bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetism and electronic structure of (La,Ba)CoO3 (abstract)
Authors: Singh David J.
Published: July 2002

The (La,Ba)CoO3 perovskites have attracted recent interest because of the discovery of giant magnetoresistance in films of this material that is possibly related to the larger effects in (La,Ca)MnO3. The present density functional studies were performed to elucidate the electronic and magnetic properties of these cobaltates and in the case of LaCoO3 the relationship to the structural parameter giving the rotation of the O octahedra. Cubic perovskite BaCoO3 is found to have a ferromagnetic ground state in agreement with experiment. Although less hybridization is expected in this system relative to manganates, the moment is still reduced noticeably from Hund’s rule value. In LaCoO3 a strong structural instability of the cubic perovskite structure against rotation of the octahedra is found. At least two metastable ferromagnetic states are found and these couple strongly to the distortion related to the instability.
Large moment fluctuations in Fe–Ni anti-Invar (abstract)
Authors: Acet M. , Wassermann E. F. , Andersen K. , Murani A. , Schärpff O.
Published: July 2002

Contrary to the Invar effect, which is a volume enhancement in a magnetically ordered state and which is characterized by a smaller than normal thermal expansion coefficient (α), the anti-Invar effect is a volume enhancement in a paramagnetic state and is characterized by a larger than normal α. Both are caused by large anharmonicities due to the presence of moment fluctuations. The Invar effect occurs only in alloys, whereas the anti-Invar effect is observed in elements like fcc-Fe and fcc-Mn as well as in their alloys with other 3D elements. This property poses the anti-Invar effect as being more fundamental than the Invar effect. In order to understand the properties of moment fluctuations in anti-Invar, we have carried out temperature dependent paramagnetic neutron scattering experiments with 3D polarization analysis on Fe100−xNix alloys (x=10, 15, 20, 25 at. %) in their high temperature fcc states (500<T<1100 K) where they exhibit anti-Invar properties. At lower temperatures, these alloys undergo martensitic transformations into the bcc phase. The results show strong magnetic scattering in the forward direction in all alloys, even at temperatures as high as 1100 K, indicating ferromagnetic correlations, although no long range ferromagnetic ordering occurs in any of these alloys. The magnetic cross section decreases with increasing temperature for x=20 and 25 at. % (500<T<1100 K), and shows the unusual behavior of remaining constant with temperature for x=15 at. % and increases with increasing temperature for x=10 at. % (700<T <1100 K). These observations are discussed by taking into account the competing effects of increasing lattice vibrational amplitude with increasing temperature, favoring the weakening of the correlations, and the anti-Invar property of the increase of the local magnetic moment with increasing temperature, favoring the strengthening of the magnetic correlations.
Spin dynamics in ferromagnetic perovskite manganites (abstract)
Authors: Hwang H. Y. , Dai P. , Cheong S.-W. , Aeppli G. , Mook H. A. , Batlogg B.
Published: July 2002

We have recently studied the spin dynamics in single crystal perovskite manganites by inelastic neutron scattering. These samples exhibit a transition from a high temperature paramagnetic insulator to a low temperature ferromagnetic metal, leading to large negative magnetoresistance near the simultaneous metal–insulator transition temperature and the Curie temperature TC. By comparing samples with different TC, we discuss systematic trends in the spin dynamics above and below TC.
Transport and magnetism in La2/3Ca1/3(Mn1−xCox)O3 and La2/3Ca1/3(Mn1−xNix)O3 (abstract)
Authors: Rubinstein Mark , Tritt Terry M. , Synder John E.
Published: July 2002

La 2/3 Ca 1/3 MnO 3 is a strong double-exchange ferromagnet where the ratio of La3+–Ca2+ ions has been chosen to maximize its favorable magnetic and transport properties. The Mn ion is unique as an effective participant in the double-exchange process. It is of interest to add Ni or Co atoms substitutionally for Mn in the form of La2/3Ca1/3(Mn1−xCox)O3 and La2/3Ca1/3(Mn1−xNix)O3 in order to determine the relative importance of the double-exchange interaction as the Mn concentration is reduced. Powder x-ray diffraction reveals we can retain the original orthorhombic structure of La2/3Ca1/3MnO3 (with up to 50% Co or Ni substitution) by a series of anneals at 1300 °C followed by repeated grindings. We have investigated the magnetization, electrical conductivity, and thermopower of these mixed transition-metal perovskites. For La2/3Ca1/3(Mn1−xCox)O3, the magnetization decreases with added Co, but ferromagnetism is retained (at least up to 50% Co). Although metallic conductivity is still present at 10% Co concentration, these Co–Mn perovskites become insulating for x⩾0.2. For La2/3Ca1/3(Mn1−xNix)O3, the metallic state is retained at least until x=0.5. The magnetic properties exhibit spin-glasslike behavior for x⩾0.25.
Micromagnetic computational standard problem (abstract)
Authors: McMichael R. D. , Donahue M. J.
Published: July 2002

Proposed solutions to a standard problem in micromagentics that were collected anonymously from a number of researchers using a variety of computational techniques are presented. This is the first time that a problem in micromagnetics has been attempted by a group. The sample problem is a 1 μm×2 μm rectangular element of Permalloy film, 20 nm thick. The material properties are specified by an exchange stiffness constant, A=1.3×10−11 J/m, a magnetization, M=8.0×105 A/m, and a uniaxial ansotropy constant, K=5.0×102 J/m3 with the easy axis directed parallel to the long edge of the sample. The requested calculations are hysteresis loops for fields directed one degree away from the long and short axes and magnetization configuration data for the remanent states. The results collected show a surprising lack of agreement. For fields applied nearly parallel to the long axis of the sample problem, calculated coercivities range from 2.4 to 33 mT, squareness values range from 0.23 to 1.00, and there is a variety of hysteresis loop shapes and mazgnetization patterns. Similar differences exist in the results for fields applied nearly parallel to the short axis of the sample. Differences in computational methods that give rise to the disparity in the results, e.g., grid geometry and size and magnetostatic field calculation methods, are highlighted.
Spin wave frequency shifts in exchange coupled ferromagnet/antiferromagnet structures: Application to Co/CoO
Authors: Stamps R. L. , Camley R. E. , Hicken R. J.
Published: July 2002

Co/CoO structures have been studied almost exclusively through measurements of hysteresis, and display an enhanced and strongly temperature dependent effective in-plane anisotropy. A recent experimental study demonstrated an alternate way of investigating effects related to the coupling across the interface by measuring frequencies of long wavelength spin waves associated with the Co film. A large increase in frequency of the low frequency spin wave in the Co was observed as the temperature was lowered through the Neél temperature of CoO. We show how these frequency shifts can be understood as an effective interface anisotropy introduced by strong exchange coupling across the Co/CoO interface. This means that spin waves in the Co also include energy contributions from the larger anisotropies experienced by spins in the CoO. The theory is presented and discussed for the Co/CoO interface and other structures.
Magnetic properties of epitaxial-doped manganite multilayer films (abstract)
Authors: Suzuki Y. , Hwang H. Y. , Cheong S. W. , Batlogg B. , van Dover R. B.
Published: July 2002

We have fabricated and studied the structure and magnetic properties of high quality single crystalline (La0.7Ca0.3)MnO3 and (La0.7Ba0.3)MnO3 films. Films were grown on SrTiO3 as well as LaAlO3 substrates. (La0.7Ca0.3)MnO3 and (La0.7Ba0.3)MnO3 films grown on SrTiO3 exhibit excellent crystallinity and magnetic properties while those grown on LaAlO3 show mediocre structural and magnetic properties. These differences indicate the importance of lattice match and structural similarity between the film and the substrate. X-ray and Rutherford backscattering analysis provide a consistent picture of the structural properties of these doped manganite films. These films enable us to investigate the crystal anisotropy using vibrating sample magnetometry and torque magnetometry. Multilayers composed of manganite layers with different dopants (e.g., calcium and barium dopants) also have excellent structural properties since there is minimal strain at the interfaces. Such isostructural single crystalline multilayers are essential for investigating exchange coupling of the doped manganites.
Structural magnetic and electrical transport properties of (Nd,Sm)0.5Sr0.5MnO3 compounds (abstract)
Authors: Mendoza W. , Peng B. , Shaheen S. A. , Rani R. , Cadieu F. J.
Published: July 2002

In recent years rare-earth manganese oxide compounds have been the object of numerous studies because of their large magnetoresistance. The (Nd,Sm)0.5Sr0.5MnO3 compounds have been reported to exhibit magnetic-field-induced first-order phase transitions as well as striction coupled magnetoresistance properties.12 We have prepared samples across the (Nd1−xSmx)0.5Sr0.5MnO3 series with x=0, 0.25, 0.50, 0.75, and 1.0 to study the physical properties of this system. The x-ray diffraction studies revealed that all samples have almost identical patterns with a few very weak additional reflections on the Nd-rich end that are due to the presence of (<5%) of the SrMnO3 phase. The observed x-ray diffraction patterns matched with the La0.8Sr0.2MnO3-type monoclinic structure (S.G. P2/c) with lattice parameters a=5.429 Å, b=5.450 Å, c=7.652 Å, and B=90.2°. The slight distortion of angle B from 90° suggests that the x-ray data may also be indexed by a closely related GdFeO3-type (S.G. Pbnm) orthorhombic structure reported for a single-crystal sample in Ref. 1. However, our observed intensity pattern matched better with the former structure. A ferromagnetic state was observed below 250 K for x=0 and the magnetic ordering temperature shifted to lower temperatures with increasing x: the x=1 sample showed magnetic ordering at temperatures below 150 K, in agreement with the published results.2 The samples exhibited a marked field dependence below the ferromagnetic ordering temperature but preliminary data on polycrystalline samples did not show a first-order transition from a metallic ferromagnetic to nonmetallic antiferromagnetic state that has been reported for a single-crystal sample with x=0 (Ref. 1).
Magnetic properties of (111) oriented Ni1−xFex alloy thin films (abstract)
Authors: Freeland J. W. , Grigorov I. L. , Walker J. C.
Published: July 2002

We present a study of changes in the magnetic properties of 4 ML face-centered-cubic (fcc) Ni1−xFex(111) thin films as a function of Fe concentration. These Ni1−xFex films were grown epitaxially on Cu(111)substrates using codeposition of Ni and enriched Fe57. The use of Cu substrates offers the ability to stabilize the fcc phase across the entire Fe concentration range making a study of the fcc phase at high Fe concentrations possible. The magnetic properties were studied by utilizing a combination of Mössbauer spectroscopy and SQUID magnetometry to gain a more complete picture of the magnetic order. For low Fe concentrations the Mössbauer data displayed only a high-spin ferromagnetic (FM) phase. However, as the Fe concentration was increased, the Mössbauer spectra showed the clear appearance of a low-spin antiferromagnetic (AFM) phase coexisting with the FM phase. Magnetometry observed the appearance of this state as a drop in the average saturation moment above x=0.65. Such a transition is in agreement with our finding that the pure Fe film is described by a coexisting FM and AFM order. Also observed where changes in the perpendicular anisotropy. The anisotropy shows changes towards favoring a perpendicular easy axis, but the system appears to have an in-plane easy axis for all concentrations.
Negative remanence in exchange coupled Co/Rh/Co sandwich structures: Experiment and model (abstract)
Authors: Theuss H. , Becker T. , Weller D. , Kronmueller H.
Published: July 2002

It is shown within a simple micromagnetic model calculation that novel magnetic hysteresis behavior including the unusual phenomenon of negative remanence can occur in asymmetric exchange coupled bi- and multilayer films. The latter denotes a magnetization state opposite to the field direction in which the sample was premagnetized. We have observed such effects in polar Kerr hysteresis measurements from perpendicularly oriented sandwich structures grown on Pt(111) buffers, e.g., Co(0.8 nm)/Rh(0.9 nm)/Co(0.8 nm). The model minimizes the total magnetic energy as function of the applied field and angles phi1 and phi2 of the two layers with respect to the film normal (easy) direction. The energy includes besides external field energy, a demagnetization term and interlayer exchange and anisotropy contributions. The general shape of the experimentally observed hysteresis loops can be reproduced, if one assumes slight asymmetries in the magnetic properties of the ferromagnetic layers. These could be caused by unequal thicknesses and/or local roughnesses which would alter the interfacial anisotropy contributions.
Magnetic moments and anisotropy energies of thin Fe films on Cu(100) (abstract)
Authors: Hunter Dunn J. , Arvanitis D. , Mårtensson N.
Published: July 2002

The attention that has been focused on the magnetic properties of Fe thin films is due to the presence of several magnetic phases the existence of which may be influenced by the preparation process and subsequent treatment. In particular, Fe grown on the (100) surface of Cu exhibits a crossover from in plane to perpendicular easy magnetization direction as the film thickness decreases below a critical threshold. Here we present an in situ characterization of this system by means of x-ray photoemission and photoabsorption experiments. These two spectroscopies combine to provide a powerful technique for depth profiling the magnetic response as a function of thickness from thin and ultrathin films. In addition photoabsorption measurements in conjunction with magnetic sun rules give access to orbital and spin magnetic moments. We find that for thin fct Fe on Cu(100) the spin moment increases from the bulk bcc value of 2.25 to 3.40 μB. This increase is linked to a relaxation in the interlayer distance and the decrease in coordination number at the surface. The orbital moment also shows an increase from 0.09 to 0.24 μB. This increase, coupled with the reorientation of the easy direction, allows a qualitative consideration of the anisotropy energy that governs this transition.
A numerical calculation of open boundary problem by applying the finite element method and boundary element method alternately
Authors: Onuki T. , Im Jee-won , Kuwahara T.
Published: July 2002

The finite element method (FEM) and the boundary element method (BEM), which have entirely different characteristics, are standard methods for the numerical analysis of physical phenomena described by partial differential equations. The FEM is suitable for the analysis of a complicated region that includes nonlinear materials, whereas the BEM is naturally effective for analyzing a very large region with linear characteristics. Therefore, considering the advantages in both methods, a novel algorithm for the alternate application of the FEM and the BEM to magnetic field problems with the open boundary is presented. This approach avoids the disadvantages of the typical numerical methods with the open boundary problem such as a great number of unknown values for the FEM and nonsymmetric matrix for the hybrid FE–BE method. The solution of the overall problems is obtained by iterative calculations accompanied with the new acceleration method.
Automatic mesh generation in finite element analysis using dynamic bubble system
Authors: Čingoski Vlatko , Murakawa Ryo , Kaneda Kazufumi , Yamashita Hideo
Published: July 2002

In this article an improved automatic mesh generation method for finite element analysis (FEA) using a dynamic bubble system is presented. The proposed method features two separate routines; one for automatic generation of a set of nodes in accordance with the desired mesh density and the Second Newton’s Law of dynamics, and one for automatic generation of finite elements according to the Delaunay algorithm and using a set of nodes generated from the previous routine. Generation of bubbles is performed frontally—starting from vertices and edges—and ending with surfaces and volumes of the analysis region after which a dynamic movement of generated bubbles is performed. The proposed method enables easy mesh density control using simple exponential functions with a very small amount of input data and very short computation time. It is applicable for automatic mesh generation of complex shapes and structures such as those usually encountered in various electromagnetic devices (rotating machines and transformers). Due to its modest requirement for input data and its ability to generate meshes with graded mesh densities, the proposed method could also be very useful for adaptive mesh refinements in FEA.
Effects of rolling conditions on microstructure and magnetic properties of thin gauged 3% Si–Fe sheets (abstract)
Authors: Kim K. Y. , Kim Y. J. , Na J. G. , Lee S. R. , Son D. R. , Woo J. S. , Lee C. S.
Published: July 2002

To match the requirements for development of transformer cores with lower iron losses, many new materials are under development including amorphous materials, 6.5% Si–Fe sheets and thin gauged 3% Si–Fe sheets. Among these materials, the thin gauged 3% Si–Fe sheets are attracting attention due to their good magnetic properties and scientific interest. Arai et al. reported that the magnetic properties of the sheets were comparable to those of the amorphous materials and (110)[001] preferred orientation of the sheets are developed by tertiary recrystallization.1 The 100 μm thick 3% Si–Fe sheets were prepared via conventional metallurgical processes including melting and casting, hot rolling to 25 mmT at 1200 °C, first cold rolling to 0.5 mmT, intermediate annealing at 800 °C for 30 min, second cold rolling to 0.25 mmT, intermediate annealing at 800 °C for 30 min, final cold rolling to 100 μm and final annealing at 1200 °C for 1 h in a vacuum of 5×10−6 Torr. Among these processes, the cold rolling process is an important one because preferred orientation of the sheets was developed in the process. Nakano et al. reported that there was an optimum cold rolling ratio to get required magnetic properties of the sheets.2 Recently, we found that the reduction rate, i.e., number of passes, as well as reduction ratio affected the preferred orientation and magnetic properties of the sheets. The number of passes in the first cold rolling process was changed from 7 to 60 and B10 values of the final sheets were changed from 1.30 to 1.84 T according to the number of passes. From x-ray experiments, it was found that intensity of (110) peak in the cold rolled and annealed sheets strongly affected the magnetic properties of the final sheets. We will discuss the relationship between the reduction rate and preferred orientation, and magnetic properties of the thin gauged 3% Si–Fe sheets.
Phase formation in annealed sputtered (Fe,Ti)–N films
Authors: Wang H. Y. , Jiang E. Y. , Wu P. , Ming S. L.
Published: July 2002

(Fe,Ti)–N thin films with titanium concentrations 3–35 at. % were prepared at room temperature by facing targets sputtering. X-ray diffraction and transmission electron microscopic studies were employed to characterize various phases observed prior to and during in situ annealing of (Fe,Ti)–N films. The films with 3–10 at. % Ti contained an α-(Fe,Ti) phase and an α′-martensite phase and the films with 20–35 at. % Ti comprised α-Fe(N), α′-martensite, and TiN phases in the as-deposited samples. The annealing temperature (TA) was increased from RT to 800 °C in increments of 50 °C. Annealing of the samples with 8 at. % Ti showed no structural changes between RT and 300 °C, but when the sample was heated to 400 °C, α′′ and TiN phases appeared in the microstructure. The samples with 25 at. % Ti also showed no structural changes when heated to 400 °C. When TA=500 °C, α′′ phase formed in the microstructure. The phases formed at 400 or 500 °C in the two samples were stable when the samples were heated up to 800 °C and followed by cooling down to RT.
Magnetic and physical microstructure of Fe16N2 films grown epitaxially on Si(001)
Authors: Brewer M. A. , Echer C. J. , Krishnan Kannan M. , Kobayashi T. , Nakanishi A.
Published: July 2002

Epitaxial Fe16N2 films were grown on Si(001) substrates with an Ag underlayer by reactive sputtering in nitrogen. Pure α′-Fe8N films were obtained which on subsequent annealing resulted in mixtures of α′-Fe8N (54%) and α′′-Fe16N2 (46%). An average moment of 1780 emu/cc, considerably larger than that of pure α-Fe (1714 emu/cc), was measured for both samples. Plan-view transmission electron microscopy of the films confirms the orientation relationship Fe16N2(001)‖Ag(001)‖Si(001) and Fe16N2[100]‖Ag[110]‖Si[100], and a small grain size (∼100 Å), while electron energy-loss spectroscopy confirms an average composition of Fe8N for both samples. Electron diffraction patterns indicate that the as-deposited α′ films already contain very small regions of ordered α′′ which were not previously detected by x-ray diffraction measurements. Mössbauer spectroscopy performed at both 300 and 16 K gave three hyperfine fields corresponding to three different iron sites for both the unannealed α′ and the annealed α′/α′′ mixtures. Lorentzian fitting of the three iron components for the α′/α′′ spectrum obtained at room temperature gave an intensity ratio of 1:2:1 (FeI:FeII:FeIII) corresponding to the expected occupancy for the three Fe sites in the Fe16N2 structure. Moreover, the pure α′ film at 300 K and both samples at 16 K showed deviation from this distribution. The three components show notable differences in the temperature dependence of their occupancies; however, all three magnetic components deviate similarly from the surface normal.
Thermodynamics of two-dimensional XXZ easy-plane quantum Heisenberg magnets
Authors: Capriotti Luca , Cuccoli Alessandro , Tognetti Valerio , Vaia Ruggero , Verrucchi Paola
Published: July 2002

We consider the quantum easy-plane (XXZ) magnet on the square lattice. In the classical case the system exhibits the Berezinskii–Kosterlitz–Thouless transition. Simulations, both for ferro- and antiferromagnets, using the method of the effective classical Hamiltonian, are made for different spin values and refer to the specific heat, the static spin correlation functions and correlation lengths in a broad range of temperatures, below and above the transition. The effects of quantum fluctuations are quantitatively evaluated and discussed.
Metamagnetic domains and dynamic fluctuations in FeBr2
Authors: Petracic O. , Binek Ch. , Kleemann W.
Published: July 2002

The mixed phase and the regime of non-critical fluctuations of the magnetic phase diagram of FeBr2 is investigated by SQUID susceptometry and light diffraction techniques. The experiments seem to evidence instability of the tricritical point as conjectured recently. The observation of stripe domains and light diffraction below and above Tm=4.6 K and the virtual continuation of the phase transition line to above Tm are in agreement with the occurrence of a critical endpoint at Tm and of a bicritical endpoint at T>Tm.
Linear magnetostriction near Curie temperature: Analytical calculations
Authors: Dominguez L. , Samojednik A. , Kulakowski K.
Published: July 2002

Analytical calculations have been performed of the linear magnetostriction λs in a t2g model band system. The Hamiltonian includes spin-orbit coupling, strain-dependent crystal field, orbital, and spin Zeeman energies. The model density of states is assumed to be six Lorenztian functions. We have calculated a ratio of a magnetostrictive critical index to the magnetic critical index η=∂ ln(λs)/∂ ln(Δ)=κ/β, where Δ is the Stoner gap, κ=∂ ln(λs)/∂ ln(ε), β=∂ ln(Δ)/∂ ln(ε), and ε is the reduced temperature. Information on many-body magnetic effects is assumed to be carried in the index β; for a given β, the coefficient η is calculated within the Stoner model of a degenerate band. Results are obtained for different numbers of electrons in the model band. Qualitative agreement with experimental data is obtained, in particular, for amorphous alloys with small magnetostriction, based on Co and Ni. For these alloys, both experimental and theoretical results are higher than the commonly accepted value of 2.0, which was derived for insulators within the Callen theory.
Transport properties of Ce2Ni2Sn and Ce2Pd2.05Sn0.95 Kondo lattice systems
Authors: Pinto R. P. , Amado M. M. , Braga M. E. , de Azevedo M. M. P. , Sousa J. B. , Chevalier B. , Étourneau J.
Published: July 2002

We report experimental data on thermoelectric power S, electrical resistivity ρ, and the magnetoresistivity of the antiferromagnet Kondo stannides Ce2Ni2Sn (TN=4.7 K) and Ce2Pd2.05Sn0.95 (TN=4.7 K). The essential features of the S(T) curves resemble those of heavy fermion systems such as CeCu2Si2: a broad and positive maximum at intermediate temperatures, followed by a sharper negative minimum at lower temperatures. S values are is considerably smaller in Ce2Pd2.05Sn0.95 than in Ce2Ni2Sn. The positive peak originates from the incoherent Kondo scattering by the excited crystal field levels of the Ce ion ground state. The negative peak might be related to the shape of the density of states associated with the Abrikosov–Suhl resonance. The change of sign in S(T) between its minimum and maximum at T* can be assigned to the fact that the Fermi level sinks below the upper band at T>T*. This behavior and the maximum observed in the ρ(T) curve at this temperature can be discussed in terms of the electron polaron model, although one must also take into account the crystal field effect. The role played by the crystal field effect, which is more important in Ce2Pd2.05Sn0.95, is analyzed for this compound, providing the magnitude of the crystal field splitting. Magnetoresistivity was also measured in both compounds. The results are consistent with the important role of the Kondo effect at low temperatures and suggest the splitting of the double degenerate bands at T>T*.
Neutron diffraction studies on Dy(Cu1−xNix)2Si2
Authors: Nashima O. , Ido H. , Ohashi M. , Onodera H. , Yamaguchi Y.
Published: July 2002

Neutron powder diffraction studies have been made for Dy(Cu1−xNix)2Si2 with x=0, 0.06, and 0.1. The diffraction patterns of DyCu2Si2 are consistent with the reported magnetic structure [k=(1/2,0,1/2)], the Néel temperature (TN=10 K) and the Dy magnetic moment (8.2 μB). The diffraction patterns of Dy(Cu0.94Ni0.06)2Si2 and Dy(Cu0.9Ni0.1)2Si2 are interpreted by the same commensurate antiferromagnetic structures with k=(1/2,0,1/2). The Néel temperatures of Dy(Cu0.94Ni0.06)2Si2 and Dy(Cu0.9Ni0.1)2Si2 are determined to be 7.5 and 6.5 K, respectively. The magnetic moments are 6.5 μB/Dy in the Dy(Cu0.94Ni0.06)2Si2 and 6.0 μB/Dy in the Dy(Cu0.9Ni0.1)2Si2. The magnetic moment decreases as the Ni concentration increases, whereas the Dy moment determined by the Mössbauer spectroscopy increases as x increases. There is a strong diffuse peak at the (1/2 0 1/2) position of Dy(Cu0.94Ni0.1)2Si2 which implies the existence of some kinds of short range ordering of Dy moments. This fact suggests that the local crystalline electric field with lowered symmetry breaks the perfect magnetic arrangement with k=(1/2,0,1/2)
Specific heat of pure Y
1−x
Pr
x
Ba2Cu3O7−δ single crystals in magnetic fields
Authors: Uma S. , Schnelle W. , Gmelin E. , Rangarajan G. , Erb A.
Published: July 2002

Specific heat measurements have been performed on single crystals of Y1−xPrxBa2Cu3O1−δ (x=0.33, 1) in the temperature range 2 K<T<50 K with magnetic fields up to 7.5 T, both parallel and perpendicular to the c-axis. The crystals were grown in BaZrO3 crucibles and therefore the possibility of crucible contamination in the crystals is reduced to a large extent. A large linear term with γ=207 mJ/mol K2 was determined for x=1 and the crystal field contribution due to Pr ion was calculated for both the samples. The transitions were found to be sharp lambda type for x=1 with the ordering temperature at 16.6 K and very broad for x=0.33 with the transition temperature below 2 K. Also, the transition temperatures increased for x=0.33 and decreased for x=1 with increasing magnetic fields indicating a different type of interaction for these two samples. The coupling in the ab plane was found to be stronger than in the c-axis.
Superconductivity, magnetism, and the charge transfer in Y0.8Ca0.2Ba2Cu3Oy (abstract)
Authors: Hejtmánek J. , Jirák Z. , Maryško M.
Published: July 2002

Highly dense ceramic samples of Y0.8Ca0.2Ba2Cu3Oy (y=6.03–6.89) have been characterized by the x-ray and neutron diffraction and investigated using the resistivity, thermoelectric power, Hall effect, and magnetic susceptibility measurements. The study showed a correlation between the occurrence of superconductivity, the structural data, and the magnetic and transport properties in the normal state. Three distinct superconducting plateaus depending on the decreasing oxygen content were revealed. First two plateaus with Tc of about 80 (y>6.75) and 50 K (6.75>y>6.50) are associated with the Ortho I and II structures which are characterized with infinite Cu–O chains. The third plateau corresponds to macroscopically tetragonal phase with Tc=25 K (6.50>y>6.40) and is possibly associated with the formation of the Cu–O–Cu dimers. All three superconducting phases exhibit sharp magnetic transitions with nearly complete diamagnetism at low temperatures. In the normal state of the superconductors, the metalliclike conductivity is accompanied by a paramagnetism with appreciable Pauli component. The hole concentrations estimated from the calculated bond valence sums for Cu2 ions in the CuO2 planes are in good agreement with a number of itinerant carriers deduced from the Hall data. The behavior of carriers is dramatically changed at the transition to the nonsuperconducting region (y<6.40). In particular, the Pauli paramagnetism and the Hall number decrease abruptly while the Cu2 bond valence sums continue to change smoothly. A rapid localization of charge carriers below y=6.40 is further indicated by the temperature activation of the resistivity and Hall constant.
New magnetic material spectroscopy (abstract)
Authors: Hogenboom D. , Widom A. , Vittoria C.
Published: July 2002

Sensitive commercial electron paramagnetic resonance (EPR) spectrometers are able to measure 10∧13 spins. Using a high Tc weak link, we were able to measure FMR on yttrium ion garnet (YIG) films which were exposed to an effective area of 3 nm by 15 micrometers. Given that this is the area of excitation and the thickness of the YIG was 0.5 micrometers, we could measure less than 10∧9 spins in a YIG film. Our weak link was fabricated by patterning a 0.5 micrometer thick film of YBCO down to a 15 micrometer wide bridge across an artificial grain boundary in an MgO substrate. Our technique uses the ac Josephson effect to generate a microwave field which couples into a YIG film which is placed in intimate contact atop the link. The frequency of this microwave field is proportional to the voltage across the weak link. The absorption of the microwaves by the YIG will affect the voltage versus current behavior across the weak link. For a YIG film of 0.5 micrometers thickness, we measured an in-plane linewidth of 125 Gauss using the weak link technique. The FMR linewidth measured by a conventional EPR technique was 40 Gauss. This implies that the excitation by the weak link may be nonuniform. We present data which illustrate this effect and numerical results for the corresponding circuit model. Our conclusions are that we have developed a viable spectroscopy to characterize extremely local magnetic interactions.
Magnetic coupling of the composite mixtures Sm2Co17/Sm2Fe17N3 and Sm2Co17/Fe
Authors: O’Sullivan J. , Rao X. L. , Coey J. M. D.
Published: July 2002

Mixing magnetically oriented 100 μm particles of coercive Sm2Co17 with 2–10 μm particles of Sm2Fe17N3 produces an enhanced hysteresis loop compared with the weighted averaged loops of the individual materials. The critical parameters involved are the sizes of the particles and the ratio of the two phases present. In 50:50 Sm2Co17/Sm2Fe17N3, two phase mixtures, an increase in the coercivity from 0.9 to 1.0 T and a small increase in remanent magnetization is observed. Similar increases also appear in 50:50 mixtures of Sm2Co17 and α–Fe. The enhanced hysteresis loops are explained by the anisotropic nature of the dipolar interaction which causes the small particles to cluster near the magnetic axes of the larger one.
Activation energy and enthalpy of decomposition for the Fe17Sm2 nitride
Authors: Cabral F. A. O. , Gama S. , Ribeiro C. A.
Published: July 2002

Decomposition studies of the Fe17Sm2N3 compound were done using a Calvet-type microcalorimeter and a simultaneous gravimetric and differential thermal analyzer (DTA). The results show that the decomposition has onset temperature of 570 °C and proceeds up to 900 °C, and presents only one well determined exothermic thermal event. The calorimetric measurement shows that the enthalpy associated with the decomposition is 69 kJ/mol. The activation energy of the process was also determined using the Kissinger method applied to the DTA signal. It was observed that there was only one activation energy for the decomposition process, with a value of 339 kJ/mol.
Jahn–Teller effects in the doubly degenerate Hubbard model
Authors: Lin H. Q.
Published: July 2002

We investigate the Jahn–Teller (JT) effects in the doubly degenerate Hubbard model by using the exact diagonalization (Lanczös algorithm) technique. This is in connection with the newly discovered “colossal magnetoresistance” perovskite-based La1−xAxMnO3 compounds. We first obtain the parameter region where the ground state of the doubly degenerate Hubbard model is ferromagnetically ordered analytically for a dimer, and numerically for larger clusters. Then the effects of static JT distortion in the magnetic ordering of the model are studied numerically.
Rippled beam free-electron laser amplifier using the axial free-electron laser interaction
Authors: Carlsten Bruce E.
Published: July 2002

A new microwave generation mechanism involving a scalloping annular electron beam is discussed. The beam interacts with the axial electric field of a TM0n mode in a smooth circular waveguide through the axial free-electron laser interaction, in which the beam ripple period is synchronous with the phase slippage of the rf mode relative to the electron beam. In this paper, we analyze the ripple motion of the electron beam and derive the dispersion relation describing the exponential growth of the rf mode. We calculate the gain for a nominal design and as a function of beam current and ripple amplitude, and show that power gain on the order of 30 dB/m of interaction is achievable. We additionally demonstrate that, under the right conditions, the interaction is autoresonant.
Laser-chemical vapor deposition of W Schottky contacts on GaAs using WF6 and SiH4
Authors: Tabbal Malek , Meunier Michel , Izquierdo Ricardo , Beau Bénédicte , Yelon Arthur
Published: July 2002

Tungsten was deposited on GaAs using a low-temperature laser-chemical vapor deposition process. A KrF excimer laser beam incident perpendicularly on a GaAs surface was found to induce metallic W formation from a gas mixture containing WF6 and SiH4 at laser energy densities as low as 25 mJ/cm2. In-situ x-ray photoelectron spectroscopy analysis shows that SiH4 plays an important role in the initiation of metallic W deposition at such low laser energy densities. Scanning electron microscopy of the W films shows a dense and regular columnar structure. Auger depth profiles show that the deposited W is pure. No impurities such as F, C, or O were observed, with a detection limit of 1 at. %, and the interdiffusion between W and GaAs is minimal. X-ray diffraction shows that the W film is mostly in the stable, highly conductive α phase, as confirmed by the low resistivity value of 21 μΩ cm. Metallic W features of 60 μm on GaAs were obtained by laser direct-projection patterning. I–V measurements show that the W–GaAs structures formed provide good quality Schottky contacts, with an average barrier height of 0.71 eV and an average ideality factor of 1.2. To our knowledge, these are the first Schottky diodes obtained using a laser based resistless projection patterning process on GaAs.
Novel fluorine-phosphate semiconductor doped glasses: Linear and nonlinear optical properties
Authors: Vaynberg B. , Matusovsky M. , Rosenbluh M. , Petrikov V. , Lipovskii A.
Published: July 2002

We have synthesized a new type of high quality semiconductor doped glass (CdSSe embedded in a fluorine-phosphate matrix) using batch technology. The linear and nonlinear optical properties of this glass are investigated and compared with borosilicate glass doped with the same semiconductor. The optical susceptibility of glass is measured using degenerate four wave mixing technique.
Enhancement of Mg activation in AlGaAs by Mg+Ar and Mg+P dual ion implantation
Authors: Hara Naoki , Suehiro Haruyoshi , Kuroda Shigeru , Takikawa Masahiko
Published: July 2002

We have investigated Mg+Ar and Mg+P dual ion implantation into AlxGa1−xAs with a wide range of the Al fraction (0⩽x⩽0.75). We characterized the electrical properties and radiation damage of implanted AlxGa1−xAs layers to clarify the effects of dual implantation. Mg+P dual implantation, which maintained a stoichiometric balance, improved the electrical properties in AlxGa1−xAs for all Al fractions investigated. On the other hand, Mg+Ar dual implantation, which introduces additional radiation damage and increases the number of group III vacancies to enhance the Mg activation, improved the electrical properties in AlxGa1−xAs with a high Al fraction but degraded them in AlxGa1−xAs with a low Al fraction. The difference between Mg+P and Mg+Ar dual implantation is due to the different mechanism of Mg activation enhancement. The effect of keeping the stoichiometric balance is valid regardless of the Al fraction, and increasing radiation damage hardly affects the activation. In AlxGa1−xAs with a high Al fraction, the resistance against implantation damage is large. Here, Mg+Ar dual implantation effectively enhances the Mg activation. Conversely, in AlxGa1−xAs with a low Al fraction, a group III vacancy is easily created by only Mg implantation. Here, radiation damage caused by Mg+Ar dual implantation is too severe to remain even after annealing and degraded Mg activation.
Spectroscopic ellipsometry study on E2 peak splitting of Si–Ge short period superlattices
Authors: Kim Y. D. , Klein M. V. , Baribeau J.-M. , Hwang S. H. , Whang K. W. , Yoon E.
Published: July 2002

We report spectroscopic ellipsometry (SE) studies on (Si)2(Ge)12, (Si)6(Ge)2, and (Si)12(Ge)2 short period superlattices (SLs) whose optical response has not been reported yet. Multilayer calculations enabled us to determine the dielectric response of the superlattice layers. We report the clear observation of splitting of the E2 peak in (Si)m(Ge)n superlattices contrary to the previous SE report that the separation was observed only in larger period SLs.
Studies of V, Nb, Cr, and Zr substituted 2:17 compounds and their carbides using neutron diffraction
Authors: Luo H. , Hu Z. , Chen M. , Yelon W. B. , Marasinghe G. K. , Ezekwenna P. C. , James W. J. , Chang W. C. , Tsai S. H.
Published: July 2002

Samples of Nd2Fe17−xTxCy with T=V, Nb, Cr, and Zr were prepared by melting of the constituent elements including C and studied by neutron diffraction. Comparing with their uncarbided counterparts, we found that the substituents transfer, more or less, from the 6c site to the 18f and 18h sites with the introduction of C atoms. This behavior appears to relate to the electronegativities between the C atoms and the early transition series elements because the 18f and 18h sites are near neighbors of the interstitial C site. However, the C effects in the V, Nb, Cr, and Zr samples are not as strong as those in Ti samples. SQUID measurements show that the Curie temperatures of these samples depend on both the interstitial C atoms and the substituents.
Application of the x-ray magnetic circular dichroism sum rules to Co/Pt multilayers (abstract)
Authors: Thompson S. M. , Henry Y. J. , Short G. R. , Flevaris N. K. , Poulopoulos P. , Angelakeris M. , Schillé J-P. , van der Laan G. , Donovan P.
Published: July 2002

The presence of perpendicular anisotropy in Co/Pt multilayers is often attributed to an enhanced orbital magnetization at the surface or interfaces of the multilayers. By the application of the 〈Lz/Sz〉 sum rule to magnetic x-ray circular dichroism spectra it is possible to probe directly the orbital to spin ratio. In this study a comparison has been made of this ratio between Co/Pt multilayers with different anisotropies and also with single films of Co with and without Pt. The multilayers were prepared in high vacuum by evaporation on Si substrates with a 2.4 nm Pt buffer and a 1.2 nm Pt capping layers, characterized by x-ray diffraction and magneto-optic Kerr effect. The magnetic circular dichroism experiments were carried out at Daresbury laboratory in 75% polarization in ±1 T perpendicular applied fields as a function of temperature from 25 to 300 K. We find that the 〈Lz/Sz〉 ratio is enhanced for a multilayer with perpendicular anisotropy compared to a similar multilayer with in-plane anisotropy.
Superconducting and magnetic properties of epitaxial high-quality Fe/Nb bilayers
Authors: Mühge Th. , Zoller I. , Westerholt K. , Zabel H. , Garif’yanov N. N. , Goryunov Yu. V. , Garifullin I. A. , Khaliullin G. G. , Tagirov L. R.
Published: July 2002

Single crystal Fe/Nb (110) bilayers with an Nb thickness dNb in the range from 140 to 650 Å and with an Fe thickness dFe in the range from 5 to 100 Å were prepared using molecular beam epitaxy (MBE) techniques. For dFe⩾20 Å a decrease of the superconducting transition temperature Tc with decreasing dNb was observed. For dFe=20 Å the FMR data revealed a decrease of the effective magnetization of the Fe layer below the superconducting transition. This magnetization behavior is attributed to a spatial modulation of ferromagnetic order due to a modification of the RKKY interaction in the superconducting state.
New aspects of Nd–Fe–B-based hydrogenation-disproportionation-desorption-recombination powders and anisotropic bonded magnets made from them: Microstructure and magnetic properties (invited)
Authors: Hirosawa S. , Uehara M. , Mino S. , Ishigaki N. , Tomida T.
Published: July 2002

Recent progress in the understanding of the texture formation mechanism in the hydrogenation-disproportionation-desorption-recombination (HDDR) process, improvements of stability with regard to thermal and structural losses, and the state-of-the-art performance of both compression and injection molded magnets made from the newly developed anisotropic HDDR powders are reported. Transmission electron microscopy observations of disproportionated Nd–Fe–Co–Ga–Zr–B alloys have revealed the existence of finely dispersed crystallites of Nd2(Fe,Co,Ga)14B which have a common crystallographic orientation. It is proposed that, upon removal of hydrogen, the hydrogen-disproportionated structure recombines from these crystallites to form textured submicron crystallites of the 2:14:1 phase. Using highly anisotropic HDDR powders, energy products (BH)max exceeding 170 kJ/m3 (21 MGOe) have been obtained on compression-molded resin-bonded magnets and 130 kJ/m3 (16 MGOe) on injection-molded ones. High coercivity HDDR powders with an intrinsic coercivity (HcJ) exceeding 1.27 MA/m (16 kOe) have also been obtained by replacing part of Nd with Dy, which enabled improvement of thermal stability. It is shown that the degradation of magnetic performance of resin-bonded HDDR magnets is prevented by eliminating pore formation during the molding process.
Micromagnetic modeling of magnetization reversal of Fe/Cr/Co exchange coupled multilayers (abstract)
Authors: Kleinefeld Th. , Schilberg M. , Theis-Bröhl K.
Published: July 2002

Multilayers consisting of Fe/Cr/Co were prepared under molecular beam epitaxial condition on MgO substrates. These kind of multilayers exhibit different types of exchange coupling with respect to the Cr spacer thickness. We have studied the samples by means of magnetooptic Kerr microscopy as well as Brillouin light scattering and additional MOKE characterization. The investigations were performed on samples with homogeneous Cr spacer thickness and also on wedge type samples. From light scattering experiments we were able to locate the ferromagnetic and the antiferromagnetic coupled regions in the wedged sample. We obtained a switching field for the antiferromagnetic coupled layers of the order of 100 kA/m. Detailed MOKE studies revealed an fourfold inplane anisotropy with collinear axes of Co and Fe layers. In order to get insight in the magnetization reversal process in such multilayers, we studied the magnetic domain dynamics simultaneously to the measurement of hysteresis loops using magnetooptic Kerr microscopy. Based on the results we developed a micromagnetic model which includes the anisotropy as well as the coupling through the Cr spacer layer. A refined numerical analysis for the different regimes of coherent rotation of the individual layers and domain wall propagation in the hysteresis loops yield the contribution of bilinear and biquadratic coupling with respect to the Cr spacer thickness. In conclusion we found oscillation of the interlayer exchange coupling with varying coupling strength.
Co/Cu multilayers with reduced magnetoresistive hysteresis (abstract)
Authors: Kubinski D. J. , Holloway H.
Published: July 2002

Co/Cu multilayers (MLs) are more sensitive [S≡(1/R)dR/dH] at the second antiferromagnetic maximum (AFM) than at the first. Unfortunately, this increase in S comes at the cost of increased magnetoresistive hysteresis (MRH). However, we find that, at 300 K, the MRH becomes very small when the Co layers are made very thin (average tCo≈3 Å), although at some cost in magnetoresistance (MR) and S. In a new class of ML structure we alternate very thin Co layers with Co layers that have conventional thickness (tCo≈15 Å). This gives the best of both worlds. The MR is comparable to that of conventional Co/Cu MLs at the second AFM (>15% at 300 K), while the hysteresis is greatly reduced. We discuss this behavior qualitatively in terms of a simple model for the field-dependent magnetization of the ML.
Magnetic domain size effect on resistivity and Hall effect of amorphous Fe83−xZr7B10Mx (M=Ni, Nb) alloys
Authors: Rhie K. , Lim W. Y. , Lee S. H. , Yu S. C.
Published: July 2002

Studies of effective permeability, core loss and saturation magnetostriction of Fe83−xZr7B10Mx (M=Ni, Nb) alloys revealed that the domain width is smallest around x=0.10. We measured the resistivity and low field Hall coefficients of these alloys and found that the maxima of resistivity and Hall coefficients occurred roughly at the same concentrations. Larger surface area of smaller domains is considered the reason.