bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of Anomalous Absorption Edge Shifts in Chromium Chalcogenide Spinels
Authors: Berger S. B.
Published: February 2004

The temperature dependencies of the optical absorption edge of magnetic chromium chalcogenide spinels have been reported.1 CdCr2S4 exhibits a blue shift while HgCr2S4 and CdCr2Se4 exhibit a red shift with decreasing temperature, especially near the magnetic ordering point. These shifts have been related to magnetic ordering. Several theoretical models for these shifts have been offered, but the opposite senses of the observed shifts has been difficult to explain. In an attempt to resolve this problem we have investigated the systems Cd1−xHgCr2S4 and CdCr2S4−xSex by diffuse reflection spectroscopy as a function of temperature. We observe a compositional dependence which we interpret using a two-transition model. One transition involves energy levels which increase in separation with decreasing temperature. This is the usual case of semiconductors and is related to phonon effects. Cr3+ crystal field transitions whose energies are relatively temperature and composition independent are seen as the temperature is decreased and the absorption edge shifts to the higher energies. The other important transition involves energy levels which decrease in separation (red shift) with decreasing temperature, particularly, near the magnetic ordering point. The energy of the magnetically shifting absorption is strongly composition dependent. The composition dependence indicates a transition involving similar initial and final states for the end members of these series. We conclude that the origin of the red shifting absorption in all chromium chalcogenide spinels is the same. Furthermore, we see no blue shift related to magnetic order in our data. This model is supported by our recent data2 on single-crystal films of CdCr2S4.
Measurement of Substrate Constraint Contributions to Magnetic Anisotropy in Thin Films of Ni–Fe and Ni–Co Alloys
Authors: Brownlow L. W. , Wilts C. H.
Published: February 2004

A method is described for the evaporation of ferromagnetic films onto a very thin water soluble polymer coating on glass substrates and their subsequent removal onto a liquid glycerin film. Determination of the change in anisotropy on removal provides a measure of the substrate constraint contribution to the anisotropy energy. Data are given for Fe–Ni alloys and Co–Ni alloys in the entire range 0% to 100% Ni. Significant changes in anisotropy upon removal were observed for almost all Ni–Fe alloys and for Co–Ni in the range 0% to 40% Co. However, essentially a zero percentage change was observed for pure Fe, 83% Ni–Fe, and the entire range 0% to 60% Ni in the Co–Ni alloys. These data are in disagreement with present theories of the constraint energy over large segments of the composition ranges.
Oblique Incidence Evaporated Iron Films
Authors: Welch R. H. , Speliotis D. E.
Published: February 2004

Magnetic hardness in thin metallic films has been sought and achieved by a number of different techniques, including oblique incidence evaporation in a vacuum. Previous studies of oblique incidence evaporation of Permalloy and iron have shown that the easy axis rotates from a direction transverse to the plane of incidence to a direction parallel to the plane of incidence, at an angle of deposition θ=65° with respect to the substrate normal. In this paper we studied the dependence of coercivity, squareness, and uniaxial anisotropy of iron films on thickness (100–1500 Å), with the angle of incidence of the vapor flux as a parameter (θ=40°–85°). We found that at all angles of incidence studied, the coercivity increases with increasing thickness, reaching a peak at about 400 Å, and decreasing monotonically for larger thickness. The uniaxial anisotropy in the transverse direction reaches a peak for θ=55°, while for θ>65° the uniaxial anisotropy in the parallel direction predominates, and increases rapidly with increasing θ. The difference between the coercivities in the transverse and parallel directions decreases with increasing thickness for θ<65° and is independent of thickness for θ>65°. The results are interpreted on the basis of chain and columnar particle growth proposed by previous authors.
Coupling Effects Between Films of Permalloy and EuO
Authors: Almasi G. S. , Ahn K. Y.
Published: February 2004

The magneto-optic Faraday effect was used to study films of permalloy (435 Å) and EuO (350–1400 Å) in intimate contact. The two films exhibit negative (antiparallel) coupling, that is, more external field is required to align the two magnetizations parallel than antiparallel. This causes the minor loop of permalloy to shift by as much as 35 Oe at 10°K. The shift is independent of EuO thickness, indicating an interfacial effect. The coupling energy is 0.13 erg/cm2, about half the value for coupled films of Fe and EuO (0.22–0.25 erg/cm2), but since the magnetization of permalloy is also half that of Fe, interface conditions for the two sets of coupled films appear to be quite similar. To establish the nature of the coupling, thin dielectric layers (Eu2O3, 11–44 Å) were introduced between the EuO and the permalloy. A 22-Å intermediate layer reduced the coupling energy tenfold. This indicates that the negative coupling between the films is due to exchange interaction. Negative exchange coupling can predict the switching behavior of composite permalloy-EuS films as well. However, the model previously used for permalloy-EuS films, involving positive exchange coupling and a quasi Bloch wall between the films, cannot explain the results of this paper.
Relationship Between Stoichiometry and Properties of EuO Films
Authors: Ahn K. Y. , Shafer M. W.
Published: February 2004

The preparation of EuO films by co-evaporation of Eu and Eu2O3 has been studied. The two most important parameters to control are the ratio r=Eu/Eu2O3 and the background pressure. The latter should be in the 10−6-Torr range, and only by starting with high density (>95%) Eu2O3 is it possible to maintain this. As r increases from 0.8 to 1.4 we observe the following: (1) A large increase in Tc is observed from 51° to 117°K; (2) the absorption a increases from 0.85×105/cm to 1.6×105/cm and the peak shifts toward the red (from 5900 to 6000 Å); (3) the longitudinal Faraday rotation 2φ (with 25° incident angle at 6328 Å) increases from 0.9×105 deg/cm to a maximum of 3.7×105 deg/cm, followed by a slight decrease; and (4) Hc (10°K) decreases sharply from 140 Oe to a minimum of 43 Oe for r=0.94, followed by a slight increase. The best films which were single-phase stoichiometric EuO, with bulk-like properties, are obtained with r=0.94. These have well-defined x-ray diffraction patterns with lattice constants a0=5.14 Å, Tc of 71°K, and the normal Faraday rotation (at 20°K with H=20 kOe) of 8.5×105 deg/cm. In films with r>0.9 the increase of Tc is attributed to the presence of Eu-metal which contributes conduction electrons to enhance the Eu2+–Eu2+ interaction. For r<0.9, films with a0=5.10 are obtained which have Tc=51°K. We propose a model where the Eu2+–Eu2+ exchange interaction is weakened by the formation of a metastable solution of Eu3+ in the EuO lattice. These new results will be compared with previously published data.
Magnetic Equation of State Near the Critical Point of Ferrimagnets and Canted Ferromagnets
Authors: Ohbayashi Kohji , Iida Shuichi
Published: February 2004

The magnetic equation of state of single-crystal ferrimagnets (YIG and MnFe2O4) and a canted ferromagnet (YFeO3) were studied experimentally. Critical indices observed for the former two cubic ferrimagnets were γ=1.32 and 1.33±0.04, β=0.35±0.02, and δ=4.6±0.2, which are nearly the same as those of ferromagnets and satisfy the scaling law prediction γ=β(δ-1). For the latter canted noncubic ferromagnet, a difference was found between the observations along different crystal axes. Along the z axis, we found that the magnetization, after subtracting the antiferromagnetic contribution from the observed magnetization, does follow the scaling law. The critical indices obtained were γ=1.9±0.2, β=0.47±0 05, and δ=5.1±0.5. Along the x axis, which is parallel to the spin axis of the antiferromagnet, complex dependence of the magnetization upon the applied magnetic field appears near the critical temperature. This phenomena can be explained by the field-induced change of the spin axis from the x axis to the z axis near the Curie point, and is quite similar to the case of Iwahashi's induced canted ferromagnetism.
Kink-Point Locus and Critical Phenomena of YIG and Nickel
Authors: Miyatani Kazuo , Yoshikawa Kazufumi
Published: February 2004

The critical point phenomena of single-crystal YIG have been studied and analyzed. A very sharply defined spontaneous magnetization has been observed from the Curie temperature down to 6° below Tc. The spontaneous magnetization has not only been directly determined by the kink-point locus, but also by a σ2 vs Heff/σ plot without any extrapolation. Taking this opportunity for checking the validity of theoretical predictions and conclusions on magnetic critical point phenomena, we have thus directly obtained the form of divergence of the spontaneous magnetization σ0, the initial susceptibility χ0, the critical isotherm σ(H)Tc and Tc, as follows: σ0(T)=160·εβ,β=0.380±0.005,χ0(T)=8.0×10−6·ε−γ,γ=1.312±0.008,σ(H)Tc=3.75·H1/δ,δ=4.42±0.05.From these results, we have concluded that the scaling law prediction γ=β(δ-1) is valid and that the magnetic equation of state is also valid. In addition, a new form for the general homogeneous function in the magnetic equation of state has been found. Replotting the data according to this new function yields a single straight line for all points both above and below Tc.
Magnetic Short-Range Order in VF2
Authors: Child H. R. , Koehler W. C. , Lau H. Y. , Stout J. W.
Published: February 2004

Body-centered tetragonal VF2 has a transition temperature at 7°K to a spiral structure in which the spins are perpendicular to the c axis and spiral along this axis with a turn angle of 96 deg/layer.1 Magnetic correlations which persist to high temperatures have been interpreted2 as evidence for relatively strong interactions between V+2 ions spaced along the c axis but weak interactions between these chains. We have measured the magnetic incoherent neutron scattering from VF2 powder and find that significant magnetic correlations persist to temperatures about 25 times the Néel temperature. Least-squares fitting of the data to a general formula for magnetic short-range order scattering was used to determine the correlation coefficients as a function of temperature. A model assuming uniaxial magnetic correlations fits the data over the temperature range studied but allowing some correlations between different chains could perhaps improve the fit at very low temperatures. Thus, VF2 appears to exhibit properties characteristic of one-dimensional ordering.
Critical Exponent β for the Magnetization of YIG
Authors: Anderson Elmer E. , Munson H. J. , Arajs Sigurds , Stelmach A. A. , Tehan B. L.
Published: February 2004

Recently Wojtowicz and Rayl suggested a method for determining β, the exponent describing the behavior of the magnetization below the critical point. We have applied this method to the study of YIG in low magnetic fields ranging from 5 to 350 Oe. The transition temperature was observed to decrease linearly with the square of the applied field (as predicted by the theory) for fields up to 50 Oe. Our data are best interpreted by proposing two values of β, namely, β=0.47 close to Tc (within 1°K) and β=0.36 for the temperature region from 1° to 120°K below Tc. Similar behavior has been reported for nickel and YFeO3. Our results add further support to the suggestion of Callen and Callen that values of β∼⅓ are not truly properties of the phase transition itself.
Eigenvalue Degeneracy as a Possible ``Mathematical Mechanism'' for Phase Transitions
Authors: Stanley H. Eugene , Blume Martin , Matsuno Koichiro , Milošević Sava
Published: February 2004

Some years ago Ashkin and Lamb noted that the phase transition in the two-dimensional Ising model with nearest-neighbor interaction was characterized mathematically by an asymptotic degeneracy of the largest eigenvalue of a linear operator. More recently Kac and Thompson showed this eigenvalue degeneracy also characterized the phase transition in the Kac model (with weak, long-range forces), suggesting that this ``mathematical mechanism'' is not restricted to systems with short-range forces. However both the Kac model and the Ising model consider the ``spins'' to be one-dimensional unit vectors assuming only the discrete values +1 and −1. We are therefore led to consider the nature of the phase transition in one of the few exactly soluble models in which the spins can assume an entire continuum of orientations-the closed linear chain of classical spins of arbitrary spin dimensionality interacting isotropically through the Hamiltonian Hν=− ∑ i=1N Ji,i+1Si(ν)·Si+1(ν),where the exchange constants Ji,i+1 are arbitrary numbers. We find for all spin dimensionalities ν that the two-spin correlation function may be expressed as ρN(ν) (γ) = (λ1(ν)/λ0(ν))r, where λ0(ν) and λ1(ν) are the largest and next-largest eigenvalues of a certain linear operator. Thus the onset of longrange order, limr→∞ρN(ν) (γ)≠0, is characterized by the degeneracy of λ0(ν) and λ1(ν). This onset of long-range order occurs at a critical temperature Tc(ν)=0, for all values of spin dimensionality ν.
Single-Crystal Magnetic Materials
Authors: Holtzberg F.
Published: February 2004

Single-crystal growth techniques are discussed in terms of their dependence on phase equilibrium, thermal stability of the desired compound, and considerations of the vapor species during the crystallization process. Ferrites, orthoferrites, garnets, chalcogenide spinels, and europium chalcogenide systems are used to illustrate some of the special problems in crystal growth.
Annealing Kinetics of the Induced Anisotropy in Ni–Fe Films: The Role of Atom Self-Diffusion in Grain Surface Regions
Authors: Roth Michel
Published: February 2004

A theory is given which seems to explain the main aspects of the annealing properties of the induced anisotropy in thin Permalloy films. The theory concerns the main anisotropy component which is very likely due to directional atom pair ordering only (the other being of magnetostrictive origin). The theory is based on the high atom mobility existing usually, in thermal equilibrium conditions, in the vicinity of the grain boundaries and grain surfaces. A kinetic law is derived for the case of annealing not producing grain growth. The phenomenon involved is in this case the evolution of the directional atom pair order in the vicinity of immobile grain boundaries and surfaces. A good agreement is found with experimental data. A second important phenomenon is the effect of the grain boundary migration during grain growth on the induced anisotropy of the film regions swept by the boundaries. This effect has been also computed in a more qualitative way and seems able to explain the irreversible anisotropy evolution observed after many annealings. Lastly the anisotropy energy measured after film deposition may also be derived, but in this case the theory needs still further refinements to give more satisfactory results.
Resonance in Ferrimagnets Containing Paramagnetic Ions with Near Crossing Energy Levels
Authors: Gurevich A. G. , Ageev A. N. , Klinger M. I.
Published: February 2004

Some new experimental results concerning the magnetic resonance in Pr3+-doped YIG are presented and discussed, together with the results for Tb3+ and Ho3+ in YIG already published, in terms of existing theories. It is shown that the magnetization directions corresponding to the near crossings of energy levels in the case of Ho3+ can lie within a certain local plane and, in the case of Pr3+, can coincide with a local axis of the dodecahedral site (for Tb3+, as Huber has found, these directions form a conical surface). The angle and temperature dependences of ΔH for Tb3+ and Ho3+ are very similar and can be explained completely in terms of slow relaxation. For Pr3+ these dependences in the directions of near crossings resemble the ``anomalous'' behavior for Yb3+ in octahedral sites. A contribution from transverse relaxation is probable in the case of Pr3+.
Phase Transitions and Magnetic Correlations in Two-Dimensional Antiferromagnets
Authors: Birgeneau R. J. , Skalyo J. , Shirane G.
Published: February 2004

Recent neutron scattering work on the planar antiferromagnet K2NiF4 has indicated that this system undergoes a true two-dimensional antiferromagnetic phase transition. In this paper we review these studies together with measurements of the magnons at low temperatures and in the vicinity of TN. New high precision measurements of the sublattice magnetization around TN are presented. It is shown that the power law M(T)/M(4.2)=0.973(1−T/92.23)0.138±0.004holds for 2×10−1 > 1-T/TN > 3×10−4. Preliminary measurements of the wavevector-dependent staggered susceptibility χαβ (q) in K2NiF4 show that only χzz, the component parallel to the anisotropy axis, has pronounced diffusive character, diverging at TN. This provides definitive proof that the phase transition is anisotropy-induced.
Magnetic and Structural Properties of Some Rare-Earth-Sn3 Compounds
Authors: Shenoy G. K. , Dunlap B. D. , Kalvius G. M. , Toxen A. M. , Gambino R. J.
Published: February 2004

The 23.8-keV Mössbauer resonance of 119Sn and susceptibility measurements have been used to study the structural and magnetic properties of CeSn3, PrSn3, and NdSn3 intermetallic compounds between 293° and 1.6°K. The deterioration of the powder samples, which yields free tin and has led to inconsistent results in the past, is clearly demonstrated in these spectra. Both PrSn3 and NdSn3 exhibit magnetic hyperfine (hf) spectra below their Néel temperatures of 8.6° and 4.7°K, respectively. In PrSn3 one-third of the Sn nuclei experience a transferred hf field of 67 kOe with the spin direction parallel to the principal electric-field-gradient axis while the remaining Sn nuclei display no magnetic splitting. This is interpreted as the result of antiferromagnetic ordering of the first kind. In NdSn3 the magnetic ordering appears to be more complex. In CeSn3 neither magnetic ordering nor the ``incipient ferromagnetism'' previously suggested is seen. The size of the quadrupole interaction and the isomer shift are nearly equal in all these compounds.
Effects of Atomic Configurational Changes on Hyperfine Interactions in Concentrated Iron-Cobalt Alloys
Authors: deMayo B. , Forester D. W. , Spooner S.
Published: February 2004

Room temperature 57Fe Mössbauer spectra were taken on atomically ordered and disordered iron-cobalt samples as a function of composition between 25 and 75 at.% cobalt. The degree of long-range order was determined in selected samples using neutron diffraction. The maximum values of the effective field (He) and isomer shift (I.S.) occur at 25 at.% cobalt where He=−365.5±0.5 kOe and I.S. = +0.037±0.005 mm/sec (relative to pure iron). In the composition range studied increasing the number of cobalt neighbors to an iron atom either by ordering or by increasing the cobalt concentration decreases both I.S. and He. The maximum decrease with ordering of I.S., He, and the line widths occurs at 50 at.% cobalt, where the decrease is about 0.02 mm/sec for I.S. and 10 kOe for He. These results indicate that local atomic configurations play a significant role in the competition between positive and negative contributions to He. The data suggest a positive spin polarization contribution at the nucleus.
Model for the Composition-Dependent Ferromagnetic to Antiferromagnetic Transition in Fe2Ti
Authors: Wertheim G. K. , Wernick J. H. , Sherwood R. C.
Published: February 2004

Mössbauer effect and magnetic susceptibility measurements on single-phase Fe2+xTi1−x confirm the finding of Nakamichi that the bulk magnetic properties are very sensitive to deviations from stoichiometry. Well below the Néel temperature antiferromagnetic titanium-rich material exhibits two narrow-line 57Fe Mössbauer spectra coresponding to the 2a and 6h sites. Ferromagnetic iron-rich material has additional components and a greatly broadened spectrum. This indicates that Fe2Ti is basically antiferromagnetic and suggests that the ferromagnetism arises from iron atoms in titanium sites which serve as nuclei for ferromagnetic clusters. These account quantitatively not only for the details of the Mössbauer spectra of iron-rich material, but also for the saturation magnetization and other features of the susceptibility data.
Al Hyperfine Fields in Ordered Fe–Al Alloys
Authors: Burch T. J. , Murphy J. J. , Budnick J. I. , Skalski S.
Published: February 2004

In the Fe–Al system a number of ordered phases are possible for compositions between 10 and 30 at.% Al. Since the aluminum site is essentially nonmagnetic, measurements of the hyperfine interaction ought to yield sensitive and detailed information about the conduction electron polarization and its dependence on long-range crystallographic order. We have studied the Al resonance in alloys containing between 14 and 28 at.% Al by NMR spin-echo spectroscopy at 4.2°K. For all compositions, the spectra of alloys which have been subjected to cold work are similar and show a single broad asymmetric distribution with a maximum at about 58 MHz which extends to 30 MHz on the low-frequency side for the higher concentrations. In the 23 to 28 at.% Al region the spectra of annealed alloys are very different having strong sharp lines at 29 and 33 MHz and a broad distribution around 58 MHz which decreases in amplitude with increasing Al content. These spectra seem to be characteristic of Fe3Al order. Since Al sites with 8 Fe nn should be in the majority, it appears that there is a large shift in the Al resonance when the alloy orders similar to the shift in the Si resonance in the Fe–Si system.
Domain Wall Structure and Magnetization Creep in Ni–Fe Films 1200–2000 Å Thick
Authors: Torok E. J. , Oredson H. N. , Simon W. J.
Published: February 2004

The wall structure and magnetization-creep behavior of homogeneous polycrystalline films 1200–2000-Å thick were examined by Lorentz microscopy. The films were then annealed in the electron microscope to enlarge the crystallites and increase the inhomogeneity. Both the wall structure and the creep process are radically different in the two cases. Homogeneous films thicker than ∼1000 Å have uniform, cross tie-free walls, but the annealed films have very pronounced cross ties, of a different variety than those in thinner films: either the cross tie or the wall is discontinuous so that the wall runs through a hole in the cross tie or the cross tie runs through a hole in the wall. In one degenerate variety, the wall consists only of Bloch lines and cross ties: the magnetization curls 360° around each Bloch line and the cross ties separate the curls. The inhomogeneous films crept by the distorting hump mechanism previously reported for conventional cross tie walls. The homogeneous films have no cross ties, but smaller humps are induced by two processes: (1) variations of coercivity, and (2) anisotropy dispersion which causes the magnetization to rotate more in some parts of the film than in others, thus causing magnetostatic charges to form on a small segment of the wall, causing local movement. Micrographs of an 1800-Å film are presented showing four different kinds of cross ties, the creep process in annealed films and the two different kinds of hump formation and creep in the homogeneous films. Creep measurements from loop apparatus confirm that the extra easy-axis field induced by magnetization rotation near the creeping walls (the field that causes creep) is much greater in inhomogeneous films in this thickness range than in homogeneous films.
Scattering Probability and Number-Phase Uncertainty in Lorentz Microscopy
Authors: Wohlleben Dieter
Published: February 2004

The probability for electrons to be scattered on a weak magnetic-field distribution is shown to have a cutoff, when Δφ, the root mean-square fluctuation of the flux of the distribution, approaches h/2e. A drastic increase of the illumination time over the classical value is required in order to detect inhomogeneities for which Δφ<h/2e.
Susceptibility and Ripple Studies in Cylindrical Films
Authors: Kehler T. P. , Coren R. L.
Published: February 2004

A short section of magnetically coated wire was used as center conductor of a section of coaxial transmission line. The complex reflection coefficient from the section was measured, at 110 MHz, as a function of an axial dc field. By relating the reflection coefficient to the surface impedance of the wire, and the surface impedance to the magnetic state of the film, the rf susceptibility χ could be determined. This technique yields absolute values of χ, so that only limited scaling can be used to obtain a fit to the theoretical curves. Two film types were used: high-dispersion thick films (∼8000 Å) and low-dispersion thin films (∼1000 Å). No fit to the experimental points could be obtained unless a complex susceptibility was assumed. The real and imaginary parts of χ and of the effective field have been calculated during the entire magnetization reversal process.
Microwave Behavior of Partially Magnetized Ferrites
Authors: Schlömann Ernst
Published: February 2004

The high-frequency permeability of partially magnetized ferrites is calculated for some simple domain configurations, comprising only ``up''- and ``down''-domains. The method used is based upon the magnetostatic approximation and neglects exchange effects, but is otherwise substantially rigorous. The components of the effective permeability tensor (ratio of average induction to average magnetic field) in general depends upon details of the domain configuration in addition to the net dc magnetization. When the dc magnetization is cycled between the two states of complete magnetization the high frequency permeability, considered as a function of the dc magnetization, in general shows hysteresis. Detailed calculations of the high-frequency permeability have been carried out for the case in which the domain configuration is cylindrically symmetric, i.e., invariant under rotation around the direction of magnetization. For any such domain configuration the two relevant components μeff and κeff of the effective permeability tensor obey the relation μeff2-κeff2=const, regardless of the number of domains and their relative size. This general theorem allows a simple derivation of the (isotropic) permeability in the completely demagnetized state, giving μeff=23{[(ω/γ)2−(Ha+4πM0)2]/[(ω/γ)2−Ha2]}1/2+13where ω is the frequency, Ha the anisotropy field, and M0 the saturation magnetization.
Low-Loss Lithium Ferrites for Microwave Latching Applications
Authors: Collins T. , Brown A. E.
Published: February 2004

Dense kiln-fired lithium ferrite ceramic blocks with values of 4πMs ranging from 3600 to 2600 G have been prepared with low microwave dielectric losses (tanδ<0.0005) and high remanence ratios [Br/(4πMs)>0.7]. These low dielectric losses were achieved by addition of MnO2 and annealing in an oxygen atmosphere after the ceramic body was fired. The lower 4πMs compositions were obtained by substituting titania for part of the iron oxide in the basic lithium ferrite formulation. A number of physical properties were evaluated and the results are presented. These include density, saturation magnetization and Curie temperature. In addition, properties useful for microwave latching applications such as the complex dielectric constant and hysteresis characteristics (i.e., remanent magnetization and coercive force) are included. The temperature variation of the hysteresis properties for selected compositions is also shown.
Magnetic Properties of Ba2Zn2Fe28O46 and Ba2Co2Fe28O46 Single Crystals
Authors: Tauber A. , Megill J. S. , Shappirio J. R.
Published: February 2004

The static magnetic properties of Ba2Zn2Fe28O46(ZnX) and Ba2Co2Fe28O46 (CoX) single crystals were measured as a function of field up to 15.5 kOe and temperature up to Tc. ZnX exhibits uniaxial anisotropy: EA=K1 sin2θ over the entire temperature range; Ha remains approximately constant ≈12.8 kOe over a large temperature range; Tc=705±3°K; nβ(calc) (H=∞, T=0)=60 μβ on the basis of a collinear spin model; nβ(exp) (H=∞, T=0)=50.4 μβ. The anisotropy of CoX is much more complex, requiring three terms. Below 416°K it exhibits an easy plane of magnetization. Above 416°K, EA=K1 sin2θ, Tc=740±4°K. The nβ(exp) (H=∞, T=0)=46 μβ in good agreement with nβ(calc) (H=∞, T=0)=47 μβ assuming a collinear spin model.
Magnetically Tunable Microwave Bandstop Filters Using Epitaxial YIG Film Resonators
Authors: Archer J. L. , Bongianni W. L. , Collins J. H.
Published: February 2004

Disk samples of epitaxially grown YIG, of significantly higher aspect ratio than are available with flux-grown YIG, can be easily fabricated for electronically tunable filter applications and are amenable to integrated circuit construction techniques and multi-channel operation. To determine the utility of epitaxial YIG resonators (190-mil diameter and 20-μm thick), bandstop filters using necked-down strip transmission line with YIG close to the stripline were tested. Tuning was achieved by varying the biasing magnetic field applied parallel to the plane of the YIG film. Adequate coupling was achieved by surrounding the transmission line and resonator with dielectric materials and by reducing the impedance level presented to the resonator. The external quality factor Qe was thus varied between 10 and 100. The devices were designed to operate from 2 to 3 GHz and exhibited an off-band insertion loss less than 1.0 dB. Maximum attenuation at center frequency decreased as the 3-dB bandwidth decreased in accordance with theory, allowing attenuation as high as 30 dB and bandwidths down to 14 MHz. The linewidth was calculated from the center frequency, Qe and the maximum attenuation, and was found to be approximately 1 Oe.
Direct Observation of the Angular Distribution of Neutrons Scattered at Small Angles by Spin Waves in Fe–Ni Alloys
Authors: Werner S. A. , Wiener (Avnear) E. , Gürmen E. , Arrott A.
Published: February 2004

Extensive measurements of the small-angle scattering of neutrons by spin waves in Fe–Ni alloys using polycrystalline samples and a polyenergetic beam have been made by Hatherly et al. Their spin-wave stiffness D was found not to agree with later measurements using the diffraction method on Fe0.5Ni0.5 and Fe0.2Ni0.8 single crystals. In an effort to resolve this disagreement we employed a new technique to observe the small-angle scattering by spin waves in these materials that uses a double-Si-crystal spectrometer, monoenergetic neutrons, and single-crystal samples. Our result for D for the Fe0.5Ni0.5 sample agrees with the previous single-crystal data. However our data on Fe0.2Ni0.8 is in closer agreement with the earlier small-angle scattering measurements. The effect of the dipole-dipole interaction on the spin-wave energy has been clearly observed in Fe0.5Ni0.5. Two discontinuities are predicted and observed to occur in the scattering profiles. However, an additional discontinuity is observed in our Fe0.7Ni0.3 single crystal, which may indicate another scattering surface of unknown origin.
Short-Range Order Effects in Paramagnetic Neutron Scattering
Authors: Tahir-Kheli R. , Malinoski F.
Published: February 2004

To study the effects of short-range order on paramagnetic lineshapes, we have computed temperature-dependent frequency moments for an exchange-coupled paramagnet with uniaxial, single-ion anisotropy, i.e., H=− ∑ f1,f2I(f1,f2)Sf1·Sf2−D  ∑ f (Sfz)2. We have used phenomenological procedures to construct the frequency and wave-vector-dependent Van Hove correlation function. The results are discussed with reference to the inelastic scattering cross section of slow neutrons from such paramagnetic systems.
Longitudinal Susceptibility χss(q, ω) of a Uniaxial Ferromagnet at T =0°K
Authors: Scalapino D. J. , Hone Daniel , Silberglitt Richard
Published: February 2004

We have recently presented a theory of zero-point effects in a uniaxial ferromagnet that is magnetized perpendicular to its easy axis. The longitudinal susceptibility χzz(q, ω) (where z is the magnetization direction) of such a system is nonvanishing only because of the zero-point deviation and therefore provides a direct probe of this feature of the ground state. Previously, significant structure was found in the uniform susceptibility χzz(q=0, ω). Here we calculate the wavevector dependence of the susceptibility, which describes inelastic neutron scattering from the system. As in our earlier work, the small expansion parameter of the theory is the ratio of anisotropy energy to exchange energy, D/J. For large momentum transfer the ω dependence of Im χzz(q, ω) consists of a peak in the two-spin-wave continuum near the single spin-wave energy εq of width 2(Dεq)1/2 and a resonance at the two-spin-wave bound state. The area under the curve χzz(q, ω) is equal to π(S−〈S2〉).
Dynamical Characteristics of the Oguchi Model
Authors: Kwon T. H. , Gersch H. A.
Published: February 2004

The Oguchi model has proved highly useful in studying the static properties of a Heisenberg spin system in the intermediate temperature range. In this paper the dynamical characteristics of a Heisenberg spin system are studied in terms of time and temperature dependent spin-spin correlation functions evaluated exactly from first principles in the Oguchi model. In particular, the wavevector-dependent frequency transform of the spin-spin correlation functions is obtained in a closed form for the cases of spins ½, 1, 32, 2, and 52 for temperatures above the transition temperature. The theory is applied to the inelastic scattering of neutrons from RbMnF3 and excellent agreement between the theory and experiment is obtained.
Promise of Magneto-Optic Storage Systems Compared to Conventional Magnetic Technology
Authors: Eschenfelder A. H.
Published: February 2004

Conventional magnetic technologies allow an extensive trade-off, in computer storage systems, of total capacity, access time to data and price. In the area of ``mass'' memories significant improvements in capacity and access time are needed that cannot be obtained using inductive recording. That technology is limited by the dimensional factors that can be achieved mechanically and the frequency response of the transducer materials. The substitution of optical transducers relieves both of these limitations so that order-of-magnitude improvements are possible. This paper includes a discussion of the computer needs, the limitations of inductive recording and an illustration in a specific product configuration of the improvement possible by the substitution of a particular magneto-optic approach.
Critical Scattering of Neutrons from RbMnF3
Authors: Lau H.-Y. , Corliss L. M. , Delapalme A. , Hastings J. M. , Nathans R. , Tucciarone A.
Published: February 2004

Measurements of the inelastic magnetic scattering of neutrons from RbMnF3 in the vicinity of the critical point are presented and compared with the predictions of the dynamic-scaling proposal of Halperin and Hohenberg. At the Néel point, the energy dependence of the scattering for various wavevectors q is well represented by a universal three-peaked function in which the component widths and separations are all proportional to q1.4±0.2, in excellent agreement with the concept of scaling as well as with the specific prediction of an exponent of 1.5. In the hydrodynamic region above TN (q small compared to the inverse-range parameter, κ) the scattering is Lorentzian in energy. The half-width at q=0 is proportional to κ1.4±0.2, once again in good agreement with the scaling prediction. Over a broad region of the q, κ plane (T≥TN), where resolution corrections can be omitted, the ``half-area'' energy widths can be represented by a simple homogeneous function of q and κ, confirming the basic assumption of dynamic scaling.
Scaling Laws for Dynamics Near the Critical Point
Authors: Hohenberg P. C. , Halperin B. I.
Published: February 2004

The static scaling laws may be generalized to dynamic phenomena by making assumptions on the behavior of time-dependent correlation functions near the magnetic critical point. At any temperature different from Tc, the correlation functions are assumed to reflect the ``hydrodynamic'' behavior of the system, for sufficiently long wavelengths and low frequencies. As the critical temperature is approached, however, the range ξ of spatial correlations in the system diverges, and the domain of applicability of hydrodynamics is reduced to very small wavenumbers, satisfying the inequality kξ≪1. The dynamic scaling assumptions relate the behavior of the time-dependent correlation functions in the ``hydrodynamic'' (kξ≪1) and ``critical'' (kξ≫1) regions. It is thus possible to predict certain features of the dynamic behavior at Tc, from a knowledge of the behavior away from Tc. Application is made to the spin-correlation function of the isotropic Heisenberg antiferromagnet, which is directly measurable by inelastic neutron scattering. Detailed predictions are presented on the form of the cross section as a function of energy and momentum transfer, and of temperature near Tc.
Enhancement of the Faraday Rotation of Iron Garnets by Bismuth and Rare-Earth Ions
Authors: Buhrer Carl F.
Published: February 2004

Ferrimagnetic single crystals derived from bismuth calcium vanadium iron garnet by gadolinium substitution are shown to exhibit a magnetic compensation temperature as well as a large Faraday rotation. Another garnet system Pr3–2xCa2xFe5−xVx O12 was studied, and it is found that the Faraday rotation increases rapidly with decreasing temperature. This suggests that the rare-earth ion contributes significantly to the rotation. This is also found to be true for Dy3Fe5O12, and it is concluded that certain rare earths as well as bismuth may be useful in achieving magneto-optical materials of improved performance.
Magneto-Optic Properties of CrTe Films Prepared by Sequential Evaporation
Authors: Comstock R. L. , Lissberger P. H.
Published: February 2004

A study was made of the magnetic properties of CrTe films relevant to magneto-optic memory applications. Polar-Faraday rotation measurements in the visible and near infrared were taken on CrTe films prepared by sequential deposition of layers of Cr–Te–Cr on cleaved mica substrates. The Faraday rotation and absorption decreased respectively from 0.5×105°/cm at 0.55 μm to 0.4×105°/cm at 1 μm and 2×105 cm−1 at 0.5 μm to 0.6×105 cm−1 at 2.5 μm. The rotation and coercive field decreased with the magnetization and vanished at the Curie temperature (∼70°C). The polar remanence ratio was 0.2. An in-plane magnetization curve had a remanence ratio of 0.8, a saturation induction (4πMs) of 1015 G and a coercive field of 117 Oe. The measurements indicate significant advantages for CrTe films in memory applications if the epitaxial growth can be improved resulting in higher polar remanence.
Low-Temperature Storage Using GaAs Lasers and EuO as Storage Medium
Authors: Fan G. , Greiner J. H.
Published: February 2004

The feasibility of a beam-addressable memory using EuO as a storage medium and GaAs laser as a transducer has been discussed in a previous paper. In this paper we shall present the results of a series of high-speed, dynamic read/write experiments and discuss the system implications of these experiments. A mechanical scanner was constructed to carry out the dynamic read/write experiment and a refrigeration system was used to provide both the cooling of the GaAs laser and the storage medium. Writing was accomplished by pulsing the laser as the light spot scanned across the film surface. The energy density required to write a 5 μ spot is below 10−2 erg. This is within the capability of GaAs lasers, hence writing time in tens of nanoseconds was easily achieved. Reading is achieved by scanning a cw GaAs laser beam across the film surface and detecting the magneto-optic signal through a cross polarizer by a PMT. Due to the large magneto-optic effect, very high-speed readout is achieved. The high-speed read/write experiment showed the feasibility of a reversible storage system which has the density and speed of an optical store. A brief comparison of this technology with that of the gas laser is given.
Magneto-Optic Properties of MnAs Films
Authors: Stoffel A. M. , Schneider J.
Published: February 2004

For polycrystalline films of MnAs the complex index of refraction, N=n—ink, and the off-diagonal component of the complex dielectric tensor, Q=Q0 exp (-iq) were determined as functions of wave-length from 0.45 μm to 0.95 μm. The complex index of refraction increases from N=1.84-i1.75 at 0.45 μm to 2.13-i3.33 at 0.95 μm. Q0 and q increase from 0.008 and 29° at 0.45 μm to a maximum of 0.0194 and 55° at 0.9 μm, respectively. The dispersion of the Kerr rotation shows a change in sign at 0.72 μm. A maximum Kerr rotation of +0.058° is found at 0.9 μm and a maximum Kerr ellipticity of −0.15° at 0.72 μm. From these data a Faraday effect | 2F | = π | NQ|/λ0 of 156 000°/cm and an absorption coefficient α=4πnk/λ0 of 4.5×105/cm were calculated at 0.8 μm. The figure of merit is f=|2F|/α=0.34°. Also calculated were the complex dielectric constant ε and the reflectivity R. A description of the film preparation technique in high vacuum (10−7 Torr) is given. The films were analysed by x-ray diffraction techniques and were found to be stoichiometric MnAs.
Enhancement of the Longitudinal Kerr Magneto-Optic Effect in Thin Films
Authors: Hoffman A. S. , Hagemeier W. R. , Cushner S. H.
Published: February 2004

This paper reports on the observation of substantial rotation and reflection enhancements with thin magnetic films evaporated on the base of high index prisms with films of suitable index on only the air side of the magnetic thin film. Angles of incidence at the prism-magnetic thin-film interface were near the critical angle for total internal reflection. The magnetic film thickness and the thicknesses and indices of the dielectrics were obtained with the help of a computer program which calculated the reflection coefficients as well as the Kerr rotations and energy mode transfer. Enhancements of rsp up to a factor of 20 were obtained.
General Critical Field Expression for the Parallel-Pumped Magnetoelastic Instability
Authors: Jackson D. R. , Orth R. W.
Published: February 2004

A general expression is derived for the critical parallel-pump-field strength for magnetoelastic waves. This expression can be used for all wave propagation directions, bias field directions, and crystal symmetries. In contrast to previous treatments, certain critical field results can be obtained without extensive numerical computation. It is found, for example, that when spin wave damping of elastic modes is dominant, these elastic modes have a critical pump field strength simply equal to twice the spin wave linewidth.
Single- and Double-Mode Parallel Pumping of Phonons in Yttrium Iron Garnet at 114 MHz
Authors: Sage J.
Published: February 2004

Previous theories of the phonon parallel-pump instability by Comstock and by Schlömann and Joseph have been based on equations of motion for the coupled magnon-phonon system. The calculations have been complicated and have not revealed the underlying physical phenomena. In this paper a simple, general, and physically intuitive theory will be described very briefly. In its fullest development this theory can be applied to a phonon of any wave vector in a crystal of arbitrary symmetry with magnetic, elastic, and magnetoelastic anisotropy and biased with a magnetic field of any magnitude and direction. Only the crossover region is excluded. A short sample calculation which yields all of the principal results of the earlier calculations will be published in the Journal of Applied Physics. In addition, experimental data which have been obtained with a 3 mm YIG sphere at pump frequencies near 114 MHz will be reported which support the theoretical results. The relation hc∼(H-HA)2, where hc is the threshold rf field, H the static internal field, and HA an effective direction-dependent anisotropy field, has been verified for [110] and [111] axes parallel to the external field. Plots of threshold power Pe as a function of frequency give phonon mode spectra from which values of elastic Q and resonant frequency f can be determined directly. These values of Q agree well with those inferred from the values of hc. Also, individual modes can be followed as a function of temperature and field. The coefficient (∂f/∂T)H agrees well with that expected from the temperature dependence of the sound velocity, and the field dependence fits the expected law (∂f/∂H)T∼hc −1. Using two signals at different frequencies, two-mode simultaneous instabilities have been observed and used to detect changes in the phonon spectrum caused by an above-threshold signal. The results explain qualitatively the dynamics of the onset of the instability and an anomalous asymmetry observed in the suppression of a weak signal near a strong one.
Compensation of Magnetostriction Effects in Iron Garnets by Manganese Additions
Authors: Dionne Gerald F. , Paul Patricia J. , West Russell G.
Published: February 2004

The presence of small amounts of Mn3+ ions in both Y3MnxFe5−xO12 and rare-earth substituted Y2.3Gd0.7Al0.4MnxFe4.6−x O12 will produce significant improvements in the hysteresis loop The samples studied were prepared by standard ceramic techniques and were found to be single phase with 10–15-μ grain diameters. The sensitivity of the remanence ratio to external stress parallel to the magnetization1,2 is reduced monotonically with x and is eliminated in both compounds at x≃0.09. In addition, remanence ratios are increased because of an apparent reduction in the effects of internal stress, and all evidence suggests a decrease in coercive forces when densities are maximized. All hysteresis-loop data, including stress sensitivity measurements, were obtained from a high-field hysteresigraph.3 The electrical resistivity decreases with x, but remains well above 1010 Ω-cm up to x=0.1 and the dielectric loss tangent is less than 2×10−4, which is highly satisfactory for microwave applications. From the results of resonance linewidth measurements compared with calculated contributions from porosity, it is concluded that anisotropy contributions to linewidths are reduced without significant increase in intrinsic linewidths. As a result, it appears that Mn3+ additions to these compounds also lower the magnetic anisotropy and, consequently, the lower limits of both linewidth and and coercive force.
Internal Stress Distribution Theory of Magnetomechanical Effects
Authors: Smith George W. , Birchak J. Robert
Published: February 2004

Using an internal-stress distribution theory of magnetomechanical effects, we calculate the vibrational stress dependence of magnetomechanical damping ψ, the effect of applied magnetic field and stress on ψ, the temperature dependence of ψ, the vibrational stress dependence of the ΔE effect, and the Rayleigh magnetic hysteresis loss law. The theory yields fairly good agreement with measurements of these parameters. The theory predicts that the maximum magnetomechanical damping ψmax is given by ψmax∼λ2E/ (JsHc), where λ is the magnetostriction. E is Young's modulus, Js is the saturation magnetization, and Hc is the coercive force.