bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-Energy Electron Beam Studies in Thin Aluminum Foils
Authors: Garber F. W. , Nakai M. Y. , Harter J. A. , Birkhoff R. D.
Published: February 2004

The transmission of normally incident electrons through aluminum, which has been vacuum deposited on a 75-Å aluminum oxide insulator has been measured for electrons of energy between 1 eV and 4 keV. Currents to the top layer (65 to 525 Å) and to the metallic bottom layer underlying the insulator were monitored with operational amplifiers in obtaining the transmission characteristic. Below 50 eV the transmission is apparently controlled by the aluminum oxide insulator. Above this energy the absorption increases rapidly until the entire beam current is absorbed in the top layer. At still higher energies the current to the top layer decreases through zero to a value of − 1 (normalized). The energy at which the top current goes to zero was found to increase as the thickness of the top layer was increased. These data were analyzed to yield the electron stopping power which was found to vary from about 3.1 eV/Å at 300 eV to about 4.5 eV/Å at 950 eV when the raw data were corrected for nuclear scattering by a modified version of the Yang scattering theory.
Ionization Model for Negative Capacitance in Low-Mobility Semiconductors such as Amorphous Chalcogenides
Authors: Rockstad Howard K.
Published: February 2004

A model to explain negative incremental ac capacitance has been developed, under the assumption that excess electrons and holes are generated in approximately equal numbers due to fairly general ionization processes such as impact ionization, localized Joule heating, or photoionization. Impact ionization is treated specifically. The motion of the excess carriers in a two-terminal bulk device gives rise to space charges which enhance the field near the electrodes. The finite times required for redistribution of the space charge and motion of the carriers through the device give rise to a lag τ of the external circuit current behind the voltage. The magnitude of the current component that lags by π/2 is proportional to ω if ωτ≪1. Thus if ωτe and ωτh≪1, where τe and τh are the electron and hole transit times, negative contributions to the ac capacitance are expected. A relation between the negative capacitance and the current-voltage characteristic is given, and this relation can provide an estimate of the order-of-magnitude of mobility for the lower-mobility charge carrier. Possible application of this model to the negative incremental capacitance observed in amorphous semiconductor switching devices is discussed, and data in the literature for a chalcogenide alloy are shown to fit the model, with an exponential dependence upon voltage. An effective carrier drift mobility of 3×10−4 cm2/V·sec is obtained.
Residual Stresses in Thin Single Crystals Bonded to an Amorphous Substrate: Silicon-Integrated Circuits
Authors: Zeyfang R.
Published: February 2004

Strains and stresses in the surface area of silicon-integrated circuits have been examined by a double-crystal x-ray diffraction method. The observed topography of the surface of the silicon single crystal is discussed with a simple model. By this nondestructive method strain fields produced by different bonding procedures can be compared and devices which are more likely to crack during temperature changes or by small mechanical loads may be identified.
Phenomenological Theory of the Hall Effect in Insulators
Authors: Mortensen O. Sonnich , Munn R. W. , Williams Digby F.
Published: February 2004

It is shown that in noncubic solids the ``Hall mobilities'' derived from Hall voltage and Hall current measurements are not identical. Their relation together with a consistency test for the measured ``Hall mobilities'' are obtained for orthorhombic symmetry. The observed quantities in Hall effect measurements on insulators are influenced by the presence of a space charge in the solid. The electromagnetic field equations are solved inside a cuboid of an orthorhombic solid by a Green function method. The results are used to relate the observed Hall voltage in the four-point-probe technique and the observed Hall current in the split-back-electrode technique to the fundamental parameters governing the Hall effect, for space-charge-limited currents under various trapping conditions. The corresponding relation for Ohmic conduction is also given. The advantages of using the Hall current method for insulators rather than the Hall voltage method are stressed.
High Electron Mobility in Zinc Selenide Through Low-Temperature Annealing
Authors: Aven M.
Published: February 2004

Electron mobility in ZnSe has been measured between 40° and 400°K. It is shown that through repeated annealing in liquid Zn the mobility maximum can be increased to 12 000 cm2/V sec. This is one of the highest mobilities measured for semiconductors with band gaps as wide as that of ZnSe (2.7 eV). The increase in mobility is mainly due to elimination of doubly charged acceptor states. The residual scattering is believed to be due, in part, to charged isolated impurities and, in part, to paired impurity dipoles.
Dynamics of Capillary Arcs
Authors: Cross R. C. , Ahlborn B. , Strachan J. D.
Published: February 2004

The gas dynamical behavior of the plasma flow induced by a capillary arc has been studied for both short and long arc pulse lengths. For a short (10 μsec) arc pulse, material evaporated from the capillary wall is ejected at high velocity from the end of the capillary into the neutral atmosphere of gas surrounding it, inducing a strong blast wave in the neutral gas. The blast wave front separates from the ejected wall material at the end of the arc pulse. The wall material itself separates into a leading hydrogen and a succeeding carbon molecule cloud when using a polyethylene capillary. With a longer (80 μsec) arc pulse, both the front velocity induced by the arc and the light output from the arc are constant for the duration of the arc.
Perturbation Theory for Electromagnetic Coupling to Elastic Surface Waves on Piezoelectric Substrates
Authors: Lakin K. M.
Published: February 2004

Starting from the fundamental acoustic and electromagnetic field equations an approximate expression is obtained for the perturbation in propagation wavenumber of a surface acoustic wave in a piezoelectric crystal. The source of perturbation is taken to be a surface spaced an air-gap distance h above the piezoelectric and is described in terms of an electrical impedance. The resulting perturbation is found in terms of the perturbing electrical impedance, an effective dielectric constant for the piezoelectric, the air-gap spacing h, and a perturbation coupling constant defined in terms of the unperturbed electric potential at the piezoelectric surface and the average acoustic power flow per unit frequency. The theory is applied to the case of a short-circuit perturbing surface and found to be in excellent agreement with certain numerical results for Y-cut Z-propagating LiNbO3 and several cuts of Bi12 GeO20. In the general case of a complex perturbing impedance, such as that exhibited by a semiconductor, the theory indicates that attenuation or gain and dispersion may be introduced by the perturbation, in close agreement with experimental observations.
Nonlinear Interaction of Microwave Electric Fields and Sound in LiNbO3
Authors: Thompson R. B. , Quate C. F.
Published: February 2004

The parametric interaction between two oppositely traveling sound waves and a nonspatially varying (k = 0) rf electric field in lithium niobate is illustrated in this paper. A pump frequency near 3 GHz is used to parametrically amplify the sound wave at one-half the pump frequency. In addition with a pump field that is sufficiently strong parametric oscillations occur within the crystal and this process forms a method of generating sound waves throughout the crystal volume. Finally, we have found that this type of parametric mixing of two sound waves can be used to obtain convolution signals in real time by monitoring the electric polarization generated within the crystal at twice the signal frequency.
Contactless Induction Method for Electric Resistivity Measurement
Authors: Chaberski Aleksander Z.
Published: February 2004

A new absolute method is developed for measuring electric resistivity in the range of 10−8 to 10+8 Ω cm of a sample of almost arbitrary shape. This method takes into account the effect of self-induced conduction and polarization current. Measurements are made with the specimen placed in a constant rotating magnetic field in such an orientation that it experiences maximum torque and has minimum moment of inertia. This can be shown to be the same orientation for which, if the specimen were free to rotate, it would reach the angular velocity of the rotating field in minimum time. This method is applicable to homogeneous and isotropic conducting materials in both solid and liquid states, including ferromagnetic substances. The mathematical formula for the measured torque is developed and the technique for measuring electric resistivity is explained.
Electrical Breakdown Measurements of Cesium Vapor in a Rhenium Electrode Diode
Authors: Jacobson Dean Leo , Robinson Lawrence Baylor
Published: February 2004

The electrical breakdown of cesium vapor was measured in a guard-ringed, variable parameter test vehicle, which had plane parallel rhenium electrodes. Experiments were conducted over a range of emitter temperatures from 1300° to 2000°K, with cesium pressures from 0.053 to 6.22 Torr and interrelectrode spacings from 0.001 to 0.07 cm. Because the breakdown occurred at relatively low voltages, the contact potential difference of the electrodes, as a function of temperature and cesium coverage, was taken into account. Paschen-type curves exhibited minima in breakdown potential as a function of spacing at high pressures. At low pressures the necessary large spacings could not be achieved. The maximum breakdown voltage was 6 V at 0.053 Torr and the minimum breakdown voltage was 0.21 V at 6.22 Torr. At low cesium pressures the effect of the emitter temperature is pronounced. When the pressure is constant, the breakdown voltage shifts as a function of emitter temperature. At high pressures, the curves do not vary with emitter temperature at constant pressure, but shift in voltage as the pressure is changed. These experiments indicate that the Paschen behavior is not followed in the case of hot electrode discharges, for all ranges of temperature and pressure.
Interaction of a Laser-Blowoff Plasma with a Strong Transverse Magnetic Field
Authors: Burnett N. H. , Smy P. R.
Published: February 2004

The interaction of laser-blowoff metal plasmas with transverse magnetic fields in the 100-kG range has been studied, the magnetic field being generated by a simple line current. Under these conditions it was found that the plasma crossed the field by means of an electrical polarization and E×B drift. An observed velocity reduction at fields greater than 10 kG has been attributed to an inhibiting effect of the magnetic field on the normal ambipolar energy transfer between electrons and ions in the expansion of laser-blowoff plasmas.
Mobile Cylindrical Magnetic Domains in Epitaxial Garnet Films
Authors: Heinz D. M. , Besser P. J. , Owens J. M. , Mee J. E. , Pulliam G. R.
Published: February 2004

The properties of cylindrical domains in wafers of bulk single crystals of various magnetic oxides have been extensively studied and described in recent months. Mobile cylindrical domains have been produced in single-crystal thin films of gallium-substituted yttrium iron garnet. The deposits are formed by chemical vapor deposition onto nonmagnetic garnet substrates greater than 1 cm2 in area. A model of magnetic anisotropy in epitaxial films is proposed and shown to be in good agreement with experimental domain observations. In a typical sample, a domain diameter of 8 μ is observed in a 6-μ-thick film at a bias field of 25 Oe. From the domain translational velocity of 5600 cm/sec, a cylindrical domain mobility of 1500 cm/sec Oe has been calculated. A fractional change in cylindrical domain diameter of −0.07%/°K has been measured at 300°K. Annealing at 1570°K for several hours did not alter the Néel temperature or the optical transmission spectrum.
Inelastic Neutron Scattering Studies of Critical Fluctuations in MnF2 Above and Below TN
Authors: Heller P. , Schulhof M. P. , Nathans R. , Linz A.
Published: February 2004

Detailed inelastic neutron scattering measurements have yielded the behavior of the scattering function S(q, ω) for both the transverse and the longitudinal fluctuations throughout the critical region of the uniaxial antiferromagnet MnF2. The results are discussed in the light of the theory of dynamic scaling. The results below TN are discussed with reference to quasihydrodynamic theories of the spin fluctuations in the ordered state.
Observations and Properties of a New Domain: Hollow Bubble
Authors: de Jonge F. A. , Druyvesteyn W. F. , Verhulst A. G. H.
Published: February 2004

A new type of domain structure, a hollow bubble or ring domain, has been studied. Calculations with zero Bloch-wall pinning force show that these ring domains can exist in platelets of uniaxial materials within a narrow region of the magnetic field normal to the platelet. Within this region the dimensions of the ring change enormously; they can be changed by a factor 2 by a field variation, which is about 100 times less than the variation needed to change the bubble radius by the same amount. We observed that the presence of a pinning force strongly influences the field region for stable ring domains and their behavior under a varying magnetic field. For a high-coercivity sample an experiment is reported, in which each cycle H1-H2-H1 of a varying magnetic field normal to the platelet diminishes the size of a ring. This experiment is in some sense the microscopic analogy to the ``bascule'' and ``reptation'' described by L. Néel.
Comparison and Discussion of Some Periodic Domain Structures
Authors: Druyvesteyn W. F.
Published: February 2004

The energy density of seven periodic domain structures in sheets of uniaxial magnetic materials placed in a magnetic field Hz perpendicular to the sheet is calculated.1 The following periodic structures are considered: I. A hexagonal bubble lattice; within the cylinders the magnetization Ms is reversed with respect to Hz. II. The negative of I; within the cylinders, Ms and Hz have the same directions. III. A similar but square lattice of bubbles and its ``negative'' (IV). V. A honeycomb structure where the prisms are the reversed domains. VI. The negative of V. VII. A strip lattice. The energy density is minimized with respect to two variables. Comparing the structures I-VII it is found that: (1) In a large field region a hexagonal bubble lattice (I) has the lowest energy. At low fields, VII has the lowest energy. (2) The structures where the reversed domains are connected with each other (II, IV, VI) have a higher energy than their negatives (I, III, V). (3) The radius of a bubble in a hexagonal lattice (I) differs at the same magnetic field considerably from the radius of an isolated bubble. (4) The energy of the honeycomb structure V is always higher than the energy of structure I. Comparing VI and II this is not the case: at higher field, structure VI has a lower energy than structure II. (5) Comparing the fields at which for I, III, V, and VII the lattice distance becomes infinite, we found the lowest field for VII and the highest for I and III. The occurrence of the different structures will of course not only be determined by the energy difference between them but also by the number of nucleation points and the possible energy barriers. The occurrence of I in zero field seems to be possible since the lattice distance for I is almost the same at zero field as at the lowest field for which I have the lowest energy. If I can occur at low fields the same holds for II, since it can be formed from I by reversing Hz. For higher fields, II will go over into VI. At a field where the lattice distance of VI becomes infinite, an isolated interconnected reversed domain will appear. This can be a hollow cylindrical domain. Changing the field in such a way that the well-known transitions occur: ring→bubble→strip→strip lattice, a transition from I→VII via different structures has been described above. Such a transition was also found experimentally. Detailed calculations will be published elsewhere.1
Reduction of the Wall-Motion Field of Polished Orthoferrite Platelets
Authors: Portigal D. L.
Published: February 2004

The wall-motion field Hc in an as-polished orthoferrite platelet is generally too large (Hc>0.5 Oe) for use in domain propagation devices without further treatment. Very low values of Hc down to less than 0.02 Oe have previously been reported by Heinlein and Pierce1 using a 1500°C anneal in a highly purified atmosphere. We find that a simple anneal at 1200°C for one hour in ambient atmosphere produces samples having Hc of 0.25–0.50 Oe, but this is sufficiently low to demonstrate device operation. The annealing treatment was successful with samples mechanically polished by various methods including samples which were also etched in 250°C phosphoric acid. Temperatures lower than 1200°C do not lower Hc sufficiently, and somewhat higher annealing temperatures lead to no further improvement. Annealing times longer than an hour do not lead to improvement in Hc and may cause surface damage. This simple technique has proven useful in obtaining samples for initial studies on domain propagation and device design.
Domain Nucleation and Boundary Effects in Thin Uniaxial Plates
Authors: Cape J. A. , Lehman G. W.
Published: February 2004

In high-mobility materials with large uniaxial anisotropy (K>2πMs2) the magnetostatic wall-energy model as applied to an infinite lamina apparently provides quantitative explanation of most observed domain structures. However, the presence of boundaries and nucleation energy barriers usually prevent the spontaneous development of the equilibrium (lowest energy)1 structures. To understand this, we argue that the confluence of any two domains (or a domain and an edge boundary), or the reverse process of separation, always involves an intermediate state of higher energy. Consequently, for example, the periodic stripe array should not be expected to spontaneously divide into a lattice of bubbles when the latter is a lower energy structure. Similarly, the repulsive interaction with the boundaries prevents the equilibrium field dependence of the lattice nearest-neighbor distance. Our observations on an epitaxial Ga–YIG film2 are shown to be entirely consistent with these qualitative considerations, while the observed domain critical fields, sizes, and spacings are quantitatively predicted by the wall-energy model. Calculations of the nucleation energy for radial growth were carried out for a semicircular (half bubble) domain at a crack and at a free edge. The latter process is even more costly energetically than volume nucleation while the former requires less than half the volume nucleation energy. At crack intersections the barrier is reduced still further. Our observations on the Ga–YIG film show that domain nucleation occurs readily at cracks and rarely at edges in agreement with the theoretical suggestions. A more complete discussion of this work will be published elsewhere.
Problem of Using Kink-Point Locus to Determine the Critical Exponent β
Authors: Arrott A.
Published: February 2004

It is shown that depending on how one analyzes measurements of the temperature dependence of magnetization near the Curie temperature it is possible to conclude that the critical exponent changes near Tc. An apparent change in β can be obtained from analyzing data generated from an equation of state in which β does not change. The mathematical and experimental difficulties in the use of the kink-point locus method are discussed.
Temperature Dependence of the Paramagnetic Resonance Linewidths in MnS and MnO
Authors: Battles James W.
Published: February 2004

The temperature dependence of the EPR linewidth in single crystal MnS and MnO have been measured at 35 GHz. The EPR in MnS abruptly stops at 152°±0.5°K as the temperature is decreased. The linewidth in MnS is proportional to (T-TN)−3/8 for 1° ≤ (T-TN) ≤ 6°K. In both MnS and MnO the linewidth is proportional to T/(T-TN) for T-TN > 10°K. In this high-temperature region the constants of proportionality are 80 and 280 Oe, respectively. An applied field of 12 500 Oe does not shift the Néel temperature in MnS.
Specific Heat of CoCl2·6H2O Near Its Néel Point
Authors: White J. J. , Song H. I. , Landau D. P.
Published: February 2004

The specific heat of a single crystal of CoCl2·6H2O has been measured with high resolution near TN = 2.291 K using the continuous heating method. Our data have been fitted to the usual asymptotic form: C/R=Aε−α+B, where α, A, and B may assume different values below and above TN. By plotting log (∂C/∂T) vs log (ε) for different values of TN we obtained values for α′, α from the slopes of linear fits. Limits on the ``unrounded critical region'' were defined by deviations from linearity on these plots. A least-square fit of the unrounded critical regions was also made. Both techniques yield the same results: the specific heat is rounded over a region about 8 mK wide, TN-Tmax = 4 mK, α′ = −0.18±0.03 below TN, and α = +0.32±0.07 above TN. We have compared our results with the earlier measurements of Skalyo and Friedberg1 on a polycrystalline sample. Our specific heat peak is sharper near TN and rises to a maximum about 7% higher, but for ε>4×10−2 the two sets of data agree quite well. Analysis of the rounding of the two peaks was made assuming the existence of subsystems having Gaussian distributions of ordering temperatures. Using this technique, we were able to fit our data in the rounded region quite well, and using the same form for the unrounded specific heat, but with a distribution half-width twice as large, we obtained good agreement with the data of Skalyo and Friedberg.
Magnetic and Electrical Characteristics of REPd3 Intermetallic Compounds
Authors: Hutchens R. D. , Rao V. U. S. , Greedan J. E. , Wallace W. E. , Craig R. S.
Published: February 2004

Magnetic characteristics of 12 REPd3 (RE = a rare earth) compounds have been studied between 4° and 300°K. Electrical resistivity measurements have been made on the Ce, Gd, and Ho compounds. All compounds are paramagnetic at room temperature; all except CdPd3, SmPd3, and EuPd3 exhibit Curie-Weiss behavior with paramagnetic moments in close agreement with that expected for the free tripositive ion. The susceptibility behavior of CePd3 is ascribed to the Kondo phenomenon (spin compensation) while that of SmPd3 and EuPd3 to the narrowness of the multiplet spacings. This interpretation for CePd3 is supported by its resistivity-temperature behavior. Evidence for magnetic ordering is found only for GdPd3, which seems to order ferromagnetically below 7°K.
Gadolinium Spin Density
Authors: Moon R. M. , Koehler W. C. , Cable J. W. , Child H. R.
Published: February 2004

Measurements of the magnetic form factor of Gd have been performed using a variety of experimental methods. Single crystals of metallic 160Gd were studied in the ferromagnetic state by both polarized and unpolarized neutron diffraction techniques. Data have been obtained for all (hk0) Bragg peaks out to sinθ/λ=1.275 and for all (0kl) peaks out to sinθ/λ=1.04. The form factor has also been measured in the metallic paramagnetic state using the polarized beam technique. In this case the shape of the spin distribution appears to be identical to that observed in the ferromagnetic state. The metallic spin density is spherically symmetric and has a radial dependence which is significantly expanded relative to Hartree-Fock calculations for the 4f electrons in Gd3+.1 It is not possible to fit the data by combining ionic 4f and atomic 5d calculated form factors. Surprisingly, the data are in good agreement with a 4f form factor2 based on Hartree-Fock atomic wavefunctions3 for atomic Gd in a 4f86s2 electron configuration. The ionic state has been investigated by measuring the paramagnetic scattering from 160Gd2O3 out to sinθ/λ=0.5 using the polarization analysis technique. In contrast with the metallic case and in accord with earlier results, we find good agreement with the Gd3+ calculation except at very small scattering angles.
NMR of CuCr2−xVxS4 Spinels
Authors: Berger S. B. , Burch T. J. , Budnick J. I. , Darcy L.
Published: February 2004

Cu and Cr NMR of CuCr2−xVxS4 spinels for x between 0 and 1 have been studied at He(l) temperatures by spin-echo techniques. The spectra are explained on the basis of a statistical distribution of Cr and V on the B sites. Our analysis of the transferred hyperfine field at the Cu nuclei fails to show clear evidence for a magnetic moment on vanadium.
Radio-Frequency Field Enhancement of the 53Cr NMR in Ferromagnetic CrBr3
Authors: Cobb C. H. , Jaccarino V.
Published: February 2004

The rf field enhancement of the 53Cr NMR in a single crystal of the uniaxial ferromagnet CrBr3 was measured, using rotary saturation, fast passage, and/or pulse turning angle techniques, in magnetic fields H0 both greater and less than the saturating field HS. With H0>HS, and H0 applied either ∥ or ⊥ to the easy direction, η exhibits the dependence on H0 expected from a domain-rotation mechanism. However, when H0<HS, the two resonances (``a'' and ``b'') observed, corresponding to nuclei belonging to Cr3+ spins oriented ∥ and ⊥ to the easy axis, respectively, have η's, intensities, rf polarizations, and H0 dependencies that are not consistent with domain rotation (for ``a'') and simple wall motion (for ``b'') enhancements. We believe that the ``a'' NMR probably arises from nuclei on the edge of the wall and the ``b'' NMR from nuclei residing in perpendicularly oriented domains (e.g., domains of closure). We demonstrate that rotary saturation measurements of η imply a distribution of maximum enhancement factors in a domain wall and that η is not strongly dependent upon the orientation of the applied rf field.
Mössbauer Investigation of Iron-Rich Fe+As and Fe+Sb Solid Solutions
Authors: Huguelet B. C. , Harper W. C. , Hummel C. L. , Kimball C. W. , Aldred A. T. , Paulikas A. P.
Published: February 2004

Mössbauer measurements have been made on iron-rich solid solutions of the Fe+As and Fe+Sb systems to investigate the origin of the increase in moment per Fe atom upon alloying. The envelope of the experimental data is not exactly fitted on the assumption of a random solid solution for models with either 2 or 4 coordination shells. The system dependence of the magnetic hyperfine field appears to correlate with the increase in average Fe moment upon alloying. The change in the magnetic hyperfine field at Fe nuclei due to nn and nnn solute atoms is similar to that found for other alloys. Quadrupolar broadening due to disorder in the 2nd or higher coordination shells leads to approximately symmetrical linebroadening.
Magneto-Medicine: Biological Aspects of Ferromagnetic Fine Particles
Authors: Nakamura T. , Konno K. , Moroné T. , Tsuya N. , Hatano M.
Published: February 2004

The behavior of carbonyl iron ferromagnetic fine particles (FFP) introduced into the vascular system has been examined by biomicroscopical observations of a rat's mesenteric small vessels, extracorporeal bypass circulating system for a dog, histological observation for a rabbit, and magnetically controlled roentgenography for another dog, rat, and rabbit. In order to make FFP aggregate and also disperse in small vessels under the magnetic field, the following conditions were found to be necessary: (1) FFP are preferably coated with a suitable kind of substance. (2) The shape of the particles should be as spherical as possible. (3) Superposed magnetic field static as well as alternating should be applied. The solubility of FFP in the human blood with chelating agents was determined in vitro. The extracorporeal bypass circulating system was considered as a useful tool for the excretion of FFP. The Iron Rings Catheter was developed to take x-ray films of the internal surface of the right ventricle.
Applications of the Mössbauer Effect in Biophysics
Authors: Johnson C. E.
Published: February 2004

The Mössbauer effect may be applied to biology in two ways: (i) to study the electronic state of an atom in a biological molecule and (ii) as a method of chemical identification. The former can be very powerful since many biological molecules have a transition-metal atom at the active center, and hence data on its chemical state and on the electron transfer which takes place during a biochemical reaction may be obtained from magnetic measurements. Mössbauer effect measurements of the hyperfine interaction of 57Fe provide detailed information on the electron distribution in iron atoms, even when the molecular structure is not known. A review is given of some molecules to which this technique has been applied. The most extensive measurements have been made on hemoglobin and its derivatives and on the iron-sulfur proteins. In the latter, pairs of iron atoms are antiferromagnetically coupled. As an example of possible applications in analysis the Mössbauer spectra of iron from a healthy and a hemosiderosis lung are compared and found to be very different.
Magnetoresistance in Cobalt Whiskers
Authors: Marker D. L. , Reichardt J. W. , Coleman R. V.
Published: February 2004

Measurements of magnetoresistance in cobalt whiskers with residual resistance ratios up to 400 have been made in the temperature range 1°–4.2°K and in magnetic fields up to 80 kOe. Field dependence curves give evidence of magnetic breakdown above 65 kOe for certain field and current orientations. Shubnikov-deHaas oscillations in the longitudinal magnetoresistance indicate the existence of small pockets of the Fermi surface that contribute directly to the oscillatory magnetoresistance with periods of 9.3×10−7 G−1, 2.8×10−7 G−1, and 8.6×10−8 G−1, respectively. The larger period of 9.3×10−7 G−1 is in approximate agreement with the period observed by Reed and Fawcett in the transverse oscillatory magnetoresistance above 70 kOe. The temperature dependence of resistivity in the range 1°–4.2°K at H̄=0 is given by ρ∝αT2 with α=15×10−12 Ω cm/deg2. At 50 kOe the coefficient of the T2 coefficient increases by 33%.
Ferromagnetism of ZrZn2
Authors: Knapp G. S. , Fradin F. Y. , Culbert H. V.
Published: February 2004

The magnetic properties of ZrZn2 and related alloys and compounds are discussed. In particular, we present a study of the pseudo-ZrZn2 alloy system ZrCu2−xAlx, where 0.76≤x≤1.5. The results of specific heat, magnetic susceptibility, and nmr measurements are presented to show that the Fermi level of ZrCu1.1Al0.9 lies at the top of a sharp narrow d-band peak that is about 0.1 eV in half-width and has a maximum value of 2.7 states (eV-Zr atom spin)−1. We also show that ZrZn2 has a sharp narrow peak in the d-band density of states at the Fermi level. This peak has a half-width of about 0.05 eV and has a maximum value of 5.4 states (eV-Zr atom spin)−1. The ferromagnetism of ZrZn2 is understandable in terms of this feature of the band structure. The occurrence of a high narrow peak at the Fermi level is the most favorable condition for the existence of itinerant ferromagnetism.
Antiferromagnetic-Ferromagnetic Transition in the Alloy System Au4(Mn1−xCrx)
Authors: Yessik M. , Noakes J. , Sato H.
Published: February 2004

The nature of the transition from ferromagnetic Au4Mn to antiferromagnetic Au4Cr has been investigated in detail in the ternary system Au4(Mn1−xCrx) by means of electrical resistivity, magnetization and magnetic susceptibility measurements. Previously published data over the composition range 0<x<0.8 have been corrected and new data for 0.8<x<1 are reported. Both the ferromagnetic moments and the Curie temperatures decrease with increasing x to a value near zero at x≈0.8 where the transition to antiferromagnetism takes place. The results indicate that the magnetic moments of both Mn and Cr are well localized and that the second-nearest-neighbor magnetic interaction plays the important role in determining whether the structure is ferromagnetic or antiferromagnetic.
Magnetic Critical Phenomena in MnP by Neutron Scattering
Authors: Minkiewicz V. J. , Gesi K. , Hirahara E.
Published: February 2004

Three-crystal inelastic neutron scattering measurements have been carried out on a single crystal of MnP in the critical region. The compound is ferromagnetic below its ordering temperature of 291.5°K. The results of the experiments have demonstrated that for T<Tc, there exists a striking dissimilarity between the profiles observed for MnP on one hand and iron and nickel on the other. We have observed for the first time the diffusive fluctuations in a ferromagnet for T<Tc. These results should be contrasted with those that were previously obtained for iron and nickel. For iron and nickel, these diffusive modes have never been clearly observed. The characteristic frequency of the critical fluctuations Γ(κ, ζ) for T=Tc is found to be reliably described by the relation Γ(0, ζ) ∝ζx, where x=2.57±0.11, and ζ is the reduced wave vector. This result is in excellent agreement with the prediction made from dynamic scaling theory. For the isotropic ferromagnet, the dynamic scaling principle predicts a critical exponent of 52.
One- and Two-Dimensional Interactions: van der Waals Compounds and their Applications
Authors: Catalano Edward
Published: February 2004

The literature on structural chemistry and complex ion chemistry was surveyed to search for existing and/or possibly molecularly engineered compounds with interactions restricted to one or two dimensions. The objective was to find real systems suitable for use as models for comparison with similarly restricted theoretical models. Promising candidates were selected from metal-metal bonded cluster compounds, linear chain compounds, and inorganic layer-structure compounds. Most promising, however, were the van der Waals or inclusion compounds, a general class in which it appears to be quite feasible to design and sythesize molecules having the desired specific properties. Channel inclusion complexes, including urea and thiourea complexes with fatty acids, thiourea complexes with halogenated hydrocarbons, and starch-iodide and benzamide-iodide complexes, inorganic complexes with urea, and thiourea and transition-metal complexes with magnesium, as well as sandwich inclusion complexes which include graphite intercalation complexes and layer lattice cyanides are promising possibilities.
Dynamic Scaling Theory for Anisotropic Magnetic Systems
Authors: Riedel E. K.
Published: February 2004

A theory of dynamic critical phenomena in anisotropic magnetic systems by Wegner and the author is reviewed and compared with recent neutron scattering experiments, especially on the uniaxial antiferromagnets MnF2 and FeF2. Particular emphasis is placed on the spin-relaxation rates as functions of the anisotropy in ferromagnets and antiferromagnets in the paramagnetic critical state. Within a generalized dynamic scaling and mode-mode approach the anisotropy parameters are treated as additional ``critical'' variables. The theory applies to the entire critical state of weakly anisotropic systems and is not restricted to a small region around the critical point. Anisotropy is interpreted as introducing a second length into the critical behavior of the system. In this case the spin-relaxation rates cannot be characterized by a single set of critical exponents for all ranges of variables even in the critical state. Instead, simple corresponding-state relations are found, in which the analytical structure of the scaling functions is related to the basic symmetry and conservation-law properties of the system. It is shown that the anisotropy parameter, and in mode-mode approximation also the isotropic material parameters, enters only the amplitudes and reduced temperature and momentum variables in those relations. Quantitative results for the spin-relaxation rates are given for the uniaxial antiferromagnets MnF2 and FeF2 and, in the isotropic limit of the theory, for the ferromagnets Fe and Ni, and the antiferromagnet RbMnF3. Recent neutron scattering results on MnF2 and FeF2 by Schulhof et al. are in good agreement with the predictions of the theory.
Photoconductivity Mechanisms in the Magnetic Semiconductor EuSe
Authors: Penney T. , Kasuya T.
Published: February 2004

In the europium chalcogenides, the 4f spin order is strongly coupled to electronic conduction. The results of static and transient photoconductivity measurements in EuSe show that there is thermally activated conduction in the paramagnetic region which increases with increasing magnetic field. These effects are similar to previous dark conductivity measurements in Gd-doped EuSe. The transient photoconductivity measurements show that it is the mobility and not the lifetime which is dependent on temperature and magnetic order. The results are explained using a magnetic impurity state model, MIS, in which spin clusters form around crystal defects. The observed distribution of long photodecay times is associated with relaxation between the MIS on a neutral complex defect and charged defect. The thermally activated conduction may be due to either hopping or quasithermal activation to the conduction band.
Absence of Ordering in Certain Isotropic Systems
Authors: Thorpe M. F.
Published: February 2004

The method of Mermin and Wagner [Phys. Rev. Lett. 17, 1133 (1966)] is used to show that one- and two-dimensional spin systems interacting with a general isotropic interaction H=12  ∑ ijn Iij(n) (Si·Sj)n,where the exchange interactionsIij(n) are of finite range, cannot order in the sense that 〈Oi〉=0for all traceless operators Oi defined at a single site i. Mermin and Wagner have proved the above for the case n = 1 with Oi = Si, i.e., for the Heisenberg Hamiltonian. The proof allows us to rule out the possibility that a small isotropic biquadratic exchange (Si·Sj)2 could induce ferromagnetism or antiferromagnetism in a two-dimensional Heisenberg system.
Wannier Representation of Electron Energy Bands and Iron Fermi Surface
Authors: Maglic R. , Mueller F. M.
Published: February 2004

For the first time Fourier inversion of the real energy bands has been performed. About 1000 inequivalent coefficients are obtained. It appears that about six nearest neighbors are important in metallic iron. The coefficients have been used to calculate FS. Spin-orbit interactions showed profound influence on the topology of FS in Fe.
Uniformity of Limits in the Calculation of the Magnetization of the Heisenberg Linear Chain
Authors: Unruh Henry
Published: February 2004

By making use of the Bogliubov inequality, a formula is derived which gives an upper limit to the magnetization of a linear chain of arbitrary length and which is subject to the Heisenberg Hamiltonian. It is found that the magnetization vanishes uniformly in the limits of an infinitely long chain and a vanishing, externally applied, field strength.
Correlation of Electrons in a Degenerate Band
Authors: Chao K. A. , Gutzwiller M. C.
Published: February 2004

The ground-state wavefunction for electrons in a doubly degenerate tight binding band has been investigated by the variational method [M. C. Gutzwiller, Phys. Rev. 137, A1726 (1965). To be referred to as G.III.] The model Hamiltonian consists of a d band with intra-atomic Coulomb interactions between both d states. The expectation values for the approximate ground state are obtained by counting the various configurations as in the earlier work on correlation in a nondegenerate band. Simple criteria are found for the occurrence of ferromagnetism and of a metal-insulator transition, using the approach of Brinkman and Rice in the latter case [W. F. Brinkman and T. M. Rice, Phys. Rev. B 2, 4302 (1970)]. If the density of states is large at the band edge, the ground state of electrons goes from paramagnetic metallic, through an intermediate ferromagnetic metallic state, to insulating as the intra-atomic Coulomb interaction is increased.
Evidence for a Dependence of the Long-Range Order Developed in Ni–Fe Thin Films on Grain Size
Authors: Williams C. M. , Schindler A. I.
Published: February 2004

Uniaxial anisotropy energy measurements on polycrystalline Permalloy films following 3He irradiation indicate that the development of NiFe and Ni3Fe long-range order in thin films has a strong dependence on the grain size. The results of these measurements show that Ku due to the development of NiFe long-range order is at least a factor of four larger in the larger-grain-size films (grain size ∼4000 Å) than in the smaller-grain-size films (grain size ∼200–300 Å); films with compositions around Ni3Fe show a much faster decrease in Ku, due to the development of Ni3Fe long-range order, in the larger-grain films than in the smaller-grain films. Such a grain-size dependence is in part attributed to the fact that the defects responsible for the ordering process can participate in more atomic replacements in the larger grains than in the smaller grains before they are annihilated at grain boundaries. Thus, the larger the crystallite, the greater the amount of long-range order developed during irradiation.
Dispersion Effects in Magnetostrictive Films
Authors: Kurlansik H. , Onyshkevych L.
Published: February 2004

The addition of 1%–5% Mn to Ni–Fe films reduces Hk by ∼3× and increases magnetostriction by ∼4×, but also raises dispersion by ∼3×, so that the product α50Ku stays within ±10% over the range of interest of from 1% to 5% Mn. Since high dispersion (α50>5°) was found to cause disturbs in memory applications, it is not advisable to use more than ∼3% Mn for a memory with 20-mil-wide film strips.
Infrared Reflectance of CdCr2S4 and CdCr2Se4
Authors: Lee T. H.
Published: February 2004

The infrared reflectance spectra of hot-pressed CdCr2S4 and CdCr2Se4 have been measured at temperatures above and below the Curie temperature. Two strong reflectance peaks are observed for these materials. These reflectance peaks are attributed to the reststrahlen bands due to the ionic bonds between the positive metal ions and the negative chalcogen ions. From the reflectance spectra, the transverse and the longitudinal optical phonon frequencies and the static dielectric constants are calculated. At room temperature, ωLO=389.9 cm−1 and 347.2 cm−1, ωTO=376.9 cm−1 and 321.6 cm−1, and ε0=9.79 for CdCr2S4; and ωLO=291.4 cm−1 and 281.3 cm−1, ωTO=288.1 cm−1 and 266.2 cm−1, and ε0=10.3 for CdCr2Se4. At liquid-nitrogen temperature, the phonon frequencies are increased approximately by 1%.
Photoinduced Changes in the Crystal Anisotropy of Si-Doped YIG
Authors: Flanders P. J. , Graham C. D. , Dillon J. F. , Gyorgy E. M. , Remeika J. P.
Published: February 2004

It is found that the cubic magnetocrystalline anisotropy of Si-doped YIG can be changed by exposing a single crystal to light at 77 or 4.2 K. The experiment is done using a (110) plate sample in a rotating sample magnetometer (RSM). Doping levels of 0.01 to 0.017 Si per formula unit Y3Fe5O12 have been studied. The experimental problem is to distinguish changes in the crystal anisotropy from changes in the induced anisotropy, since both are influenced by light. This separation is possible by analysis of the RSM outputs at different frequencies and phase angles. We observe changes in the quantity (−32K1−14K2); the changes are a few percent in magnitude, occur within a few seconds or tens of seconds after the light is turned on, and persist for long times (many minutes) after the light is turned off. For low levels of Si doping, irradiation at 77 K produces large changes in the part of the cubic anisotropy attributable to the Fe2+ present. The measured change in anisotropy for all samples is an increase in magnitude at 77 K, but a decrease at 4.2 K. The effect is not observed in Ca-doped YIG, confirming previous conclusions that ferrous ions are principally responsible for photoinduced anisotropy effects.
Theory of Photoinduced Uniaxial Anisotropy in Silicon-Doped Yttrium Iron Garnet
Authors: Alben Richard , Dillon J. F. , Gyorgy E. M. , Remeika J. P.
Published: February 2004

The polarization dependence of the photodetachment cross section for Fe2+ ions on ``a'' sites in YIG:Si is shown to provide a mechanism which explains part of the photoinduced uniaxial anisotropy in this material. Using a crystal-field theory, we have obtained quantitative predictions for the relative anisotropy in the cross section and found that a 5%–10% population imbalance between differently oriented sites is expected for light polarized along the [110] direction. We also report new experimental measurements which characterize the magnetization and the light polarization dependence of the uniaxial anisotropy. The measurements show a substantial polarization-independent photomagnetic anneal, not included in the theory. However, the polarization-dependent inducible anisotropy is in general accord with the proposed mechanism.
Optical and Magneto-Optic Properties of Polycrystalline CdCr2S4
Authors: Moser F. , Ahrenkiel R. K. , Carnall E. , Coburn T. , Lyu S. L. , Lee T. H. , Martin T. , Pearlman D.
Published: February 2004

The optical absorption spectrum of hot-pressed polycrystalline CdCr2S4 has been measured between 700 nm and 50 μ, with particular emphasis on the region of weak absorption between 700 and 1200 nm, in which Faraday rotation spectra were also obtained. In this region, two series of phonon-assisted transitions have been observed, involving phonon energies of 0.041 and 0.046 eV. These energies are similar to the optical phonon energies obtained from infrared measurements on the same crystals. At 4.2°K, and with an applied magnetic field of 3000 Oe, the Faraday rotation reaches a value of about 1.6×104 deg per cm near 750 nm. At 80°K, the rotation is decreased by about a factor of two. These crystals have very low optical scatter or impurity absorption, resulting in high transparency in the near infrared. At 4.2°K, the figure of merit is about 60 deg dB−1 in this spectral region, and decreases to about 30 deg dB−1 at 80°K.
Magnetoreflectance Studies of CdCr2Se4 and CdCr2S4
Authors: Ahrenkiel R. K. , Moser F. , Lyu S. , Pidgeon C. R.
Published: February 2004

Specular reflectance and reflectance-magnetodichroism measurements on single-crystal and polycrystalline CdCr2Se4 and CdCr2S4 have been made at liquid-helium temperatures. The specular reflectance is generally characterized by a single strong peak near the band edge. The magneto-reflectance is quite rich in structure extending from the band edge to about 5.0 eV.
Thermopower of Dilute Alloys of Cerium in Lanthanum
Authors: Grobman Warren D.
Published: February 2004

We have measured a large negative peak in the thermoelectric power S of the La–Ce Kondo system at low temperature. This result is qualitatively different than that of Sugawara. Measurements were made in the range 4.2 K≤T≤350 K for four compositions: 0.03-, 0.5-, 2-, and 5-at.% Ce. The size of the peak was in the range −1.5 to −2. μV/K and occurred at ∼20 K. The shape of the curves, and the temperature at which the peak occurred, were nearly independent of composition for T<150 K, while the curves were somewhat sample dependent at higher temperatures. While significant amounts of metallic impurities were present, they probably do not contribute a peak in the thermopower. However, on the basis of these measurements one cannot rule out the possibility that the peak is intrinsic to La.