bluuebunny/crossref_metadata_2025 huggingface dataset:
Anisotropic Behavior in Superparamagnetic Systems
Authors: Yasumori I. , Reinen D. , Selwood P. W.
Published: April 2005

The relative magnetization of an assembly of fine, ferromagnetic, single-domain particles which show uniaxial magnetic anisotropy has been formulated as a function of ρ and λ, the ratios of magnetic energy of particle moment in field and anisotropy energy to thermal energy, respectively, under various situations for particle orientation. Strict formulation for the system where the anisotropic symmetry axes of particles are distributed in thermal equilibrium gives the simple Langevin equation without any influence of anisotropy. In the case, however, of particles with fixed random orientation of symmetry axes the observed decrease of relative magnetization was deduced, while it was found that the Langevin equation is still valid as low and high field approximations. Several simpler cases where the symmetry axes of particles are constrained on a plane parallel or perpendicular to the applied field were also treated and the deviations from the Langevin equation were estimated. These results confirm Bean and Livingston's view about the behavior of the system in low and high field extremes, but show that Knappwost and Rust's formulation, which was developed to explain the anomaly in the magnetization of fine cobalt particles, is not reasonable, suggesting sources of anisotropy other than those of magnetocrystalline origin.
Generation of Elastic Waves by Transient Surface Heating
Authors: White R. M.
Published: April 2005

When the surface of a body is subjected to transient heating (e.g., by electron bombardment or rf absorption) elastic waves are produced as a result of surface motion due to thermal expansion. This process is analyzed, with particular emphasis on the case of an input heat flux varying harmonically with time, to relate the elastic wave amplitude to the characteristics of the input flux and the thermal and elastic properties of the body. Experiments performed with both electron impact and rf absorption verify the proportionality of the stress wave amplitude and the absorbed power density, and correlate well with the thermal and elastic properties of the heated medium. Comparison of the elastic wave stress amplitude with radiation pressure shows that the former may be much greater than the latter, as experiments have demonstrated. When a barium titanate crystal was used to detect the elastic waves produced, heating by a single 2-μsec pulse of electrons or microwave radiation produced easily detectible signals at power levels down to 2 W/cm2, corresponding to a computed peak surface temperature rise of about 0.001°C, and produced piezoelectric crystal voltages ranging from about one to more than 60 mV/kW/cm2 absorbed power density.
Planar Stress Field of a Dislocation in an Anisotropic Plate
Authors: Chou Y. T.
Published: April 2005

An analysis is presented of the stress field of a dislocation over the central plane of an infinite anisotropic plate with the dislocation lying in the plane. Basic expressions for such a field are given. The stress decreases monotonically as the thickness of the plate decreases. Thus, as expected, the internal stress field in the plate is weaker than in the bulk material for both isotropic and anisotropic cases. Anisotropy may either enhance or reduce the effect of thinning on the relative strength of the stress field (σ/σinf., where σinf. is the stress in the infinite medium) when compared to the isotropic case. Criteria for differentiating the two opposite effects are presented.
Applicability of Minimum Rate of Entropy Production to the Solidification of Lamellar Eutectics
Authors: Tiller W. A.
Published: April 2005

The minimum rate of entropy production, during the one-dimensional freezing of a lamellar eutectic at velocity V has been calculated using a well-defined model. This principle is found to be equivalent to the maximum interface temperature principle at small V but not at large V. Since it does not provide the correct relationship between lamellar spacing and freezing velocity over the entire experimental range, there is considerable doubt as to its physical significance when applied to this case.
Effects of Tensile Stress on the Domain Structure in Grain-Oriented 3.25% Silicon Steel
Authors: Gniewek J. J.
Published: April 2005

A Kerr magneto-optic apparatus has been used to observe the effects of tensile stress on the domain structure in grain-oriented 3.25% silicon steel. Unlike the powder pattern technique, the Kerr apparatus reveals the complete surface domain structure without regard to the type of domain wall, and enables observation of the domain structure changes upon application of small stresses. The behavior under stress of a new pattern, termed the ``chevron,'' has been noted.
Ferromagnetism in V-Fe and Cr-Fe Alloys
Authors: Nevitt M. V. , Aldred A. T.
Published: April 2005

Saturation magnetizations of V-Fe and Cr-Fe alloys were measured and the results used to determine the average atomic magnetic moment. The linear rate of change of the moment with solute concentration, dp̄B/dC, was found to be −3.286 Bohr magnetons per atom in V-Fe α-phase alloys. Transformation of an approximately equiatomic alloy to the CsCl-ordered α′-phase resulted in a decrease of about 0.4 Bohr magneton in the moment. Subsequent transformation to the σ-phase lowered it by a factor of three. In Cr-Fe solid solution alloys, dp̄B/dC was found to be −2.36 Bohr magnetons per atom.
Interacting Partial Dislocations in GaSe
Authors: Basinski Z. S. , Dove D. B. , Mooser E.
Published: April 2005

Electron microscopical observations of glide dislocations lying in the basal plane of the layer structure GaSe are described. The dislocations are dissociated into partials separated by stacking fault ribbons as wide as 1 μ. Network-like grids formed from sets of ribbons lying in different glide planes were observed and are analysed in detail. A discussion is given of the contrast effects produced by dislocation dipoles contained in the grids.
Coupling of Electromagnetic and Quasi-Static Modes in Plasma Loaded Waveguides
Authors: Auld B. A. , Eidson J. C.
Published: April 2005

This paper presents a coupled mode formalism which predicts the effects of rf magnetic fields on the propagating modes of the plasma loaded waveguide. In addition, this formalism provides a simple physical interpretation of the propagation characteristics in terms of the uncoupled modes, which are taken to be the waveguide modes and quasi-static plasma modes, respectively. The method of obtaining coupled mode equations is given and two typical calculations using the theory are discussed.
Heat Transfer to Boiling Helium
Authors: Reeber M. D.
Published: April 2005

The coefficient of heat transfer between a smooth vertical silver surface and liquid helium is typically 0.020±0.005 W cm−2 °K−1 for convection and increases in the pool boiling range to 0.22 W cm−2 °K−1 for 0.040 °K superheat at 4.0°K. The heat transfer rate for pool boiling decreases with decreasing temperature between 4.0° and 2.5°K in proportion to the latent heat of vaporization and inversely as the molar volume of the vapor at the pressure and temperature of the experiment. The superheat in the steady state is determined by the volume rate of gas production at the surface. The hysteretic nature of the heat transfer curves in the boiling range suggests a nucleation mechanism strongly dependent on the thermal history of the specimen.
Scattering of Plane Compressional Waves by a Spherical Obstacle
Authors: Pao Yih-Hsing , Mow C. C.
Published: April 2005

The scattering of plane compressional waves by a spherical obstacle in an elastic solid, which was investigated by Ying and Truell is examined further. For a rigid inclusion, the boundary conditions are redefined to take into consideration the motion of the inclusion inside the solid. By a proper limiting process, it is shown that the solutions for a rigid insert, a fluid sphere, a cavity, or an obstacle in a fluid are all derivable from the general results of an elastic inclusion. The rates of energy scattering due to a small rigid obstacle (a«λ) are found to be inversely proportional to the fourth power of wavelength.
Drift and Diffusion of Activation in Oxide-Coated Cathodes
Authors: Okumura Koji , Hensley Eugene B.
Published: April 2005

Two experiments pertaining to oxide-coated cathodes are described. One of these involves the drift of activators in oxide-coated cathodes under the influence of an electric field and the other involves the diffusion of these activators in the absence of an electric field. The drift experiment revealed a temporal nonuniform increase in the activation of the conductivity. However, it failed to show any significant spatial nonuniformity. Pulse measurements of the current-voltage characteristics showed the origin of the temporal nonuniformity to be associated with nonohmic properties of the coating which were related to the Nergaard effect. The results of the diffusion experiment indicated a diffusion coefficient at 1000°K for the crystalline defects undergoing diffusion of (1.5±0.8)×10−7 cm2/sec and an activation energy of the diffusion of 0.67±0.4 eV. Comparison of this diffusion coefficient with data obtained from the literature makes possible the identification of the mobile defects involved in activation processes in oxide-coated cathodes as being barium vacancies. On the bases of these results, a ``mobile acceptor model'' for oxide-coated cathodes is presented which suggests that activation is achieved by the removal of the highly mobile acceptors (barium vacancies) which compensate the relatively immobile donors (oxygen vacancies) and not by an increase in the density of the donors as has usually been assumed. Nergaard's theory explaining the millisecond decay of thermionic emission from oxide-coated cathodes is modified in terms of the mobile acceptor model, and the kinetic processes involved in the activation of oxide-coated cathodes are discussed.
Transient and Temperature Response of a Distributed, Thermally Activated System
Authors: Macdonald J. Ross
Published: April 2005

After a brief survey of methods of representing the transient response of a distributed linear system, a general analysis is presented of the temperature dependence of relaxation times of a thermally activated system. It is assumed that either the pre-exponential factor τd (or its inverse, the vibrational frequency vd) or the activation energy E may be separately distributed or that they both depend linearly on a structure factor S and so are similarly distributed. Further, τd, E, and parameters of their distributions are taken temperature independent. A two-parameter, generalized, truncated exponential probability density function is chosen for the distribution of S and transformed to yield a power-law distribution for G(τ), the over-all relaxation-time distribution function. Expressions yielding the time and temperature response of the system are then derived from G(τ). The specific transient response considered is that following the imposition of a constant forcing function at t=0 and may represent the charge and current response of a dielectric system after application or removal of a constant voltage, the strain and rate-of-strain response of a mechanical system after constant stress is applied or removed, or the stress and rate-of-stress response on application and maintenance of a constant strain. Representative results are found to agree quantitatively or qualitatively with available transient and temperature responses for a wide variety of materials under electrical or mechanical stimulation. In particular, the current transient response or stress relaxation response is constant for measuring times less than the shortest relaxation time of the system, then exhibits one or two regions with t−(1+ρi) behavior, and, finally, decreases very rapidly when the measuring time exceeds the longest time constant of the system. The theory leads to specific possibilities for the temperature behavior of ρi and to reasons why it is usually found to be less than unity in magnitude. Finally, conditions are analyzed for which the time-temperature superposition law of rheology applies.
Quasi-One-Fluid Theory of Wave Propagation in a Fully Ionized Plasma
Authors: Willett Joseph E.
Published: April 2005

A complete set of linearized two-fluid equations for the perturbation of a viscous, heat conducting, fully ionized plasma is transformed into an equivalent set of equations in the one-fluid quantities. The ``quasi-one-fluid'' formulation thereby obtained is applicable to a wider class of phenomena than the magnetohydro-dynamic formulation and differs from the latter because of terms corresponding to current inertia, Hall effect, space charge, diffusion, unequal electron and ion temperatures, viscous electric effects, and a viscous force arising from nonuniform currents. Dispersion relations are obtained for waves of small amplitude. Included are equations showing the effects of viscosity on electron electrostatic, ion electrostatic, and electromagnetic waves in the absence of a static magnetic field, and Alfvén waves propagating along a magnetic field.
Atom Ejection in Low Energy Sputtering of Single Crystals of bcc Metals
Authors: Anderson G. S.
Published: April 2005

The spatial distribution of atoms ejected from low index planes of the bcc metals V, Fe, Nb, Mo, Ta, and W have been studied in detail for bombardment by Hg+-, Xe+-, Ar+-, and Ne+-ions of energies up to 800 eV. The principal atom ejection directions are the 〈111〉 and 〈100〉, which are the directions of focuson propagation in bcc crystals. Other atom ejection directions were observed in which momentum focusing is not possible. These are interpreted to be the result of 〈111〉 focusons (directed outwardly with sufficient energy) intercepting surface atoms which are not in normal atom sites. For bombardment of (100) surfaces the center spot ([100] ejection) appears above a certain minimum energy which increases in the order Ne, Ar, Xe, Hg. This is true for widely different masses of the target atoms, and seems to be related to the spatial distribution of momenta in the crystal. For heavier bombarding ions the momenta tend to be directed more in the forward direction and the center spot becomes less pronounced than for lighter bombarding ions.
Spin-Wave Instability in Hexagonal Ferrites With a Preferential Plane
Authors: Schlömann E. , Joseph R. I. , Bady I.
Published: April 2005

The large magnetic anisotropy field that is encountered in many planar ferrites markedly affects the rf magnetic field at which spin-wave instability sets in. When the rf magnetic field is applied perpendicular to the dc field, spin-wave instability can occur through the first-or second-order process. At ferromagnetic resonance, the first-order process dominates if the frequency is smaller than a characteristic frequency ωc. This characteristic frequency has been calculated for ellipsoidal samples. For most shapes, the anisotropy field tends to increase ωc, thus favoring the occurrence of the first order process. In addition, the anisotropy tends to decrease the critical field at which spin-wave instability sets in. Assuming the spin-wave relaxation rates to be equal in the two situations compared, we find that for large anisotropy the second-order threshold is reduced approximately in the ratio of (2/ζ)½ for relatively high frequencies; and in the ratio of (ω/ωMζ)½ for relatively low frequencies where ζ=Ha/4πM is the anisotropy field in units of the saturation magnetization. In parallel pumping, the threshold field is reduced in the ratio of 1/(1+ζ) at low biasing fields.
Magnetic Memories—Capabilities and Limitations
Authors: Rajchman Jan A.
Published: April 2005

The capabilities and limitations of magnetic computer memories are discussed. Comparison of microferrites and thin films shows that in the near future both approaches are likely to provide 100-nsec memories of several thousand words. A gradual increase in storage capacity, beyond today's few million bits, may result from batch fabrication of magnetics, but a significant increase requires methods of batch fabricating both the magnetic and the associated semiconductor devices. Capacities of hundred millions or billions of bits are likely to be attained sooner by superconductive techniques. Content addressable or associative memories of the magnetic and superconductive types are discussed briefly.
Spin-Density-Wave Mechanisms of Antiferromagnetism
Authors: Overhauser A. W.
Published: April 2005

The salient properties of spin-density-wave (SDW) states of an electron gas are reviewed. Antiferromagnetism in metals can arise from SDW's whether or not the metal contains localized magnetic moments (with spin degrees of freedom). If there are no local moments, a SDW ground state of the conduction electron gas is required, containing one or more collective-electron magnetization waves. Large amplitude SDW's are observed in Cr and, very likely, small amplitude SDW's will eventually be observed in a number of other metals, including some of the nontransition elements. Localized d or f electrons can be antiferromagnetically ordered by SDW's even though the conduction electron gas, in which they are imbedded, has of itself a nonmagnetic ground state. Exchange interactions between the localized-electron spins and the spin polarization density of excited SDW's are responsible for excitation of the latter, which in turn orient and order the former. Indirect exchange interactions (Ruderman-Kittel) between localized spins are shown to be equivalent to, and derivable from a multiple SDW excitation of the electron gas. However, the indirect-interaction model is inappropriate for a discussion of long range order, since the SDW excitations are then concentrated in a few discrete waves. Observed effects such as truncation of the Fermi surface by repopulation of k space, the energetic importance of commensurateness between the SDW wavelength and lattice constant, and the occurrence of multiply periodic states of long range order are natural and immediate consequences of SDW theory.
Mn55 Nuclear Magnetic Resonance in Manganese Ferrite
Authors: Heeger A. J. , Ghosh S. K. , Blocker T. G.
Published: April 2005

The observation of the nuclear magnetic resonance of Mn55 in ferromagnetic MnFe2O4 is reported. The resonance frequency in a sphere is 586.0 Mc in an external field of 2730 Oe; and the linewidth is found to vary from 1.7 Mc at 1.65°K to 2.2 Mc at 4.2°K. The nuclei involved have been identified as associated with Mn2+ ions located on A sites in the spinel lattice. The double resonance technique used in these experiments allows the direct measurement of the nuclear spin-lattice relaxation time T1. The magnitude of T1 at 1.65°K is found to be 1.5×10−3 sec.
Nuclear Resonance and the Hyperfine Field in Dilute Alloys of Nickel in Iron
Authors: Streever R. L. , Bennett L. H. , La Force R. C. , Day G. F.
Published: April 2005

The nuclear magnetic resonance absorption of Ni61 has been observed in 1% Ni61−99% Fe and 2% Ni61−98% Fe solid solutions at 85.4 Mc at room temperature. The linewidth is about 700 kc in the 1% sample at room temperature with some broadening in the 2% sample. At 77°K the resonance occurs at 89.1 Mc with nearly the same linewidth as at room temperature. The intensity of the resonance signal was found to decrease in an externally applied steady magnetic field. The room temperature hyperfine field at the Ni61 nucleus is 558 kOe, assuming a nuclear moment of 0.3 nm, or 186 kOe, assuming a moment of 0.9 nm. If one assumes that the hyperfine field at the nickel nucleus is nearly proportional to the local moment, a resonance frequency of 85.4 Mc implies a local atomic magnetic moment of about 2.0 Bohr magnetons at the nickel atom, somewhat less than that in pure iron.
Domain Walls in Thin Ni–Fe Films
Authors: Middelhoek S.
Published: April 2005

The energies of Bloch and Néel walls are calculated as functions of the film thickness and the angle through which the magnetization turns in the wall. It appears that in addition to the well-known Bloch—Néel wall transition as a function of the film thickness, Bloch—Néel wall and cross tie—Néel wall transitions occur when the angle through which the magnetization rotates is changed by an external field. The new calculations together with a review of previous work give a comprehensive picture of the domain walls in thin Ni—Fe films.
Longitudinal Permeability in Thin Permalloy Films
Authors: Torok E. J. , White R. A.
Published: April 2005

Longitudinal permeability is defined as μL=(∂BL/∂HL)HT=const, where BL is the component of applied field parallel to the easy axis, and HT is a bias field applied perpendicular to the easy axis of a magnetic film with uniaxial anisotropy. The longitudinal permeability is also the hard axis resonance amplitude in the low frequency limit.

The longitudinal permeability is calculated using three different models. The first, that of an ideal single domain film, yields an infinite value when the bias equals HK. Considering the effect of inhomogeneity in easy axes, resulting in many noninteracting domains, gives a maximum μL/μT=22α90−23, where μT=4πM/HK and α90 is so defined that 90% of the film has a local easy axis within α90 degrees of the average easy axis. Including variation of HK within a film lowers μL/μT at HT=HK to 22α90−23−34α90−1Δ9012, where Δ90 is defined so that 90% of the film has an HK within Δ90 HK of the average HK of the film.

Comparison with experimental curves for films with measured values of α90 and Δ90 shows that the last model yields good qualitative agreement and correctly predicts the behavior of μL/μT with finite drive fields, but quantitatively the experimental maxima are 20% to 60% lower than the theoretical curves. This difference is attributed to wall energy terms which are not included as part of the regular calculation.
Relationship Between Uniaxial, Inverted, and Rotatable Initial Susceptibility Permalloy Films
Authors: Flanders P. J. , Prutton M. , Doyle W. D.
Published: April 2005

A set of experiments have been performed to examine the relationship between various types of Permalloy films which are usually regarded as being quite separate. The three types are (i) uniaxial films, (ii) inverted films, and (iii) rotatable initial susceptibility (RIS) films. It is postulated from the results of these experiments that the three films differ by the KV fraction of randomly oriented material present. At one extreme, the uniaxial films have a very low random KV fraction, while at the other extreme, RIS films are completely random arrays. The inverted films can be regarded as being intermediate between these two types. The origin of the high remanence observed in the inverted and RIS films is theoretically attributed to the effects of magnetostatic interactions between adjacent regions with different easy directions.
Magneto-Caloric Effects in Gadolinium Trichloride
Authors: Garton G. , Leask M. J. M. , Wolf W. P. , Wyatt A. F. G.
Published: April 2005

Measurements are reported of the temperature changes which accompany isentropic magnetization of GdCl3 at temperatures between 1.2° and 4°K with fields between 0.5 and 8.4 kG. At the Curie temperature 2.20° K temperature changes of ∼0.5°/kG were found. Using the results of earlier zero-field specific heat measurements, an entropy diagram was constructed, and from this the variation of specific heat with temperature in various fields calculated. The general behavior is well explained by a simple molecular field theory based on high temperature susceptibility measurements.
Crystallographic Transitions in Several Chromium Spinel Systems
Authors: Wold A. , Arnott R. J. , Whipple E. , Goodenough J. B.
Published: April 2005

The systems NixFe1−xCr2O4, CuxNi1−xCr2O4, and CuxCo1−xCr2O4 have been prepared and their crystallographic properties studied. From consideration of Jahn-Teller effects, it was shown that A-site Cu2+ is more effective than A-site Fe2+ in stabilizing a tetragonal (c/a<1) distortion at A-site Ni2+. In addition considerably more Cu2+ is required on the A site to distort CoCr2O4 from cubic symmetry to c/a<1 than is required to change the distortion of NiCr2O4 from c/a>1 to c/a<1.
Entropy Changes of Ferromagnetic-Antiferromagnetic Transitions from Magnetic Measurements
Authors: Flippen R. B. , Darnell F. J.
Published: April 2005

The thermodynamics of magnetic processes in several compounds undergoing ferro- or ferrimagnetic to antiferromagnetic transitions are studied using critical field-transition temperature data obtained with a miniature-coil, pulsed-field technique. The zero-field transition entropy change in each material is calculated by means of the magnetic Clapeyron equation. The results are discussed in terms of the Kittel theory of exchange inversion. The variation of exchange constant α with lattice parameter c is found to vary over only a small range, and is roughly proportional to the reciprocal of the sublattice magnetization.
Magnetic Properties of a Single Crystal of Manganese Phosphide
Authors: Huber E. E. , Ridgley D. H.
Published: April 2005

Magnetic measurements on a single crystal of MnP indicate strong ferromagnetic interactions at room temperature and cast doubt on previous ferrimagnetic or canted spin models. A new first-order transformation has been observed at 50°K below which MnP is metamagnetic, i.e., it exhibits a ferromagnetic-antiferromagnetic transformation which is a function of applied field and temperature. One possible explanation is that there are competing antiferromagnetic-ferromagnetic interactions that are temperature-dependent.
Metamagnetism of Siderite (FeCO3)
Authors: Jacobs I. S.
Published: April 2005

A uniaxial antiferromagnet with high anisotropy should show a sharp transition to a ferromagnetic configuration in fields sufficient to overcome the antiferromagnetic exchange. Examples of this restricted type of metamagnetism are rare, but are here extended to ferrous carbonate in its natural mineral form, siderite. Magnetic susceptibility measurements on powder and oriented crystals at 10 kOe between 4.2° and 300°K confirm in detail the high anisotropy and indicate a Néel point at 38°K for this mineral. Pulsed field magnetization measurements to 200 kOe at 4.2°K exhibit a metamagnetic transition toward ferromagnetism starting near 100 kOe for both the powder and c-axis oriented crystal. The transition in this crystal is nearly complete at 200 kOe where the magnetization approaches a saturation of about 6μB per iron ion. From an elementary statistical model of the broad transition, the critical field for pure FeCO3 is estimated at about 200 kOe. These data and other factors lead to a conclusion that the predominant exchange interaction in FeCO3 is antiferromagnetic.
Square Loop Properties of Lithium-Nickel-Zinc Ferrites
Authors: West Russell G.
Published: April 2005

Results of an experimental investigation to obtain square-loop materials for use in multiaperture logic cores at elevated temperatures are described. Compositions in the lithium-nickel-zinc ferrite system were found to exhibit well-developed hysteresis loops and squareness ratios of 0.96 at temperatures in excess of 300°C. For certain processing conditions, the squareness ratio of lithium ferrite increases when substitutions of Ni2+ are made for Li1+ and Fe3+ in the system Li0.5−x/2NixFe2.5−x/2O4. A maximum in the 25°C squareness ratio of 0.90 is observed for x=0.04. Further improvement of technical hysteresis loop properties are obtained by introducing Zn2+ according to the formula Li0.48−y/2ZnyNi0.04 Fe2.48−y/2O4. A maximum in the 25°C squareness ratio of 0.957 is observed for y=0.05. The substitution of Zn2+ decreases the coercive force, and increases the switchable magnetic induction.
Effect of Surface Pinning on the Magnetization of Thin Films
Authors: Davis J. A. , Keffer F.
Published: April 2005

The pinning of surface spins introduces an effective gap into the spin-wave spectrum of thin films, causing the magnetization vs temperature curve to lie well above that obtained from use of periodic boundary conditions. The results of detailed calculations are presented for the case of partial surface pinning in a simple cubic lattice, spin ½. For films of greater than 6 atomic layers, and for a surface anisotropy field greater than one-tenth the surface exchange field, the magnetization is essentially indistinguishable from the Bloch T32 curve below T/Tc∼0.2. This may account for recent measurements by Neugebauer on nickel and by Hoffmann on iron.
Effect of Surface Roughness on Magnetic Properties of Films
Authors: Prosen R. J. , Gran B. E. , Kivel J. , Searle C. W. , Morrish A. H.
Published: April 2005

The conditions of the surface have been found to have a profound influence on the magnetic properties of thin Permalloy films. Quantitative measurements of the surface area were determined by adsorption of a C14-labeled surfactant. Static hysteresis curves, torque curves, and ferromagnetic resonance spectra have been measured and related to surface roughness. ``Biaxial Anisotropy'' has been observed for films deposited on unidirectionally scratched substrates. The linewidth of the uniform precessional mode with the magnetic field applied parallel to the plane of the film is increased with increasing surface roughness.
Magnetic Anisotropy in Pyrrhotite
Authors: Bin M. , Pauthenet R.
Published: April 2005

The direction of easy magnetization in pyrrhotite deviates from the (001) plane below 205°K. We have measured the thermal variations of the anisotropy constants and the spontaneous magnetization between 4.2° and 292°K and determined the fraction of Fe3+ ions on each sublattice.
Detection of Low-Intensity Magnetic Fields by Means of Ferromagnetic Films
Authors: West F. G. , Odom W. J. , Rice J. A. , Penn T. C.
Published: April 2005

This paper describes a sensitive field magnetometer which utilizes a ferromagnetic film as the sensitive element. Typical vacuum-evaporated films of Ni–Fe of approximately 80% Ni composition are used. These films are chosen to have small directional dispersion in the uniaxial anisotropy. It is well known that in the absence of other magnetic fields the application of a cyclically varying field (of amplitude greater than the anisotropy field) perpendicular to the average anisotropy axis causes the film to split into domains parallel to that axis. The net flux which cuts a sensing coil, the axis of which is parallel to the domains, remains zero during such cyclic magnetization. If there is a small component of field along the anisotropy axis, however, the net flux is proportional to that field. The induced voltage in the sensing coil indicates the direction and magnitude of the axial component of an ambient field. This principle forms the basis of the present device.

In practice, a sinusoidal rf (4 Mc) field drives the film along the perpendicular direction. A modulation field is applied along the average anisotropy axis. The output of the sensing coil is a modulated rf signal which is diode-rectified and phase-detected. In the absence of an ambient field along the anisotropy axis, the detected signal is of a frequency twice that of the modulation field. An ambient field along the axis causes the fundamental frequency of the modulation field to appear. The amplitude of the fundamental is proportional to the ambient field for small fields, while the phase is determined by the direction of the field. Fields having frequencies from dc to roughly 0.1 the modulation frequency can be detected in this manner. The smallest field so far detected by this device is of the order of 10−6 Oe in a dc to 1 cps bandpass.
Waffle Iron—a New Memory Structure
Authors: Smith J. L.
Published: April 2005

The waffle iron is a short bit-length, closed flux-path, magnetic storage device. It consists of a high permeability base plate into which orthogonal slots have been cut, leaving a sequence of rectangular posts, giving the surface the appearance of a miniature waffle iron. To complete the array, a wiring pattern is placed into the slots and an overlay of square-loop magnetic material is laid across the tops of the posts. A closed flux-path structure consisting of a short length of this square-loop material and a longer length of high permeability material is thus formed by each pair of posts. Because of the structure of the device almost complete machine manufacture seems feasible.

Many experimental models of both destructive and nondestructive read memories have been built. All of the models feature the ``two-core per bit'' type of operation, and consequently, have good operating margins. One model, which has been tested extensively, has a 0.030-in. bit length. It is a linear select memory and has an output of greater than 100 mV with a duration of 0.25 μsec for a read current of 0.8 A. The write requirements are 0.3-A word and 0.15-A bit current with an allowable variation on each of ±35%. Time to write is 2 μsec. A second model which has been tested extensively is an electronically alterable, nondestructive read memory of the bimagnetic material type. The read repetition rate is greater than one megacycle and the write time is 5 μsec. With a bit length of 0.020 in. the read, word-write, and bit-write currents are 0.35 A, 0.45 A, and 0.3 A, respectively. The word- and bit-write currents have an allowable variation of ±35% each.

Experiments with memory arrays using the waffle-iron technique have shown that bit lengths of 0.005 in. or less and read-write cycle times of less than 0.5 μsec are feasible. With such short bit lengths, both the packing density and the field efficiency are high, (approximately 1000 bits per square in. and 100 Oe per A). Consequently, the waffle-iron structure is suitable for high-speed memory applications, where small size and potentially low drive currents are of prime importance.
Rotating-Field Technique for Galvanomagnetic Measurements
Authors: West Forrest G.
Published: April 2005

The isothermal galvanomagnetic behavior of an isotropic conductor in a magnetic field can be described by the vector equation E=ρ⊥j+n(j·n) (ρ∥−ρ⊥)+ρH(n×j),relating the electric field E to the current density j. The resistivities ρ∥ and ρ⊥ are those which result when the magnetic induction is parallel to j, and normal to j, respectively; ρH is the Hall resistivity. The vector n is a unit vector which has the direction of the induction. When j is directed along the x axis, and n is made to rotate in the x-y plane with angular frequency ω, one obtains an electric field with ac components Ẽx=12jx(ρ∥−ρ⊥) cos2ωt,Ẽy=12jx(ρ∥−ρ⊥) sin2ωt.These equations describe the magnetoresistive effects in a rotating magnetic field. On the other hand, if n is allowed to rotate in the x-z plane, the y component of the electric field becomes Ẽy=ρHjxsinωt. This equation describes the Hall effect in a rotating field. By measuring the ac components of E having frequencies ω and 2ω when jx is known, ρ∥−ρ⊥ and ρH can be determined.

This rotating-field technique is being used to study the galvanomagnetic properties of ferromagnetic films of rectangular geometry. The films are placed in the field of a magnet rotating at a frequency of 20 cps. When the magnetic field is in the plane of the film, the magnetoresistive equations apply, as has been verified by measuring Ẽx and Ẽy using a wave analyzer. No components having a frequency other than 2ω (40 cps) were detected. The absence of a component with frequency ω shows that no ``Umkehreffekt'' is present in the magnetoresistance. If the wave analyzer is replaced by a high-gain amplifier having a narrow band-pass at 40 cps, it is possible to detect a magnetoresistive change (ρ∥−ρ⊥)/ρ∥=10−6 in a typical film of 10-Ω resistance. A particular advantage of this method is that it is not susceptible to small variations in ρ∥ or ρ⊥ due to fluctuations in temperature. Thus, measurements as a function of temperature are easily and rapidly obtainable.

The utility of the technique is demonstrated by measurements of ρ∥−ρ⊥ as a function of composition in Ni-Fe alloy films having fcc and bcc structures near 30% Ni. The effect of allotropic transformations (fcc→bcc) on magnetoresistance in these films is also presented. It has been found that this transformation occurs with greater difficulty in films on glass substrates than in the bulk alloys.
Low Flux Density Ferrite Memory Cores
Authors: Weisz R. S. , Brown D. L.
Published: April 2005

Previously it has been shown that lowering the flux density Bs of ferrite memory cores has useful effects. Since hysteresis losses are proportional to Bs, lowered Bs allows higher repetition rates to be applied without deterioration of square-loop properties. Lowered Bs also contributes to lower delta noise Δ2, which is an important factor limiting the practical size of a coincident-current core array. The chief problem encountered in preparing low Bs materials is in simultaneously maintaining adequate squareness of the hysteresis loop, low coercivity, and high Curie temperature Tc.

A new series of materials is reported in which tin or indium substitutes for a portion of the iron in conventional magnesium manganese ferrites. For example, the composition 70 MgFe2O4·15 Mg2SnO4·15 Mn3O4 has Bs = 800 G, or approximately ⅓ that of usual square-loop ferrites. Typical 30-mils o.d. cores of this composition switched with a full drive of 580 mA at a risetime of 0.1 μsec have uV1 = 33 mV, dVz = 4 mV, Δ2 = 0.3 mV, tp = 0.18 μsec, ts = 0.37 μsec, IB = 350 mA, and Tc = 160°C. The Δ2 is less than half that produced by conventional ferrite memory cores switching at the same speed.
Precision Measurement of Magnetic Flux Changes
Authors: Haakana C. H. , Noakes J. E. , Werner S. A. , Koch F. B. , Arrott A.
Published: April 2005

Systems which employ two amplifiers—one to amplify, the other to integrate—for precise and/or sensitive determination of magnetic flux changes are described and their operating characteristics reported. One system measures flux changes in time intervals of 10−2 to 10−1 sec using moderately broad band amplifiers and gated circuits. This system discriminates against dc thermals with a gated self-balancing potentiometer, and combines sensitivity and precision with the convenience of rapid readout on a digital voltmeter. The sensitivity of the system is 10−7 V sec, the accuracy better than one part in 104, and combined with pickup coils, permits detection of changes of 10−3 G. This system has been applied to measurements of saturation magnetization, Curie temperature, and susceptibility of iron and its dilute alloys. An alternative system uses a highly stable, dc chopper amplifier in which case the flux changes are produced in seconds and can be followed for minutes. This method has been essential for measurements of the change of magnetization of iron and nickel with pressure where many seconds are necessary to insure isothermal conditions. Sensitivity is the same as above. In another system for the measurement of the temperature dependence of susceptibility, a null coil is wound on the surface of a cylindrical sample which is oscillated near a pair of detector coils at 6 cps. The coil output is amplified, chopped in phase with the signal and integrated. This system has a sensitivity of Δχ = 10−10 and has been used to obtain the temperature dependence of the susceptibility of vanadium.
A uhf Delay Line Using Single-Crystal Yttrium Iron Garnet
Authors: Eggers Frieder G. , Strauss Walter
Published: April 2005

An investigation has been made to determine the properties of yttrium iron garnet at room temperature as a delay-line transducer material. Preliminary results show a specific delay of 2.5 μ sec/cm and an attenuation of 0.4 dB/cm in the frequency range 0.5 to 1.0 Gc/sec. At low input power the insertion loss is constant whereas at high input power the output power is approximately constant; the high-power behavior appears suitable for sonic limiting devices. A method has been found for suppression of unwanted echoes. Evidence from low power level insertion loss measurements indicates that it may be possible to construct a YIG delay line with a 3-dB bandwidth greater than 100 Mc/sec and at the same time an insertion loss less than 60 dB.
Temperature Dependence of Localized Moments in Metals
Authors: White J. A. , Clogston A. M.
Published: April 2005

Anderson's theory of localized magnetic states in metals is extended to finite temperatures assuming the metal-impurity states are populated in accordance with Fermi statistics. A procedure is given for determining the magnitude of the local moment at any temperature. If when unmagnetized at elevated temperatures the local state lies at the Fermi level, it will become magnetic at a critical temperature TC=k−1π−12Δ(1−πΔU−1)12, if kTC is small compared with 2Δ, the width of the state. Below TC the moment is then μ=2μBkU−1[3π(TC2−T2)]12. If the local state is unmagnetized at T = 0 and lies appreciably above or below the Fermi level, it may first become weakly magnetic before demagnetizing again at high temperatures. In general the temperature variation is greatest when Δ and U, the Coulomb repulsion between a pair of electrons localized in the state, or/and μ itself are small.
Domain Studies in Hematite with Polarized Neutrons
Authors: Pickart S. J. , Nathans R. , Brown P. J. , Alperin H. A.
Published: April 2005

Hematite (α-Fe2O3) has been the subject of numerous magnetic investigations aimed at determining the relation of the ``parasitic'' ferromagnetism to the basic antiferromagnetic structure. In the present study, Bragg scattering of polarized neutrons from natural and synthetic single crystals was used to obtain information about the antiferromagnetic domains, which were not directly observed by previous methods. Two types of measurement were performed: the dependence upon initial polarization (polarization ratio) of mixed magnetic-nuclear reflections, which gives information about antiferromagnetic domains whose spins have components along the polarization direction; and analysis of the final polarization from purely nuclear reflections, which gives information about the domains with spin components perpendicular to the polarization direction. These measurements show that at room temperature, where the spins lie within the basal plane (perpendicular to the rhombohedral [111] axis), two types of antiferromagnetic domains exist: the spins lie in (at least) three easy directions and within each of these there are 180° domains. Response of the polarization ratio of the (210) reflection to an external field indicates that each of the trigonal domains becomes nearly a single 180° domain type in fields of the order of 100 G. The final polarization of neutrons reflected from the (111) plane demonstrates that in zero field the distribution of the trigonal domains is isotropic, and that in increasing fields the spins turn 90° to the field direction. These results are consistent with a canted moment model such as proposed by Dzyaloshinski and Moriya. However, attempts to observe this canted moment directly, by the polarization dependence arising from the contribution of the parasitic ferromagnetism to nuclear reflections, have so far failed, suggesting that if such a moment exists it must have a different spin density distribution from the Fe3+ ion. In the low temperature phase, where the spins are along the [111] axis, the crystal contains a more nearly equal distribution of the two 180° domain types, which cannot be altered by an external field, but the degree of single domainness can be increased by cooling from a more nearly single domain condition above the transition. On cycling through the transition, however, the crystal retains no memory of its previous antiferromagnetic domain configuration. These investigations will be more fully reported elsewhere.
Neutron Diffraction Investigation of Pt-Based Alloys of the First Transition Series
Authors: Pickart S. J. , Nathans R.
Published: April 2005

Polarized neutron diffraction measurements show that the radial spin density of the Mn atom in ferromagnetic MnPt3 is contracted relative to the Mn2+ distribution, while that of Cr in ferrimagnetic CrPt3 is more expanded. An antiferromagnetic structure was detected in the ordered CrPt phase at room temperature in which the spins are antiferromagnetic within each layer.
Magnetoelastic Sensitivities in Evaporated and Electrodeposited Permalloy Films
Authors: Wolf I. W. , Crowther T. S.
Published: April 2005

When a magnetostrictive Permalloy film is stressed in the direction of its preferred axis of magnetic orientation, an effective anisotropy is induced in the film causing a change in measured Hk. Magnetoelastic measurements were made on evaporated and plated films 100–4000 Å thick and varying in composition from 70% Ni, 30% Fe to 85% Ni, 15% Fe. Composition was determined by x-ray fluorescence. It was found that the magnetoelastic coupling constant B = (2/M) (dHk/de) was significantly less for plated than for evaporated films of the same composition. There is evidence of both intercrystallite or internal crystallite slip and film-to-substrate slip, but neither is large enough to account for the observed differences.
Internal Field, Dispersion, Creeping, and Switching Speed of Coupled Films
Authors: Chang H. , Yelon A. , Voegeli O.
Published: April 2005

Coupled films were made either by sandwiching striplines between two films on different substrates, or by multilayer evaporation. The first structure was found to be inadequate in reducing the demagnetizing field because the airgap was appreciable. The switching speed of coupled films was calculated with the Landau-Lifshitz equation, taking into account the magnetostatic coupling energies. Nearly identical rotational switching speeds were found for single and coupled films. The results were verified by measurements. Skew, dispersion, and creeping were measured. It was found that coupled films were less susceptible to the word field disturb from neighboring lines. It was also found that the orthogonal drive scheme could be realized, using coupled films with small diameter and large thickness, which would prevent their use as single films.
Flux Reversal by Néel Wall Motion
Authors: Copeland John A. , Humphrey Floyd B.
Published: April 2005

Domain wall motion is expected to be the fundamental flux reversal mechanism for thin permalloy films in the low drive region H<HK. The velocity of a wall when a switching field H is applied is given by V=G(H-H0), where G is the mobility and H0 is the starting field. For films with thicknesses on the order of few hundred angstroms, where Néel walls are expected, the mobility is limited by intrinsic damping to a value much smaller than would be expected if only eddy current damping were present. Calculations have been made using a value for the damping constant, α=.015, obtained from coherent rotation studies, and the experimental values for the thickness of the wall obtained by Fuchs in order to obtain G as a function of film thickness. The mobility was measured for seven permalloy films ranging from 128 to 495Å thick. The agreement between the calculations and experiment was within 30%, indicating that the value of α for a 100-Å film is nearly the same as for thicker films.
Direct Observation of High-Speed Magnetization Reversal in Films
Authors: Conger R. L. , Moore G. H.
Published: April 2005

An optical strobing apparatus has been developed which makes it possible for the first time to observe magnetization reversal in thin films with reversal taking place during a period of the order of a microsecond. Studies of the details of magnetization reversal are important for understanding how reversal takes place in films. In addition, work on strobing of magnetic films will be of value in developing nondestructive magneto-optic readout for thin-film computer memories.