bluuebunny/crossref_metadata_2025 huggingface dataset:
Exchange-Enhanced Paramagnetism and Weak Ferromagnetism in the Ni3Al and Ni3Ga Phases; Giant Moment Inducement in Fe-Doped Ni3Ga
Authors: de Boer F. R. , Schinkel C. J. , Biesterbos J. , Proost S.
Published: February 2004

Measuring the magnetization of various alloys in the Ni3Al and Ni3Ga phases, we found that going to higher Ni concentrations within these phases, a transition from strongly exchange enhanced paramagnetism to weak ferromagnetism takes place at 74.5 and 76 at.% Ni, respectively. The shape of the reciprocal magnetic susceptibility vs temperature curves measured between 4.2° and 300°K is of the Stoner type and suggests for these alloys a density-of-states curve varying appreciably in the Fermi region leading to a low effective Fermi energy. This conclusion is confirmed by the strong curvature of the magnetization in high magnetic fields. These results and the fact that an increase of the magnetic field applied on ferro-magnetic samples effects a large increase of the magnetization even at the lowest temperatures (4.2°K) and in the highest fields (200 kOe) led us to a description in terms of the itinerant electron model. The use of this model seems justified by the fact that in agreement with calculations by Edwards and Wohlfarth, the magnetic data of both paramagnetic and ferromagnetic compositions fit nicely on straight lines in σ2 vs H/σ plots, this being so for all temperatures (down to 4.2°K) and all magnetic fields between 4 and 200 kOe. Combining the magnetic data and low-temperature specific-heat data, we obtained some information on the bandshape at the Fermi level. We cannot conclude, however, to more than a nice qualitative agreement with the Stoner model at this moment. As would be expected in highly polarizable metals and alloys, small amounts of Fe dissolved both in paramagnetic and in ferromagnetic alloys in the Ni3Al and Ni3Ga phases lead to giant moment inducement.
Magnetomechanical Damping in the Temperature Range of 78°–300°K
Authors: Frank R. C. , Schroeder C. W. , Johnson B. G. , Swanson P. A.
Published: February 2004

The damping of mechanical vibrations associated with the stress induced movement of magnetic domain boundaries was measured in pure nickel, pure iron, and in an iron-3.7% silicon alloy over the temperature range of 78°–300°K. The damping was characteristically amplitude dependent with maximum values at specific values of the shear strain. The damping for nickel, as expressed by the fractional energy loss per cycle, had maxima that decreased from 0.58 at 78°K to 0.36 at 300°K. The damping maxima for iron decreased only slightly from 0.20 at 78°K to 0.16 at 300°K. However, in the case of the iron-3.7% silicon alloy the damping maxima increased from 0.19 at 78°K to 0.28 at 300°K. The results are discussed in terms of the recently proposed mathematical model of magnetomechanical damping by Smith and Birchak. Much of the variation of the damping with temperature can be accounted for by variations of the magnetostriction with temperature.
Magnetic Field Effects in the s-d Exchange Model of a Localized Impurity
Authors: Bloomfield Philip E. , Hecht Robert , Sievert Paul R.
Published: February 2004

We use an equation of motion approach to the problem of a spin ½ localized impurity in a metal in a magnetic field. The symmetries of the zero field are no longer present, and we find it necessary to define seven Green's functions. A t-matrix formalism is developed after truncating the equations of motion. We find that six of the Green's functions are expressible in terms of four t's that satisfy coupled nonlinear singular integral equations. Our treatment takes account of the difference in g values of the conduction electrons and impurity electrons, and determines the electron magnetization self-consistently. The corresponding Hilbert boundary value problem cannot be solved by the procedures which were useful in zero field. Instead, we numerically solve our system of finite-field integral equations by an iterative procedure starting from the exact zero-field results. We obtain results which are qualitatively similar to experiment and exhibit the freezing out of the spin-flip scattering process and a subsequent suppression of the compensated state as the magnetic field increases.
Coherent Exchange Scattering of Low-Energy Electrons by Antiferromagnetic Crystals
Authors: Palmberg P. W. , De Wames R. E. , Vredevoe L. A. , Wolfram T.
Published: February 2004

Coherent exchange scattering has been observed by low-energy electron diffraction from antiferromagnetic NiO. Below the Néel temperature, low-energy electron diffraction from the (100) surface, exposed by cleavage in ultrahigh vacuum, has revealed one-half order beams that correspond to the magnetic periodicity; these diffraction beams disappear at the Néel temperature. The intensity of the additional half-order ``magnetic'' beams relative to that of the integer order beams is 2%–6%. Because direct dipole-dipole and spin-orbit interactions are much too weak to produce beams of the observed intensity, the occurrence of half-order beams is attributed entirely to exchange scattering. The interpretation of the experimental results is discussed with the aid of a Heisenberg model Hamiltonian. Theoretical expressions for the elastic exchange cross section are in accord with the experimental data. The temperature dependence of the intensity of the exchange scattering is qualitatively explained by a molecular-field model which includes surface corrections.
Instabilities of Magnetostatic Modes in Axially Magnetized YIG Rods
Authors: Bini M. , Millanta L. , Rubino N. , Tognetti V.
Published: February 2004

Experiments have been made of magnetostatic wave propagation through an axially magnetized, [100]-oriented, unpolished YIG rod of circular cross section, at increasing incident power levels, and instability phenomena have been noted. The transmitted power was observed as a function of frequency and the power level was increased until instabilities occurred in the form of oscillations superimposed on the transmission spectrum. The instabilities started first in the cutoff region of the magnetostatic volume modes (f=γHde) and extended towards higher frequencies for increasing incident power. The onset occurred for incident power values of a few tens of mW (a few mW into the crystal). For very high values of incident power (∼1 W), strong transmission below γHde was also noted. The oscillations were then examined at fixed microwave frequency, both in the time and in the frequency domain and were found to be of the order of the hundreds of kHz, and to be highly unstable, as shown by jitter in the time representation and random noise in the spectral analysis. The instability effects seemed to be rather insensitive to the sample orientation. The results are discussed in the light of existing theories.
Magnetoelastic Wave Instability Threshold in an Axially Magnetized YIG Rod
Authors: Van de Vaart H.
Published: February 2004

The instability threshold has been derived for transversely pumped magnetoelastic waves in YIG for the case where the equilibrium position of the magnetization vector is in an arbitrary direction with respect to the crystal axes. It is shown that under these conditions the threshold has a finite value for magnetoelastic waves propagating parallel to the direction of the internal field, and that the threshold is minimum near crossover. It is also shown that this minimum in the threshold is lowest when the internal field is rotated ∼15° with respect to one of the 〈100〉 axes of YIG in a {100} plane. These results satisfactorily explain the experimental observations reported earlier.
Magnetization Creep of Cross-Tie Walls
Authors: Torok E. J. , Lo D. S. , Oredson H. N. , Simon W. J.
Published: February 2004

Cross-tie walls are known to have lower creep thresholds than non-cross-tie walls. This indicates that a special creep mechanism exists for cross-tie walls but does not for non-cross-tie walls. Such a mechanism is described: In the presence of a pure dc easy-axis field the magnetization antiparallel to the field buckles more, while the magnetization on the other side of the wall buckles less. This causes magnetostatic charges on the wall which cause the main wall itself to buckle in a zig-zag fashion with bends at the Bloch lines between the cross-ties and at the Bloch lines in the cross-ties. When a hard-axis field is applied, the Bloch lines between the (fixed) cross-ties move along the wall, a situation resulting in magnetostatic charges causing the apex of the zig-zags to move along with the Bloch lines. This distorts the symmetrical zig-zags into asymmetrical ones having alternately long and short legs. At this point, each segment of the zig-zag lies at a different angle from the easy axis than it did before the hard-axis field was applied. When the hard-axis field is relaxed, the zig-zags become symmetrical again, and if a hard-axis field of opposite polarity is applied, the zig-zags lean the other way, i.e., the short segments become long and vice versa. This forces each segment to move with each hard-axis pulse. Electron micrographs are presented which show all these processes clearly, and magnetostatic calculations of the stray field are made for each step of a typical case.
Dynamic Kerr Observations of High Speed Flux Reversal and Relaxation Processes in Permalloy Thin Films
Authors: Kryder M. H. , Humphrey F. B.
Published: February 2004

Sequences of dynamic Kerr photographs with 10 μ resolution have been taken during high-speed flux reversal and as the magnetization relaxes from a state of dynamic equilibrium to the final state of static equilibrium upon termination of the drive field. Equipment used consisted of a sampling Kerr magneto-optic camera with a Q-switched ruby laser for a light source. The 10 nsec exposure time was synchronized with a variable time delay between application of the magnetic field and actuation of the camera stable to within 3 nsec. The results show that, with zero transverse field, longitudinal propagation of diffuse transverse boundaries and nucleation of partially reversed regions are processes which are involved in intermediate and high-speed switching, respectively. The velocity of the diffuse boundary varies approximately as H5, and the mobility is as large as 1.4 cm/Oe-μsec. With a transverse field, the low-speed reversal is similar to diffuse boundary propagation and the higher-speed processes are rotational in nature. The transition from the propagational phenomenon to the rotational phenomenon is abrupt. Blocking is easily observable and is found to be important in the rotational process. Furthermore, it has been found that, if the reversing field is terminated prior to saturation, the flux either continues to reverse or relaxes to a less-switched condition, in times of the order of 1 μsec. Hence, conclusions drawn from interrupted pulse experiments may be in considerable error. A complete description of the results of this investigation will be published in the May, 1969 issue of the Journal of Applied Physics.
Effect of the Weak Ferromagnetic-Antiferromagnetic Transition on the Electron Transport Properties of Hematite
Authors: Ransom C. L. , Hanton J. P.
Published: February 2004

The effect of the weak ferromagnetic-antiferromagnetic transition on the electron transport properties has been observed in synthetic, single-crystal, magnesium-doped hematite (nominally 0.5% replacement of iron). Separate measurements of the transverse voltage were taken with current flowing along the [111] direction and within the (111) plane. The predominate results are: (1) The activation energy of the transverse voltage is quite different for current flow along the [111] direction and in the (111) plane. The energies are −1.3 and 1.3 eV, respectively. (2) The transverse voltage produced for current along the [111] direction is altered as the crystal undergoes the weak ferromagnetic-antiferromagnetic transition at the Morin temperature (Tm), whereas no change is observed at Tm for transport in the (111) plane. (3) No normal or anomalous Hall effect is observed for current flow in either the [111] direction or the (111) plane; however, a second-order dependence upon the applied magnetic field is observed for the transverse voltage. The observed results are interpreted in terms of a model which considers the effect of the magnetic state upon the electron-transfer process within the crystal. The electron transfer energy is assumed to be a function of cos(θ/2), where θ is the angle between the quantization directions (determined by the large internal magnetic fields) of the iron ions, between which the electron is assumed to hop.
Exponent Inequalities for Critical Point Spin Correlation Functions
Authors: Fisher Michael E.
Published: February 2004

Simple but rigorous derivations are presented for the inequalities2−η≤dγ′/(2β+γ′)≤dγ′/(2−α′),2−η≤d(δ−1)/(δ+1),γ≤(2−η)ν.(Proofs of the first two inequalities have been given by Buckingham and Gunton.1) Here d is the dimensionality and α′, β, γ, γ′ and δ are the exponents characterizing a ferromagnet near its critical point,2 while η and ν describe the decay of the corresponding spin-spin correlation functions2 Γ2(r)=〈S0zSrz〉, according to ∑ |r|<RΓ2(r)∼R2−η(T=Tc,H=0,R→∞),κ1∼(T−Tc)ν,(T→Tc,H=0),where the inverse range of correlation κ1 may be defined by κ1−2=[Σrr2Γ2(r)]/[ΣrΓ2(r)]. The corresponding exponent ηE for the energy-energy correlation function (essentially Γ4(r, a)−[Γ(a)]2, where Γ4(r, a)=〈S0zSazSrzSr+az〉) is proved to satisfy 2−ηE≤dα′,2−ηE≤dαc/(1+ζ+αc),where the specific heat CM at T=Tc diverges with the magnetization as M−αc while the energy derivative | ∂U/∂M |Tc varies as Mζ. (Mean Field or classical values are αc=0, ζ=1.) The proofs are based on the ``intuitively obvious'' properties: (a) positivity, namely, Γ1=〈S0z〉≥0,Γ(r)−Γ12≥0,Γ4(r, a)−[Γ2(a)]2≥0, and (b) monotonic increase of Γ1, Γ2 and Γ4 under increase of magnetic field and decrease of temperature. These properties are known3 to be rigorously valid for Ising models of arbitrary spin, lattice structure and ferromagnetic coupling (Jij≥0). Their validity for real magnets together with the experimental observation δ≤4.7 leads to the significant conclusion η≥0.05.
Minimum Polarity Models in the Theory of Magnetic Properties of NiCu Alloys
Authors: Kirkpatrick S. , Velický B. , Lang N. D. , Ehrenreich H.
Published: February 2004

The success of minimum polarity over rigid band models in describing the electronic structure of NiCu alloys is ascribed to the fact that the random scattering potential is strong enough to produce nearly independent Ni and Cu subbands. Calculations of magnetic properties using a model density of states whose first three moments are exact are in good agreement with experiment and essentially better than previous results.
New Semi-Hard Magnetic Alloys of Co–Fe–Be System
Authors: Sasaki Tatsuji , Matsui Yuji , Henmi Zenzo
Published: February 2004

New semi-hard magnetic alloys, which have a square hysteresis loop and a low value of magnetostriction, are presented here. These alloys consist of about 90% Co, about 1% Be, and the balance Fe. We can attain a coercive force between 10 and 30 Oe by controlling the Be content. The Co content affects the stress sensitivity of the hysteresis-loop shape. One example is a tape 0.126 mm wide and 0.011 mm thick containing 88.5% Co, 1.3% Be, and 10.2% Fe, finally annealed at 800°C for 10 sec. This tape exhibits a coercive force of about 30 Oe, a squareness ratio of about 0.9, and very small change in the hysteresis-loop shape with 20-kg/mm2 tensile stress. In a sample containing more than 0.7% Be, CoBe precipitates and the grain size is much smaller than in a sample without Be. This fact contributes to the increase of coercive force with Be content. Moreover, the drawn wire has a 〈111〉 fiber texture, and the rolled-flat tape has a {110} 〈111〉 texture. These textures do not change until at least 900°C annealing temperature, and the squareness ratio of the hysteresis loop along the wire or the tape axis remains high. One of these alloys is now used for information storage in a computer memory device named CTM (Composite Tape Memory).
Exchange Narrowing: Systematic Discrepancies between Theory and Experiment
Authors: Gulley J. E. , Silbernagel B. G. , Jaccarino V.
Published: February 2004

The strongly exchange-narrowed Mn++ EPR in single-crystal MnF2, KMnF3, and RbMnF3 has been studied at X band with T≫TN. The half-widths at half-maximum ΔHexp of the nearly Lorentzian line profiles are 260±10, 59±3, and 58±3 G, respectively. From the accurately known values of exchange and lattice parameters we have calculated the second and fourth moments using the full dipolar Hamiltonian and obtained the half-width at half-maximum ΔHth of an assumed truncated Lorentizan line profile. For MnF2, KMnF3, and RbMnF3 we find ΔHth=112.5, 20.8, and 20.8 G with ΔHexp/ΔHth=2.3±0.1, 2.8±0.2, and 2.8±0.2, respectively. Similarly, a comparison between theory and experiment for the exchange-narrowed hyperfine-broadened 19F NMR in these same materials yields ΔHexp/ΔHth=1.48±0.04, 2.20±0.11, and 2.10±0.10 for MnF2, KMnF3, and RbMnF3, respectively. The systematic nature of the discrepancies suggests that the decay of the spin autocorrelation functions are inadequately characterized by a single decay constant and that the stochastic processes are more complex than have hitherto been assumed.
Magnetic Properties and Collective Excitations of Systems with Singlet Crystal-Field Ground State
Authors: Cooper Bernard R.
Published: February 2004

We discuss the magnetic behavior of rare-earth systems where the crystal-field-only ground state of the rare-earth ion is a singlet. For such materials there is a threshold value for the ratio of exchange to crystal-field interaction necessary for magnetic ordering even at zero temperature. The magnetization process for singlet ground-state systems is first discussed from the effective field point of view. We then treat the theory of the collective excitations and the relationship of their behavior to the magnetic ordering process. A discussion is given of the present experimental situation and of a variety of promising possibilities for future experiments to increase understanding of such systems. The techniques discussed are neutron diffraction and scattering, susceptibility, high-field magnetization, nuclear magnetic resonance, specific heat, and paramagnetic resonance experiments. The discussion of the experimental situation emphasizes the role of the rare-earth—Group V compounds. The system TbxY1−xSb illustrates the behavior obtained when, by alloying, one increases the exchange beyond the critical value necessary for magnetic ordering.
Magnetic Behavior of Intermetallic Compounds of Beryllium
Authors: Wolcott N. M. , Falge R. L. , Bennett L. H.
Published: February 2004

The corroboration by NMR of the ferromagnetism of CrBe12 has prompted an NMR investigation of the magnetic properties of other beryllium intermetallic compounds. Magnetic-moment measurements of these materials show complex magnetic behavior at 4.2°K. All samples show strong paramagnetic moments and many exhibit remanence. To provide more information on this magnetic behavior, the 9Be resonance was observed in the following samples, listed in approximate order of their magnetic strength at 4.2°K: CrBe12, ZrBe2, MnBe8, FeBe11, MnBe12, NbBe3, ZrBe13, TaBe3, CrBe2, Nb2Be17, TiBe12, NbBe12, VBe12, MoBe12, TaBe12, WBe2, Ta2Be17, WBe12. Magnetization measurements are also reported for AgBe12, FeBe12, CoBe12, PtBe12, PdBe12, and AuBe12. Both the Knight shifts and the linewidths show correlation with the moment measurements. Most of the samples show small magnetic moments, positive Knight shifts, and small linewidths, ΔH, less than (800 A/m=10 Oe). ZrBe2 and MnBe8 have larger moments, negative values of K (∼−0.03% at 300°K) and larger ΔH [∼(1.2 kA/m=15 Oe) at 300°K]. CrBe12, clearly ferromagnetic, with the largest moment (of the compounds studied) has the most negative value of K (−0.19% at 300°K) and the largest ΔH [(1.83 kA/m=23 Oe) at 300°K]. From the temperature dependence of the linewidths, it appears that NbBe3 and CrBe2 may be magnetic, although much weaker than CrBe12. Anisotropic Knight shifts and first-order quadrupole effects are noted.
Soft X-Ray-Emission Spectroscopy as a Tool for Probing Energy Bands
Authors: McAlister A. J.
Published: February 2004

Soft x-ray-emission spectra are produced by electrons initially in valence-conduction-band states dropping into vacant core levels. Naively, one expects this process to yield a reasonable picture of the electron distribution in energy. This expectation is readily susceptible to theoretical test, because emission spectra (including transition probabilities) are rather simple to calculate in the one-electron approximation, once the initial hard work of the band calculation has been done. The most serious interpretational problems are many-body in nature, namely, shielding of the initial-state core vacancy, and lifetime of the final-state-valence-conduction-band hole (the latter problem being common to other deep-band techniques as well). Band-theory calculations of the emission spectra of the light metals are now becoming available and indicate that these phenomena affect them only slightly. Presently available work on the first transition series metals strongly suggests that many-body effects do not greatly distort these spectra either, but detailed calculations have not yet been done. Pertinent light metal results will be briefly presented, and experimental spectra of Fe and Ni discussed in the light of our present understanding.
Field and Temperature Dependence of 153Eu Spin-Lattice Relaxation in Ferromagnetic EuO
Authors: Guenther B. D. , Christensen C. R. , Daniel A. C. , Teaney D. T.
Published: February 2004

Nuclear spin-lattice relaxation times, T1, of 153Eu have been measured in an irregularly shaped single crystal of EuO over the temperature range of 1.2∘≦T≦4.2∘K and magnetic field range of 0≦H0≦14 kOe for the crystal orientation H0⊥Hrf∥〈110〉 by use of the spin-echo technique. The relaxation process was found to satisfy T1 ∝ T−n, where n increases from approximately 2–3 as H0 increases from zero to 10 kOe. The T1 is essentially field independent up to H0≈2 kOe, Above this field there is a general increase in T1 but with an intermediate peak value at H0≈6 kOe. The analysis of these data is complicated by the irregular shape of the sample and the high saturation magnetization, 4πM0=24 kOe, for EuO. Within these limitations these results have been compared with the theory developed by Honma for EuS.
Grain-Size Effects on Switching Properties of Lithium Ferrite
Authors: Babbitt R. W. , Sands G. , Dunlap A.
Published: February 2004

This study was to determine the effects of grain size on the switching properties of lithium ferrite and to establish the best grain size for a fast switching speed, with a minimum increase of coercive force. Lithium ferrite (Li0.5Fe2.5O4) powder prepared by four preparation techniques; flame spraying, commercial techniques, fluid-bed reaction, and Li2CO3+Fe2O3 mixture, was densified by hot pressing or conventional sintering. There was a variation in the grain growth and densification characteristics of the powders, but the magnetic properties were similar for a given grain size. The magnetic properties which were evaluated as a function of grain size were squareness, switching coefficient, coercive force, and switching time. These properties were evaluated for a 0.7 to 15.0 μ grain-size range in order to establish the effects of grain size on switching speed and coercive force. It is shown that samples with a reduced grain size have twice the switching speed of larger grain-size samples with little or no increase of coercive force.
Effects of Composition and Heat Treatment on the Stress Sensitivity and Magnetic Squareness Ratio of Molybdenum Permalloy Tape
Authors: Grupen W. B. , Chin G. Y.
Published: February 2004

Some present day memory applications, for example, the Piggyback Twistor,1 require a magnetic material that possesses a coercive force in the vicinity of 0.5 Oe, a low-stress sensitivity and a square hysteresis loop. A study was made of several modified molybdenum permalloys (79%–82% Ni, 12%–17% Fe, 4%–6% Mo) for possible use in these applications. Tapes having dimensions 3.2 by 0.26 mil were made from these alloys by roll flattening 1.1 mil diam wires that had been heat treated (600°–900°C, 3 sec) at a diameter of 2.2 mil. The roll-flattening operation results in a widening of the tape without appreciable elongation. The stress sensitivity of these tapes, as determined by changes in 60 Hz coercive force and squareness ratio with tension, was found to depend on crystallographic texture and on nickel composition. The zero-stress-sensitivity composition increased from about 79% to about 80.5% Ni when the as-flattened tape was annealed (700°–900°C, 15 sec) to reduce the coercive force to the desired value. X-ray examination showed that the tape texture changes from a cold-worked {110} 〈11̄1〉 texture to a recrystallized {001} 〈100〉 texture. The {001} 〈100〉 intensity is influenced by the 2.2 mil wire heat-treatment temperature; a maximum was observed for 650°C. This treatment also corresponds to a maximum in the squareness ratio of the tape. Detailed studies of the strain-insensitive alloys, which contain 80.5% Ni, showed that the squareness ratio increased with molybdenum content for tapes which had received the same processing treatment and, hence, had the same {001} 〈100〉 texture. These observations are consistent with published data, indicating that for the investigated alloys, the first magneto-crystalline anisotropy constant is positive and increases in magnitude with molybdenum additions. In addition, a 6% Mo-80.5% Ni-13.5% Fe alloy was processed to tape of 2.4 by 0.5 mil cross section, by flat-drawing 2.45-mil wire through a diamond die. The flat-drawing process results in an elongation of the tape without appreciable widening. The as-drawn texture consists of {110} 〈11̄2〉+{110} 〈001〉+{112} 〈111̄〉 components, and transforms to a sharp {110} 〈001〉+{210} 〈001〉 texture upon annealing. The large 〈001〉 intensity along the tape axis resulted in an extremely square hysteresis loop (Br/Bs∼1.0).
Origin of Threshold-Response Delay in Ferrite Memory Cores
Authors: Kmetz A. R. , Barker R. C.
Published: February 2004

Very slow low-level switching, recently predicted to explain the anomalous delay in threshold response of MgMnZn-ferrite memory cores, has been observed. Switching is approximately logarithmic with time over seven decades. Field and temperature dependences are given. The apparent absence of disaccommodation and of detailed history dependence indicate that this aftereffect has its origin in thermal fluctuations rather than diffusion. When aftereffect switches a critical amount of flux, near 1% of remanence, a spontaneous transition occurs to a mode of switching with a wall area-mobility product 104 larger. A model which relates this transition to the effects of surface volatilization of Zn during firing is proposed.
Dilute 57Fe Mössbauer Studies in Cu–Ni Alloys
Authors: Swartzendruber L. J. , Bennett L. H. , Watson R. E.
Published: February 2004

The 57Fe Mössbauer effect has been used to study the properties of iron as a dilute impurity in Cu–Ni alloys. Using either 0.5 at.% Fe as an absorber or 57Co as a source in Cu–Ni alloys, a doublet is observed. This is presumed to be a small, partially resolved magnetic hyperfine field (based on evidence presented elsewhere) and we examine the implications of this for Cu–Ni alloys which are normally considered nonmagnetic. Previously, the hyperfine field at 4°K was reported as undergoing a rapid decrease in value near the equiatomic composition: from 243 kG at 55 at.% Ni to 0 kG at 45 at.% Ni (both alloys are ferromagnetic at 4°K). The present work extends this study and finds that the magnetic field drops instead to the small value of 10 kG. This field is almost independent of Ni concentrations between 10 and 45% Ni (i.e., in both ferromagnetic and paramagnetic regions). It is temperature independent between 4.2° and 198°K, and is the same for either the source or absorber (0.5%). The Mössbauer results suggest an invartype effect occurs in this alloy system.
Optical Studies of Ho3+ Ions in YGaG and YIG
Authors: Johnson L. F. , Dillon J. F. , Remeika J. P.
Published: February 2004

Optical absorption and emission spectra are used to determine the energy levels of the 5I8 ground state and 5I7 excited state of Ho3+ in diamagnetic yttrium gallium garnet (YGaG) and ferrimagnetic yttrium iron garnet (YIG). The results for YGaG are consistent with the D2 site symmetry of the dodecahedral site in the garnet structure. Energy levels in YIG that are separated by site inequivalency are identified from spectra observed as the Fe3+ magnetization is rotated in the (11̄0) plane. Stimulated emission is observed from Ho3+ in YGaG at 2.086 and 2.114 μ and from Ho3+ in YIG at 2.086, 2.089, and 2.107 μ. Each of the lines in YIG is composed of two superimposed electronic transitions. The exchange field in YIG makes possible magnetic tuning of coherent emission over a range of 70 Å. This paper is a brief description of work that will be treated in full elsewhere.
Spin Diffusion and Propagating Modes in Anisotropic Paramagnet
Authors: Tahir-Kheli Raza A.
Published: February 2004

Approximate nonlinear, integral equations for the spectral functions of the longitudinal and the transverse correlations are derived. From these, the rate at which local fluctuations in magnetization density diffuse is calculated for nearest-neighbor interactions in an anisotropic paramagnet, H=− ∑ i,jI0(ij)[SizSjz+γ(SixSjx+SiySjy)],with sc and bcc structures. At high temperatures, the diffusion constant is found to be D0=(13)γ2zI0a2[πs(s+1)/3z(1+γ2)]1/2,where a is the nearest-neighbor distance and z is the coordination number. For the isotropic case, i.e., γ=1, the structure of the spectral function is examined for the existence of propagating modes. It is concluded that at high temperatures i.e., βJ(0)≪1, no observable propagating modes with adequate thermodynamic weight exist. It turns out, however, that near Tc for Ka>[χ(0, 0)]−1/2, propagating modes, EK, with adequate thermodynamic weight are possible. One finds EK∼(Ka)5/2 and D0K2∼χ−1/4(0, 0) (Ka)2.
Angular Dependence of the Ferromagnetic-Resonance Field and Linewidth in Metal Plates
Authors: Vittoria C. , Barker R. C. , Yelon A.
Published: February 2004

We have calculated the 25 GHz ferromagnetic-resonance field and linewidth for a metal plate, whose properties correspond to those of Ni, with its static magnetization at an arbitrary direction with respect to the plane of the plate, and with an rf field applied at normal incidence. We assume an isotropic Landau-Lifshitz damping parameter, metallic conductivity, and either full pinning or no pinning of the magnetization at the surface. We find that the resonance linewidth varies strongly with angle, and that the value of linewidth and its angular dependence are strong functions both of plate thickness and of pinning condition. The shift in resonant field from its value for an insulator also depends strongly upon these parameters, but the angular dependence of the field is relatively small. The results, based on the isotropic Landau-Lifshitz relaxation model, do not account for the linewidth anisotropy observed in single-crystal nickel platelets.
Theory and Method for Magnetic Resonance Measurements
Authors: Soohoo R. F. , Christensen P.
Published: February 2004

The boundary-value problems involving a usual spherical ferromagnetic sample and commonly used cavity geometries are virtually impossible to solve so that perturbation theory must be used to interpret the results of resonance measurements. However, we have succeeded in solving the problem of a spherical sample in a spherical cavity under the assumption of uniform and circularly polarized magnetic field within the sample. Using the results of this calculation, it is possible to measure all components of the intrinsic susceptibility tensor of the medium. Furthermore, they enable the determination of the limits of validity of perturbation theory with regard to sample size, dielectric constant, and susceptibility. The calculation is self-consistent in that uniform and circularly polarized magnetic field within the sample is assumed and verified for a certain range of parameters by results of the calculation itself. Dimensional resonance, characterized by cavity-like resonances within the sample, is found to occur if the sample has a high susceptibility. In the case of ferromagnetic materials with small linewidths, this dimensional-resonance effect can be sufficiently serious as to render the usual interpretation via perturbation theory completely invalid. A measurement technique for determining the magnetic properties of a sample by use of our theory is described. It has been found that use of our solution makes it possible to relax the restriction of very small sample size as implied by perturbation theory. The use of a larger sample is highly desirable, since the change in frequency and Q would be greater, and less refined instruments would be required for making the measurements.
Two-Magnon Scattering Amplitudes for Pits of Arbitrary Shape
Authors: Sage Jay P.
Published: February 2004

The quantum mechanical technique of Sparks, Loudon, and Kittel for determining the two-magnon scattering amplitudes F(k, k′) in H = ΣF (k, k′) bk+bk′ induced by a spherical pit in an infinite crystal, is extended to pits of arbitrary shape. By expressing the energy in terms of a charge-charge interaction rather than as a field-moment interaction, the selection of relevant terms and the inherent symmetries are much clearer. Two terms which are linear in pit distribution can be solved exactly and explicitly for any collection of pits of arbitrary shapes and sizes. For scattering from the uniform precession the result is F(0,k)=(2πMγℏ/V)[ ∫ VPexp(ik·r)dτ](3 cos2θk−1),and reduces to the Sparks et al. result for a single sphere. The integral has a simple interpretation in terms of Huygens wavelets from each missing spin which interfere according to the phase exp(ik·r). A third term which is not linear in pit distribution can be solved explicitly only for a general ellipsoidal pit. It reduces to a constant addition of between −2 and +1 to the 3 cos2Θk−1 factor. For a spherical pit it vanishes. An explicit solution for scattering from modes other than the uniform precession can also be obtained.
Raman Scattering by Magnetic Excitations in RbNiF3
Authors: Chinn S. R. , Zeiger H. J. , O'Connor J. R.
Published: February 2004

An investigation of Raman scattering of argon laser light (λ=5145 Å) from the hexagonal ferrimagnet RbNiF3 revealed at T≅15°K an intense, broad, symmetrical line shifted ∼510 cm−1 from the laser excitation energy. At higher temperatures as its amplitude decreased, the line broadened and its shift decreased. It was still intense at the Curie temperature (139°K), and remained observable to temperatures near 200°K. This line, assumed to arise from magnetic excitations, is best described in the ordered state as due to two magnon scattering, and at Tc and above due to scattering from a cluster. From the computed magnon modes and energies, optical absorption data, and susceptibility measurements, we determine a set of exchange constants for RbNiF3. These constants are then used to relate computed magnetic properties with the results of NMR and other measurements. Comparison of the Raman shifts with the predictions of a molecular field model are used to obtain estimates of the short-range order persisting above Tc.
Lorentz Microscopy of Recorded Magnetization Patterns
Authors: Speliotis D. E. , Morrison J. R.
Published: February 2004

We have used the technique of transmission electron microscopy for the static observation of recorded magnetization patterns in high-coercivity films of cobalt and iron. Such magnetization patterns consist of a series of oppositely magnetized regions, and are characterized by the length of these magnetized regions and by the spatial variation of the magnetization in the transition regions in which the magnetization reverses from one direction to the opposite direction. Magnetic bit patterns of different wavelengths were recorded with conventional high-resolution transducers and by magnetic transfer from high-coercivity masters. The Foucault method using the objective aperture diaphragm was found particularly applicable for observing the oppositely magnetized regions, resulting in good contrast for wavelengths as small as 2 μ. For the observation of the magnetization transition regions, the defocus mode was found better applicable. Measurements of the length of the transition region by this method are in good agreement with our theoretical calculations using harmonic analysis to determine the resulting demagnetization.
Methods for Improving the Signal-to-Noise Ratio of Photon- and Electron-Beam-Accessed Magnetic-Film Memory Systems
Authors: Smith D. O.
Published: February 2004

Methods are described for reducing shot-noise in magnetic-film memories which are to be read magneto-optically with the auxiliary aid of an electron beam. In such memories the principal source of shot noise is from the light which is used to illuminate the memory array. The following methods of reducing array shot noise are considered: (1) selective background, (2) magneto-optical balance, (3) temperature control of magneto-optical spectra in rare-earth iron garnets (REIG). It is concluded that the use of REIG spectra appears to offer not only the best, but also a satisfactory solution to the array shot-noise problem.
Theory of Localized Magnons and Magnon-Exciton Interactions in Antiferromagnetic Perovskites
Authors: Parkinson J. B.
Published: February 2004

The antiferromagnetic perovskites have a simple cubic-magnetic structure with nearest-neighbor exchange interactions only. The perfect-crystal Green functions can be easily expressed in terms of tabulated integrals, so that they are particularly suitable for making quantitative comparison between theory and experiment. This is done for exciton-magnon interactions in pure crystals, and for magnons associated with impurities. Electric-dipole absorption in the pure crystal can occur when an exciton is created on one magnetic ion and a spin deviation on its nearest neighbor. The resulting magnon sideband reflects the interaction between the exciton and magnon which are in close proximity. If the exchange between a pair, when one is electronically excited is J′, and the normal exchange is J, then satisfactory agreement with the experimental lineshape can be obtained for J′=0.5J in RbMnF3. The positions of localized magnons are calculated as a function of impurity-host exchange, and compared with experimental results on Ni2+ in RbMnF3. The symmetry of the defect system is also considered, showing that additional terms are allowed. The most important of these is probably electric-dipole absorption in the far infrared, creating two localized magnons.
Elasto-Optic Properties of Bi12GeO20, Bi12SiO20, and SrxBa1−xNb2O6
Authors: Venturini E. L. , Spencer E. G. , Ballman A. A.
Published: February 2004

The elasto-optic matrix elements for longitudinal waves in four new single crystals have been measured. Piezoelectric bismuth silicon oxide and bismuth germanium oxide, cubic point group 23, show an elasto-optic strength of 6.0 and 6.6, respectively, relative to fused silica based on an equal acoustic power criterion discussed below. Ferroelectric crystals from the strontium barium niobate system, tetragonal point group 4mm, were also studied. One with a Sr to Ba ratio of 3:1 showed an elasto-optic strength of 26; another with a ratio of 1:1 had a strength of 5.8.
Temperature Coefficients of the Work Functions of the (110) and (100) Surfaces of Tungsten Single Crystals and of Polycrystalline Tungsten Foil
Authors: Hopkins B. J. , Lee T. J. , Williams C. B.
Published: February 2004

A modification of the Kelvin contact potential difference technique has been used to measure the temperature coefficient of the work function of three tungsten surfaces over the temperature range from 77°–1300°K. Two of the surfaces were polished and etched single-crystal planes having the (110) and (100) orientations, the third was a sample of polycrystalline foil. The results obtained were foil(11±1)×10−5 eV/∘K(110)(6.3±0.6)×10−5 eV/∘K(100)(9.1±0.4)×10−5 eV/∘Kand these are in qualitative agreement with predictions based on the temperature dependence of the electron smoothing component of the surface double layer.
Normal and Anomalous Dember Effect
Authors: Chattopadhyaya S. K. , Mathur V. K.
Published: February 2004

Using Gärtner's equations, effect of sample thickness on the Cember potential (VD) for strong and weak absorption is discussed. It has been found that for strong absorption, as the thickness of the sample increases, VD tends to a saturation value which decreases as the front-surface recombination velocity increases. For weak absorption, initially the behavior of VD depends upon front and back surface recombination velocities, but for large thickness it becomes independent of them and increases linearly with thickness. When front-surface recombination velocity is large as compared to back surface recombination velocity, a sign reversal can take place. Expressions are obtained for VD(i) when surface recombination is dominant, or (ii) when bulk recombination is dominant, for weak and strong absorption in the case of thin and thick specimens. A physical explanation of anomalous Dember effect is given by considering the contribution of the concentration gradients set up by front, back, and bulk recombinations. It appears that simultaneous measurements of Hall angle and ratio of PME and Dember potential can be helpful in measuring electron-hole mobility ratio.
Time Behavior of Laser Modes in GaAs Platelet Lasers
Authors: Keune D. L. , Rossi J. A. , Holonyak N. , Dapkus P. D.
Published: February 2004

Uniformly doped GaAs platelet lasers (77°K) are studied on a time basis and are shown to lase without significant delay. Data are presented showing that the turn-on time and wavelength of longer wavelength laser modes are controlled by electron-hole pair density and not the usual impurity effects.
Computer Solutions of Lorentz-Field Amplification in a Bounded Semiconductor Medium
Authors: Golden J. R. , Chang K. K. N.
Published: February 2004

A Lorentz field is generated when moving free carriers in semiconductors interact with a crossed magnetic field. A set of computer results show a strong interaction with high-order transverse modes in a parallelplate wave guide. As a result of this interaction, unstable solutions which lead to the generation of growing waves have been found.
Estimation of Electronic Influences on Vibrational Entropy in Dilute-Solid-Solution Alloys
Authors: Passoja D. E. , Chao K. A. , Ansell G. S.
Published: February 2004

The excess entropy measured in dilute solid solutions of Al–Ag, Al–Zn, and Pb–Tl is compared to a theoretical vibrational entropy model. This theoretical model is based upon the localized influences which solute atoms produce in dilute solid solutions. The excess entropy, of vibrational origin, is found to arise principally from the mass and valence differences between solute and solvent atoms. Use of various types of potentials is made to calculate the ``force constants'' between the impurity atoms and the nearest-neighbor atoms in the solid solutions.
Superconducting Studies in A-15 Pseudo-Binary Systems
Authors: Blaugher R. D. , Pessall N. , Patterson A.
Published: February 2004

Superconductivity studies have been performed on some A-15 pseudo-binary systems to determine whether low-temperature annealing, conducive to atomic ordering, will markedly raise the superconducting transition temperature. The following systems were examined for this behavior: Nb3Ge–Nb3Al, Nb3Sn–Nb3Al, and Nb3Ga–Nb3Al. Greater attention was devoted to the Nb–Al–Ge system in view of the recent observation of superconductivity above 20°K in this system. We have also observed transition temperatures approaching 20°K in nearly single phase samples of Nb–Al–Ge after low-temperature heat treatment at 700° and 800°C. It is suggested that crystallographic long-range ordering is responsible for the high Tc's observed in this system. Attempts to confirm this hypothesis by determining the degree of long-range order were unsuccessful. The other A-15 systems which we investigated also exhibited an increase of Tc with low-temperature heat treatment; however, none exceeded 18°K.
Prediction of Transient Stability Limits for Composite Superconductors Subject to Flux Jumping
Authors: Iwasa Y. , Weggel C. , Montgomery D. B. , Weggel R. , Hale J. R.
Published: February 2004

A computer model is described which is capable of predicting the maximum current, the ``threshold current'', which can be carried without quenching a Nb–Ti copper composite superconductor undergoing flux jumping. The model utilizes standard equations governing the generation and flow of heat in a wire to predict its voltage-versus-time response as a function of current. Calibration of the model for any specific thermal environment is done by means of a simple test procedure involving measurement of the response following the triggering of a flux jump in a small noninductively wound solenoid of one to four layers. The combination of theoretical and experimental techniques indicates the value of adequate copper cross section and good thermal environment, and suggests the superiority of porous fabric or cadmium foil over more conventional interleaving materials, and of thin insulation over the alternatives of thick insulation or none at all.
Steady-State Flux Jumping in Superconducting Niobium Titanium Tubes in Superimposed ac and dc Magnetic Fields
Authors: Gandolfo David A. , Dubeck Leroy , Rothwarf Frederick
Published: February 2004

Steady-state flux jumping has been observed for a thin-walled tubular sample of NbTi subjected to coaxial superimposed ac and dc magnetic fields. By steady state we mean flux jumping which occurs reproducibly each cycle. The magnitude Δh of the field shielded by a sample in the critical state decreases with increasing field rate. A rate-dependent effective resistivity ρeff that exists just prior to a given flux jump has been calculated from Δh and the field rate. It is found that ρeff increases nonlinearly to a value of about 5×10−9 Ω·cm at h = 7×105 Oe/sec in a background field of 10 kOe. Such a ρeff is characteristic of the flux-creep state.
Magnetization Measurements in Nb–Ti Sheets
Authors: Taylor Clyde E.
Published: February 2004

It may be possible to achieve stable confinement of large dc levitated superconducting rings for controlled thermonuclear research by surrounding the ring with superconducting walls. The requirements for such confining walls are given in terms of calculated field strength and surface current density for a superconducting ring between two flat superconducting plates. The ``spring constant'' of such a magnetic suspension has been calculated as a function of geometry. Flux penetration data have been obtained for 0.005-in.-thick Nb–Ti sheets in several layers and are compared with the requirements for design of stabilizing walls.
Fabrication of Tunnel Junctions on Niobium Films
Authors: Mullen L. O. , Sullivan D. B.
Published: February 2004

High background currents, often observed in tunnel junctions when the barrier is formed by oxidizing niobium, probably stem from the fact that one oxide of niobium (NbO) is not an insulator. The fabrication process described in this paper uses an active layer of gas adsorbed on the niobium surface that reacts with the upper film (e.g., lead) and forms the junction barrier. This technique avoids the problem above and results in tunnel junctions with low background currents.