bluuebunny/crossref_metadata_2025 huggingface dataset:
Aging and Stabilization of Electrodeposited Cylindrical Magnetic Thin Films
Authors: Chang J. T. , Gianola U. F. , Sagal M. W.
Published: April 2005

The effects of aging cylindrical magnetic film elements are described. The films are 1 μ thick, circumferentially anisotropic, and electrodeposited upon a 125-μ (0.005-in.) -diam wire substrate. The values of skew and dispersion undergo considerable changes even when the films are stored at room temperature. The results of aging at 55°C under a variety of applied magnetic fields are described. Measurements are given for skew, dispersion, anisotropy, and coercive fields. Under worst case conditions, all parameters experience changes several times their initial values. The changes tend toward terminal values, and the degradation of the important magnetic parameters is clearly influenced by the transverse fields during the aging process. Experimental data showing a possible stabilization of the film by field annealing in an easy direction field following electrodeposition are discussed.
Nuclear and Ferromagnetic Relaxation in MnFe2O4
Authors: Heeger A. J. , Blocker T. G. , Ghosh S. K.
Published: April 2005

The Mn55 nuclear spin-lattice relaxation time, and the ferromagnetic resonance linewidth have been studied in the same single crystal of ferrimagnetic MnFe2O4 as a function of the temperature. The low-temperature linewidth is accurately described in terms of a ``slow'' relaxation theory based on the low-lying levels of the Fe2+ impurities present. The NMR T1 results indicate that the dominant nuclear relaxation mechanism arises from the complex frequency pulling, brought about by ``slow'' relaxation in the ferromagnetic spin system.
Nuclear Antiferromagnetic Double Resonance in MnCO3
Authors: Shaltiel D. , Fink H. J.
Published: April 2005

The resonance of the low-frequency branch of MnCO3 shows that the anisotropy field in the (111) plane consists of an approximate temperature-independent term equal to 0.98±0.04 G and a temperature-dependent term equal to (8.6±0.3)/Tn. The temperature-dependent anisotropy field originates from the hyperfine interaction and is proportional to the nuclear polarization which is inversely proportional to the nuclear spin temperature Tn. The nuclei form two sublattices that are coupled antiparallel to the electronic sublattices. This coupling shifts the nuclear frequency from that calculated from its hyperfine field by a factor proportional to the nuclear polarization. At 4.2°K we have observed indirectly the nuclear resonance at 538 Mc/sec by measuring its effect on the electron resonance. It is 102 Mc/sec lower than the frequency we would expect if the nuclear and electron system are decoupled. For intermediate nuclear power levels we have observed two electron resonances at 6 Gc/sec. One of the lines corresponds to the unshifted resonance line the position of the other line was dependent on the nuclear frequency only. This shift is due to partial saturation of the nuclear spin system which saturates at about 640 Mc/sec.
Antiferromagnetic Resonance in CoF2, NiF2, and MnCO3
Authors: Richards P. L.
Published: April 2005

The far-infrared transmission spectra of CoF2, NiF2, and MnCO3 have been measured from 3 to 100 cm−1 using the techniques of Fourier-transform interferometry. Antiferromagnetic resonance branches have been found in CoF2 at 28.5 cm−1 and 36 cm−1 at temperature T≈0, each of which splits in a magnetic field along the c axis. These resonances have been studied as a function of field, temperature, and uniaxial stress. Antiferromagnetic resonance at 31.1 cm−1 and ferromagnetic resonance at 3.33 cm−1 in NiF2 at T≈0 have been studied as a function of field and temperature, and the results are compared with Moriya's theory. Antiferromagnetic resonance has been observed at 4.13 cm−1 in MnCO3 at T≈0.
Sendust Sheet-Processing Techniques and Magnetic Properties
Authors: Helms H. H. , Adams Edmond
Published: April 2005

A procedure has been developed whereby the brittle magnetic alloy of silicon, aluminum, and iron (Sendust) can be produced in sheet from through metal powder rolling techniques. Ring laminations stamped from this sheet at 0.014 in. thickness have the following dc properties: μ20=16 130; μmax=36 625; Hc=0.062 Oe; Br=3450 G; Bm=8960 G. High frequency (<1 Mc/sec) loss values were lower than in laminated molybdenum Permalloy cores of the same thickness. The nominal Sendust composition is 9.5% silicon, 5.7% aluminum, and balance iron.

This new process should make possible more extensive use of Sendust in applications that can take advantage of its remarkable intrinsic characteristics such as high initial permeability, high resistivity, low magnetostriction, and low magnetocrystalline anisotropy.
Ferrimagnetic Resonance of Rare-Earth-Doped Iron Garnets
Authors: Van Vleck J. H.
Published: April 2005

After a brief introductory sketch of the analogs in paramagnetic relaxation, the longitudinal and transverse (so-called ``slow'' and ``fast'' relaxing) mechanisms of line broadening as well as shifting by rare-earth impurities are described and compared, particularly for Kramers doublets. The longitudinal process is operative only if the rare-earth ions are anisotropically coupled to the iron magnetization, either because of the crystalline field or anisotropy in the exchange coupling tensor itself. Unless the anisotropy is small, the longitudinal process predominates when the splitting of the RE energy levels by the crystalline and exchange fields is large compared to the ferrimagnetic resonance frequency and to the relaxation frequency, as is usually the case at temperatures well below the Curie point. The longitudinal mechanism describes quite well the behavior of Yb in yttrium iron garnet except for one peculiar as yet unexplained peak in the linewidth at one angle at low temperatures. The line broadening with Eu comes from the excited state J=1, and its variation with temperature and frequency is explicable with the longitudinal model. The ``giant anisotropy frequency shifts'' with Tb are presumably caused by abnormally small rare-earth energy intervals for certain angles. The k=0 mode is mixed with other k modes by the rare-earth impurities. This effect is not ordinarily a major cause of line broadening, but may be for Tb at low temperatures.
Natural Spectrum of Polycrystalline YIG as a Function of Density
Authors: Holm J. D. , Morrish A. H.
Published: April 2005

The initial permeability of polycrystalline yttrium iron garnet has been measured as a function of density at frequencies from 5 kc/sec to 7.5 Gc/sec. The samples were prepared by grinding very pure single crystals into a powder and then pressing and sintering. The samples of highest density have two absorption peaks that occur at approximately 1 Mc/sec and 200 Mc/sec. This is in very good agreement with observations on polycrystalline samples prepared with coprecipated powders but not with those prepared with raw oxides. The separation between the two absorption peaks decreases as the sample density decreases, until ultimately only one peak is observed in a sample with 72% density. The results are interpreted in terms of the changing sample porosity.
An Analog Computer Study of Unstable Spin Waves
Authors: Morgenthaler Frederic R.
Published: April 2005

This paper presents some of the results concerning second-order instabilities obtained by analog computation. In addition to the direct information they provide, one of the great virtues of the computer results is to indicate forms of solutions that are possible. For example, an interesting ``capture'' phenomenon, whereby the uniform precession forces certain spin waves into phase synchronization and hence unstable growth, once discovered, can be understood by inspection of the equations. The presence of certain relaxation oscillations can be explained at least qualitatively.
Computer Study of Transient Spin-Wave Buildup in Ferrites
Authors: Harker K. J. , Shaw H. J.
Published: April 2005

The equations of motion for spin waves in a spherical ferrite sample, together with equations for the uniform mode, have been programmed on a digital computer. The equations for the uniform mode are general and allow for arbitrary amplitude of the mode. The spin-wave equations are solved to first order in the spin-wave amplitudes, so that the parametric couplings which lead to first-order and second-order spin-wave instabilities are included. These equations have been solved for the case of transient buildup of the uniform mode in a magnetic field whose amplitude rises linearly with time. With rapidly rising fields, it is found that the uniform mode attains amplitudes far exceeding the limits of the usual small-signal analysis. The parametric buildup of spin waves accompanying the uniform mode is traced as a function of time, and the effect of these spin waves upon the uniform mode is considered. This defines the conditions under which useful transient devices can be designed for the generation of microwave energy with pulsed magnetic fields, which can take advantage of the large rf magnetization in ferrites when operated well beyond the usual steady-state saturation limits.
Wall Motion by Reverse Néel Walls in Thin Films
Authors: Patton Carl E. , Humphrey Floyd B.
Published: April 2005

Néel walls are presumed to exist as domain boundaries in thin films with thicknesses below 500 Å. Low-drive switching with a superimposed transverse field Ht may take place by reverse wall motion (Ht·Mwall<0) or normal wall motion (Ht·Mwall>0). Wall velocities are determined by intrinsic damping rather than eddy current losses, for films in this thickness region. Reverse wall mobilities have been measured by interrupted pulse flux reversal experiments, in agreement with calculations based on the Landau-Lifshitz equation and uniaxial anisotropy. With h=Ht/HK=0.1, normal wall mobility is about 20% greater than reverse wall mobility.
Torque and Rotational Hysteresis in fcc Single-Crystal Cobalt Films
Authors: Doyle William D.
Published: April 2005

The Stoner-Wohlfarth coherent rotation model is extended to the case of biaxial anisotropy. The rotational hysteresis Wr as a function of reduced field h is zero for all values of h except 0.25≤h≤1, and the maximum value of Wr/K=2.38 at h=0.25. The rotational hysteresis integral W equals 1.54. Measurements on fcc single-crystal Co films (−K=4−6×105 ergs/cm3) prepared by epitaxial evaporation onto the (100) face of single-crystal MgO slabs held at 400°C yield results in wide disagreement with the theory. While the expected angular symmetry of the torque is observed, the model does not correctly predict the magnitude of the torque nor the fields at which specific types of angular dependence will be observed. The rotational hysteresis begins in every case before the predicted onset and continues beyond the expected cutoff. The maximum value of Wr/K and W are found to vary from 1.0 to 1.6 and from 2.9 to 1.7, respectively. It is concluded that the magnetization reversal process in single-crystal Co films is not coherent and that, as in the case of polycrystalline films, noncoherent rotations or wall motions must be investigated. This situation in single-element, single-crystal films calls for a reappraisal of the importance of compositional inhomogeneities and local crystalline anisotropy in polycrystalline alloy films.
Critical Magnetic Scattering of Neutrons in Iron
Authors: Passell L. , Blinowski K. , Brun T. , Nielsen P.
Published: April 2005

The intense small-angle scattering of low-energy neutrons by magnetic materials near the Curie temperature has been shown by Van Hove to be directly related to the appearance of large scale fluctuations in the spin density. We have measured the angular and energy distributions of 4.27-Å neutrons scattered at small angles in iron and determined the spin correlation range 1/κ1 and a parameter Λ associated with the lifetime of the fluctuations. Our results confirm the recent observation of Jacrot, Konstantinovic, Parette, and Cribier that the scattering is not elastic even at the Curie temperature. We have found that κ12 varies as (T−Tc)43 for 0°<T−Tc<∼30°C as predicted by Domb and Sykes and by Gammel and Marshall and also that our measured values of κ1 are in fair agreement with those predicted theoretically by Kocinski.
Magnetic Transitions in Dysprosium Aluminum Garnet
Authors: Ball M. , Wolf W. P. , Wyatt A. F. G.
Published: April 2005

Measurements of the susceptibility, internal energy, entropy, and magnetization of dysprosium aluminum garnet in the region of the Néel temperature are compared with recent theoretical calculations for Ising models. In general, good agreement is found but there are some quantitative discrepancies which are probably due to the neglect of long range dipolar forces in the theories.
Low-Temperature Properties of Dilute Ferromagnets in Copper
Authors: Klein Michael W.
Published: April 2005

The low-temperature properties of dilute magnetic impurities in copper are studied, using an extension of the recently developed statistical theory of Klein and Brout. A Ruderman-Kittel (RK) interaction is assumed to exist between the magnetic impurities. The probability distribution of the effective field about an impurity, due to all other impurities of the system, is obtained. The variation of the probability distribution as a function of the impurity concentration, temperature, and J S2, where J is the strength of the Ruderman-Kittel interaction and S is the magnitude of the impurity spin, is briefly discussed. Using this distribution function, we show that the model explains the low-temperature specific heat of dilute copper-cobalt and copper-iron. Copper-magnanese has been explained previously by the same model. The fact that the Ruderman-Kittel potential explains the low-temperature specific heat of all three substances gives additional support to the theory that this potential, rather than Overhauser's spin-density waves, is the correct interaction mechanism between dilute magnetic impurities in a nonmagnetic conductor. The detailed shape of the probability distribution and its variation with temperature is in qualitative agreement with recently obtained Mössbauer data on dilute Cu–Fe.
Specific Heat and Longitudinal Correlation Function for the Heisenberg Ferromagnet
Authors: Tahir-Kheli Raza A. , Callen Herbert B.
Published: April 2005

The correlation function of the z components of the spins in a ferromagnet is calculated as a function of the interspin distance, temperature, and applied magnetic field. The formalism used is that of two-time, temperature-dependent Green functions, with the decoupling approximation introduced by Callen. The results accordingly agree with those obtained by spin-wave theory at low temperatures, and remain approximately valid up to the region of the Curie temperature. Knowledge of the correlation function for nearest neighbors enables one to calculate the energy and thence the specific heat; the results are also relevant to elastic neutron scattering.
Self-Consistent Correlation Method (S.C.C.M.) Applied to Ideal Ising and Heisenberg Magnetic Systems
Authors: Clapp P. C.
Published: April 2005

The theory to be described here may be classified as a cluster theory of ferromagnetism. It is presently restricted to lattices with nearest-neighbor interactions, but is unrestricted in spin magnitude. Its point of departure from previous cluster theories is that it represents the effect of the other spins of the lattice on the cluster as a temperature-dependent exchange coupling, rather than as a molecular field. A second significant departure is to impose a consistency condition on the spin-spin correlation functions of the cluster rather than on the spin averages as in previous theories. This new cluster theory can be motivated from quite fundamental considerations, and achieves a significant improvement over previous cluster theories, especially in the region just above the Curie temperature.

Given a particular lattice, the cluster chosen for application of the theory consists of a central spin pair plus the additional spins and bonds necessary to form all the next most direct linked paths between the central spin pair. In the cluster Hamiltonian the central coupling has the constant value J while all the other couplings are taken to have a temperature-dependent value J′ (T) which will be evaluated self-consistently. J′ (T) can be understood physically as the direct coupling J plus an additional coupling caused by the correlative effect of spins outside the cluster acting on the cluster spins. The S.C.C.M. Hamiltonian can be derived more rigourously by writing the density matrix for the entire spin system and formally summing over all spin variables except those of the cluster. From this reduced density matrix it is possible to infer the form of the corresponding Hamiltonian. Apart from small terms which vanish at high temperatures, this is the S.C.C.M. Hamiltonian.

The method of determining J (T) for any particular case brings out the second feature of the theory. By demanding that the correlation between the central pair be equal to that of any other nearest neighbor pair, J (T) can be uniquely determined for any temperature. Since the average energy, the free energy, and the specific heat of the lattice are related to the nearest-neighbor correlation function, these properties are immediately obtainable from the cluster calculation. If the correlation function is calculated with an external magnetic field added, then the dc magnetic susceptibility and average magnetization can also be determined.

A complete presentation of the theory and results for the Heisenberg and Ising square and simple cubic lattices of spin one-half are contained in a paper by this author to be published elsewhere.
Remarks on the Oguchi Transformation
Authors: Tahir-Kheli Raza A. , Callen Herbert B.
Published: April 2005

It is shown that the Oguchi transformation in the theory of ferromagnetism is a low-temperature spin-wave approximation, whose validity depends on the implicit assumption that the kinematical interaction can be ignored. This limitation has the consequence that the theory of Oguchi and Honma, if accepted at higher temperatures, predicts an infinite Curie temperature.
Neutron Diffraction Study of Magnetic Stacking Faults in Antiferromagnetic K2NiF4
Authors: Plumier René
Published: April 2005

The unusual width of the (101) and (102) magnetic reflections of antiferromagnetic K2NiF4 shows the existence of stacking faults in the piling up of the antiferromagnetic sheets. The probabilities of such faults at various temperatures are determined and show an increase with temperature. It is then shown that lowering of the free energy is obtained by considering exchange striction from a slight orthorhombic distortion; ungrouped stacking faults lead to an additional lowering of the free energy.
Magnetic Structure Investigations at the Nuclear Center
Authors: Bertaut E. F. , Roult G. , Delapalme A. , Bassi G. , Mercier M. , Murasik A. , Van Qui Vu , Aleonard A. , Pauthenet R. , Chevreton M. , Jansen R.
Published: April 2005

The following structures are briefly described MnYO3, β-FeNaO2, GeNi2O4, and Cr3X4 (X=S, Se, Te) MnYO3. In this remarkable hexagonal layer structure there are fivefold coordinated Mn atoms. Their spins order below 46°K forming angles of 120° in the layer. Two different models may be inferred from powder data. The ambiguity is lifted by the fact that a weak ferromagnetism is observed. The ferromagnetic and the ``120° model'' must belong to the same representation which is here the identity representation.

β-FeNaO2. Each iron spin has 4 antiparallel neighbors. The antiferromagnetic axis is 0z. From group theory it is concluded that the weak ferromagnetism must be along 0y.

GeNi 2 O 4. Ni is on octahedral sites (B sites). Spin-ordering sets in below 15°K with doubling of the unit cell. The propagation vector of the antiferromagnetic arrangement is k0=[½½½]. The spin-value is S=1.1. The true symmetry is rhombohedral.

Cr 3 X 4. Cr3S4 has a huge magnetic unit cell. The structure is antiferromagnetic. In Cr3Se4 the same large magnetic cell is observed, but at low temperatures a second antiferromagnetic mode is superimposed. In Cr3Te4 a ferromagnetic structure is observed at room temperature, but below 80°K an antiferromagnetic configuration is superimposed which explains that the magnetization at saturation is inferior to the sum of the moments of the individual atoms.
Magnetic Structures in FeCr2S4 and FeCr2O4
Authors: Shirane G. , Cox D. E. , Pickart S. J.
Published: April 2005

The magnetic structures in FeCr2S4 and FeCr2O4 have been investigated by the powder neutron diffraction technique. From room temperature diffraction patterns, it is established that both compounds are normal spinels with sulfur and oxygen parameters u=0.384 and 0.386, respectively. The FeCr2S4 specimen exhibits a Néel temperature of 180°K, with σ0=1.6 μB. The magnetic intensities of this compound at 4°K are in good agreement with those based on a simple Néel model with μFe=4.2 μB and μCr=2.9 μB. The magnetic properties of FeCr2O4 are, on the other hand, more complex. The moment σ0 is considerably lower than the value expected from the Néel model. Moreover, the crystal becomes tetragonal with c/a<1 around 135°K. The magnetic reflections at 4°K show satellite peaks characteristic of a cone spiral structure described in the theory of Menyuk, Dwight, Lyons, and Kaplan for tetragonal spinels. However, a quantitative comparison of observed and calculated intensities indicates that some modifications of the simple cone model are necessary. The satellite reflections disappear at 35°K, and the spin arrangement is collinear between this temperature and the Néel temperature at 80°K.
Magnetic Susceptibilities of Mo-Tc Binary Alloys
Authors: Lam D. J. , Van Ostenburg D. O. , Pracht D. W.
Published: April 2005

Magnetic susceptibilities of binary Mo-Tc alloys have been measured in the temperature range from 300°K to the superconducting transition temperature. The room-temperature magnetic susceptibility increases from pure Mo (82.5×10−6 emu/g-atom) to a small maximum (96.5×10−6 emu/g-atom) at the equiatomic composition. Further additions of Tc produce a decrease which reaches a minimum at 70 at. % Tc (χ=87.3×10−6 emu/g-atom). Beyond this composition there is another increase to the value of 120.8×10−6 emu/g-atom for the pure Tc. No Curie or Curie—Weiss type of temperature dependence of the susceptibility was observed in all the alloys in the temperature range investigated. The results are discussed in relation to the band theory of metals and some generalizations about the d-band form of the three transition metal series having the same electron concentrations are made.
Rare-Earth—Conduction Electron Interaction in 4d and 5d Laves Phases Compounds
Authors: Shaltiel D. , Wernick J. H. , Jaccarino V.
Published: April 2005

A systematic investigation has been made of the sign and magnitude of the g shifts of Gd impurities in the AB2 intermetallic Laves phase compounds. Gd was partially substituted in the A site for the elements Sc, Y, La, Th, U, and Zr. The B sites are occupied by elements of the 4d and 5d group whose positions in the periodic table are such as to correspond to an almost filled d band (e.g., Ru, Os, and Rh). Negative Gd g shifts, Δg, up to 7% were observed; the highest was that found in LaRu2. It was found that the closer a B element was to having a filled d shell the larger was the g value. It appears, in keeping with our previous EPR measurements on Gd in Rh-Pd, Pd-Ag alloys and in Cu5Ca intermetallic compounds, that without exception the g shifts are negative when the d band is nearly filled in the host alloy and that the g value has a minimum as a function of the atomic position in the host d band metal. In very fine powders of the hard superconductors (Ce0.99Gd0.01) Ru2 and (Th0.99Gd0.01) Ru2 no change in the Gd g shift was observed in going from the normal to the superconductor state. For the superconductor (La0.15Ce0.85) Ru2 the La Knight shift was also observed not to change in going from well above to well below the superconducting transition temperature. In both experiments Ho≪the upper critical field.
Critical Alternating Currents in Superconductors
Authors: Young F. J. , Schenk H. L.
Published: April 2005

Critical alternating currents are studied experimentally in 0.96-cm (⅜-in.) lengths and coils of superconducting wire. Critical current density vs frequency are measured for three short Nb-25% Zr wires having the same amount of cold work. For the 5-, 10-, and 20-mil (125-, 250-, and 500-μ) specimens, the critical ac density varies from 290 to 170 kA/cm2, 200 to 110 kA/cm2, and 107 to 54 kA/cm2, respectively, as the frequency is varied from 0.02 to 4.5 kc/sec. A short length of Nb–Ti wire 9 mils in diameter has a critical ac density that varies from 260 to 130 kA/cm2 as the frequency ranges from 0.01 to 4.5 kc/sec. In a 15-mil Nb3Sn specimen, the critical ac density is constant at 90 kA/cm2 from 0.01 to 4 kc/sec.

In a tightly wound Nb-25% Zr coil, the critical ac density is found to vary inversely with the one-fourth power of frequency. In a very loosely wound coil of the same material, the critical ac density varies inversely with the cube root of frequency. In all cases investigated, except for pure niobium, the exponent on the frequency varies from −0.25 to −0.36. In a Nb coil, no variation of critical ac density is observed below 200 cps. Above 200 cps, it behaves in the same way as the other coils tested. The data indicate that the critical ac density occurs such that the coil or short wire always becomes normal at the same level of power dissipation.
High-Temperature Heat Capacity of Ferromagnets Having 1st- and 2nd-Neighbor Exchange: Application to EuS
Authors: Wojtowicz Peter J.
Published: April 2005

This paper describes the derivation of exact power series expansions of the high-temperature magnetic-heat capacity of Heisenberg ferromagnets having both first- and second-neighbor exchange interactions. The calculations employ the general diagramatic techniques developed by Rushbrooke and Wood as extended by Wojtowicz and Joseph to include the second-neighbor interaction. All mixed coefficients for terms through the fifth power of the inverse temperature have been computed for arbitrary spin and general lattice structure. The series expansions have been used to obtain certain information on the nature of the exchange interactions in ferromagnetic EuS. By the use of a spin-wave analysis Charap has been able to deduce a set of values for the exchange interactions which simultaneously fit the low-temperature magnetization and heat capacity data. It has been found that these same values substituted into the present theory are capable of reproducing the high-temperature heat capacity data of Moruzzi and Teaney. This result lends considerable support to the hypothesis that EuS is very nearly an ideal Heisenberg ferromagnet.
Sublattice Magnetizations in Lithium Ferrite as a Function of Temperature
Authors: Prince E.
Published: April 2005

We have made neutron diffraction measurements of the intensities of the (111) and (331) reflections from a single crystal of ordered lithium ferrite, LiFe5O8. These intensities are proportional to the value of the function [2MA(T)+2MB(T)]2/[2MA(0)+2MB(0)]2, where MA(T) and MB(T) are the magnetizations of the ions on tetrahedral and octahedral sites, respectively. Combining this intensity data with the net magnetization measurements of Rado and Folen, we have determined the individual magnetizations of the A and B sublattices. A comparison of these sublattice magnetizations with calculations based on molecular field theory, using parameters determined from the magnetic measurements, revealed that the actual values of these magnetizations at temperatures greater than about 0.4 Tc are significantly higher than the values predicted by the theory. Furthermore, a systematic investigation of the effects of varying the ratios of the A-A and B-B molecular field parameters to the A-B parameter revealed that, although a linear function of these ratios can be determined quite precisely from the curve of net magnetizations, the individual ratios can be varied over a rather wide range with little effect on the shape of the curve. It happens, however, that the curves of neutron diffraction intensities as a function of temperature are even less sensitive to variations in the values of these ratios than are the curves of net magnetization. It is possible that a better fit could be obtained between the experimental data and the theoretical curves by postulating a temperature dependence of the parameters. However, an examination of the temperature variation of the (333) and (115) reflections reveals that the oxygen parameter changes very little over the temperature range of interest. For this reason a large variation in the values of the molecular field parameters appears unlikely. It appears, therefore, that the molecular field theory is inadequate to account for the fine details of the magnetization temperature curves in this substance. There are indications that the inclusion of higher-order terms in the A-B exchange energy will improve the agreement.
Exchange Coupling of Neighboring Mn++ Ions in Mn++:ZnS Crystals
Authors: Brumage William H. , Yarger Charles R. , Lin Chun C.
Published: April 2005

The magnetic susceptibilities have been measured over the temperature range of 20°–800°K for five ZnS crystals doped with Mn++ in varying concentrations from 0.5% to 7.5%. Above 77°K the magnetic data show considerable deviation from Curie's law which can be explained by the antiferromagnetic coupling between the Mn++ ions situated at the nearest neighboring cation sites in the ZnS crystal. To account for the experimental results quantitatively, it is assumed that the numbers of clusters of various types are determined by statistics of random distribution and that the exchange operator can be taken as the simple isotropic form −2J (s1·s2). The observed susceptibilities can then be fitted to the theoretical values using J=−9±1 cm−1. Additional terms of the exchange operator such as the next-nearest neighbor interaction, anisotropic exchange, and the biquadratic coupling term are discussed.
Metamagnetism and Exchange in Ferrous Bromide
Authors: Jacobs I. S. , Lawrence P. E.
Published: April 2005

The highly anisotropic behavior of several ferrous ion antiferromagnets (FeCl2, FeCO3) has suggested Ising-model behavior. Sharp field-induced (metamagnetic) transitions to a ferromagnetic (or nearly saturated paramagnetic) configuration have been observed. We find this transition in a single crystal of anhydrous FeBr2, occurring at a field of 31.5 kOe at 4.2°K, using pulsed fields to 200 kOe along the c axis. The spontaneous magnetization per Fe2+ ion is 4.05±0.2 Bohr magnetons. A differential molar susceptibility between 70 and 200 kOe of 0.8±0.2×10−2 is interpreted as a Van Vleck ``temperature-independent'' paramagnetism in qualitative agreement with expectation. Susceptibility—temperature measurements along the c axis reveal an asymmetric shape characteristic of recent Ising-model calculations. In the region just above the Néel point (11°K), the magnetization shows an initial upward concavity with field and a subsequent inflection point. This destruction of residual intermediate range order had also been predicted in Ising-model calculations. The critical transition field determines the antiferromagnetic interlayer exchange and an estimate is made of the ferromagnetic intralayer exchange. The values for FeBr2 are compared with those for FeCl2, revealing a considerable drop in the ferromagnetic parameter with change of anion from Cl to Br. The antiferromagnetic exchange of FeBr2 is stronger than its ferromagnetic exchange in contrast to previous experience with layer structures.
A Measurement of Spin-Wave Dispersion in MnF2 at 4.2°K
Authors: Low G. G. , Okazaki A. , Stevenson R. W. H. , Turberfield K. C.
Published: April 2005

Antiferromagnetic spin waves have been observed in a single crystal of MnF2 at 4.2°K by the inelastic scattering of neutrons. Measurements were carried out using a twin rotor time-of-flight spectrometer having a mean incident neutron wavelength of 3.00 Å and a resolution (half-width at half-height) of 0.06 Å. Spin waves were excited in the {010} plane of the crystal and the dispersion plotted for propagation in the 〈100〉 and 〈001〉 directions. The observed dispersion curves can be accurately described by spin-wave theory using the following values for the parameters concerned: J1=0.32°K (ferromagnetic), J2=−1.76°K (antiferromagnetic), |J3|<0.05°K, and an anisotropy field at zero wave vector HA=1.06°K, where J1 is the exchange coupling between nearest neighbours (in the 〈001〉 direction), J2 acts between next nearest neighbours (〈111〉 directions), and J3 refers to third-neighbour coupling (along the 〈100〉 and 〈010〉 directions). A small effect arising from the wave vector dependence of HA has been calculated and taken into account in the evaluation of these parameters. The values quoted are in excellent agreement with those predicted from measurements of perpendicular susceptibility, spin-wave resonance, paramagnetic resonance in (Mn, Zn)F2, and from paramagnetic neutron scattering at room temperature.
Power Spectra Measurements on Ultralow-Noise Beams
Authors: Hammer J. M.
Published: April 2005

The results and experimental details of S-band noise figure and noise spectra measurements made on electron beams that yield traveling-wave-tube noise figures below 4 dB are presented. A simple technique involving the observation of the change in noise figure with electron gun-helix separation in a special traveling-wave tube (TWT) is employed. A positive correlation between current and velocity fluctuations contributes heavily to the reduction of noise in the ultralow-noise TWT. This conclusion supports the general results, but not necessarily the details of the theory of noise on multivelocity beams. In contrast to the theoretical assumption that the important source of multivelocity flow is the cathode temperature, the measurements give evidence that, in the ultralow-noise TWT, the flow of importance is due to transverse beam slippage in the finite beam that produces what may be termed a ``quasi-multivelocity'' flow with a velocity spread which may be wide enough to mask the thermal spread. The measurements also raise the possibility that a cyclotron wave-space-charge wave interaction occurring beyond the cathode-first anode space contributes to the noise behavior.
Diffusion and Glass Transition in Simple Liquids
Authors: Naghizadeh Jamshid
Published: April 2005

The free volume theory of diffusion for hard spheres, earlier developed by Cohen and Turnbull, is modified for simple van der Waals type liquids. The modified theory fits the self-diffusion data for argon, krypton, and xenon fairly well and predicts glass transition for these liquids at approximately one-third of their normal boiling points. It is found that a more accurate model for the free volume diffusion in liquids should include a redistribution energy for voids arising from the nonlinear behavior of the pair potential. The agreement found by Cohen and Turnbull between their hard sphere theory and the self-diffusion data of some large organic molecules is interpreted as evidence that the simple Lennard-Jones potential does not accurately describe their interaction with each other. This situation is reflected in the failure of these molecules to obey the same corresponding states relationship as the simple liquids of argon, krypton, and xenon.
Hall Effect Investigation on Lithium-Diffused Gallium Arsenide
Authors: Fuller C. S. , Allison H. W.
Published: April 2005

Hall effect results on GaAs crystals into which Li is introduced at 500°C, as well as at higher temperatures, and subsequently removed in Ga or in air at 500°C are reported. Four acceptor ionization energies: 0.023 and 0.11 eV (previously reported), 0.05 eV and 0.14 eV are found. The two levels 0.023 and 0.05 eV are attributed to Li self-pairs. The 0.11- and 0.14-eV levels are associated with Cu possibly already present in the GaAs crystals. The 0.023-eV level is found in crystals produced by the floating-zone process at all temperatures of the Li diffusion. The 0.023-eV level is also found in the horizontal Bridgman crystals when diffusion is above ∼650°C. Below ∼500°C, however, the 0.05-eV level predominates in Bridgman crystals and the 0.023-eV level is not observed. The origins of the various levels are discussed.
Spherulitic Crystallization from the Melt. II. Influence of Fractionation and Impurity Segregation on the Kinetics of Crystallization
Authors: Keith H. D. , Padden F. J.
Published: April 2005

The influence of impurity segregation on the kinetics of spherulitic crystallization is examined. Radial growth rates over a range of temperatures are reported for a number of polymers containing various proportions of impurity species of widely varying molecular weight. Variations of growth rate with molecular weight indicate that transport processes within regions extending some distance from solid-liquid interfaces play a major part in controlling growth kinetics. In the unusual case of a polymer containing impurities of very low molecular weight, extended radial diffusion of impurities may even give rise to parabolic growth [(radius) ∝ (time)½] in place of linear growth as normally found. Conventional interpretations fail to account satisfactorily for dependence on molecular weight, and are clearly inadequate in that they deal with transport processes only as ``jump'' processes at these interfaces. Complications that transport processes within the body of the melt introduce into the problem of formulating an absolute theory of growth kinetics are discussed.
Quantitative Determination of Preferred Orientation
Authors: Bragg Robert H. , Packer Charles M.
Published: April 2005

Solutions are presented to the problems which usually prevent accurate x-ray determinations of preferred orientation. The relationship between observed and corrected intensities and the angle dependent absorption factors are treated, including the case of displaced and distorted line profiles caused by low absorption in thick specimens. The absorption factors are derived experimentally from measurements of the scattered (Compton) background radiation without recourse to calculations or tables. Two simple methods, either by varying the wavelength λ or the order n of the x-ray reflection, span the region of the pole figure in which data are unreliable or unobtainable if only one wavelength or order is used. Random samples are not required for normalizing the data to ``times random'' units. The intensity for a random sample can be calculated from the corrected data itself and the equation Ihkl=[[double integral operator]Ihkl(α,β) sinαdαdβ]/2π where (α,β) are the coordinates of a point on the usual polar net. It is suggested that textures can generally be represented by Gaussian or superposition of Gaussian orientation distributions.
Diffusion of Chromium in Nickel
Authors: Murarka S. P. , Anand M. S. , Agarwala R. P.
Published: April 2005

Diffusion of chromium in pure nickel has been studied by radioactive tracer and residual activity technique. The diffusion coefficient (in units of cm2/sec) in the range 600°−900°C is given by DCr/Ni=0.03 exp(−40 800/RT) and in the range 350°−600°C is given by DCr/Ni=5.45×10−9 exp(−13 700/RT).

The bimodal diffusion behavior has been explained on the basis of solid solubility of chromium in nickel, which falls very rapidly from 1000°C downwards. The entropy of activation for high-temperature diffusion falls very well on the entropy vs activation energy curve for lattice diffusion in nickel.
A Backward-Wave Surface Mode in a Plasma Waveguide
Authors: Carlile R. N.
Published: April 2005

The phase characteristics of a backward-wave pass-band mode have been measured in a plasma waveguide, consisting of a circular waveguide that is coaxial with the positive column of a mercury-vapor discharge. The mode has been identified as an n = 1 surface mode similar to that described by Trivelpiece.

It is shown that zero-temperature plasma-waveguide theory predicts this mode accurately. This theory predicts that a simple relation exists between the lower cutoff frequency of the n = 1 mode and the electron plasma frequency. It is demonstrated that the calculated electron plasma frequency obtained from this relation, where the cutoff frequency is determined from a measurement of the phase characteristics, agrees well with an independent measurement of electron plasma frequency by the well-known cavity-perturbation method.

Both the n = 1 and symmetric n = 0 modes were excited by a novel coupler called the double-ring coupler.

Noise measurements on the n = 1 and symmetric n = 0 modes indicate that the output signal-to-noise ratio is independent of input signal power and is about 7 dB for the n = 1 mode and varies from about 7 to greater than 30 dB for the n = 0 mode. A simple model of the plasma is proposed which explains all the noise observations reported here. Crawford and Lawson independently proposed a similar model.
Multimoding in Lasers
Authors: Pantell R. H.
Published: April 2005

It is well established that multimoding occurs in lasers within the homogeneous linewidth of the active medium. The oscillations at more than one frequency can be explained on the basis of the spatial or time dependence of the population difference. A threshold condition for multimoding is obtained in terms of the maximum power density from the laser with a single frequency output.
Ferromagnetic Resonance in Multidomain Thin Films
Authors: Hasty Turner E.
Published: April 2005

A thin Ni-Fe film, having a uniaxial magnetic energy of the form EK = K sin2θ, usually exists as a single domain. If a magnetic field of sufficient magnitude is placed perpendicular to the easy axis of magnetization and then removed, the film splits into a large number of 180° domains. When this simple multidomain structure occurs, it is theoretically possible to excite two ferromagnetic resonance modes by applying a small magnetic field perpendicular to the easy axis. The first of these, a low-frequency mode (0–10 Mc/sec) is excited by an rf field perpendicular to the domain walls; and the second, a high-frequency mode (0–25 Gc/sec) is excited by an rf field parallel to the domain walls. The high-frequency mode has been observed in a number of films having anisotropy fields from 3–5 Oe. The experimental results are compared with theoretical relations for frequency as a function of applied field. In addition, it is possible to estimate the domain wall separation from measurements of the high-frequency mode. These measurements indicate that typical domain widths range from 30 to 50 μ. A difference is observed in the position of the absorption peak when measured in an increasing field than when measured in a decreasing field. This hysteresis is explained by the Bloch-Néel wall transition theory which was used to explain the constricted M-H loops. Attempts to observe the low-frequency mode were not successful.
Ultrasonic Relaxation Loss in SiO2, GeO2, B2O3, and As2O3 Glass
Authors: Strakna R. E. , Savage H. T.
Published: April 2005

Measurements of the temperature dependence of the longitudinal and shear ultrasonic attenuation from 5 to 50 Mc/sec were made in SiO2, GeO2, B2O3, and As2O3 glass. In each case a relaxation loss with a distribution of relaxation times was found. The median activation energy of the relaxation process is about 500 cal/mole in SiO2, 2500 cal/mole in GeO2, 5400 cal/mole in As2O3, and 6300 cal/mole in B2O3. The results are interpreted in terms of a two-bond-length model of glass where the relaxation loss is associated with bridging oxygen atoms which have two stress-sensitive equivalent equilibrium positions. A relationship between the relaxation loss mechanism and the viscosity of these glasses is assumed in this study. A calculation of the activation energies based upon this assumption agrees well with the values determined experimentally.
Analysis of High-Power Effects in Ferrimagnetics from the Point of View of Energy Transfer. Part I. First-Order Instabilities
Authors: Kemanis G. , Wang S.
Published: April 2005

Spin-wave instabilities in ferrimagnetics in the past have usually been analyzed by considering the perturbed equations of motion of the normal variables of the spin system. In this paper the spin-wave behavior is analyzed without the use of the normal mode development. The properties of the kth spin wave—its energy and the phases and amplitudes of its transverse magnetization components—are derived using real variables exclusively. Then the criteria for all the first-order instability thresholds are derived from the power balance of the spin system. The physical processes which lead to spin-wave instabilities are discussed.
Piezoresistivity of Manganin
Authors: Bernstein David , Keough Douglas D.
Published: April 2005

The change in resistance of a class of alloys called Manganin has been measured as a function of peak shock-wave pressure up to 190 kbar. The pressures are obtained from explosively generated plane shock waves in a material of known Hugoniot equation of state. The Hugoniot of Manganin, which has been measured up to 359 kbar, is used with the resistance measurements to calculate the change in resistivity with volume. The results are compared with previous measurements.
Electron Tunneling through Asymmetric Films of Thermally Grown Al2O3
Authors: Pollack S. R. , Morris C. E.
Published: April 2005

The current through Al-Al2O3-metal structures was studied in detail and a description based upon electron tunneling is presented. It is shown that: the trapezoidal energy barrier model of Simmons adequately accounts for the details of the current-voltage characteristic over 9 current decades; there is a built-in voltage across a thermally grown oxide film of 0.92 V, in agreement with Mott's theory; the vacuum work function of the counterelectrode determines the barrier height at the oxide-counterelectrode interface; and the electron affinity of Al2O3 is 1.58 eV.
Electrical Effects due to the F Center in the Potassium Halides
Authors: Maycock J. N.
Published: April 2005

The electrical conductivity of the additively colored potassium halides has been investigated as a function of color density and temperature. These studies show that the crystals have a conductivity lower than that of the pure crystals for temperatures outside the intrinsic range. The experimental thermal dissociation energy of an F center is compared with the theoretical values.
Photocontrolled Surface Conductance in Anodized InSb
Authors: Mueller R. K. , Jacobson R. L.
Published: April 2005

Anodized p-type InSb, cooled to the extrinsic conduction range in the dark, was found to have an n-type inversion layer at the surface. Illumination changes the n-type inversion layer to a p+ accumulation layer; any intermediate surface condition can be obtained by controlled exposure. A method for determining the surface conductance of the inversion layer is described. This method utilizes a high-impedance grain boundary barrier to avoid shunting of surface conduction by bulk conduction. The highest surface conductance on unilluminated anodized samples was found to be 100 μmhos/square.

Spectral response measurements show that photons in the energy range 0.8 to 3.5 eV are effective. The effect is believed due to photoemission of electrons from the InSb into the oxide. Transient response measurements for light on and off conditions on n- and p-surfaces support this model.