bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical Constants of the Eu Chalcogenides Above and Below the Magnetic Ordering Temperatures
Authors: Güntherodt G. , Schoenes J. , Wachter P.
Published: February 2004

The optical constants of the Eu chalcogenides between 1 and 6 eV have been obtained by means of a reflection technique for single crystals and transmission measurements for evaporated thin films. The films have been cooled below the magnetic transition temperature and the transmission was measured with linearly and circularly polarized light in external magnetic fields. In addition the Faraday effect of thin films in the energy range of 1 to 3.5 eV has been observed. Below the Curie point the lowest energy absorption peak is split into three polarization-dependent maxima and the absorption peak between 4 and 5 eV into a doublet. The data suggest transitions from the localized 4f7 states into the spin-split states, respectively, 4f6(7FJ)5d(t2g and 4f6(7FJ)5d(eg). The specific Faraday rotation of EuS at 2.2 eV is 550 000°/cm, the largest rotation yet observed for a nonmetallic compound.
Magnetic and Electrical Properties of the CuCr2Se4−xBrx System
Authors: Miyatani K. , Minematsu K. , Wada Y. , Okamoto F. , Kato K. , Baltzer P. K.
Published: February 2004

The system CuCr2Se4−xBrx has been prepared both in powder and in single-crystal forms. The lattice parameter increases linearly from 10.325 to 10.405 Å as x increases from 0 to 1. The magnetic moment also increases linearly from 5 μB/mole to 6 μB/mole, and the ionic configuration Cu1−x2+Cux1+[Cr23+]Se4−x−2Brx1− applies well to this system. The Curie temperature suddenly drops over 100° in the small range 0.98≲x≲1.0. This sudden change in Tc is unusual and is one of the most significant facts found in this work. It has also been found that both the electrical resistivity and Hall effect (p-type) change very sharply near x=1 and that the system becomes semiconductive only for 0.98≲x≲1. A strong correlation is thus found between the Curie temperature and the electrical properties (electrical resistivity ρ and anomalous Hall coefficient R′). Replotting the electrical resistivity and the Hall coefficient as a function of the Curie temperature for arbitrary composition x, we found a simple relationship between them. ρ∝R′∝exp[−(2.5±0.3)×10−2·Tc]. This relationship is satisfied over the whole system, 0≥x≥1 (covering both the metallic and semiconductive ranges). Here, the temperature dependence of ρ is not large and the above equation covers the measured temperature range, 77° to 300°K. The Hall voltage for the semiconductive range is found to be proportional to the magnetic moment. On the other hand, the Hall voltage for the metallic range takes its maximum value at the Curie temperature and saturates in high fields. Therefore, R′ in the above equation was chosen as the maximum value of the curves of R′ versus T with x as a parameter. These values of R′ correspond to points of maximum scattering in the (R′, X, T) surface. This indicates that both ρ and R′ are dominated by the magnetic exchange interactions. These properties are qualitatively understood in terms of the itinerant d-hole carriers of the Cu2+ ions which exchange couple to the localized spins of the Cr3+ ions and enhance the magnetic exchange interactions in the system (Carrier Enhanced Magnetic Exchange). Nevertheless, no theory yet exists to explain the remarkable relation described above. A more complete description of this research will be published elsewhere.
Energy Spectra of Magnetic Semiconductors: Eu Chalcogenides and NiO
Authors: Kasuya T.
Published: February 2004

The energy spectra of EuO and EuSe have been calculated based on atomic spectra and the pure ionic crystal model. The relative position of each level is consistent with that proposed previously by Kasuya and Yanase to interpret magnetic, optical and electrical properties of Eu chalcogenides. Furthermore, quantitative agreement with experiment is satisfactory. A similar calculation for NiO leads to the following predictions: (1) For Li-doped samples conduction occurs in the 3d band. (2) The first band-to-band transition is from 3d8 to 4s states in contrast to the accepted assignment 2p to 3d9. The latter transition corresponds to the large broad absorption peak near 16 eV.
Transport in Gd-Doped EuO
Authors: von Molnar S. , Shafer M. W.
Published: February 2004

Transport properties of 2% Gd-doped EuO with Tc≈123°K have been investigated. The resistivity anomaly in the neighborhood of Tc can be explained satisfactorily by critical scattering theory. Estimates of the exchange interaction between conduction electrons and localized spins are found to vary from 4.9×10−2 eV in Gd :EuO to 4.3×10−2 eV in Gd :EuS.
Formation of Reversed Domains in Plate-Shaped Ferrite Particles
Authors: Holz A.
Published: February 2004

The coercive force of fine-particle magnets consisting of hard ferrite particles in a nonmagnetic matrix is always considerably less than 2K/IS. A coherent rotation model is not adequate to describe this behavior. It is suggested that reversed domains come into existence at places where strong stray fields are present. Plate-shaped particles are considered with a large ratio of diameter to thickness with the easy axis perpendicular to the surface. At the edge the magnetization is forced to incline due to the large horizontal component of the stray field. The magnetization of the platelets is described by three parameters. Numerical minimization of the total energy, which is carried out for BaFe12O19 and SmCo5, shows that at a certain field, Hn, an antiparallel oriented domain comes into existence. Hn is positive (parallel to the initial magnetization direction) for particle thicknesses d>1.5 μ for BaFe12O19 and for d>1.8 μ for SmCo5. For d<345 Å (BaFe12O19) coherent rotation becomes energetically more favorable. This model shows that formation of reversed domains in a wide range of plate-shaped particle thicknesses determines the coercive force.
Ferromagnetic Resonance Transmission in the Iron Group Metals
Authors: Phillips T. G.
Published: February 2004

This paper describes a two-cavity microwave transmission experiment, carried out at a frequency of 35 GHz and a temperature of 4.2°K. A resonant transmission signal has been observed through thin sheets of the ferromagnetic metals permalloy and nickel when placed as the common end wall of the two TE110 cavities. The transmitted signal occurs at an external magnetic field value appropriate to the antiresonance condition. It is better defined for the Permalloy sheets, due to the smaller line-width, and in this case the resonance line shows a structure which is associated with an interference effect between electromagnetic waves and spin waves. A general theory of microwave transmission in ferromagnets is briefly presented, which includes spin-wave and boundary condition effects, and an indication is given of the origin of the structure observed in this experiment as well as of further effects to be expected in experiments of this type.
ENDOR Study of the Unpaired Spin Density in KZnF3:Mn2+
Authors: Jeck R. K. , Krebs J. J. , Folen V. J.
Published: February 2004

ENDOR techniques were used on KZnF3:Mn2+ to investigate spin polarization of the electrons which contribute to nearest- and next-nearest neighbor Mn2+ superexchange interactions in the anti-ferromagnetic isomorph, KMnF3. Spin unpairing contributes measurably to the superhyperfine (shf) interaction with 1st and 2nd-nearest fluorine neighbors of Mn2+ ions in KZnF3. The shf constants at 4.2°K for the 1st neighbor F− nuclei are As=54.26±0.03 MHz and Aσ−Aπ=0.68±0.03 MHz, using r=2.065 Å for the Mn2+–F− separation. From the above results the fraction of unpaired spin in the 2s and 2p orbitals of these F− ions is calculated to be fs=0.60% and fσ−fπ=0.26%. For the 2nd neighbor F nuclei at (a0/2, 0, a0), a0 being the lattice constant, we find the components of the shf tensor (in MHz) to be Bxx=0.38, Byy=−0.30, Bzz=1.60, ½(Bxz+Bzx) = 1.04, with Bzx≅Bxz. These values reflect an isotropic component Bs=0.56 MHz which is 2.5 times larger than that found by Davies in the less covalent salt KMgF3:Mn2+. This indicates the importance of covalency (as opposed to overlap) contributions to the spin unpairing at more distant neighbors of Mn2+ ions in perovskite fluorides.
Nuclear Magnetic Resonance Studies of Magnetic Impurities in Metals
Authors: Narath Albert
Published: February 2004

The application of nuclear magnetic resonance techniques to the study of magnetic impurity states in metals is reviewed. Two types of experimental information are considered. The first concerns resonance shifts and spin-lattice relaxation rates of isolated impurities, while the second concerns the effects of impurity-impurity interactions on these quantities. The theory of exchange-enhanced hyperfine interactions in metals is discussed for both elemental metals as well as impurities in metals. It is shown that the available low-temperature impurity data, which span a large range of 0°K susceptibilities, can be accounted for by treating the intra-atomic Coulomb and exchange interactions in the Hartree-Fock approximation. This even applies to two alloys, Au(V) and Mo(Co), which have previously been treated as Kondo systems. The present results therefore appear to remove the distinction between magnetic and nonmagnetic regimes of the magnetic impurity problem.
Theory of the Static Stability of Cylindrical Domains in Uniaxial Platelets
Authors: Thiele A. A.
Published: February 2004

A theory of the static stability of circular cylindrical domains in uniaxial magnetic platelets has been developed. The predictions of the theory agree well with experiments and provide requirements which link material properties to domain geometry. The theory is developed by assuming a model in which the domains have cylindrical walls of zero width and by then calculating the size and stability of the domains, using a straightforward although somewhat lengthy energy method. It is found that in order for domains to exist Ku⪞2πMs2, where Ku is the uniaxial anisotropy constant and Ms is the saturation magnetization. However, the model is most accurate when Ku≫2πMs2, although in this case the domains tend to have a low mobility. A formula relating domain size to the material parameters, the plate thickness and the applied bias field is obtained. More important, static stability considerations indicate that when all parameters except the bias field are held constant the domains are stable only over a 3:1 diameter range and at best a 1.6:1 bias field range. Cylindrical domains are found to be most stable for device applications when the domain radius is equal to the plate thickness which is in turn approximately equal to σw/πMs2, where σw is the wall energy density.
Neutron Diffraction Study of the Kondo Effect in Cu–Fe
Authors: Stassis C. , Shull C. G.
Published: February 2004

Polarized neutron techniques have been used to sense the effect of the localized spin in a 1000-ppm Fe in Cu single crystal on the intensity of Bragg scattering. The coherent magnetic scattering that results from the localized magnetization induced by 15 000 Oe of applied field strength has been studied in the (111) Bragg reflection in the temperature region of 2°–25°K The measured localized field-induced magnetization was found to have the same temperature dependence as the bulk magnetization indicating the absence of any large field-induced magnetization in the compensating conduction electron cloud. The Kondo temperature was found to be approximately 10° K by fitting the data with the predictions of the spin fluctuation model.
Temperature Dependence of the Impurity Magnetic Moment for Mn in Cu above the Kondo Temperature
Authors: Bensel J. , Gardner J. A.
Published: February 2004

The susceptibility of dilute solutions of Mn in Cu has been measured between room temperature and the melting point. The deviation of the susceptibility from Curie behavior is in qualitative agreement with a model calculation of the perturbation of impurity susceptibility by the s·d interaction. The coupling constant ρJ is known independently and the remaining parameter is chosen to fit the data near the melting point. Qualitative agreement is good, although quantitative deviation is found. An apparent inconsistency between experimental and theoretically expected value of the fitting parameter is noted.
Instability of an Isolated Straight Magnetic Domain Wall
Authors: Hagedorn F. B.
Published: February 2004

An isolated straight magnetic domain wall along the y axis results when a thin slab of magnetic material in the xy plane is exposed to a nonuniform z axis magnetic field of the form Hz=βx, if β is sufficiently large. The behavior of this magnetic system as β decreases is discussed. A magnetic energy calculation shows that sinusoidal deviations from the straight wall become energetically favorable at a critical value βc. The wavelength of this distortion at βc and βc are both expressed in terms of magnetic material properties. Experimental observations of this instability in YbFeO3 are given, and the magnetic parameters determined this way are in reasonable agreement with those obtained by using other methods. Effects of material inhomogeneieties are discussed, and the transition from the initially distorted straight wall to the demagnetized state of stripe domains is shown.
Domain Wall Dynamics Measured Using Cylindrical Domains
Authors: Rossol F. C. , Thiele A. A.
Published: February 2004

An experimental technique is presented for the measurement of the translational dynamic properties of a cylindrical domain held stable in a thin plate of uniaxial magnetic material by a bias field normal to the plate. A second field HROT, rotating in the plane of the plate at a frequency ω, saturates and rotates the magnetization in a circular Permalloy film parallel to and in close proximity with the plate. Interaction of the domain with the Permalloy disk pulls the domain underneath the edge of the disk and causes it to follow HROT around the circumference. The force driving the domain is proportional to the sine of the angle ψ by which the angular position of the domain lags behind HROT. For the rare-earth orthoferrites, this drive force is balanced by a viscous drag proportional to ω and inversely proportional to wall mobility. For a sample of the mixed orthoferrite Sm0.55Tb0.45FeO3, measurements of ψ at frequencies up to 225 kHz by stroboscopic observation of the domain showed the linear dependence of sin ψ on ω predicted by the viscous damping model and yielded a wall mobility of 970 cm sec−1 Oe−1 in agreement with measurements on the same sample using a previously published technique.
Spin-Wave Excitations in Ho0.5Er0.5
Authors: Holden T. M. , Powell B. M. , Stringfellow M. W. , Woods A. D. B.
Published: February 2004

The propagation of spin waves in the conical and spiral magnetic phases of the alloy Ho0.5Er0.5 has been studied by neutron inelastic scattering. The spin-wave energies are comparable with those observed in Ho and Er. The spin-wave dispersion curves at 4.2°K were analyzed in terms of the Fourier transformed exchange energy, J(q) - J(0), and an effective anisotropy parameter. The linewidths and energies of certain spin-wave modes were studied as a function of temperature. As the temperature was raised the spin-wave energies decreased. The linewidth of the distribution at Q = (0, 0, 2π/c) increased sharply on raising the temperature toward the cone-to-spiral transition. No broadening was observed for the mode of the same wave vector in Ho over the same temperature range.
Magnetic Anisotropy Effects on the Hall Resistivity of Rare-Earth Metals
Authors: Cullen James R. , Rhyne James J. , Mancini Ferdinando
Published: February 2004

Results are presented for the normal and anomalous Hall coefficients R0 and Rs in the rare-earth metals Tb, Dy, Ho, and Er in the paramagnetic temperature range. Above TN the anomalous coefficients are essentially temperature-independent, indicating a dominant spin-disorder scattering mechanism. Measurements with H along 〈0001〉 gave a constant value of R0=−3.6×10−12 Ω-cm/G for all elements of the series; Rs, on the other hand, varied by more than a factor of 8. This variation cannot be explained on the basis of an isotropic theory of spin disorder. The coefficients for H applied in the basal plane showed changes in sign of both R0 and Rs occurring between Dy and Ho and a marked decrease in magnitude of Rs throughout the series. A model is developed to include the effects of the large magnetic anisotropy on the anomalous coefficient. The model includes the contribution from both intrinsic and extrinsic spin-orbit couplings. The magnetic anisotropy has a significant effect on Rs, and leads to better agreement with experiment.
Effect of Uniaxial Stress on the Magnetic Ordering Temperatures of Gadolinium and Dysprosium Single Crystals
Authors: Bartholin Henri , Bloch Daniel
Published: February 2004

Previous measurements on the effect of a uniaxial stress up to 150 bar on the Curie temperature of Gd1 and on the Néel temperature and the antiferromagnetic to ferromagnetic transition temperature of Dy2 are reported. The measurements were made with the stress applied along each of the three principal axes of single-crystal samples. The results are discussed in terms of current models of the magnetic ordering in rare-earth metals.
Magnetic Form Factor of Thulium Metal
Authors: Brun T. O. , Lander G. H.
Published: February 2004

The magnetic form factor of thulium at 4.2°K has been measured with polarized neutrons. The low-temperature magnetic structure of thulium1 is the 4+, 3− ferrimagnetic configuration, in which the spins are aligned along the c axis. The net ferromagnetic component is 1μβ per atom. With the exception of the two innermost reflections the experimental observations are in excellent agreement with the 4f12 form factor. The anisotropy in the magnetization density has been confirmed by using an elevated counter technique. For the low-angle reflections an additional contribution to the form factor is present. We have attributed this contribution to a 5d-like conduction polarization. A calculation using free-atom wave functions gives a 5d form factor that is in reasonable agreement. However, the magnitude of the 5d-like contribution is six times greater than that generally regarded as the conduction electron polarization in thulium. This discrepancy could be explained if some modification of the 4f wave functions occurs due to the exchange coupling; in this case the division into a localized 4f moment and a conduction-electron moment is incorrect.
Effect of Pressure on the Néel Temperature of MnCO3, CoCO3, and FeCO3
Authors: Srivastava Vishnu C.
Published: February 2004

The effect of pressure on the Néel temperature of MnCO3, CoCO3, and FeCO3 is reported up to 10 kbar. Also the compressibilities of these materials have been measured at 4.2, 77, and 300°K. The shift of the Néel point has been measured through the mutual inductance of an astatic coil assembly containing the sample. The measured values of dTN/dP=+0.68, +0.11, and +≤0.1°K/kbar for MnCO3, CoCO3, and FeCO3 respectively. Combining the compressibility measurements with dTN/dP the value of magnetic Grüneisen constant γm=d log TN/d logV has been estimated for these compounds. The values of γm are found to be −11±4, −2.3±2, ≤ −5.3±3 for MnCO3, CoCO3, and FeCO3 respectively. Within the experimental error the value of γm for CoCO3 and for FeCO3 is close to −3.3, the value which Bloch has found for a number of 3d transition metal compounds showing superexchange coupling. However, the value of γm for MnCO3 is relatively higher and seems to be a combination of the volume dependence of at least two magnetic exchange interaction constants for the nearest and next nearest neighbors.
Magnetic Interactions in Gadolinium Orthoferrite
Authors: Cashion J. D. , Cooke A. H. , Martin D. M. , Wells M. R.
Published: February 2004

GdFeO3 has been shown by other workers to behave as an antiferromagnet at temperatures below 650°K, the ordering pattern is GxFz. We have measured the canted Fe moment at low temperatures and we have observed its influence on the paramagnetic susceptibility of the Gd3+ ions. At 1.47°K the Gd3+ ions order antiferromagnetically along the crystallographic a axis and at low temperatures and high magnetic fields ``spin flop'' is observed. The specific heat has been measured between 0.5° and 12°K but its form is not easily explained.
Itinerant Electron Ferromagnetism of Hexagonal Cobalt
Authors: Wohlfarth E. P.
Published: February 2004

The results of two energy band calculations based on different potentials are reviewed. A variety of magnetic and other properties is discussed on this basis. Due to the uncertainties in the potentials only an approximate value of the exchange splitting for cobalt may be derived, namely ΔE=1.05±0.30 eV. Comparison of the ratio of the effective interaction energy to the d-band width is made for iron, cobalt, and nickel and is about ⅙ for all three metals.
Room-Temperature Ferromagnetic Materials Transparent in the Visible
Authors: Wolfe R. , Kurtzig A. J. , LeCraw R. C.
Published: February 2004

The only known crystals which have a spontaneous ferromagnetic moment at room temperature and are transparent well into the visible spectrum are ferric borate, FeBO3, and ferric fluoride, FeF3. Each of these rhombohedral materials is a canted antiferromagnet with a small moment lying in the plane perpendicular to its unique axis. The Curie temperatures are 348° and 363°K and the room-temperature moments are 115 and 40 G, respectively. Both materials contain ferric ions in a slightly distorted octahedral environment. Both are green with a series of absorption bands in the visible, a fundamental absorption edge in the violet (FeBO3) or ultraviolet (FeF3), and a large Faraday rotation per unit absorption at room temperature, as high as 14°/dB at 5250 Å for FeBO3 and 16°/dB at 4050 Å for FeF3. Recent measurements of the magnetic, optical, magneto-optical, and microwave resonance properties of these materials are discussed. Potential applications of room temperature transparent ferromagnets include microwave magneto-optical modulation of visible lasers with very low modulation power per unit bandwidth, optical deflection and isolation, magneto-optic displays, and holograms. Each of these applications is discussed along with the limitations imposed by the optical absorption and birefringence of the available materials. Birefringence interferes with the Faraday effect in most noncubic crystals. Methods for minimizing this interference are described. Finally, requirements for useful transparent ferromagnets are defined and guidelines for finding new materials are suggested.
Lattice Anisotropy in Antiferromagnetic Chromium
Authors: Steinitz M. O. , Schwartz L. H. , Marcus J. A. , Fawcett E. , Reed W. A.
Published: February 2004

Thermal-expansion and magnetostriction measurements on single-Q, single-crystal chromium give the lattice parameter ratios in the transverse spin-density-wave phase, through the spin-flip transition, and in the longitudinal spin-density-wave phase. The spin-flip transition is found to be accompanied by anisotropic discontinuities in length, resulting in a net volume dilatation of 1.4×10−6 with decreasing temperature. The Q direction is found to be the longest axis at 300°K and to bcome the shortest axis below about 220°K.
Evidence for Impurity Clustering and Matrix Polarization in Nickel-Palladium Alloys
Authors: Ferrando W. A. , Segnan R. , Schindler A. I.
Published: February 2004

We have measured the hyperfine fields at 57Fe nuclei diffused as impurities in various nickel-palladium alloys, using the Mössbauer effect technique. In addition to the Ni–Pd matrix polarization, we find evidence of impurity clustering polarization within the same matrix. The impurity polarization effects are reduced by long term anneal of the samples. The Curie temperature of the Ni–Pd matrix exhibits a sharp rise at about 2% nickel concentration, indicating that the onset of ferromagnetism occurs at this composition in agreement with previous magnetization data. The saturation hyperfine field shows a small variation in the concentration dependence, with broad maximum centered around 45% of nickel.
Magnetic and Magneto-Optical Properties of Some Rare-Earth and Yttrium Orthochromites
Authors: Tsushima K. , Aoyagi K. , Sugano S.
Published: February 2004

From the magnetic susceptibility measurement on YCrO3 above TN=141°K, two exchange constants J1 and J2 are estimated to be J1=−8.7 cm−1 and J2=−1.0 cm−1. In DyCrO3 weak ferromagnetism is observed along the a axis, with TN=146°K. When temperature goes below 5°K, the direction begins to rotate towards the b axis. In SmCrO3 a spin reorientation is found. The direction of weak ferromagnetism changes from the c axis to the a axis when the temperature is lowered below the reorientation temperature Tr=35≈40°K. The magnitude of the weak ferromagnetic moment of Cr3+ decreases rapidly below Tr. Using the values of J1 and J2, we estimate the magnon energies at high-symmetry points on the Brillouin zone surfaces in YCrO3. By comparing these magnon energies with the optically observed sidebands of the R lines, one may assign the sidebands to the combined excitations of one exciton and one magnon, and of one exciton and two magnons. The observed temperature dependence of these sidebands supports this assignment. The zero-magnon absorption lines due to Cr3+ ions were also observed in DyCrO3. However, the polarization and the magnetic field dependence of the lines are found to be quite different from those in YCrO3.
Optical Absorption Spectrum of ErCrO3
Authors: Meltzer R. S. , Moos H. W.
Published: February 2004

We present a study of the optical absorption spectrum of ErCrO3 as a function of temperature and magnetic field. Three types of processes are observed: (1) creation of a Cr+3 R-line exciton, (2) excitation of the Kramers-split Er+3 single-ion states, and (3) the simultaneous excitation of a Cr+3 exciton and a spin-flip of the Er+3 ground state. At 9°K there is a discontinuous change in the Cr+3 and Er+3 spectra due to a Cr+3 spin reorientation. The selection rules on the Davydov-split Cr+3 exciton states are shown to yield information about the magnetic symmetries. The magnetic dipolar contributions to the Er+3 Kramers splittings are calculated and shown to be insufficient to explain the large observed splittings, which implies an additional Er+3–Cr+3 interaction and a possible Er+3–Er+3 interaction.
Spectroscopic Observations of Antiferromagnetic Erbium Oxide
Authors: Bloor D. , Dean J. R. , Stedman G. E.
Published: February 2004

The far-infrared and visible spectra of erbium oxide have been observed above and below the transition to the antiferromagnetic state at 3.4°K. Absorptions due to electronic transitions between levels of the ground-state multiplet appear in the far-infrared spectrum, they are split into several components below the Néel temperature. This structure can be interpreted in terms of transitions for ions on two inequivalent sites. The changes in the visible absorption spectrum, together with changes in phonon frequencies, are attributed to the presence of the exchange fields and a magnetostrictive expansion of the crystal lattice in the ordered state. This data and the results of crystal field calculations using the superposition model indicate a small value for gz for C2 site ions. Electron spin resonance spectroscopy and other methods give a much larger value for gz. This discrepancy has not yet been resolved.
Light-Induced Increase in Domain-Wall Stiffness in Si-Doped YIG
Authors: Lems W. , Metselaar R. , Rijnierse P. J. , Enz U.
Published: February 2004

The initial susceptibility of Si-doped YIG is reduced by irradiation with light. This effect is explained by a simple two-center model, a strongly anisotropic center being formed from a less anisotropic one by light-induced electron transfer. Domain walls are pinned by these strongly anisotropic centers, of density n. If n is small enough, the stiffness will be proportional to n, since by local wall deformations each center within the wall can be made to occupy a site of lowest energy. For larger n this is no longer possible: instead, the optimum wall position will be determined by the statistical fluctuations of the center distribution, and the stiffness will be proportional to n1/2.
Magnetic Properties of GdIG Films Grown by Chemical Vapor Deposition
Authors: Stein B. F.
Published: February 2004

GdIG films with thicknesses from 0.3 μ to 3.0 μ have been grown epitaxially on the (100), (110), (111), and (320) faces of yttrium aluminum garnet substrates by chemical vapor deposition in a reactor consisting of two concentric tubes. The influence of substrate temperature, transport rates of iron and gadolinium chloride, partial pressures of oxygen, water vapor, and HCl, and substrate position on film growth have been determined. It is shown that diffusion between the film and substrate causes a variation in the compensation temperature through the film volume. This results in an asymmetry in the coercive force about the compensation temperature and anomolous behavior in the Faraday-effect hysteresis loops. These observations are discussed in terms of several possible models.
Evidence of Long-Range Order in 3He-Irradiated Permalloy Films
Authors: Williams C. M. , Schindler A. I.
Published: February 2004

Room-temperature measurements of the uniaxial anisotropy energy Ku have been made on a series of vacuum-deposited thin polycrystalline Permalloy films following 3He particle irradiations at temperatures of 125°, 200°, 250°, and 300°C. The results indicate the development of two types of long-range order, in addition to the so-called Fe–Fe atom pair short-range directional ordering: an NiFe-type order similar to that reported for neutron-irradiated bulk Permalloy, which enhances Ku for compositions around 50% Ni–50% Fe, and the well-known Ni3Fe-type order, which develops at the expense of Fe–Fe atom pair ordering, for compositions in the neighborhood of 75% Ni–25% Fe. The maximum value of Ku obtained due to NiFe long-range order is at least an order-of-magnitude less in the thin film than in the neutron-irradiated bulk Permalloy; also, the irradiation temperature at which Ku ceases to be enhanced by the development of NiFe long-range order in the thin film is well below the ordering temperature reported for bulk samples. For films with compositions off the NiFe and Ni3Fe stoichiometry, the temperature dependence of Ku was found to be in good agreement with the (Tc-Ta) behavior as suggested for Fe–Fe atom pair short-range directional ordering.
Ordering in Two- and Three-Dimensional Diluted Heisenberg Antiferromagnets
Authors: Breed D. J. , Gilijamse K. , Sterkenburg J. W. E. , Miedema A. R.
Published: February 2004

The ordering temperatures of the magnetically diluted antiferromagnetic systems K2MnpMg1−pF4 and KMnpMg1−pF3 are presented. From a magnetic point of view these systems can be looked upon as being simple quadratic and simple cubic, respectively. Near p=1, (d/dp)[TN(p)/TN(1)] is clearly larger than 1, as is theoretically expected for diluted Heisenberg systems. The curves for TN as a function of the concentration show that the critical concentrations for ordering at T=0 are clearly different for the two magnetic systems; they may be equal to the values predicted for the percolation problem. The low-field perpendicular susceptibility strongly depends on p, at the lower temperatures it can be approximately described by a kind of Curie-Weiss law. Extrapolating the Curie-Weiss behavior down to T=0, one obtains values for the inverse perpendicular susceptibility that, if plotted as a function of the concentration of magnetic atoms, show a clear similarity to the concentration dependence of the ordering temperature itself. Some results of the two-dimensional nearly Ising system K2CopMg1−pF4 are also presented.
Critical-Point Behavior of FeBO3 Single Crystals by Mössbauer Effect
Authors: Eibschütz M. , Pfeiffer L. , Nielsen J. W.
Published: February 2004

Mössbauer effect (ME) measurements of the hyperfine (hf) interaction in single crystals of FeBO3, have been used to study the behavior of the sublattice magnetization near the Néel temperature. The ME spectra show that the two nearly antiferromagnetic sublattice moments lie within the easy plane, in agreement with symmetry considerations for weak ferromagnets with calcite structure. From about 0.94≤T/TN≤0.9996 the hf field fits the equation H(T)=H0D(1-T/TN)β with β=0.354±0.005, D=1.208±0.001 and TN=348.35°K to ±0.01°K precision and to ±0.2°K absolute accuracy.
Dynamical Aspect of Magnetic Critical Phenomena
Authors: Kawasaki Kyozi
Published: February 2004

Dynamical behaviors of magnetic systems near the critical points in the nonhydrodynamical regime are considered. For this purpose we construct a kinetic equation to describe the time evolution of critical fluctuations that corresponds in a sense to the Boltzmann equation of dilute gases. A graphical technique is developed to deduce the self-consistent equation for time-correlation functions of critical fluctuations from this kinetic equation, and the properties of the time-correlation functions of spin fluctuations for the Heisenberg isotropic and planar ferro- and isotropic antiferromagnets are discussed. The results are then applied to the high-frequency sound attenuation in magnetic systems.
Hyperfine Interactions in the 5d Transition Series
Authors: Freeman A. J. , Mallow J. V. , Bagus P. S.
Published: February 2004

Hyperfine interactions at nuclei of 5d transition ions have been studied by means of spin-unrestricted Hartree-Fock (SUHF) calculations. For the divalent ions Hf2+ through Pt2+, the core polarization field per unpaired spin is constant to within 9%, varying from −700 to −760 kG. Comparison is made with the few NMR, EPR, and Mössbauer experiments available to date. Fair agreement is found between theory and experiment, indicating the need for more experiments and additional information about relativistic contributions to the hyperfine interaction.
Anomalous Temperature Dependence of the Hyperfine Field at 119Sn Nuclei in Co and Co–Ni Alloys
Authors: Huffman G. P. , Dunmyre G. R.
Published: February 2004

The temperature dependence of the hyperfine field for 119Sn nuclei in Co is anomalous as it decreases much faster with increasing temperature than does the magnetization. For Sn119 in Ni, on the other hand, the hyperfine field-temperature dependence is quite normal. We have performed Mössbauer studies from 4.2°K to the Curie point on alloys of composition Co1−xNix (x=0.4, 0.6, 0.8) that contain 1 at.% 119Sn. The anomalous temperature dependence decreases in a fairly uniform fashion as the composition changes from Co to Ni. A possible explanation of this behavior is that the Sn impurity atom and some fraction of its nearest neighbors may act as a spin polarized unit which behaves like a localized moment in the ferromagnetic host. If one assumes that, the anomalous temperature dependence can be explained by a molecular field model.
Magnetization-Ripple Internal Fields in Thin Magnetic Films Measured by Ferromagnetic Resonance
Authors: Hoper J. H.
Published: February 2004

The internal fields presumably due to magnetization ripple are measured using parallel ferromagnetic resonance techniques. At these frequencies (>400 MHz) the ripple does not have time to follow the average magnetization, hence its full internal field can be detected. In low-frequency techniques (<10 KHz) only the pinned ripple walls significantly contribute. To properly measure the resonance condition, the point at which the phase angle between the average magnetization and the rf field is 90° is detected. The resulting internal fields have the sign and power dependences which agree with Harte's nonlinear ripple theory. It was assumed that the internal field Hi is the following function of the uniform effective field on the average magnetization, Hu; Hi=K/Hun. From film to film, the measure of n's ranged from 0.4 to 1.6. Therefore, according to Harte's model, the physical size of the inhomogeneities causing magnetization ripple can range from the size of the crystallites to many microns. The measurements also indicate that the total effective field on the average magnetization, Heff=Hu+Hi, versus Hu has a minimum. This is contrary to Harte's non-linear theory. The discrepancy is attributed to severe pinning of the ripple walls, which occurs for small values of Hu. When this happens, the equilibrium conditions on which Harte bases has calculations are no longer true.
Polycrystalline Ferrite Films for Microwave Applications Deposited by Arc-Plasma
Authors: Harris D. H. , Janowiecki R. J. , Semler C. E. , Willson M. C. , Cheng J. T.
Published: February 2004

The arc-plasma spray deposition has been successfully applied to fabrication of planar ferrite microwave integrated circuits. Powdered commercial sintered microwave ferrites were deposited at rates up to 2 mils/min on 1 sq in. dielectric substrates by arc-plasma spray deposition. Process controls produced densities >99% theoretical and also maintained stoichiometric agreement between deposited films and starting material. Annealing expanded <0.1 μ grains to useful sizes of 1 to 20 μ and controlled cation distribution. Special annealing techniques were necessary for ferrites having volatile products such as Ni–Zn compounds. Magnetic and microwave properties of several deposited Mg–Mn ferrites and CeYIG agreed closely with bulk values. Remanence ratios up to 0.87 were noted. Practical application of arc-plasma deposition required material compatibility studies at high temperature. These considerations often superseded microwave properties as criteria for material section.
Ferrochromite Films for Microwave Applications
Authors: Braginski A. I. , Oeffinger T. R.
Published: February 2004

We previously mentioned the possibility of preparing polycrystalline Mg–Mn ferrochromite films by Chemical Transport Deposition (CTD) using HCl as a reaction gas.1 The thermodynamic calculations, however, indicated that the ferrochromite chemical transport should be enchanced when using chlorine Cl2 in place of HCl. The aim of this paper is to report the effect of Cr3+ substitution on the deposition rates and on the properties of deposits grown in Cl2 containing atmosphere. The objective was to evaluate the feasibility of CTD-grown films for C- and S-band ferrite device applications. We performed our experiments using source samples belonging to the system Mg1.2Mn0.2Cru-Fe1.6−uO4 where 0≤u≤1.2. Film deposition rates were decreasing slightly with increasing u. Rates of 10 to 50 mg h−1 cm−2 were, however, achieved in the u range of interest. Typical thickness of films was 250 to 400 μ. The 4πMs of deposits was controlled by Cr3+ substitution down to zero at u=1.1 (25°C). The ΔH and tanδε were measured at X band. The dielectric losses were rather low (tanδε≅10−3) and independent on u. The ΔH was increasing with u due to increasing compositional nonuniformity. Even though minimization of this effect was not attempted, for films having 4πMs values between 2300 and 1500 G (0≤u≤0.65) the ΔH was between 220 and 350 Oe. We concluded that ferrochromite films with not too high Cr3+ content may be useful for technical applications.
Ferrite Film Circulator
Authors: Hartwig C. P. , Readey D. W.
Published: February 2004

A ferrite film circulator is demonstrated for use in microwave integrated circuits. An analysis describes the performance of the circulator as a function of the film thickness. Specifically, it is found that the maximum possible bandwidth is proportional to the ratio of film thickness to overall thickness. Thus, a minimum film thickness is required to provide the bandwidth necessary for the device to be useful. Practical limits on impedances realizable with microstrip put another lower limit on thickness.
Microwave Properties of Fine Grain Polycrystalline Yttrium Iron Garnet
Authors: Patton Carl E.
Published: February 2004

Parallel pump and subsidiary absorption thresholds at 9.1 GHz and effective linewidth (ΔHeff) at 9.9 GHz have been measured for polycrystalline YIG samples with average grain diameter a0 from 1 to 30 μ. The spin-wave linewidths extrapolated to zero wavenumber, ΔHk→0, vary as a0−1. The ΔHk for a0=1 μ shows an inverse k dependence. A simple model in which the spin-wave transit time across individual grains limits ΔHk explains the a0−1 dependence but not the observed k dependences for ΔHk. The high-field ΔHeff is 5.5 Oe for a0=1 μ and varies as a0−1/2. These ΔHeff are significantly larger than single-crystal linewidths. Two-magnon interactions cannot contribute to the high-field ΔHeff. A recently proposed magnetostriction mechanism is consistent with the a0−1/2 dependence, but the predicted magnitudes are much smaller than observed.
Ferromagnetic Resonance and Nonlinear Effects in Ferrites with Uniaxial Anisotropy
Authors: Dixon Samuel , Weiner Maurice , Coin Thomas R. Au
Published: February 2004

A study of the ferromagnetic resonance properties in single crystal Zn2+–Ti4+ substituted barium ferrite (BaZnxTixFe12–2xO19) with uniaxial anisotropy has been performed over the frequency range 23.2–37.6 GHz. By varying the amount of zinc and titanium substitution, the anisotropy field can be varied from 17 000 to 5000 Oe. This investigation concentrated on the composition in which x=2.25. These samples had an anisotropy field of 6590 Oe and a saturation magnetization of 2300 G. Cavity perturbation techniques were used with polished spherical samples 15–20 mils in diameter. The measured resonance linewidth along the easy axis was 43.2 Oe at 23.2 GHz and increased to 82.5 Oe at 37.6 GHz. Extrapolation of resonance data gave γ=2.75 MHz/Oe. As the incident rf power was increased at 36.1 GHz, nonlinear effects were observed. The threshold field for spin wave excitation was 0.86 Oe. Using an extension of nonlinear spin-wave theory to include uniaxial anisotropy, an intrinsic spin-wave linewidth of 0.97 Oe was calculated.
Spin-Wave Damping and Hydrodynamics in the Heisenberg Antiferromagnet
Authors: Harris A. B. , Kumar D. , Halperin B. I. , Hohenberg P. C.
Published: February 2004

The Dyson-Maleev formalism is used to calculate the decay rate of antiferromagnetic spin waves at low temperatures and long wavelengths. Various regimes must be distinguished depending on the relation between the wavevector k, the temperature T, and the anisotropy energy. For the isotropic system the relevant parameters are (a) the incident energy, (b) the thermal energy, (c) the deviation from linearity (``curvature energy'') of thermal spin waves, and (d) the curvature energy of the incident spin wave. In the anisotropic case the damping of the k=0 mode has the same dependence on spin-wave energy as in the isotropic system. In all cases, the decay rate is small compared to the frequency, which implies that the spin waves are appropriate elementary excitations for small k and T, and that they interact weakly among themselves in this limit. For k→0 with T fixed, the decay rate is proportional to k2 in the isotropic system. This agrees with an earlier hydrodynamic prediction and contradicts previous microscopic calculations. In this low-k limit the full spin-spin correlation function is calculated, and it agrees with the hydrodynamic form proposed earlier. The possibility of experimental verification of these predictions is briefly discussed.
Kinetic Theory Calculation of the Spectral Density for RbMnF3
Authors: Reiter G. F.
Published: February 2004

An approximate analytic expression for the spectral density in a Heisenberg paramagnet, obtained from a kinetic theory suggested by the author, is used to calculate the frequency and wavevector-dependent neutron scattering cross section in RbMnF3. The theoretical predictions are compared with measurements of the neutron scattering cross section at 3.5TN, for various wavevectors throughout the Brillouin zone, and are found to be in good agreement.
Promise of Optical Memories
Authors: Rajchman Jan A.
Published: February 2004

A concept based on optical techniques may provide a long-sought ideal: a mass memory with archival permanence yet electronic accessibility. It is based on hologram storage on an erasable medium and compound addressing by pages and within pages. An array of light valves composes the page to be stored. The direction of illumination of the page composer, resulting from the deflection of a laser beam, determines the location on the storing medium at which a hologram of the page will be recorded (and the previous record erased). Addressing a record by a reference beam projects the stored pattern on a sensor array. The page composer and sensor array are physically combined with an electrically addressable transistor memory. In operation, the transistor memory acts as the internal memory of a computer that can be filled and emptied by a single pulse of illumination. A memory of 1010 bits with 105 pages accessible in microseconds and 105 bits per page accessible in fraction of a microsecond appears possible in the following demonstrated respects: holograms in films of MnBi, acoustic deflection, integrated sensor and memory arrays, and optic arrangements. There are resonable expectations for light shutter arrays and sufficient pulse power from lasers.
Magneto-Optic Properties of Quenched Thin Films of MnBi and Optical Memory Experiments
Authors: Chen D. , Aagard R. L. , Liu T. S.
Published: February 2004

Magnetic, magneto-optic, and optical properties of thin films of MnBi prepared in the quenched high-temperature phase have been measured. The saturation magnetization is reduced from normal phase by 23% and the coercive force is more than doubled at room temperature. In the visible spectrum region, there is no significant change in the optical absorption, but the strong wavelength dependence of the magneto-optic Faraday rotation in the normal films is essentially eliminated. At the 6327-Å HeNe-laser wavelength the room-temperature Faraday rotation is reduced to one-half of that of the normal films. These properties coupled with the low Curie temperature of 180°C make the quenched MnBi films well suited for optical memory applications. Experimental results of laser Curie-point writing, erasing, and reading using the quenched films have been obtained. In comparing with the normal films, the laser writing power is reduced by a factor of four; unheated spots are not disturbed by the erasure field; and the margin between the writing temperature and the decomposition temperature is much increased. The measured activation energy and annealing time constant associated with the transformation of this high-temperature phase to the normal phase indicates that the thermal stability of the quenched MnBi films is lacking for memory applications.
Optical Scanning for a Magneto-Optic Memory
Authors: Hunt R. P. , Magyary Andrew K. , Dickey Baron C.
Published: February 2004

To make a viable magneto-optic memory system a suitable scanning device is required. A mechano-optical scanner is discussed which is capable not only of digital laser beam recording onto magnetic films deposited onto a tape substrate, but also of wideband magneto-optic detection by means of the longitudinal Kerr effect. A signal-to-noise ratio in excess of 30 dB is demonstrated for a 1-MHz band-width with a packing density (including guardband) of 7 megabits/in2.