-Mc cavity. The rf threshold field required for the onset of nonlinearity exhibited a definite dependence upon the resistivity, increasing by a factor of three as the resistivity was decreased to 390 Ω-cm. The initial permeability spectrum maintained its characteristic form and no significant change was observed in the intrinsic magnetic properties over the range of resistivity values. Data are presented showing the observed relationships between the magnetic and dielectric properties and the dc resistivity. The application of this technique to improved peak power performance in a C-band wye-junction circulator is discussed.
Eddy-Current Losses Calculated From a Domain Model and Observed Densities of Mobile Walls
Authors: Wycklendt Daniel A.
Published: April 2005

Recent measurements involving the superposition of small audio-frequency field increments on silicon iron sinusoidally magnetized at low frequencies indicate that the number of mobile domain walls varied during the cycle. This experimental technique has been considerably refined to provide a better measure of the number of active domain walls. Based on a simple domain model, a calculation of the number of active walls can be made from the experimental data.

The principal objective of this study is an explanation of the anomalies in the loss-frequency curve of grain-oriented silicon iron magnetized to high flux densities at low frequency. The data obtained from the superposed incremental magnetization experiment can be directly applied to a calculation of the eddy-current losses with a simple domain model. Since no better information is available, it is necessary to take the speeds of the walls to be equal to complete the calculation. With this assumption and the audio-frequency data, the losses are then calculated point-by-point over a cycle of magnetization. Comparing the calculations to loss measurements, 60 percent of the anomalous eddy-current loss and some of the downward curvature of the measured loss-frequency curve are accounted for in the 0–30 cps range. Considering the assumptions made, this result gives plausibility to the domain model and the interpretation of the audio frequency magnetization data.
Eddy-Current Effects with Rapidly Varied High Magnetic Fields
Authors: Elder Tait , Barte William
Published: April 2005

Eddy-current broadening of magnetic hysteresis loops in iron and Alumel wires has been studied with oscillatory fields of varying amplitude (100–14 000 Oe) which change as rapidly as 7×105 Oe/sec in the irreversible part of the loop. Under conditions where eddy currents dominate, coercive forces which are 50 times the static coercive force of a few oersteds can be observed. Experimental data obtained at 77°K and at 300°K with both materials agree qualitatively with a theoretical expression derived by Brown under the simplifying assumption that the outer skin reverses in magnetization before the inner core magnetization changes. The coercive force in this expression increases as the square root of the product of conductivity, cross sectional area, saturation magnetization, and rate of change of field.

When the irreversible part of the loop is traversed at some fixed rapid rate, the observed coercive force increases with increasing amplitude or duration of the immediately preceding magnetic field. This effect on coercive force of the previous aligning field amplitude and duration is attributed to flux-reversal delays similar to those denoted as magnetic viscosity by Mitkevitch. Such delays have also been interpreted in terms of domain wall nucleation time by Becker.
Thin Soft Magnetic Tapes in Small High-Frequency Fields
Authors: Boll R.
Published: April 2005

The complex permeability of iron-nickel and Permalloy alloys in tape thicknesses down to 2 μ has been measured at frequencies up to 1000 Mc. Toroidal cores with different heat treatments and in the cold-rolled state were used.

Curves of complex permeability (Cole-Cole diagrams) differ more and more from the classical eddy-current curve with decreasing thickness. It is concluded that spin relaxations and resonances are effective as known from ferrites. The effects are clearer in alloys with low rather than high initial permeability; and are strongly marked in thin cold-rolled tapes and magnetic films.

Curves of complex permeability are analyzed by means of the equations for domain-wall motions and domain rotations, and the significant parameters are discussed.

These measurements brought new knowledge about the limiting frequency and the loss factor of thin tapes. The limiting frequency tends clearly towards an upper limiting value with decreasing thickness. This is in contrast to the classical eddy-current theory. The upper limiting frequency is identical with the ``gyromagnetic'' limiting frequency as calculated by Snoek, which is accordingly not valid for ferrites only, but for metals as well. The frequency-dependent part of the loss factor tgδ has a lower limiting value. This loss minimum and the upper limiting frequency are really reached at annealed tapes of some microns, and with dust cores of fine powders.

Further improvements are possible by using special anisotropies. An example is the magnetic films which have a direction of hard magnetization and a plane of easy magnetization. Cold-rolled and half-hard thin tapes with Isoperm-like hysteresis loops show effects in similar direction.

The results show that a tape thickness of 3–6 μ is normally sufficient to applicate metals in the Mc range. Further decreasing of the tape thickness seems successful only if reduced to the order of 1000 Å and also using the strong shape anisotropy used with the magnetic films.
Some Magnetic Structure Properties of Terbium and of Terbium-Yttrium Alloys
Authors: Koehler W. C. , Child H. R. , Wollan E. O. , Cable J. W.
Published: April 2005

Neutron diffraction studies have been made on single-crystal and polycrystalline specimens of terbium. Earlier magnetic and thermal measurements have indicated a transformation to an ordered magnetic state at approximately 230°K, and a subsequent order-order transformation at approximately 220°K. The neutron measurements show that in the narrow antiferromagnetic region, the magnetic structure of terbium is a helical structure. The interlayer turn angle varies from 20.5° per layer at the Néel point to 18.5° per layer at the lower transition. At this lower temperature the structure transforms, in the absence of any external applied field, to a classical ferromagnetic structure in which the moments are in, or nearly in, the planes perpendicular to the hexagonal axis. At very low temperatures the magnetic moment per atom is very nearly 9.0 Bohr magnetons, the value expected for the ordered tripositive ion. Neutron diffraction measurements have also been made on a series of alloys of yttrium and terbium in order to study the influence of magnetic dilution on the magnetic properties of the rare-earth metals. Introduction of yttrium into terbium reduces the Néel temperature and broadens the range of existence of the helical phase. At 30-at. % yttrium and above, the spontaneous transformation to the ferromagnetic state is not observed, for zero applied field, even for temperatures as low as 4.2°K. As in pure terbium the interlayer turn angle ω of the helical structure in the alloys varies with temperature, but the slope of the ω vs temperature curve becomes smaller with increasing yttrium concentration.
Thermodynamic Self-Consistent Theory of Indirect Exchange in Metals
Authors: Kaplan T. A.
Published: April 2005

A theory of indirect exchange forces (so-called s-d or s-f exchange) is given, based on Peierl's statistical mechanical variational principle. We consider a set of determinantal states, | M1…MN〉 = | M〉, where | Mi| ≤S, the localized spin, and N is the number of localized spins, the ith spin being quantized along some direction ẑi; in each of these determinants, the conduction electrons occupy general band orbitals φk which are constant over the set. The energies, EM=〈M | H | M〉, are of course functionals of the φk; varying the partition function with respect to the φk then leads to Hartree-Fock-like (HF) equations for the φk which involve the thermal averages M̄i of the localized spins, which in turn are given as functions of the φk. A particular type of solution, a spiral, is shown to exist insofar as symmetry is concerned; in connection with this proof a generalization of the Bloch translational symmetry theorem is given. Neglecting entirely the band-band exchange terms (which would give rise to Overhauser's spin-density waves), the spiral wave vector is the same as in the well known indirect ``s-f'' exchange theory in the limit M̄i→0 (i.e., as the Néel temperature is approached). For finite M̄i, the present approach provides a reasonable generalization, giving the expected energy gaps due to the magnetic long range order. It is shown that the corresponding band structure exhibits spin-density waves even though their source in Overhauser's theory has been eliminated. The temperature dependence of the gaps plus the implied changes in the Fermi surface gives rise to temperature-dependent effective exchange integrals which cause a thermal variation in spiral wavelength.
Microwave Absorption in Gadolinium-Yttrium Alloys
Authors: Popplewell J. , Tebble R. S.
Published: April 2005

The paramagnetic, antiferromagnetic, and ferromagnetic states of the gadolinium-yttrium alloy system have been examined by microwave resonance techniques. The paramagnetic g values are found to be concentration dependent ranging from 1.96±0.03 in pure gadolinium to 2.13±0.05 in the 3.7% gadolinium alloy. The results, in particular the experimentally determined g shifts, are discussed in terms of the Yosida development of Zener's proposal that the coupling between d (or f) spins is brought about by exchange interaction with the polarized conduction electrons.
Magnetic Properties of Some Divalent Europium Compounds
Authors: McGuire T. R. , Argyle B. E. , Shafer M. W. , Smart J. S.
Published: April 2005

The compounds of divalent europium form an attractive series for magnetic investigations because the magnetic ions are in S states and the crystal structures are generally simple. Moreover, true ferromagnetism, which is quite unusual in compounds, has been observed in EuO by Matthias, Bozorth, and Van Vleck.

We have prepared samples of EuS, EuSe, EuTe, EuCO3, EuTiO3, EuZrO3, EuSO4, and EuCl2, and studied their magnetic properties in the temperature range 4–300°K. The sulfide, selenide, and telluride all have the same NaCl crystal structure as EuO; we find that the first two also become ferromagnetic at low temperature, while EuTe is paramagnetic, or possibly antiferromagnetic, at 4.2°K. The Curie temperatures for EuS and EuSe are about 18° and 7°K, respectively. The saturation moments of EuS and EuSe extrapolated to 0°K correspond to 6.8 and 6.7 μB per Eu++ ion, respectively, in good agreement with the expected value of 7 μB. None of the other compounds were ferromagnetic at liquid helium temperatures, and all exhibited θ values very near to 0°K. A tentative interpretation of the exchange interactions in the rocksalt compounds is made using a molecular field treatment.
Superconductivity in Lead-Base Solid Solution Alloys
Authors: Livingston J. D.
Published: April 2005

Magnetization as a function of applied magnetic field has been measured at 4.2°K for binary solid solution alloys of In, Tl, Sn, Hg, Na, and Bi in Pb. For pure Pb, superconductivity is destroyed when the field penetrates abruptly at the thermodynamic critical field HC. With sufficient alloying element in solution, the field, instead, first penetrates at HFP(HC). Increasing solute concentration decreases HFP and increases HN in a manner predicted by the ``negative surface energy'' theories of Abrikosov and others. Annealed specimens approach reversible magnetic behavior and exhibit little trapped flux. Plastic deformation is found to increase HFP, the area under the ascending magnetization curves, the hysteresis, and the trapped flux, but has little effect on HN. The results indicate that the high critical field of these alloys results from the lowered surface energy of a superconducting-normal interface and not from any specific filamentary features of the microstructure. Dislocations do, however, increase the magnetic hysteresis and, it is inferred, the current-carrying capacity of these alloys.
Electrical Resistivity of Ferromagnets at Low Temperatures
Authors: Goodings D. A.
Published: April 2005

The contribution to the electrical resistivity of ferromagnetic metals which is associated with the magnetic behavior has been studied theoretically at low temperatures. As in previous theoretical work this resistivity ρmag is assumed to be due to scattering of conduction electrons by spin waves through the s-d interaction. An expression valid for a general lattice is presented which is shown to account for the form of the temperature dependence of ρmag for Fe, Co, Ni, and Gd. However, estimates of the magnitude of the resistivity arising from this interaction indicate that it may be from one to two orders of magnitude too small to account for the experimental results.
Absorption Contrast Effect in bcc Metals
Authors: Bronisz S. E. , Douglass Dana L.
Published: April 2005

Contrast figures of unusual form have been observed in bcc metals by transmission electron microscopy. These figures appear to be caused by the anomalous absorption of electrons.
Application of the Simplified Cylindrical Distribution Function: Orientation in Linear Polyethylene
Authors: Milberg M. E.
Published: April 2005

The simplified cylindrical distribution function has been applied to the determination of the degree of orientation in a moderately oriented linear polyethylene fiber. The function has been calculated using the first two and ten lines of the scattering pattern and, though poorly resolved, yields useful orientation information. It is concluded that the greater the knowledge of the molecular structure of the material under investigation the less the amount of intensity data required to produce a useable function. Although the technique is applicable to both crystalline and noncrystalline materials, the standard methods, applicable only to crystalline materials, are simpler. Therefore, the method can be considered to apply mainly to moderately oriented noncrystalline fibers.
Ionic Conductivity and Time-Dependent Polarization in NaCl Crystals
Authors: Sutter P. H. , Nowick A. S.
Published: April 2005

When a constant electric field is applied to a dielectric, the current density per unit field decreases with time from an ``initial conductivity'' to a ``final (or steady-state) conductivity.'' A study is made of this time-dependent polarization effect for ``pure'' NaCl crystals in the range from 50° to 200°C. The validity of Ohm's law and the superposition principle is demonstrated for these crystals; this establishes the linearity of the formal equation which relates polarization, electric field, and their time derivatives. Also studied are the effect of prolonged current flow, the effect of deliberate introduction of an air gap between the crystal and one of its electrodes, and the effects of impurities, deformation, x-ray irradiation, and annealing. It is concluded that the results are not consistent with the well-known space-charge polarization theory of Joffé, according to which the buildup of space charge occurs because of blocking of the current carriers at one or both electrodes. Rather, it is necessary to regard the time-dependent polarization as a dielectric relaxation phenomenon. According to this viewpoint, the final conductivity, and not the initial value, represents the true ionic conductivity. Possible relaxation mechanisms are discussed in terms of defect clusters and charged jogs on dislocations.
Phonon Assignments in GaAs and CdS
Authors: Spitzer William G.
Published: April 2005

Previously, five phonon energies were used to explain 16 peaks in the measured lattice absorption spectrum of GaAs. It is shown here that the five phonon energies are not unique and that a different set of values is also compatible with the available infrared data both with respect to the absorption peak positions and the temperature dependence of the absorption. A similar difficulty is also noted for some previously published work on CdS. It is suggested that difference band absorption measurements might show which sets of phonon energies are correct.