bluuebunny/crossref_metadata_2025 huggingface dataset:
Metal-oxide-metal tunneling junctions on Ta and Nb: Background conductivity resulting from different oxide barriers
Authors: Brunner Michael , Ekrut Horst , Hahn Artur
Published: February 2003

Two different ways of preparing the oxide barrier in tunneling junctions on Ta and Nb yield different conductance-voltage characteristics. While a ’’dirty’’ preparation method with atmospheric air as an oxidation medium leads to strongly polarity-dependent but temperature-independent characteristics, we find rather symmetric but strongly voltage-dependent and temperature-dependent (hence ’’low-quality’’) characteristics when oxidation is performed in a pure oxygen dc plasma discharge. High or low quality of the junctions in the former and latter case, respectively, are also reflected in the superconducting characteristics. The results are tentatively interpreted in terms of band bending in the oxide on account of ionic surface charges in the dirty case.
Electrical and optical properties of tin oxide films doped with F and (Sb+F)
Authors: Shanthi E. , Banerjee A. , Dutta V. , Chopra K. L.
Published: February 2003

This paper presents the structural, electrical, and optical properties of F- and (Sb+F)-doped tin oxide films prepared by spray pyrolysis technique. Resistivity as low as 5.5×10−4 Ω cm with high optical transmission (≳80%) and high infrared reflection (∼90%) have been obtained in F-doped tin oxide films. The figure of merit ΦTC = T10/Rsh (52.6×10−3Ω−1 at 0.65μm) of these films is the highest amongst the results reported on doped tin oxide films. The variation of mobility with doping concentration has been analyzed to understand the electron–conduction mechanism. The Drude theory has been used to explain the optical properties near the plasma edge.
Exact magnetic field of a helical wiggler
Authors: Park S. Y. , Baird J. M. , Smith R. A. , Hirshfield J. L.
Published: February 2003

Exact solutions are presented for the magnetic field of single and double current-carrying helical windings. The latter is a configuration used widely in free electron lasers. Differences are shown between the exact field and the simple form commonly assumed in analyzing free electron laser interactions.
Ion beam mixing at nickel-silicon interfaces
Authors: Averback R. S. , Thompson L. J. , Moyle J. , Schalit M.
Published: February 2003

Ion-beam mixing at Ni-Si interfaces was studied as a function of dose, temperature, and dose rate. In the temperature range 10–443 K, the thickness of the mixed Ni-Si layer grew proportionally to the square root of dose for 250-keV Ar+ and 280-keV Kr+ irradiations. The apparent diffusion coefficient for mixing during irradiation comprises temperature-dependent and independent contributions. The activation enthalpy for the temperature-dependent part is ∼0.09 eV. Varying the dose rate by a factor of ∼20 had little effect upon mixing at either 80 or 368 K. Analysis of these results using chemical rate theory showed that the low activation enthalpy for diffusion was not associated with point-defect motion. More likely the temperature dependence of mixing in Ni-Si is associated with the effect of temperature on compound formation. The results for mixing at 10 K in Ni-Si were compared with those in Ni-Al, at 10 K, to study the effect of crystal structure on displacement mixing.
Self-modulation in the argon-ion laser
Authors: Ramsey J. M.
Published: February 2003

The irradiance from a low-gain argon-ion laser is observed to be self-modulated at harmonics of the longitudinal mode spacing (82 MHz). The laser is made to oscillate at five discrete frequencies between 408 and 737 MHz by controlling resonator losses.
Optimization of a model internal combustion engine
Authors: Aizenbud Boris M. , Band Yehuda B. , Kafri Oded
Published: February 2003

We optimize the operating conditions of a model internal combustion engine to obtain maximal efficiency. The model engine consists of a cylinder equipped with a piston containing a working fluid, coupled to a heat bath and heated by internal combustion with a rate of heating that is very weakly dependent on the temperature and pressure of the working fluid. We consider operation of the piston under the influence of an external pressure. The model engine includes valves allowing the exhaust and intake of the working fluid. It is found that the optimal operation has an optimal compression ratio and an optimal energy to be exhausted with the working fluid. The results are demonstrated with a numerical example. Also presented are optimizations for this system of the average power, and optimization of the work for a given specified average power.
Fabrication and testing of single-crystal lanthanum hexaboride rod cathodes
Authors: Hohn F. J. , Chang T. H. P. , Broers A. N. , Frankel G. S. , Peters E. T. , Lee D. W.
Published: February 2003

Single-crystal lanthanum hexaboride (LaB6) rods 1 mm in diameter by more than 150 mm in length have been fabricated by the laser-heated floating zone process. The procedure is described and the effects of process parameters such as power input and pulling and feeding rates on the geometry of the molten zone and on the quality of the grown rods are discussed. By seeded growth, the orientation of the rod axis can be selected; crystals having a rod axis corresponding to 〈100〉, 〈110〉, 〈111〉, 〈210〉, 〈211〉, and 〈321〉 were prepared during this study, each with a uniform diameter and having high crystalline quality. These crystals have been used for evaluating the performance of the indirectly-heated-type LaB6 rod cathode. An electron optical test column allows the angular emission pattern, the crossover intensity distribution, the tip temperature, and the on axis brightness of the cathode to be observed and measured. Symmetries in the angular emission and the crossover are related to the LaB6 lattice structure, and these patterns are accurately reproducible. At operating temperatures of 1800 K, the angular emission is smooth and the crossover a uniform round spot. Brightness measurements of the six different orientations show that the brightness increases with increasing temperature and eventually levels off to a space-charge limitation. The use of 〈100〉-oriented rod cathodes with a cone tip of less than 2 μm radius gives the best results and an increase in brightness of a factor of about three as compared to sintered material of the same geometry can be achieved.
Isotope fractionation in secondary ion mass spectrometry
Authors: Shimizu N. , Hart S. R.
Published: February 2003

We report a systematic ion microprobe study of the isotope fractionation effects produced during the sputtering of pure metal targets. The metals studied were B, Mg, Si, Cr, Ni, Cu, Ge, Mo, Ag, Sb, Re, Tl, and Pb; isotopic ratios were determined with typical precisions of 0.1%–0.3%. Detailed studies show that the data are free of possible instrumental biases, interference effects, etc. In all cases, the secondary ions produced by sputtering are enriched in the lighter isotope, and the degree of this enrichment is a function both of the spatial location and the energy of the extracted ions relative to the sputtering site. The maximum observed L/H enrichment factors vary from 6.5%/amu for B to 0.6%/amu for Pb, and are observed to show an approximate MH/ML [rather than a (MH/ML)1/2 ] dependence on mass. For masses greater than 50, the mass dependence of this fractionation can be closely described by an ionization model modified after Schroeer et al. [Surf. Sci. 34, 571 (1973)]; it is improbable that the observed fractionation is related to differential sputtering effects. Use of ion microprobes for isotopic research in geochemistry/cosmochemistry will require careful control of this isotopic fractionation process if a precision of better than 1%–2% is sought.
Ground-state dissociative attachment rates in HCl and Cl2
Authors: Sze R. C. , Greene A. E. , Brau C. A.
Published: February 2003

Ground-state dissociative attachment rates in HCl and Cl2 are obtained in the electron energy ranges from 0.7 to 5 eV. The results are compared with previous cross-section data convoluted with Boltzmann-code calculations for the electron energy distribution.
Competing processes in long term accelerated aging of double heterostructure Ga1−xAlxAs light emitting diodes
Authors: Zipfel C. L. , Saul R. H. , Chin A. K. , Keramidas V. G.
Published: February 2003

Two competing processes have been observed in bias aging of double heterostructure (DH) Ga1−xAlxAs light emitting diodes (LEDs) at elevated temperatures. These high radiance LEDs are used as sources in optical data links. In the past, only a single aging process, slow degradation in light output, has been reported for dark line defect (DLD)-free LEDs of similar type. In contrast, we report for the first time, a second process which gives a gradual increase in light output. As temperature and current are increased this new process increasingly masks the degradation process, until at 270 °C there is a net increase in light output of 50% after about 300 h of aging. We show that the two processes have different time dependences and activation energies, and that their effects on the light output during aging can be separated mathematically. A model for the new process is presented in which nonradiative traps anneal out in a recombination-enhanced defect reaction. This leads to higher injection efficiency and light output at fixed current. Operating lifetimes increase due to the second process; for example, at 25 °C mean-time-to-failure (MTTF) increases by a factor of three to 9×107 h.
Boron ion implantation in Hg1−xCdxTe
Authors: Baars J. , Hurrle A. , Rothemund W. , Fritzsche C. R. , Jakobus T.
Published: February 2003

Boron was implanted in p-type Hg1−xCdxTe with 0.23⩽x⩽0.28. The implantation was carried out using fluences between 1×1013 and 7×1016 B+ cm−2 and ion energies of 120 and 350 keV with the samples either at room temperature or at 80 K. For fluences of more than 1×1013 B+ cm−2 a saturation of the electron concentration at a level of 3×1018 cm−3 was obtained. No carrier freezeout was found in the implanted layers at temperatures between 10 and 300 K. The n-type conductivity persisted after annealing for 30 min at 150 °C and disappeared after annealing at 300 and 500 °C for 30 min. The boron concentration profiles analyzed by secondary ion mass spectrometry were found to be unaffected by annealing. The results suggest that the n-type conduction is caused by implantation damage and that the implanted boron is bound in immobile complexes and does not become effective as a donor.
Determination of the b1⇄b2 dynamic transition of potassium chloride with Manganin gauges
Authors: Rosenberg Z.
Published: February 2003

The dynamic b1⇄b2 phase transformation of KCl was investigated in a series of plane wave experiments. Commercial Manganin gauges, used as in-material stress gauges, recorded stress histories of the shock loaded KCl specimens. Using calibration curves for the gauge’s response under shock loading and unloading the values of the b1⇄b2 transformation stresses were determined. It was found that the b1→b2 transition is at 2.34 GPa while the reverse b2→b1 transition is at 1.61 GPa. These values are in good agreement with values given by other workers using static and dynamic methods.
Dispersion of the piezobirefringence of GaAs due to strain-dependent lattice effects
Authors: Feldman Albert , Waxler Roy M.
Published: February 2003

The piezobirefringence of GaAs has been measured over the wavelength range 3.5–10.6 μm. A small yet significant dispersion is found which is attributed to the strain dependence of the transverse optic phonon. The main contribution to the dispersion appears to be due to the strain-induced anisotropy of the transverse effective charge. The data are in reasonably good agreement with the theory of Humphreys and Maradudin. The strain-induced relative anisotropies of the transverse effective-charge and the high-frequency photoelastic constants k∞11−k∞12 and k∞44 have been calculated on the basis of a two-parameter fit to the photoelastic dispersion.
A line-source electron beam annealing system
Authors: Knapp J. A. , Picraux S. T.
Published: February 2003

A line-source electron beam annealing system has been developed for directed-energy beam processing of materials. The electron gun in this system produces a sheet beam which is focused at the sample into a line that can be swept laterally for area annealing. The system provides well-characterized beam dwell times in the range 5 μs–2 ms, a heretofore neglected time regime. We demonstrate line-source annealing by the system for the case of (100) epitaxial regrowth of 1250 Å of amorphous As-implanted Si with dwell times as short as 150 μs. Calculations of temperature profiles and the corresponding regrowth thicknesses for solid phase epitaxy are presented for Si as a function of beam power density and dwell time. Measured power density and dwell time thresholds for layer regrowth are consistent with solid phase epitaxy of the amorphous layer. The results show the utility of the system in exploring regrowth at temperatures near the melt.
Tunneling characteristics of amorphous Si barriers
Authors: Meservey R. , Tedrow P. M. , Brooks J. S.
Published: February 2003

Tunnel junctions using a barrier of amorphous silicon (a-Si) between normal and superconducting metals were studied at temperatures from 300 to 0.45 K. These junctions were reliably made by depositing on a 77 K glass substrate Ni or Au, a-Si 60 to 100 Å thick, and then Al. It was demonstrated that the dominant conduction process was elastic tunneling by the presence of structure caused by the superconducting energy gap of Al, and by comparing measurements of the voltage, temperature, and barrier thickness dependence of the conductance with theory. The effective barrier heights were grouped close to 2×10−2 eV. A semiquantitative argument suggests that the barriers controlling the elastic tunneling are much the same as those controlling the phonon-activated variable range tunneling at higher temperatures. Although a-Si barriers can be formed reliably and have low leakage, the low barrier height leads to large nonlinearity even at low voltages.
Capacitance/voltage studies on etched and anodized single-crystal n-CdSe
Authors: Frese Karl W.
Published: February 2003

The influence of redox couples on the measured flatband potentials of n-CdSe was investigated. It is concluded that flatband shifts are due to interface surface state-redox electrolyte carrier exchange that leads to variable interface state charge. Interface state densities ∼2×1012 cm−2, were determined with thin anodically grown selenium layer. A band diagram for the CdSe/Se electrode is presented.
Equation of motion of a spin vortex in a two-dimensional planar magnet
Authors: Huber D. L.
Published: February 2003

An equation of motion for a spin vortex in a two-dimensional easy plane magnet is presented and discussed. The theory is based on earlier work on the dynamics of magnetic domains and includes the effects of vortex-vortex interactions, (weak) applied fields, and dissipation. A possible approach to detecting vortices is also discussed.
A neutron study of the spin reorientation transition in Gd
Authors: Cable J. W. , Koehler W. C.
Published: February 2003

The temperature dependence of the spin orientation in Gd has been reexamined with particular emphasis on the spin-reorientation phase transition near 235 K. The spin orientation and the relative magnetization were determined in the 9–310 K region from Bragg intensity measurements on the (100) and (002) reflections from an isotopic single crystal of 160Gd. At low temperatures, the angle between the spin and the c axis is 40°. This angle ϑ increases with increasing temperature up to 71° at 130 K and then remains constant out to 220 K where it begins a precipitous decrease on approaching the reorientation transition at Tr⩾235 K. Unexpected intensity behavior was observed both below Tr, where the relative magnetization dips about 3% below the smooth curve in the 220–234 K region, and above Tr, where Bragg intensity remains at the (002) position out to about 285 K. Since the anisotropy constants of Gd indicate that ϑ = 0 above Tr, we attribute this (002) intensity to long wavelength orientation fluctuations and assume that only the average ϑ is zero above Tr. In spite of this complication, the transition exhibits Landau behavior in that the order parameter sin ϑ follows a (Tr−T)1/2 dependence in the range 223–234 K.
Effective field model for Ising ferromagnets: Influence of triplet correlations
Authors: Taggart G. B.
Published: February 2003

We use an effective field model which explicitly incorporates correlations to determine the critical temperatures for Ising ferromagnets on the common lattices. We specifically retain only the first nontrivial odd correlation function, i.e., the triplet correlation function, while neglecting higher order correlations. Within this approximation, assuming the equivalence of triplet correlations involving both central and noncentral sites in the nearest neighbor cluster yields values for the critical temperature below the Bethe approximations for z⩾4. Likewise, assuming the inequivalence of triplet correlations involving the central and noncentral sites yields values of the critical temperature only slightly higher than the Bethe approximation for z⩾4. For lattices with four- and six-fold coordination this equivalence and inequivalence of triplet correlation functions is sufficient to cause the critical temperatures to approach their two- and three-dimensional values. For instance, for quadratic lattices we obtain values for the critical temperature of 2.5284 and 2.9796 as compared to the exact and series results of 2.2692 and 2.7044 for the simple quadratic and diamond lattices, respectively, and the Bethe approximation of 2.8854.
Simple isotropic compressible ferromagnets
Authors: Nicoll J. F.
Published: February 2003

The Sak model isotropic compressible n-component ferromagnet is a simple system with a rich behavior. For n≠1, Goldstone modes are present. To these may be added the possibility of Fisher-renormalized exponents for positive specific heat exponent α or additional corrections-to-scaling for negative α. Furthermore, physically realizable systems always have a first-order phase transition for positive α and may have one even for negative α. Earlier work is extended to the entire thermodynamic region in such a way that all of these possibilities are encompassed in a single expression. In contrast to other approaches, the Goldstone modes, Fisher renormalization, and first-order transitions are handled in a uniformly valid crossover function which also resums the usual Wegner expansion to incorporate the transition to mean-field. Although formally correct to 0(ε), the result is exact for n = ∞ and vanishing effective rigid system coupling constant. In the special case of n = 1, 0(ε2) results are obtained. The methods employed should be applicable to other systems with first-order phase transitions.
MCRG study of d-dimensional random field Ising models
Authors: Rasmussen E. B. , Novotny M. A. , Landau D. P.
Published: February 2003

A Monte Carlo Renormalization Group method has been used to study d-dimensional hypercubic Ising models in random fields (±H). For d = 4 our results are consistent with classical critical behavior. For d = 3 we find a sharp transition with critical exponents which are apparently modified by the random field. We find no sharp transition for d = 2.
Heat capacity of Mn1−x ZnxF2 for x<0.1
Authors: Salamati-Mashhad H. , Dixon G. S. , Martin J. J.
Published: February 2003

The heat capacities of crystals of Mn1−x ZnxF2 have been measured by an ac technique for five Zn concentrations less than 10%. Particular attention has been paid to the region of the Néel temperature. TN(x) is found to be well represented by a linear relation for this range of concentrations. The peaks in the heat capacity are found to be rounded by the Zn dilution. The critical behavior outside the region of obvious rounding is consistent with a crossover from Heisenberg-like to Ising-like behavior for ΔT/TN⩽(HD/HE)1/φ where HD and HE are the effective dipole and exchange fields, respectively, and the crossover exponent φ = 1.3.
General-temperature series expansion for Ising systems
Authors: Lee Felix , Westwanski B. , Wang Y. L.
Published: February 2003

A linked-cluster expansion technique for spin systems has been used to obtain the free energy, the magnetization, and the susceptibility series for three Ising systems: (1) The Ising model; (2) The Blume-Capel model; and (3) The Blume-Emery-Griffiths (BEG) model. Each Hamiltonian is first divided into a single-ion potential and a term describing the interaction of the spin fluctuations. Only the latter is treated as perturbation in the series expansion. In the parametric phase our series reduces to the exact high-temperature series. However, in the ordered phase the same series can be used to study the low temperature behavior, such as the variation of the magnetization with temperature. The analysis of the eighth-order series for systems (1) and (2) show results in excellent agreement with the previous ones obtained from the high-temperature and low-temperature series. The series analysis for the BEG model shows quite different behavior from that predicted by the mean-field approximation.
Electrochemically induced coloration of liquid crystal materials
Authors: Nakamura Kenichi , Kaneko Shuzo , Ito Yasuo , Hirabayashi Hiromitsu , Ogura Katsuyuki
Published: February 2003

It is found that liquid crystal materials such as 4−n-octyl-4′-cyanobiphenyl (8CB) exhibit a reversible color change induced by an applied electric current when a small amount of electrolyte is dissolved into them. Such a coloration occurs in the liquid crystalline phase as well as in the isotropic phase. Their optical transmission spectra, and dependences of electrolyte concentration and applied voltage on the coloration, are measured in the isotropic phase. The changes in optical transmission and liquid crystal texture during a coloration and bleaching sequence are measured in the nematic, smectic, and cholesteric phases. A mechanism of coloration is discussed and an application of such an electrochromic liquid crystal cell to a laser beam writing is proposed.
Noise characteristic of helium-cadmium laser
Authors: Chiu M. S. , Tseng Y. G.
Published: February 2003

Our experiments have revealed that the noise of the He-Cd laser is caused not only by the moving striations but also by the wall-effected He pressure difference in the laser tube as well as the cataphoresis-effected concentration wave. And these three factors influence each other. By using a He diffusion return path, the output at 4416 Å increases from 60 to 110 mW, and the noise ratio decreases from 4% to 1.3% at 4416 Å, and to 0.9% at 3250 Å.
Determination of the recombination coefficients in undoped (In,Ga)(As,P) from transient optical output analysis of (In,Ga)(As,P)-InP double heterostructure LED’s
Authors: Heinen J. , Albrecht H. , Weyrich C.
Published: February 2003

The transient optical output of (In,Ga)(As,P)-InP light-emitting diodes with unintentionally doped active layers was analyzed experimentally and theoretically to determine the coefficients of radiative and nonradiative recombination mechanisms. Two models are given which describe both the transient behavior of these diodes and their output saturation. As a result, a linear radiative term plays an important part. On the other hand bimolecular radiative recombination, is much smaller than for (Ga,Al)As diodes. Nonradiative terms may be attributed to Auger recombination.
On the determination of the spatial distribution of deep centers in semiconducting thin films from capacitance transient spectroscopy
Authors: Zohta Yasuhito , Watanabe Miyoko Oku
Published: February 2003

It is pointed out that a widely used simple formula may give rise to serious errors in profiling deep level concentrations from capacitance transient experiments. A correction formula is derived based on the space-charge analysis.
Dynamic magnetic response near TC
Authors: Heinrich B. , Arrott A. S.
Published: February 2003

Linear response theory is applied to the frequency dependence of the ac magnetic susceptibility of iron whiskers and nickel platelets in the region of the Curie temperature. It is shown that a particular form of a magnetic memory function can be used in formulations based on either or both magnetic viscosity and/or relaxation toward equilibrium. This memory function corresponds to a relaxation rate that follows a power law in time as proposed by Ngai and White.
Magnetism of surfaces and interfaces
Authors: Freeman A. J. , Wang Ding-Sheng , Krakauer H.
Published: February 2003

Recent experimental advances in the study of surfaces have raised important questions about our fundamental understanding of these phenomena. One important consequence of this has been the development of theoretical/computational methods for accurately determining the electronic structure and properties of surfaces and interfaces. This talk reports on theoretical determinations of the magnetic properties of free surfaces [e.g., Ni(110)] and overlayers [e.g., Ni on Cu(001)] based on self-consistent spin polarized energy band determinations of the energy dispersion and spatial character of surface states. Particular attention is paid to surface state effects on surface spin polarization, magnetic moments, and exchange splittings. Detailed results of charge and spin densities and layer projected density of states are presented. Comparisons are made to relevant photo-emission and other experiments, the nonexistence of magnetically ’’dead’’ layers is described, and comparisons with earlier results1 on coherent modulated Cu/Ni structures are given.
Electronic structure and magnetism of actinide compounds: 6d versus 5f electrons (invited)
Authors: Reihl B. , Mårtensson N. , Vogt O.
Published: February 2003

We have studied the electronic structure of cleaved (100) single crystals of UAsxSe1−x, UxTh1−xSb, UxY1−xSb, and USbxTe1−x with high resolution (ΔE<150 meV) photoemission using synchrotron radiation 10⩽hν⩽130 eV. In contrast to the general belief we find significant differences in the photoemission density of states near EF for these compounds. We conclude that these differences are mainly due to valence d electrons and that U has a quasi-localized 5f3 configuration in resonance with itinerant 6d states. This explains the quite similar magnetic behavior of UxTh1−xSb and USbxTe1−x, both of which change from highly anisotropic 〈100〉 antiferromagnets (x≳0.85) to 〈111〉 ferromagnets (x≲0.85), i.e., with an increasing number of 6d electrons. Namely, the smaller ordered moment in UTe and the gradually decreasing moment in UxTh1−xSb for 1.0⩾x⩾0.3 are attributed to the 6d moments which are polarized antiparallel to the 5f moments.
Magnons at the Curie temperature
Authors: Poling D. A.
Published: February 2003

Random phase approximations (RPA) have very successfully treated spin wave excitations in Heisenberg ferromagnets at low temperatures. The role played by these magnons at the order-disorder transition, however, has been a topic which has eluded RPA theories to date. In light of recent data1, the idea of magnons at the Curie temperature and above has become more difficult to refute. This adds incentive to attempt to model interacting magnons at the Curie temperature. This work examines some attempts to formulate higher random phase approximations and discusses why they fail as they approach the transition temperature. A new interpretation of some work by Parmenter and author2 is presented. The nature of the approximations made in that work is discussed, and an attempt is made to eliminate incorrect contributions from the energetically disfavored paramagnetic state to the correct magnon renormalization. A solution is presented which has the proper low temperature behavior as demonstrated by Dyson3 restored and the reentrant behavior eliminated. An examination of how close this model comes to behaving correctly is performed, and a comparison is made to some recent work by Parmenter4.
Dyson Maleev transformation in ferromagnets with single ion planar anisotropy
Authors: Rastelli E. , Tassi A.
Published: February 2003

We show that the Dyson Maleev (DM) transformation can be applied to the Heisenberg ferromagnet with single ion planar anisotropy, although the resulting bilinear part of the bosonic equivalent normally ordered Hamiltonian is not hermitian. With respect to the approaches adopted up to this moment, that is, the Holstein Primakoff (HP) transformation and the matching matrix elements (MME) method, the DM transformation offers relevant advantages because it generates a finite number of interaction potentials, so we are able to obtain the second-order perturbation contributions of the excitation energy, a practically impossible result to get in the HP and MME frame. In this communication we discuss the problem and show the main results; details of the very long, cumbersome calculations will be given elsewhere.
Ladder approximation for uniaxial metamagnets
Authors: Rastelli E. , Tassi A.
Published: February 2003

The ladder approximation for metamagnets with exchange, single ion uniaxial anisotropy, and small interplane coupling is worked out. The thermal renormalization and damping of the uniform mode are given in explicit form. This result is applied to FeCl2, improving previous analysis where the self-energy of a ferromagnet was extended to metamagnetic systems. We find that the aforementioned approach is good in itself. However, we obtain non-negligible improvements by taking into account the damping effects neglected in the previous approach (for instance, a spurious divergence in the self-energy is taken off). The fitting of the energy gap and the magnon lifetime with experimental data indicates that the single ion anisotropy is pre-eminent with respect to the exchange one.
Abstract: Magnetic solitons’ contribution to the specific heat of (CH3)4NMnCl3 in an external magnetic field
Authors: Borsa F.
Published: February 2003

It has been shown theoretically that linear magnetic systems with planar anisotropy should display nonlinear excitations, i.e., sine-Gordon solitons upon application of a magnetic field perpendicular to the chain axis. Experimental evidence for ID magnetic solitons has been presented for TMMC from neutron scattering and NMR measurements.1 The classical statistical mechanics of this system predict a soliton contribution to the free energy and thus to the specific heat.2 In order to test experimentally the thermodynamic relevance of magnetic solitons, I performed measurements of specific heat in single crystal TMMC in an external magnetic field up to 10 Tesla, applied both perpendicular and parallel to the chain. The measurements were performed with an adiabatic calorimeter in the temperature range 1.5–15 °K. The results show an extra contribution for H⊥c not present for H∥c. This contribution displays a broad maximum which scales approximately as H/T in agreement with the theory. The maximum occurs just above the peak in the specific heat which is observed in correspondence to the three-dimensional transition temperature, and it can be clearly resolved only for H⩾5.0 T. The soliton energy obtained by fitting the experiments to the classical theory is Es = 2.0 H for H = 5.39 T and Es = 1.8 H for H = 10 T to be compared with the theoretical value of Es = gμBHS = 3.35 H and with the value obtained by neutron scattering at H = 3.2 T, i.e., Es = 2.6 H. The discrepancy between theory and experiment is discussed in terms of renormalization corrections and of a possible soliton instability occurring for fields between 3 and 5 T.      a)Permanent address: Institut di Fisica, Universita di Pavia, 27100 Pavia, Italy.  1J. P. Boucher, L. P. Regnault, J. Rossad Miguod, J. P. Renard, J. Bouillot, and W. G. Stirling, J. Appl. Phys. 52, 1956 (1981).  2K. M. Leung, D. Hone, D. L. Mills, P. S. Riseborough, and S. E. Trullinger, Phys. Rev. B 21, 4017 (1980).
Field-induced ’’exchange flips’’ in a randomly diluted antiferromagnet (invited)
Authors: King A. R. , Jaccarino V. , Sakakibara T. , Motokawa M. , Date M.
Published: February 2003

High field magnetization measurements of the randomly diluted, anisotropic antiferromagnet FexZn1−xF2 have revealed the behavior of novel single-spin ’’exchange-flips’’ as well as the concentration-dependent spin-flop field HSF. Measurements were made in pulsed field up to 550 kOe in the High Magnetic Field Laboratory in Osaka University on samples from pure FeF2 to beyond the percolation limit. The exchange flips occur when the applied field H0 exceeds the effective exchange field HE = nHE/z at sites on the down-sublattice with n magnetic neighbors. Below either HSF or a ’’crossover’’ field Hcr, hysteretic behavior is seen with respect to the direction of field sweep, whereas above HSF or Hcr it is absent. Further, Hcr is marked by an anomalous peak in dM/dH versus H0. These effects are explained by a transition to a disordered state above Hcr, which possesses no long-range antiferromagnetic order. A classical-spin computer simulation has been found to accurately reproduce these effects.
The role of Sz-nonconserving interaction in the spin dynamics of impure antiferromagnetic insulators
Authors: Hone Daniel , Thayamballi Pradeep
Published: February 2003

In some antiferromagnetic insulators with dilute substitutional impurities, the magnon spectrum includes spatially localized states in the energy gap below the magnon band. With increasing magnetic field these may become degenerate with a piece of the continuum. If Sz is conserved, the band states and the localized states are orthogonal by symmetry, and the local mode cannot decay into the continuum. We examine here the contribution of Sz-nonconserving interactions to the magnetic resonance linewidth of the local mode as it enters the continuum. The effect of the site local orthorhombic anisotropy in Mn:FeF2 is calculated, in particular, and is shown to be a reasonable source of the observed width at the single available experimental point.
Single crystal magnetic studies on CuCl2⋅TMSO, a Heisenberg 1-d ferromagnet
Authors: Landee C. P. , Willett R. D. , Waldner F.
Published: February 2003

CuCl2⋅TMSO is one of a series of linear chain spin 1/2 compounds with predominantly ferromagnetic interactions in the paramagnetic state. Initial studies conducted on powders showed the exchange interaction J to be large (J/k = 39 K) with significant antiferromagnetic interchain exchange (J′/J = −0.02). This paper reports the results of single crystal susceptibility, magnetization, and antiferromagnetic resonance studies on this material. The orthorhombic compound orders antiferromagnetically at 3.3 K with the easy axis canted to the chains. Spin-flop transitions are observed upon the application of small fields (<5 kOe) within the ac plane. A molecular field model estimates the antiferromagnetic exchange field and anisotropic field to be 2×10−2 and 10−3 as large as the ferromagnetic exchange field. AFMR measurements indicate a four-sublattice arrangement, with the moments inclined alternately ±55° to the chain axis. A four-sublattice model is proposed for the magnetic structure.
Magnetic properties of single-crystal {110} iron films grown on GaAs by molecular beam epitaxy (invited)
Authors: Prinz G. A. , Rado G. T. , Krebs J. J.
Published: February 2003

Single-crystal {110} Fe films, grown for the first time by molecular beam epitaxy on GaAs, have been studied by a variety of techniques in order to determine the dependence of the magnetic properties upon film thickness L and quality, and an overview of these results is presented. The dependence of the ferromagnetic resonance (FMR) field upon its orientation in the (11̄0) plane was measured at 16.4 GHz and shows that the magnetically easy axis is [110] for L<50 Å and [001] for L≳150 Å. A theory of FMR which incorporates magnetocrystalline surface anisotropy is outlined. It successfully interprets the thickness dependence of the FMR data in the ultrathin (L≲50 Å) regime and shows them to be surface dominated. FMR data at 9.2 GHz, which contain both aligned and nonaligned resonance branches, are presented as a function L. In addition, the dependence of the branches on frequency f for 8 GHz<f<40 GHz along [001] and [110] are discussed.
Domain lattice FMR in magnetic garnet thin films
Authors: Myers E. C. , Bi S-Y. , Charap S. H. , Artman J. O.
Published: February 2003

We report on domain lattice FMR observations made on thin film specimens of magnetic garnet prepared by LPE. The resonance data were taken on (111) films over a frequency f range extending from 1 to 4 GHz. The applied magnetic field H was directed along [1̄10]. The specimens were set on a microstrip microwave transmission line, thus permitting concurrent observation of the domain lattice pattern from above. The input parameters needed to predict the positions of the two FMR modes obtaining for a bubble or stripe lattice were gathered from independent observations which included domain geometry data. The fit to our previously published theoretical expressions, in which the domain aspect ratios are taken into account, is reasonable. The observed FMR mode separations, which are as little as one-half of the predicted values, may be a more sensitive test of the theory.
Measurement of magnetization of YIG films on gadolinium-gallium garnet substrates from ferrimagnetic resonance and ferrimagnetic antiresonance
Authors: Dewar G. , Murthy V. R. K. , Shone M. , Belt R.
Published: February 2003

The radio-frequency magnetic properties of (111) oriented YIG films epitaxially grown on gadolinium-gallium garnet substrates have been investigated using microwave transmission and reflection techniques. Measurement of the microwave frequency and applied magnetic field at ferrimagnetic resonance indicate that the g-factor is 1.997±0.003 and 4πM−2Ku/M+4K1/3M = 1700 G. In addition to FMR, a broad feature ∼100 G wide, observed in both transmission and reflection, is argued to be associated with the zero of the real part of the permeability (ferrimagnetic antiresonance, FMAR). The YIG magnetization is directly proportional to the separation in magnetic field between FMR and FMAR and is in the range 1560–1660 G for these films. The measurement of both FMR and FMAR is a new method of determining the magnetization of as-grown nonconducting, ferrimagnetic films. The uniaxial anisotropy does not complicate the determination of the magnetization. This method is applicable to any film for which the magnetization is significantly larger than the FMR linewidth and/or the magnetostatic mode spectrum excited.
Abstract: Theory of ferromagnetic resonance and static magnetization in ultrathin crystals
Authors: Rado George T.
Published: February 2003

A theory of the static magnetization orientations ϑ0 and of the ferromagnetic resonance fields Hres in ultrathin (50 Å) monocrystalline films is presented. Both ϑ0 and Hres are shown to depend crucially on the magnetocrystalline surface anisotropy, and a generalization of this anisotropy is proposed. The equation-of-motion of the magnetization is combined with the general exchange boundary condition, and realistic approximations are introduced in order to obtain closed algebraic expressions for ϑ0 and Hres. The spatial dependence of the oscillations in the presently calculated resonance modes do not represent spin waves but nondissipative exponential decays perpendicular to the film surfaces. The film thicknesses 2L are so small that the films are ’’beyond spin wave cutoff.’’ It is shown that the angular dependence of Hres differs strongly from that predicted by the uniform mode theory and that Hres is a linear function of 1/L rather than independent of L. An extension of the theory to somewhat thicker films is also presented. A full account of this work has been submitted to Physical Review B.
Superconductivity in CeCu2Si2 (invited)
Authors: Lieke W. , Rauchschwalbe U. , Bredl C. B. , Steglich F. , Aarts J. , de Boer F. R.
Published: February 2003

The specific heat, dc magnetization, and resistivity have been measured on a number of samples of CeCu2Si2, which was previously found to assume a novel superconducting state below 0.5 K [F. Steglich et al., Phys. Rev. Lett. 43, 1892 (1979)]. By comparing the transition temperatures of samples which have undergone different heat treatments, we demonstrate that small fractions of spurious phases hinder the onset of superconductivity but are gradually removed with annealing. With a very pure sample obtained in this way, we have fully confirmed all the characteristics of the superconducting phase transition reported before on less pure samples. However, even after this heat treatment, the samples show clear signs of striking inhomogeneities, manifested by a static Meissner effect of bulk samples representing typically only a few percent of the volume. An increase of this Meissner signal up to 60 vol% is observed after powdering the samples. In addition to measurements in the superconducting state, the intrinsic value of the low-temperature susceptibility in the normal state was determined by measurements on a pure sample and is discussed in connection with the large electronic specific-heat coefficient γ.
Thermal variations of the conduction electron polarization in HoAl2
Authors: Boucherle J. X. , Schweizer J.
Published: February 2003

The magnetization density of the ferromagnetic compound HoAl2 has been investigated by polarized neutron diffraction. At low temperature the bulk magnetization corresponds to 9.3μB per formula but the form factor analysis leads to a 4f magnetic moment of 8.6μB/Ho. It reveals the existence of a large polarization of the conduction electrons, whose spatial extension is compatible with 5d electrons. We have studied the variations of this polarization when the temperature increases. At 25 K, well below the Curie temperature (32 K), most of the polarization has disappeared. At this temperature the magnetization under the experimental field of 46 kOe has only slightly decreased compared to 4.2 K. It is then clear that the decrease of the 5d polarization cannot be directly related to the variation of the 4f magnetic moment.
Study of the magnetization reorientation in HoCo5
Authors: Decrop B. , Deportes J. , Givord D. , Lemaire R. , Chapert J.
Published: February 2003

A noncollinearity of the magnetic structure in HoCo5 during the magnetization reorientation has been evidenced through magnetization Mössbauer and neutron diffraction measurements. Such a configuration permits a minimization of exchange interactions and holmium and cobalt anisotropies which are competing.
Magnetic moment distribution in iron vanadium alloys
Authors: Mirebeau I. , Parette G.
Published: February 2003

Neutron diffuse scattering measurements were performed on FeV alloys from 1 to 20 at. % vanadium. The nuclear cross sections were previously investigated in comparison with NMR experiments and exhibit a very strong atomic short range order (SRO). The magnetic cross sections are analyzed here in terms of the localized moment model of Marshall using the known atomic SRO parameters. The data indicate a strong negative moment on the impurity site, almost constant at about −1.1μB up to 5 at. % V. It then tends to zero as the Vanadium concentration increases. The moment values are in very good agreement with previous unpolarized neutron experiments on dilute alloys. The magnetic perturbation terms on the neighbor sites are shown to have similar trends as shown by the SRO parameters. The values of the magnetic cross sections for K = 0 are in good agreement with the values of (dμ/dc)2 derived from magnetization measurements when c⩽4 at. %V. They differ strongly for higher concentrations, which suggests that neutrons probe a more restricted region around the impurity than magnetization measurements.
Magnetic excitations and lattice vibrations in uranium rock-salt structure compounds
Authors: Holden T. M. , Buyers W. J. L. , Svensson E. C. , Jackman J. A. , Murray A. F. , Vogt O. , DuPlessis P. de V.
Published: February 2003

The inelastic magnetic and lattice vibrational scattering of neutrons from several uranium pnictides (UN, UAs, and USb) and chalcogenides (US, UTe) have been measured in recent years. The character of the low-temperature magnetic scattering depends on the lattice constant. The compounds with the largest lattice parameters (USb, UTe) exhibit several branches of well defined magnetic excitations. The other compounds exhibit only a broad, featureless frequency response at low temperature, which may be extremely weak, as in new data presented here for US. Nonetheless, intense paramagnetic scattering is observed above the ordering temperature, with a marked 2d anisotropy in all the pnictides. A force-constant analysis of the phonon dispersion relations for these materials has been carried out. We show here that UTe, US, and UAs have negative values of the elastic constant C12, as has been found in certain mixed valence compounds. As expected from the increase in ionic size, the compressibility increases with the lattice parameter in each series; the chalcogenide compressibility is greater than that of the corresponding pnictide.
Paramagnetic scattering studies of the short-range order above TC in 3d transition metal compounds and pure iron
Authors: Brown P. J. , Deportes J. , Givord D. , Ziebeck K. R. A.
Published: February 2003

For 3d metals the atomic moments derived from Curie-Weiss susceptibility are often significantly larger than the value obtained in the ferromagnetic state from the saturation magnetization. Indeed the Heisenberg model is known to be inappropriate for metallic systems which in general are better described by band theory. The ground state properties of metallic magnets, particularly the occurrence of nonintegral moments, are well accounted for using the Stoner theory, but the model is less satisfactory at elevated temperatures. The persistence of spin waves above TC and the observation of paramagnetic moments indicate that the 3d band remains split in the paramagnetic phase in contradiction with mean field theory. A direct way of establishing the existence of paramagnetic moments is by the paramagnetic scattering of neutrons. The technique is particularly enhanced if polarization analysis is employed to obtain a unique estimate of the magnetic cross section. In the present paper, we report the results of recent polarization analysis measurements on transition metal compounds and on pure iron. The magnitude of the atomic moments were found to be temperature independent, remaining close to the saturation magnetization values. Considerable short-range order persisting up to many times TC was observed and it is these correlations which give rise to the large paramagnetic moment extracted from the Curie constant.
Observation of spin waves in Pd(1.5% Fe)
Authors: Lynn J. W. , Rhyne J. J. , Budnick J. I.
Published: February 2003

Inelastic neutron scattering measurements have been carried out on the ’’giant-moment’’ alloy system Pd(1.5% Fe), which is in the dilute ferromagnetic regime. Below the Curie temperature of 67 K relatively well defined spin wave excitations have been observed in the small wave vector region (Q⩽0.14 Å−1). The dispersion of these excitations is consistent with the quadratic relation E = DQ2 expected for an isotropic ferromagnet, with D = 40 meV Å2 at a temperature of 40 K. With increasing temperature the spin waves are found to renormalize in energy and broaden rapidly with both increasing Q and increasing temperature.
Muon spin rotation in iron and its dilute alloys
Authors: Arrott A. S. , Heinrich B. , Stronach C. E. , Lankford W. F.
Published: February 2003

Muon spin rotation measurements are reported for iron and its dilute alloys. The average local field seen by the muon diffusing among interstitial sites is corrected for the Lorentz cavity field to extract the average hyperfine field. The rate of change of hyperfine field with solute concentration at room temperature is given for iron alloyed with N, Al, Si, Ge, Ti, V, Cr, Mn, Co, Ni, Mo, and W. The origin of strain sensitivity of the average local field is discussed using results on the effects of applied fields on polycrystalline and single crystal samples.