bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of the magnetization in high fields in the diluted antiferromagnet Fe46%Zn54%F2
Authors: Lederman M. , Hammann J. , Orbach R.
Published: July 2002

Magnetization measurements on the diluted antiferromagnet Fe46%Zn54%F2 were performed in fields up to H≂8 T. The difference between the Néel temperature TN = Tc (H=0) and the temperature where (δM/δT)H is maximum scales as H2/φ, with φ=1.46(2) and φ=1.50(2) for zero-field-cooled (ZFC) and field-cooled (FC) procedures, respectively. The temperature where the ZFC and FC curves split from one another, Teq(H), yields a larger φ=1.56(4). The variation of the magnetization in the FC case, for T < Teq(H), is interpreted as being internal to the domains, while the larger value of (δM/δT)H in the ZFC case compared to the FC is attributed to the critical behavior in the random-field Ising model (RFIM) regime around the transition temperature Tc(H).
Magnetic properties of submicrometer-sized nickel particles prepared by a low-temperature nonaqueous reductive technique employing polyols (abstract)
Authors: Nanna L. C. , Bush R. L. , Arajs S. , Rasmussen D. H. , Skodzinsky M. J.
Published: July 2002

A simple method of producing small nickel particles (<1 μm) of narrow size distribution has been explored. The particles have been formed by stirring and heating (with reflux) a solution of nickel(II) chloride hexahydrate, sodium hydroxide, and a polyol (ethylene, tetraethylene, or polyethylene glycol) both in the presence and absence of an added surface active compound (polyvinylpyrrolidone). The polyol functions as a nonaqueous solvent and ultimately as a reducing agent through its thermal decomposition, reducing the nickel hydroxide suspension precipitated from the nickel(II) chloride hexahydrate with sodium hydroxide in solution. Sodium hydroxide serves also to catalyze the reductive reaction. Chief parameters influencing the mean size, distribution of sizes, and morphology of particles produced by this method are temperature of the reaction, selection of the solvent/reducing agent, concentration of the added surfactant, and concentrations of the reactants. The particles have been examined by scanning and transmission electron microscopies and x-ray diffraction. Magnetic measurements have been performed at room temperature in a vibrating sample magnetometer. Special consideration has been given to the coercivity and reduced remanence.
Fe-Zr sputtered thin films for perpendicular magnetic recording media
Authors: Nakagawa Shigeki , Tanaka Hiroki , Nitta Michio , Naoe Masahiko
Published: July 2002

Fe-Zr sputtered thin films with large perpendicular magnetic anisotropy were prepared by facing targets sputtering (FTS) apparatus. Perpendicular anisotropy was clearly observed at Zr content around 10 at. %. The substrate temperature Tsub of ∼100 °C resulted in the large perpendicular magnetic anisotropy energy Ku of 1.2 × 106 erg/cc and the high perpendicular coercivity Hc⊥ of 1.0 kOe in the Fe-Zr sputtered film with the saturation magnetization 4πMs of 6.7 kG. It was also shown that Tsub higher than 200 °C caused the decline of Ku and the lowering of 4πMs. It is concluded that Fe-Zr sputtered film may be applicable for the media in the perpendicular magnetic recording system.
Analysis of magnetic multilayer anisotropy using an ultralow-compliance-torque magnetometer
Authors: Pechan Michael J. , Bait Michael E. , Paul Bradley D.
Published: July 2002

A torque magnetometer has been designed to make measurements with 0.01-dyn cm resolution. The system was constructed with a Cambridge Technologies 20-nm/dyn-force transducer and was calibrated with a current-carrying coil. A novel analysis technique is presented to determine anisotropy constants in thin-film samples at fields much less than the material’s saturating field. Results from a Fe/Cu multilayer sample are reported.
Automatic measuring system for microregion magnetic field distributions by means of a SEM
Authors: Chen Hougyou , Liu Yan , Yang Jiongfeng , Yi Jingzeng
Published: July 2002

An automatic measuring system used to measure the microregion magnetic field distribution by means of a SEM has been built. The physical principle is to use the interaction of Lorentz force between electron beam and sample magnetic fields to test the magnetic field. A precisely focused electron beam resulting from a SEM with 5000–20 000-V accelerating voltage is used as the magnetic field detector. The image collection system is formed of PCP (photocon plate), camera, and a high-speed image processing interface. By using a scanning control interface, the selected-area, adjustable step-length electron beam scanning can be completed. The spatial resolution of this measuring system is better than 0.1 μm. The magnetic field sensitivity for measuring VHS mode video magnetic head is about 20 Oe. Using a soft magnetic ring with a little gap, we calibrate the measuring system and the error is <10%. The lower cost of this system makes it very practical.
Measurements of nonreciprocal optical effects on magnetic and superconducting materials using a fiber-optic gyroscope (abstract)
Authors: Spielman S. , Fesler K. , Eom C. B. , Geballe T. H. , Fejer M. M. , Kapitulnik A.
Published: July 2002

We have modifed a fiber-optic gyroscope based on the Sagnac interferometer to measure nonreciprocal phase shifts. The instrument has a sensitivity of better than 1 μrad and is insensitive to any reciprocal phase shifts. Thin films of high-temperature superconductors (HTSC) have been measured in search for nonreciprocal effects below Tc due to ‘‘Anyon superconductivity’’ ground state. No nonreciprocal phase shift was observed in any of the measured samples.1 The Faraday effect in various magnetic thin films (e.g., EuO) have been measured using the instrument showing a great sensitivity to submonolayers of the materials.
Two- to three-dimensional recoupling in AgMn/Cu multilayers
Authors: Stubi R. , Bass J. , Cowen J. A.
Published: July 2002

The return from ‘‘quasi-two-dimensional’’ to three-dimensional (3D) behavior of the spin-glass Ag0.91Mn0.09 is studied in multilayer samples of Ag-Mn layers of width 4 nm alternated with Cu interlayers of width 1 nm ≤ Wil ≤ 60 nm. For Wil ≳ 30 nm, the SG layers are magnetically decoupled; as Wil decreases below 30 nm, the quasistatic spin-freezing temperature Tf gradually rises toward the bulk spin-freezing temperature Tbf. When normalized to Tf/Tbf, the Ag-Mn data increase with decreasing Wil in a fashion similar to previous data for Cu-Mn. The data are compared with three different equations.
Picosecond spin-polarized photoemission studies of magnetic materials (invited) (abstract)
Authors: Meier F. , Vaterlaus A. , Aeschlimann M. , Guarisco D.
Published: July 2002

With pulsed lasers as light sources, a new and important element is added to photoemission: time resolution. In order to make use of the high light intensities available, the experiment should measure the spin polarization of the total photoyield since this quantity is insensitive to space-charge effects. The following examples are discussed: (1) For amorphous GdTbFe, a magneto-optical storage material, it is shown that the speed of thermomagnetic writing critically depends on whether the initial temperature of the sample is above or below the compensation temperature. (2) The response of the magnetic order to pico- and nanosecond heating pulses has been measured in iron and other materials. It is found that the spin polarization of the electrons emitted from Fe by 30-ps pulses does not depend on the pulse energy even if melting occurs. Accordingly, ferromagnetic relaxation in Fe takes place on a time scale which is considerably longer than 30 ps. (3) Using optical spin orientation, the dynamics of the band bending induced by high-intensity laser pulses has been studied in a p-GaAs/Cs Schottky barrier. The buildup time for the photovoltage created by the electron-hole pairs excited in the band-bending region is of the order of several hundred picoseconds.
Influence of adsorbates on surface magnetization and Stoner excitations of Ni(110) (abstract)
Authors: Kirschner J. , Hartung J.
Published: July 2002

The change of the electronic structure and magnetic properties of Ni brought about by adsorption of reactive gases has received much attention over the past years. A number of studies using a variety of techniques has lead to conflicting results concerning the behavior of the exchange-split conduction bands, while there is general agreement that, e.g., oxygen adsorption leads to a strong decrease of the surface magnetization. We use high-resolution spin-polarized electron-energy-loss spectroscopy on Ni(110) to study the spectral distribution of Stoner excitations in the near-surface region upon adsorption of CO and O2. As shown previously, the spin-dependent part of the energy-loss spectrum provides a measure of the exchange splitting at the Fermi surface via the excitation of Stoner pairs in a two-electron exchange process. Our results show a drastic reduction of the surface magnetization even for very small exposures (≤0.5 L), in agreement with previous work. The spectral distribution of Stoner excitations, however, does not change its shape. It is concluded, that the decrease of the surface magnetization is not due to a gradual reduction of the exchange splitting upon gas adsorption. Alternative mechanisms (e.g., the formation of a paramagnetic or antiferromagnetic surface layer) are discussed.
Simultaneous measurement of dynamic strain and magnetization in metallic glass ribbons
Authors: Vohra S. T. , Bucholtz F. , Dagenais D. M. , Koo K. P.
Published: July 2002

A novel technique that simultaneously measures the dynamical magnetic and magnetostrictive properties of transversely annealed metallic glass ribbons (2605S-2) is reported. Magnetostriction was measured with a fiber-optic interferometer while magnetization was obtained with an induction probe. Below resonance, close agreement was found in the values of the anisotropy field Ha as obtained by each method. Near resonance, the equilibrium coherent rotation model (CRM) is no longer applicable and the dynamics of the sample must be included explicitly in order to derive magnetomechanical parameters under ac drive conditions. Certain parameters were found to depend on the magnitude of the ac excitation field suggesting that nonlinear magnetization must also be considered in the CRM.
Generation of a transverse field for annealing amorphous alloys (abstract)
Authors: Silgailis J.
Published: July 2002

In amorphous alloys, the direction and magnitude of the magnetic field during annealing has a profound effect on the induced magnetic properties. Some applications require the annealing field to be at right angles to the ribbon being annealed. Only cumbersome methods exist for generating a field other than along the length of the ribbon. The purpose of this paper is to demonstrate a new and improved method for generating a field 30°–90° away from the long axis of the ribbon. Two sheets of electrical steel are bent into a square tube such that one of the sides is inside the annealing furnace with the edge of the tube along the length of the furnace. The two sheets of steel are separated by a spacer to permit an amorphous ribbon to be passed between the sheets. The steel sheets are lap-joined outside the furnace and enclosed in a coil. Magnetic field is set up in the electrical steel by means of dc current in this coil. The field strength in the steel is adjusted by means of the electrical current and the direction by twisting the whole tube. The amorphous alloys have a much lower coercivity and a much higher permeability than common electrical steel, thus, the induced field will cover the required range and the magnetic flux will be transferred to ribbon during annealing.
Choice of object functions for inverse problem optimization in electromagnetics (abstract)
Authors: Hoole M. Ratnarajan R. , Hoole S. Ratnajeevan H.
Published: July 2002

In the analysis of inverse problems in electromagnetics, the object function that is defined not only is a minimum at the optimum, but also zero. As a result, the Newton algorithm which reaches the neighborhood of the optimum rather quickly, degenerates because of the division of a zero by a zero. This paper takes up two issues. First, we investigate the use of an alternative function that gives the optimum not by a minimum but when it is zero. This allows the Newton algorithm to be used without restriction. Second, we take up the case where the usual function with zero minimum is used, but the second derivative (the Hessian) is nonsingular; now the design parameters are sought using the second derivative of the object function when the computation by the first method becomes sluggish. It is further shown that the second derivative of the object function is naturally computable from the finite-element trial functions and the solution.
Critical spin fluctuations of Ni monolayers at the Curie temperature: A magnetic resonance study in UHV (invited)
Authors: Li Yi , Baberschke K. , Farle M.
Published: July 2002

Measurements of the critical behavior of the magnetic resonance linewidths ΔHpp from thin Ni(111) films on W(110) ranging in thickness from 15 to 40 Å are presented. A huge peaklike broadening of ΔHpp is observed over a narrow temperature interval near the thickness-dependent Curie temperature TC(d). This peak results from the critical ‘‘speeding up’’ of the relaxation rate Γ ∝ ΔHpp near TC due to spin fluctuations. This finding is discussed in relation to results on Gd monolayers, which show no critical broadening of the resonance linewidth. From the peak in ΔHpp, we directly determine TC(d), e.g., TC(15 Å)=512 K, and the shift exponent λ=1.42±0.3 for the thickness dependence of TC. Below TC, ΔHpp is analyzed in terms of the Landau–Liftshitz equation of motion, which yields ΔHpp ∝ (1 − T/TC)−β, with β the critical exponent of the spontaneous magnetization. A power law with β ranging from β=0.38 (bulk Ni) to 0.29 (15 Å) is found for ≊ 10−3 ≤ (1 − T/TC) ≤ 6 × 10−2.
Magnetization stability in the magneto-optical readout of TbFeCo disks
Authors: McDaniel T. W. , Finkelstein B. I.
Published: July 2002

An experimental and computer simulation study of the stability of thermomagnetically written domains during magneto-optical (MO) readout has been conducted. Using standard TbFeCo disks from various sources, the variation in signal amplitude and jitter versus readout laser power, bias field, and irradiation time was measured. With this data, safe regions for readout operation over the life of an erasable optical storage device can be mapped out. To gain insight into the mechanisms of thermomagnetically induced domain instability and the associated degradation in carrier-to-noise ratio and jitter performance, a micromagnetics simulation with its companion laser heating model was applied to cases of long-term MO readout. The graphic displays of the micromagnetics model exhibit the onset of induced domain-wall irregularity associated with the increase of readout noise and jitter. In extreme cases, noticeable mark size changes corresponding to partial erasure effects are predicted.
Improvement in the sensitivity of the magneto-optical media
Authors: Hatwar T. K. , Stinson D. G.
Published: July 2002

Future magneto-optical drives will spin disks at higher speeds to reduce latency and increase data rate. Since optimum recording power increases approximately linearly with disk velocity, and due to limited available power from laser diodes, high sensitivity magneto-optic media will be required. We have enhanced the writing sensitivity of conventional rare-earth transition metal alloy based TbFeCo media with the addition of a fourth element, such as Zr. We studied the static and dynamic performance of TbFeCo-Zr media as a function of Zr addition, as well as Tb, Fe, and Co concentrations. We found that alloying TbFeCo with about 5-at. % Zr, the writing sensitivity was increased by more than 40%. At higher Zr concentrations (≳15 at. %), the media becomes sufficiently sensitive to restrict read powers to unacceptably low levels, the domain boundaries become ragged giving rise to high noise levels, and carrier levels decrease due to reduced Kerr rotation. Using an AlN antireflection layer on TbFeCo-Zr5 media, a CNR of 60 dB in a 30-kHz resolution bandwidth was obtained at 3.5-μm mark lengths. An additional advantage of alloying with Zr is that Zr oxidizes readily and segregates to the surface to form a protective layer enhancing the environmental stability of the media significantly. Thus a high sensitivity, high performance, and high corrosion resistance magneto-optical disk can be prepared using TbFeCo-Zr media.
A new method for reducing hysteresis of torque sensors: Cross magnetic shaking
Authors: Sasada I. , Nagano E. , Harada K.
Published: July 2002

Cross magnetic shaking, a new method for reducing hysteresis, is proposed for shaft torque sensors based on the magnetostrictive effect of the steel shaft. In this method, an auxiliary magnetic field (shaking field) is superposed perpendicularly to the exciting field while differences in permeabilities along lines of tension and of compression are detected by the field at the shaft surface. Hysteresis measured for a quench-hardened steel shaft of 25 mm diameter under the applied torque from −490 to 490 Nm was reduced by about nine times and sensitivity increased by a factor of 2.
Calculation of the static magnetization of metamagnetic La2CuO4
Authors: Coffey Dermot
Published: July 2002

Interactions in La2CuO4 are predominantly antiferromagnetic. However, a strong field-induced transition is seen in the static magnetic susceptibility which points to the existence of a net ferromagnetic moment in the CuO layers. Here, this net ferromagnetic moment is incorporated into a description of the CuO layers by the inclusion of a modified Dzyaloshinskii–Moriya term in the Hamiltonian. The spin-wave spectrum in the presence of an applied field is calculated with this Hamiltonian and the ground state is determined as a function of applied field from the free energy. A calculation of the static magnetization as a function of applied field perpendicular to the CuO layers is used to determine the parameters of the model. Comparison is made with experiment.
ac susceptibility studies on lead-substituted Bi-Sr-Ca-Cu-O superconductors
Authors: Hammer A. , Srinivasan G.
Published: July 2002

The temperature, frequency, and ac field dependence of the ac susceptibility components χ′ and χ″ have been studied in polycrystalline samples of Bi2−xPbxSr2Ca2Cu3Oδ with x values ranging from 0.2 to 0.3. The data do not show any evidence for the presence of low-Tc phases in the samples and perfect diamagnetism is observed at about 80 K. Data on χ″ vs T show the expected intrinsic peak near Tc and a coupling peak at a lower temperature Tp. For a specific field and frequency, Tp is found to increase with increasing lead substitution. The activation energy for flux creep, determined from the frequency dependence of Tp, is much smaller than the values reported for Y-Ba-Cu-O superconductors. The ac field H dependence of Tp is found to follow the relation 1 − Tp/Tc∝Hq with q =1/2. The estimated q value does not compare favorably with the values reported for Y-Ba-Cu-O compounds.
Magnetic properties of Fe/Cr and Fe/Cu superlattices (abstract)
Authors: Fullerton Eric E. , Schuller Ivan K. , Parker F. T. , Grimsditch M. , Sowers C. H. , Robertson W. , Kumar Sudha , Beliën Ph. , Bruynseraede Y.
Published: July 2002

Both the static and dynamic magnetic properties of sputtered Fe/Cr and Fe/Cu superlattices are compared. The static magnetization is measured by vibrating sample and SQUID magnetometry as well as conversion electron Mössbauer spectroscopy. The spin-wave spectra are measured by Brillouin light scattering (BLS). Although the structure of these two systems as determined by x-ray diffraction are similar, the magnetic properties differ greatly. In Fe/Cr, the Fe layers couple antiferromagnetically (AF), which can be correlated with an enhanced magnetoresistance. Both the strength of AF coupling and the size of the magnetoresistance are found to have a maximum value for Cr thicknesses of 11 Å and increase with decreasing Fe thickness. Broad spin-wave spectra are observed by BLS and there is little indication of surface anisotropy. In contrast, Fe/Cu shows no indication of AF coupling of the Fe and a perpendicular surface anisotropy is observed. Very well-resolved spin-wave modes showing the band structure expected for a ferromagnetically coupled magnetic superlattice are observed in the BLS spectra. The relationship between the interface structure and the varying magnetic properties will be discussed.
Exchange coupling of ferromagnetic layers across nonmagnetic interlayers (abstract)
Authors: Vohl M. , Wolf J. A. , Grünberg P.
Published: July 2002

The problem of exchange coupling of ferromagnetic layers across nonmagnetic interlayers is of fundamental importance in layered magnetic structures. It is also of importance where these structures are considered for applications. Previously a method has been described how to obtain quantitative information on interlayer coupling from Brillouin light scattering (BLS) from spin waves.1 Here we would like to present new results obtained from samples with wedge-shaped interlayers and discuss the strength of the coupling as a function of the interlayer material and thickness and the growth conditions. In situ characterization of the growth was performed by means of Auger spectroscopy, RHEED, and SPALEED investigations. In some cases like for Fe layers separated by Cu the coupling appears to depend strongly on the crystallinity of the interlayer. Epitaxial interlayers decouple the ferromagnetic films stronger than polycrystalline ones. This observation and its relation to the sometimes observed antiferromagnetic-type interlayer coupling will be discussed.
Magnetic properties of titanomagnetites Fe3−xTixO4 (0≤x<1)
Authors: Kakol Z. , Sabol J. , Honig J. M.
Published: July 2002

Magnetization measurements have been carried out for Fe3−xTixO4 single crystals in the range 0≤x<1. The calculated cubic K1 anisotropy constant passes through a minimum at x≊0.2 and rises abruptly for larger x. This behavior can be correlated with progressive increase in Fe2+ occupancy of the tetrahedral interstices for x≳0.2.
Magnetic structure of UPb3 and UIn3: A 119Sn Mössbauer study (abstract)
Authors: Yuen Tan , Lin C. L. , Perez I. , Crow J. E. , Bykovetz N.
Published: July 2002

The temperature dependence of Mössbauer spectra and magnetic susceptibility has been measured for U(In1−xSnx)3 and U(Pb1−xSnx)3. The low temperature 119Sn Mössbauer spectra of U(Pb1−xSnx)3 for 0.02≤x≤0.85 and U(In1−xSnx)3 for 0.02≤x≤0.5 show the existence of magnetic hyperfine field at the Sn site. The results for both systems indicate that the antiferromagnetic structure for the Pb- and In-rich pseudobinary alloys is not as previously reported based on interpretation of neutron diffraction data. A magnetic structure is proposed with a magnetic unit cell of (2a,2a,2a) (where a is the lattice constant of crystal unit cell) and the U magnetic moments alternating between the eight possible (1+,1+,1+) directions for both systems. The new structure is consistent with both neutron diffraction and Mössbauer results.
SEMPA studies of CoNiCr and its correlation with noise at different write currents (abstract)
Authors: VanZandt T. , Browning R. , Lee Sang Y. , Khan Mahbub R. , Pressesky J. L. , Duan S. L. , Heiman Neil
Published: July 2002

Recently, there has been much interest in the noise characteristics of recording media.1 These characteristics have not been completely explained by current theoretical models.2 This paper reports on the noise characteristics as a function of write current and its correlation with written bits as observed by SEMPA.3–5 CoNiCr film sputtered on Al-based NiP substrate was used for this study. The grain structure of this sample was obtained by high-resolution SEM, and the magnetic microstructure was observed by utilizing the SEMPA technique. The saturation magnetization MS and coercivity HC for this film are 731 emu/cc and 794 Oe, respectively. The noise measurements show that as the write current is increased from zero, a well-defined noise maximum is observed at a write current of about 8.5 mA o-p. With further increase in write current, noise voltage decreases to a minimum at about 14 mA o-p, and then rises again showing a secondary, broad noise maximum at a write current of about 34 mA o-p. The SEMPA images of the written transitions at those three write currents of 8.5, 14, and 34 mA o-p have been obtained. These SEMPA images show a strong correlation with the noise behavior of this media. At the first noise maxima, where the signal amplitude is high, the magnetized regions show some cross links and the transition region shows irregularities. For the intermediate write current where noise is low, the cross links between the magnetized regions are minimal. At the secondary maximum, where the readback signal amplitude is very low, the magnetized regions are heavily overlapped and there is practically no identifiable transition regions as observed by SEMPA. The micromagnetic structure of a reverse dc erase track at the noise maximum has also been observed by SEMPA and will be reported along with the noise data.5 The details of this paper appear in Ref. 5.
Be-Cr-doped high-corrosion-resistant magneto-optical disks
Authors: Horiai N. , Ohashi T. , Yoshida S. , Miura M. , Tokushima T. , Fujii T.
Published: July 2002

The addition of Be to TbFeCo thin-film layers results in the gradual deterioration of carrier-to-noise ratio (CNR) and optimized recording power Pw compared with that of Cr. Be and Cr composite-doped disks, whose CNR maintain over 54 dB at a recording frequency of 1 MHz, prevent bit-error-rate (BER) levels from increasing in accelerated environmental tests. It is noted that even prior to environmental stability testing, BER of Be-Cr composite-doped disks is less than that of nondoped disks. From the results obtained by Auger electron spectroscopy analysis, it is explained that this difference is caused by fluctuations in the reflections of disks resulting from varying thicknesses of extra layers formed as a result of reaction between TbFeCo and SiN. Finally, through the write-erase cycle test and differential scanning calorimetry analysis, it has been demonstrated that Be-Cr-doped disks possess a higher thermal durability than nondoped disks.
Magnetic and electrical properties of the ferromagnetic dense Kondo system CeSix (abstract)
Authors: Hill P. , Willis Frank , Ali Naushad
Published: July 2002

Measurements of resistivity, susceptibility, and magnetization for polycrystalline samples of the alloys CeSix (1.60≤x≤2.00) in the temperature range 4–300 K show that samples with x≤1.85 order ferromagnetically while those with x≥1.90 do not. Magnetization measurements using a SQUID magnetometer show Tc to remain fairly constant at 10.1 K for x≤1.70, rising gently to 11.1 K at x=1.80, then decreasing rapidly with increasing x. Magnetization measurements on x≤1.80 show the presence of an anomalous second peak at a slightly higher temperature, which has only been observed in single crystals of CeSix for x=1.70 and 1.71.1 CeSix with x=1.90 shows interesting resistive behavior with characteristics of both the magnetic and nonmagnetic samples. The room temperature resistivity is more than a factor of 100 greater than the other members of the series, which vary from each other by no more than a factor of 3, indicating that this sample is very close to the critical concentration for the magnetic–nonmagnetic transition in this series. High-temperature anomalies are seen in the resistivities of samples with x≥1.90, however, no magnetic correlation is observed.
Magnetic superexchange in YIG and Ca2+:YIG
Authors: Lehmann-Szweykowska A. , Wojciechowski R. , Pust L. , Wigen P. E. , Batra S.
Published: July 2002

Starting with the Anderson periodic Hamiltonian for a system of the narrow- and wide-band electrons, an explicit expression for the superexchange coupling between spins of localized electrons is derived and analyzed. The superexchange coupling is based on the fourth-order p-d hybridization. One special aspect of the problem is systematically discussed at T=0 K. It is shown that the presence of compensating holes produced in Ca2+:YIG by the valence-uncompensated doping, can result in a decrease in the strength of the superexchange coupling and, depending on the concentration of the doping Ca2+ ions, a change of the sign of the coupling constant is obtained. Numerical results are calculated both for the charge-transfer and Mott–Hubbard models for insulators
Anisotropy energy of Y2Fe14B, Y2Co14B, Y2Fe14−xCoxB, and La2Co14B
Authors: Ukai T. , Yamaki K. , Takahashi H. , Mori N.
Published: July 2002

The anisotropy energies of Y2Fe14B, Y2Co14B, Y2Fe14−xCoxB, and La2Co14B are discussed with reference to the band model. The energy bands exclusive and inclusive of the p bands in B are formulated in consideration of the spin-orbit interaction and exchange splitting. The anisotropy constant K1 is estimated through the difference between the calculated electronic energies with the magnetization parallel to [001] and [100]. In Y2Fe14B, the calculated K1 is much the same as the experimental result, and in Y2Co14B, it agrees with the experimental result. In Y2Fe14−xCoxB, the calculated K1 inclusive of the p bands in B is much the same as the experimental result. Finally, the anisotropy energy of La2Co14B is discussed by taking into account the 4f bands just above the Fermi level.
Magnetic phase transitions in α-CeS2
Authors: Klimenko A. G. , Ishikaev S. M. , Tagaev A. B. , Vasilyeva I. G. , Karpenko M. M. , Fischer P. , Furrer A. , Staub U.
Published: July 2002

Physical properties of platelike single crystals (size up to 3×0.2×2 mm3, b axis perpendicular to the plate) of α-CeS2 (monoclinic, space group P21/c) have been studied and magnetic ordering was found below 7 K. Specific heat Cp(T) has a λ-type anomaly at TN=6.7 K with entropy sm=0.94R ln 2 and an additional maximum at 6.0 K (S′m ∼ 0.02R ln 2). Crystal-field splitting of the 2F5/2 state of Ce3+ into three doublets with energy levels 0, 230, and 415 cm−1 was obtained from Cp(T). Magnetic properties were studied using SQUID and vibrating sample magnetometer. Paramagnetic susceptibility obeys a Curie–Weiss law with anisotropic effective magnetic moment and Curie–Weiss temperature: P=2.1 μB, Θ=+7.8 K for b axis and P=1.5 μB, Θ=−0.8 K in directions perpendicular to the b axis. Complicated magnetic behavior is observed in low fields (0.1–100 Oe) below TN (strong dependence of dc susceptibility on thermomagnetic prehistory, cooling rate and applied field, remanent and memory phenomena). Magnetization curves at 4.2 K show a sharp metamagnetic transition at Hc=5 kOe for a field along the b axis and linear dependence M(H) up to 30 kOe for the perpendicular direction. Neutron scattering experiments are in progress to establish details of the magnetic structure and the crystal-field splitting.
Dynamics of magnetic domains and walls
Authors: Slonczewski J. C.
Published: July 2002

General equations are proposed to describe the simultaneous rotations of the magnetization vectors and the displacements of curved domain walls in one pair of magnetostatically coupled magnetic films separated by a variable distance. Leakage-field energy is written in the ‘‘transmission-line’’ approximation. The effects of dissipation and the constraint of flux continuity across a domain wall are handled by d’Alembert’s virtual work principle. The result is a set of coupled equations of the following kinds: (1) dynamic torque balance at each point inside a domain, (2) wall-domain constraint due to flux continuity, (3) boundary condition on domain magnetization which depends on instantaneous wall positions, and (4) wall velocity. Within certain limitations these equations apply to the core of an inductive magnetic recording head.
Existence limits of stable and unstable domain-wall motion
Authors: Fang T. T. , Thiele A. A.
Published: July 2002

We obtain the entire region of existence of one-dimensional steady-state lossless domain-wall motion for the case of orthorhombic anisotropy with an in-plane field applied at an arbitrary angle. We compute the unstable solutions by applying the method used to obtain the stable solutions. Solutions with a nonzero in-plane field may be classified by their correspondences to the four Walker solutions: two stable (Bloch-wall-like) and two unstable (Néel-wall-like) solutions. All three previously identified breakdown mechanisms in stable solutions, domain-wall breakdown, and breakdown in the near and far asymptotic regions, are again found in unstable solutions. The existence limits of stable and unstable solutions are found to share certain boundaries. We find at any given velocity and field the number of solutions is zero, two, or four. We find an analytic solution which occurs within solution regions and as a boundary of the regions.
A generalized harmonic map equation for micromagnetics (abstract)
Authors: Kotiuga P. R.
Published: July 2002

Static micromagnetic configurations are determined by minimizing the sum of anisotropy, demagnetization, exchange, and external interaction energies subject to the constraint that the magnitude of the magnetization vector is constant throughout a sample. If this constraint is not explicitly imposed in a numerical or analytical investigation, a Lagrange multiplier type of term must be appended to the energy functional. In a region R, two candidates are λ∫R(1 − ‖m(r)‖2)2 dV and ∫Rλ(r)(1 − ‖m(r)‖2)dV. The first expression is a Higgs or penalty term in the terminology of physicists and numerical analysts. The penalty parameter λ is given a physical meaning through the Ginsburg–Landau theory.1 The second expression is avoided by physicists and numerical analysts since the scalar Lagrange multiplier λ(r) represents an unwelcomed proliferation of variables. Following the theory of harmonic maps, the scalar Lagrange multiplier field is eliminated in this paper and a generalized harmonic map equation for micromagnetics is obtained. This equation has no extraneous phenomenological constants and enables one to ignore the original constraint in numerical solutions. It also provides a means for tying together results concerning singularities of harmonic maps2 with Block point phenomena believed to exist in VBL memories. This, in turn, enables one to make general inferences about the energetics of Bloch points which are not possible through purely topological investigations or simulations involving specific geometries.
Statistical model for ferromagnetic hysteresis (abstract)
Authors: Pinto Marc A.
Published: July 2002

An application of the Landau theory of phase transitions to the construction of a thermodynamical equation of state for a uniaxial ferromagnetic crystal is presented. This equation of state is taken as starting point for the construction of a phenomenological constitutive law for bulk ferromagnetic hysteresis. This law describes both alternating and rotational hysteresis, reversible and irreversible magnetization, complex minor and major loop behavior, anhysteretic magnetization, and the Rayleigh relationship for weak fields. A geometrical interpretation, which was discussed briefly earlier (see Ref. 1), is given and it is shown that this law can be understood as an extension of the oft-cited Preisach model to the vectorial case. It is also shown that this law compares favorably with other recent extensions (see Ref. 2). Last, the mathematical problems raised by the solution of the Maxwell field equations in the presence of such constitutive laws are discussed.
Spin canting in γ-Fe2O3 particles (abstract)
Authors: Parker F. T. , Foster M. W. , Margulies D. , Berkowitz A. E.
Published: July 2002

It has been reported1 that the spin canting observed in γ-Fe2O3 is confined to a thin surface layer. We have examined by Mössbauer spectroscopy acicular γ-Fe2O3 particles, ∼250 Å in diameter, before and after coating them with 57Fe, which provided a surface layer 1.3 Å thick, if uniform. Measurements were made in applied fields Ha to 60 kOe collinear with the γ-ray direction at temperatures T from 4.2 to 296 K. For the coated particles, 50% of the signal arose from the adsorbed 57Fe. For a given Ha and T, the relative heights of the Δm=0 transitions were the same for coated and untreated particles. This indicates that the spin canting, which is visible at 4.2 K even at 60 kOe, is a bulk phenomenon and is not confined to a thin surface layer. The spectra were fit with a distribution p(H) in hyperfine fields H. A tailing to low-H values from the main (core) peaks was observed for both sets of particles, but was more extensive for the treated particles. Thus the tailing presumably is due to surface Fe with weaker exchange coupling. Fitting2 the magnetic moment vs Ha curves of the untreated sample, we obtained a paramagnetic high-field susceptibility term which increased nearly linearly with decreasing T, as did the Mössbauer canting. Potential implications of these results for magnetization reversal in these particles will be discussed.
Site assignment in CoTi-substituted barium ferrite from Mössbauer spectroscopy (abstract)
Authors: Agresti D. G. , Hai Ou , Hong Y. K. , Chung B. J. , Joo H. Y.
Published: July 2002

As part of our effort to evaluate substituted barium ferrites for particulate magnetic recording,1 we have obtained room-temperature Mössbauer spectra for BaFe12−2xCoxTixO19 (CoTi), with x=0, 0.15, 0.30, 0.45, 0.60, 0.75, 0.80, 0.85, and 0.90. With increasing x, resolution steadily worsens as the 12-K lines broaden and change shape, but monotonic trends are evident for all spectral features, suggesting a gradual change in site occupation and local atomic environment. Following our earlier work on the well-resolved CoMo spectra (up to x=2.0),2 we have successfully fit a five-site Lorentzian model for x<−0.45, however with the 12-K peak width varying independently of the other sites. A three-site model fits the main features of all spectra with a nearly linear decrease with x of hyperfine field (ionic moment) of ∼13%, 11%, and 9%, respectively, for the 4fvi, 4fiv, and 12-K sites, while the 12-K spectral area and hence site occupation increases; this is in contrast to CoMo, where the fields are independent of x and the 12-K area decreases with x. The correct fitting model is of course more complex than either of these, but is greatly clarified by the use of magnetically oriented absorbers with the gamma rays passing parallel to the magnetization direction. This procedure removes lines 2 and 5 from the sextet components and allows the full shape of 12-K lines 1 and 6 to be observed. We will discuss the results obtained in this way and relate them to trends in the bulk magnetic properties of CoTi-substituted barium ferrite.
Neutron scattering studies on lanthanum cuprates (invited) (abstract)
Authors: Hayden S. M. , Aeppli G. , Cheong S-W. , Mook H. A. , Fisk Z. , Rytz D.
Published: July 2002

The results of recent inelastic neutron scattering investigations of the magnetic excitations in La2CuO4 are reviewed. Experiments were performed using a wide range of incident neutron energies. Our study of the paramagnetic phase is consistent with a picture for the magnetic dynamics which shows conventional spin waves at higher energies and an overdamped response due to spin-wave interactions at lower energies. The evolution of the magnetic dynamics with doping in the La2−x(Ba,Sr)xCuO4 system is discussed.
Domain formation in exchange-coupled double layers
Authors: Mergel Dieter
Published: July 2002

Exchange-coupled double layers consisting of two ferrimagnetic layers of different composition of the general system GdTb-FeCo were experimentally studied by magnetic reversal experiments using the Faraday effect. A novel type of magnetic reversal experiment was developed in which one of the layers is in a partly demagnetized state when the other layer is reversed. In order to interpret the resulting complicated Faraday hysteresis curves, phenomenological domain formation models are developed, including the following processes which are not possible in single layers: creation/annihilation of interface wall; reversal of the magnetization against (antiparallel to) the external field; vertical motion of the interface wall; competing reversal processes in the two layers. A method is described to estimate the distribution of compensation temperatures in a layer from reversal curves of double layers.
Effects of epitaxial strain in Er/Lu thin films
Authors: Beach R. S. , Borchers J. A. , Erwin R. W. , Rhyne J. J. , Matheny A. , Flynn C. P. , Salamon M. B.
Published: July 2002

A series of c-axis Er films ranging in thickness from 400 to 9500 Å were grown on lutetium base layers to investigate the effects of epitaxial growth on the magnetic properties of bulk erbium. Neutron diffraction studies show that the basal plane lattice parameter of Er approaches the smaller Lu value as the films are made thinner. Below TN the phase angles of the Er spin modulation are diminished from bulk Er values in the thinner films. For all but two 800-Å films, there is evidence from bulk magnetization and neutron data of a first-order transition to a conical ferromagnetic state, similar to bulk Er. These phenomena are compared to the properties of Er films grown on yttrium, whose basal plane spacing exceeds that of Er. In these systems the turn angles are greater than bulk and the ferromagnetic phase is suppressed. This contrasting behavior is consistent with predictions from a magnetoelastic energy model of epitaxial constraint developed to describe the Er/Y systems.
Development of high power cw KCl:Li (F+2)A color center lasers
Authors: Afzal Robert S. , Schneider Irwin
Published: July 2002

We have developed a KCl:Li (F+2)A color center laser which produces more than 1.2 W of cw laser power, over a factor of 3 greater than the highest power previously reported. This paper contains the results of this study, and presents guidelines for preparing laser crystals to routinely and reproducibly generate these high powers.
The effect of heat treatments on the structure and composition of Al(0.8% Si)/TiW/polycrystalline Si system
Authors: Berger S. , Komem Y. , Weiss B. Z.
Published: July 2002

The effect of heat treatments on the structure and composition of a system consisting of a conductive outer layer made of Al(0.8% Si) thin film, TiW(30:70 at. %) thin film as a diffusion barrier, and a polycrystalline Si substrate was studied. It was established that heat treatments at temperatures ranging between 400 and 500 °C led to the diffusion of Si and Al through the TiW layer, following which Al diffused into the polycrystalline Si, while Si diffused into the Al film. The silicides of TiSi, TiSi2, and intermetallic compounds of Al3Ti and WAl12 were formed at the Al/TiW interface as a result of the 30-min heat treatment at temperatures ranging between 450 and 500 °C.
Excitonic photoluminescence line shape due to interfacial quality in quantum well structures in a magnetic field
Authors: Lee Johngchul , Sanders G. D. , Bajaj K. K.
Published: July 2002

We present a theory correlating the excitonic photoluminescence linewidth in quantum wells with the microscopic structure of the interface in the presence of a magnetic field. The interface is described in terms of microscopic fluctuations δ1 and δ2 where δ1 is the local fluctuation in the well width and δ2 is the lateral correlated extent of the fluctuation. We use Lifshitz’s theory of disordered alloys to determine the probability distribution of fluctuations of the well size over the effective extent of the optical probe, namely, the exciton. The line shape is then calculated from this distribution. We have evaluated the fullwidth at half maximum (σ) for both the heavy-hole exciton and the light-hole exciton as a function of the well size and interface parameters δ1 and δ2 in the presence of a magnetic field in GaAs-Al0.3Ga0.7As and In0.53Ga0.47As-InP quantum well structures. We find that for a given set of values of well width and interface parameters, the application of the magnetic field reduces the effective size of the exciton and thus increases the linewidth.
Influence of excited states on the energy loss of fast ions in a hydrogen plasma
Authors: Kärcher B. , Peter Th.
Published: July 2002

Stopping power calculations of fast ions penetrating a hydrogen plasma target in local thermodynamic equilibrium at arbitrary temperatures are performed. Excited state contributions to the energy loss are included in the framework of the Bethe formalism. Average ionization potentials for the excited ions are given in a quasiclassical approximation. It is shown that the net effect is an enhancement of the stopping power compared to the energy loss when assuming all atoms to be in their ground state.
Effect of laser annealing on electrical and optical properties of n-mercury cadmium telluride
Authors: Dawar A. L. , Roy Savita , Nath Tirlok , Tyagi Sanjay , Mathur P. C.
Published: July 2002

Single crystals of n-Hg1−xCdxTe (x=0.18) were grown by the Bridgman technique. The bulk single crystals were irradiated with laser pulses of various energy densities. A pulsed laser (Nd:YAG) capable of producing 10-ns pulses of 0.53-μm wavelength (frequency doubled) with varying energy densities (2–50 mJ/cm2) was employed. dc conductivity and Hall coefficient measurements were made on the single crystal using the van der Pauw technique in the temperature range 77–300 K, for both as-grown and laser-irradiated samples. Also, transmission measurements of the samples were taken at room temperature. Both electrical and optical studies showed that laser irradiation introduces additional defects in mercury cadmium telluride (MCT), and its quality deteriorates instead of improving as observed in many other semiconductor materials. We found that laser irradiation increases free-carrier concentration and decreases the band gap of MCT.
The thermal stability of Al/Ti-Ta metallization on Si
Authors: Ben-Tzur M. , Eizenberg M. , Greenblatt J.
Published: July 2002

The thermal stability and the interfacial reactions in the metallization system of Al/Ti-Ta/Si for T≤550 °C were studied by x-ray diffraction, transmission electron microscopy, and Auger electron spectroscopy. The analysis of this complex system was made possible by a systematic study of its subsystems. Bilayers of Al/Ti and Al/Ta, tri-layers of Al/Ti/Ta and Al/Ta/Ti, and finally Al on alloy films of Ti-Ta were deposited on Si(100) and studied. The refractory metals interactions with Al started at much lower temperatures than those with Si. In the case of the bilayer systems, Al/Ti/Si and Al/Ta/Si, the onset of interaction with Al was at 300 and 350 °C for Ti and Ta, respectively, resulting in the formation of Al3Ti and Al3Ta. The corresponding temperatures for silicide formation were 500 and 700 °C. For the tri-layer systems the Al overlayer reacted with the top refractory metal at 300–350 °C, while the lower metal reacted with the Si substrate at the corresponding temperature for silicide formation. For the alloy samples, reactions at the Al/refractory alloy interface at 300–350 °C resulted in a mixture of Al3Ti and Al3Ta. The majority of the Ti-Ta film, and especially its interface with Si, remained intact at annealing temperatures lower than 500 °C. For the Ta-rich compositions studied (Ti20Ta80 and Ti50Ta50) at 500 °C Si diffused through the Ti-Ta alloy to the outer region of the contact forming Ti and Ta disilicides, while Al penetrated deeply into the Si substrate. Most of the Ti-rich alloy, Ti80Ta20, however, remained essentially intact even after 500 °C 30-min anneal. At this stage a shallow contact was obtained by the formation of a very thin silicide layer at the substrate interface, while penetration of Al to the substrate was prevented by a limited interaction of Al with Ti and Ta to form the corresponding aluminides.