bluuebunny/crossref_metadata_2025 huggingface dataset:
Distinction between electron and hole traps in semi-insulating GaAs
Authors: Kiliulis R. , Kazukauskas V. , Bourgoin J. C.
Published: July 2002

Traps are commonly characterized in semi-insulating materials using a thermally stimulated current technique that cannot discriminate between electron and hole traps. We describe a combination of several thermally stimulated techniques that allow us to distinguish electron traps from hole traps. It is applied to semi-insulating GaAs where the EL2 defects present in this material introduce holes in the valence band when they are transformed into their metastable states. The nature, donor or acceptor, of the traps detected by thermally stimulated current has been verified by thermally stimulated Hall mobility and thermoelectric effect current measurements. The results obtained allow us to precisely determine the role of acceptor defects in the compensation mechanism of this material.
Magnetic impurities and clusters on Ag, Pd, and Pt surfaces (abstract)
Authors: Stepanyuk V. S. , Wildberger K. , Lang P. , Zeller R. , Dederichs P. H.
Published: July 2002

The modern STM technique allows to manipulate the structure of small clusters on surfaces and to produce one- or two-dimensional nanostructures. It can be speculated that due to the reduction of coordination number many of these clusters containing transition elements are magnetic. In this paper we will show that small clusters of 3d, 4d, and 5d atoms are magnetic on different substrates. Our calculations are based on density functional theory and a newly developed KKR Green’s function method for defects at surfaces. We calculate the magnetic properties of several linear chains (C) and plane islands (I) of 4d and 5d adatoms. In particular we consider linear chains of 2 (dimers, C2), 3 and 4 adatoms (C3 and C4), being oriented in (110) direction, as well as three compact islands with 4, 5, and 9 adatoms (I4, I5, and I9). The moments of the dimers are in general reduced by the interaction in comparison to the moments of single adatoms. In most cases both a ferromagnetic and an antiferromagnetic configuration exist, with the ferromagnetic one being more stable at the end and the antiferromagnetic one at the beginning of the series. For the linear chains quite large moments are obtained, but the behavior with size is nonregular. The large moments obtained for all four chain structures indicate that also infinite chains of these atoms should show appreciable moments. For compact islands (I4, I5, and I9) the hybridization effects within the cluster are even larger. For Ag(001) we find only appreciable moments for the Ru and Rh nanostructures.
Effect of nitrogen interstitial in α-Fe crystalline on the magnetic soft properties of FeTaN thin films
Authors: Chang W. C. , Wu D. C. , Lin J. C. , Chen C. J.
Published: July 2002

The effect of nitrogen interstitial in α-Fe crystalline on the magnetic soft properties of Fe87.5Ta12.5Nx films has been investigated. The magnetic soft properties can be improved by carefully controlling the nitrogen flow ratio during sputtering and the postannealing to result in a film with fine grain size and low magnetostriction. Using x-ray diffraction technique, the nitrogen content in the α-Fe crystalline of FeTaN film can be estimated from the effect of lattice expansion of α-Fe crystalline due to nitrogen interstitial. It is found that the film with about 7.5 at. % N2 in α-Fe crystalline exhibits the best magnetic soft properties of Br=1.6 T, Hc=0.2 Oe and μi(1 MHz)=3400, respectively. X-ray diffraction analysis is found to provide a proper and an easy way to identify the suitable condition for growing excellent magnetic soft FeTaN thin films.
Simulations of inhomogeneous magnetization processes in ultrathin films with growth-induced roughness (abstract)
Authors: Moschel A. , Zangwill A. , Stiles M. D.
Published: July 2002

Magnetization in ultrathin films depends in a very sensitive way on the morphology of the surface of the films. Using a X–Y model with enhanced anisotropy at the step edges, we have calculated hysteresis loops in different directions at zero temperature for nominally vicinal and flat films with growth-induced surface roughness. For the vicinal films the influence of the width of the terraces and the strength of the anisotropy at the edges is studied. For the ‘‘flat’’ films the influence of the shape and the size of 2D and 3D island roughness is investigated. Some interesting properties of the coercive field and the remanent magnetization are discussed and compared to some experimental results.
Early development of neutron scattering by magnetic materials (invited) (abstract)
Authors: Shull C. G.
Published: July 2002

Neutron scattering has established itself as an important tool in studying magnetic materials. The early history of this development will be outlined, ranging from the earliest studies of paramagnetic materials to those of ferromagnetic and antiferromagnetic materials.
Neutron scattering as a probe of unconventional superconductivity in YBa2Cu3O7 (invited) (abstract)
Authors: Keimer B.
Published: July 2002

We have carried out a variety of neutron scattering experiments on the high temperature superconductor YBa2Cu3O7 which elucidate the unconventional microscopic properties of the cuprates and illustrate the power and versatility of neutron scattering as a probe of novel materials. Small angle magnetic neutron diffraction is thus far the only technique capable of revealing the structure of the vortex lattice in the cuprate superconductors in magnetic fields of several tesla. We have shown that in YBa2Cu3O7 the vortex lattice in this field range has an oblique structure when the field is applied along the c axis (perpendicular to the CuO2 layers). Recent theoretical work has attributed this unusual structure to the unconventional (d-wave) pairing state realized in the cuprates. We will also discuss the rich behavior of the vortex lattice when the field is tilted with respect to the c axis. We have used inelastic neutron scattering to elucidate the magnetic and lattice vibrational excitations in YBa2Cu3O7. Magnetic and phonon scattering were separated both by detailed calculations of phonon dynamical structure factors and by neutron polarization analysis. A novel resonant magnetic excitation at wave vector q=(π/a,π/a) and energy ℏω=40 meV appears in the superconducting state. The simplest model for this resonance relies on quasiparticle pair creation and implies a sign reversal of the energy gap function on the Fermi surface. Alternative models will also be discussed. We have also observed a strongly wave vector dependent renormalization of the energy and lifetime of a specific phonon in the superconducting state of YBa2Cu3O7. Since the phonon self-energy is sensitive to the anisotropies of the energy gap and Fermi surface via the electron–phonon interaction, our data contain important information about these parameters.
Nonmonotonic length dependence of switching field of nanolithographically defined single-domain nickel and cobalt bars (abstract)
Authors: Kong Linshu , Chou Stephen Y.
Published: July 2002

Previously, the switching field of nanolithographically defined single-domain permalloy and nickel bars was found to increase monotonically as the bar aspect ratio increases or as the bar width decreases. In this work, we will show, for the first time, that when the bar width is fixed, the switching field of nanolithographically defined single-domain nickel and cobalt bars changes nonmonotonically with the bar length (hence the aspect ratio). The isolated Ni and Co bars were fabricated using electron-beam lithography and a lift-off process, and have a thickness of 35 nm, a fixed width of 100 nm, and a length varying from 200 nm to 5 μm. The magnetic force microscopy (MFM) showed that all the as-fabricated bars, except for the Ni bars with a length less than 500 nm, are single domain. The switching field of the single-domain bars was found to increase with the bar length first, then decrease after reaching a peak. The peak switching field and corresponding bar length are 640 Oe and 1 μm for Ni, and 1250 Oe and 2 μm for Co. This behavior, which clearly deviates from Stoner-Wohlfarth model, suggests that two different mechanisms should be responsible for the magnetization reversal process in different bar lengths. When the bar length is small, the exchange energy can be much larger than the magnetostatic energy to keep the bar switching quasicoherently and the switching field increases with the bar length. However, when the bar length is large, the exchange energy cannot overweigh the magnetostatic energy for quasi-coherent switching and therefore the lowest switching energy state would be incoherent switching which involves multidomain (or vortex) reversal leading to an decrease of switching field. The detailed analysis will be presented in the paper.
Neutron diffraction determination of the nuclear spin ordering in Cu and Ag at nano- and subnano-K temperatures (invited)
Authors: Steiner M. , Metz A. , Siemensmeyer K. , Lounasmaa O. V. , Tuoriniemi J. T. , Nummila K. K. , Vuorinen R. T. , Clausen K. N. , Lefmann K. , Rasmussen F. B.
Published: July 2002

The spontaneous nuclear spin ordering in the simple diamagnetic metals Cu and Ag has been studied by neutron diffraction using the spin dependent part of the nuclear cross section. Simple antiferromagnetic ordering of type I has been found in zero field for these fcc-systems below 60 and 560 pK for Cu and Ag, respectively. The ordering in an applied field has been investigated and the phase diagrams have been determined: a very complex phase diagram with different magnetic structures, strong hysteresis, and time dependence was found for Cu, while in Ag the phase diagram is very simple. Comparison with ab initio calculations allows a rather deep insight into the interactions leading to the ordering.
The effect of physical structure on δI curves of longitudinal thin film (abstract)
Authors: Walmsley N. S. , Hart A. , Parker D. A. , Dean C. , Chantrell R. W.
Published: July 2002

Conventionally, the simulation of thin films has been carried out using an HCP structure to represent its physical structure. Such an idealized structure has been shown to overestimate the effect of interparticle coupling by using an homogeneous exchange coupling scheme. A more realistic approach to represent physical structure has been undertaken by generating a system of grains which lie on a radially isotropic structure and have a nonuniform volume distribution. Interaction effects can be shown by computing δI curves formed through the comparison of the remanence curves using the Wohlfarth relation. To generate an isothermal remanence curve (IRM), a realistic ac erased state is necessary which has been carried out by using a simulated annealing technique. A comparison has been made to determine the effect of physical structure on the bulk properties by computing hysteresis loops, remanence curves, and δI curves. To reduce statistical error these have been averaged over four different sets of easy axes. The loops indicate that an irregular physical structure leads to an increase in the coercivity and decrease in the squareness. The dc-demagnetization curves show an increase in remanent coercivity; this can be attributed to the random physical structure decreasing the size of magnetically correlated regions within the microstructure. The effect of the physical microstructure on the δI curves will be discussed in full in the paper.
Micromagnetics of uneven ultrathin film trilayers
Authors: Arrott A. S.
Published: July 2002

For ferromagnetic sandwiches where two iron layers are separated by a middle layer, the strength of the coupling sharply varies with each change in the thickness of the middle layer. If the interfaces were sharp on the atomic scale, one could observe this directly, or else one can use micromagnetics to take into account the actual morphology of less than ideal interfaces as shown by Slonczewski. As an approximation the average magnetization in each layer is found from a phenomenological energy that contains the competition between bilinear and biquadratic exchange constants that depend on surface roughness. More detailed micromagnetic calculations using relaxation methods to treat spatial variations of the magnetization in each layer are given for Fe–Cr–Fe sandwiches. The approximate calculations give jumps in the magnetization at critical fields, but these are smeared out in the more detailed calculations. For the special case of equivalent layers, the phenomenological expression for the energy can be used to obtain analytic expressions for the field dependence of the magnetization.
The protective coatings of NdFeB magnets by Al and Al(Fe)
Authors: Qin C-D. , Li A. S. K. , Ng D. H. L.
Published: July 2002

NdFeB permanent magnets are coated by two different methods, dip coating and evaporation coating, with Al. It is found that an intermetallic reaction has occurred and formed a dominant phase of FeAl2.5Bx at the interface between the Al coating and the magnet material. Such reaction enhances strong bonding between the coating and the magnet. When Al(Fe) is used for the coating, no detectable intermetallic phase is observed, and therefore the amount of magnet material is conserved. We have studied the effect of the intermetallic phase on corrosion protection of the magnet. It is found that this phase is less resistive towards HNO3 solution than pure Al; but it is more resistive towards NaOH and NaCl. The investigation of the changes of microstructures of the coatings under different tempering conditions have also been carried out. The results show that the heat treatment at 500 °C for 10 min for a ∼ 2 μm thick Al coating by evaporation can produce an optimal layer of intermetallics. If the temperature is too low, the coating will not adhere to the magnet or if too high, it will fully react with the magnet and produce a rough coating surface.
Fabrication and magnetic properties of metal/cobalt ferrite composite thin films
Authors: Na J. G.
Published: July 2002

To improve the saturation magnetization (Ms) of cobalt ferrite thin films, metal/cobalt ferrite composite thin films were prepared by controlling the sputtering conditions such as oxygen concentration in sputtering gas, substrate temperature, composition of the thin films. With decreasing the oxygen ratio from 50% to 5% in sputtering gas and increasing the substrate temperature from room temperature to 400 °C, the Ms of the thin films was increased and the coercivity (Hc) decreased. The result was attributed to deposition of the metal/cobalt ferrite thin films. With increasing the metal ratio in the thin films, preferred orientation of (111) of cobalt ferrite was worsened in the thin films. The metal deposited in the thin films was identified as cobalt with (002) preferred orientation. In higher cobalt content than the stoichiometric composition of CoFe2O4, the thin films with high Ms and Hc could be deposited in the wide substrate temperature range of 200–400 °C. We can prepare the metal/cobalt ferrite composite thin films with Ms about 580 emu/cm3 and Hc 1700 Oe by controlled the sputtering conditions. The thin films are thought to be applicable to the magnetic recording media with high recording density.
Hall-effect characterization in semiconductor heterostructure junctions using polarized laser signal
Authors: How Hoton , Fang Ta-Ming , Lu Feng , Ahern Brian
Published: July 2002

We describe a new technique which exploits the ac Hall effect in the characterization of layered semiconductor structures. The method employs laser signals in the presence of a dc magnetic bias field. Upon incidence the polarization of the optical signal is rotated via a Lorentz force due to the ac Hall effect. As such, the reflected waves carry information on the Hall mobility of the charge carriers. The calculations show that ac Hall reflection warrants sufficient intensity to be measured. Our theory is complete in the sense that depth profiling has been explicitly incorporated in the formulation.
Electronic structure and exchange interactions in fcc Fe
Authors: van Schilfgaarde M. , Antropov V. P. , Harmon B. N.
Published: July 2002

We apply a new first principles approach for evaluating the spin dynamics for arbitrary magnetic systems. The method is implemented within the local density approximation and applied to γ-Fe, a frustrated system where we obtain new low-energy magnetic configurations.
Magnetic ordering in clustered Fe-based materials synthesized from laser plasma (abstract)
Authors: Blyakhman Yuri , Polushkin N. I. , Akhsakhalyan A. D. , Gusev S. A. , Salashchenko N. N. , Semenov V. G.
Published: July 2002

Using Mössbauer spectroscopy and the magneto-optical Kerr effect, we investigate how magnetic order arises and develops in Fe/C and Fe/Cr layered mixtures prepared by pulsed laser deposition. Two different magnetic transformations are found to occur in these systems under their thermal annealing. The first is the paramagnetic–ferromagnetic crossover that takes place at annealing temperatures of 420–570 K in dependence on the average iron content. Varying the latter quantity, we also observed the same crossover to the ferromagnetic state. The second transformation is magnetic hardening at 700–800 K. The specific feature of both transformations is that they occur within narrow ranges of annealing temperature (<10 K). The first transformation is supposed to result from an additional iron enrichment of Fe-rich clusters and is associated with magnetic phase transition near the Curie temperature. The cluster enrichment was directly observed with transmission electron microscopy and was also shown in small angle x-ray scattering patterns of the compositionally modulated structures. The observed sharpening of the magnetic behavior near the Curie temperature is presumably a manifestation of the clustered structure. The second transformation is found to be induced by the onset of crystallization and the formation of particles of the new Fe-rich phase.
Computational calculations of magnetic relaxation and viscosity in small magnetic grains
Authors: Hernàndez J. M. , Zhang X. X. , Tejada J.
Published: July 2002

In this article we present a phenomenological model which simulates very well the mag- netic relaxation behavior experimentally observed in small magnetic grains and single domain particles. In this model, the occurrence of quantum tunneling of magnetization below a certain temperature is taken into account. Experimental results for different materials are presented to illustrate the most important behavior deduced from our model.
Lattice effects on the magnetoresistance in doped manganese perovskites (abstract)
Authors: Hwang H. Y. , Cheong S-W. , Radaelli P. G. , Marezio M. , Palstra T. T. M. , Batlogg B.
Published: July 2002

A detailed study of doped LaMnO3 with fixed carrier concentration has revealed a direct relationship between the Curie temperature Tc and the average ionic radius of the La site 〈rA〉, which is varied by substituting rare earths of different ionic radii for La. With decreasing 〈rA〉 magnetic order and significant magnetoresistance occur at lower temperatures with increasing temperature hysteresis, and the magnitude of the magnetoresistance increases dramatically. The predominant structural effect of decreasing 〈rA〉 is to decrease the Mn–O–Mn bond angle, which is accompanied by slight variations in the Mn–O bond distance. These results demonstrate that the notion of ‘‘double exchange’’ must be generalized to include changes in the Mn–Mn electronic hopping element as a result of microstructural changes induced by composition, temperature and pressure variations.
Ozone assisted, block-by-block, molecular beam epitaxy of La–M–Mn–O (M=Ca,Ba,Sr) thin films and La–M–Mn–O/Dy–Ba–Cu–O multilayers (abstract)
Authors: Vas’ko V. A. , Kraus P. A. , Achutharaman V. S. , Nordman C. A. , Goldman A. M.
Published: July 2002

We report the growth of thin (<100 nm) La–M–Mn–O(M=Ca,Ba,Sr) films by ozone assisted, block-by-block, molecular beam epitaxy on single crystal SrTiO3, and LaAlO3, substrates. The films were characterized by RHEED, high resolution x-ray diffraction, x-ray reflectively, RBS, Ion channeling, SEM, and STM. These films show a very high degree of crystalline perfection as evidenced by very low rocking curve widths (<0.06 deg), extremely low surface roughness (of the order of one unit cell) and the observation of thickness fringes using x-ray diffraction. Careful magnetic, transport and magnetotransport measurements of these films will be presented. Multilayer structures of La–M–Mn–O and Dy–Ba–Cu–O/La–Sr–Cu–O superconductors have also been fabricated. The effect of interfacial chemistry on the transport properties of such layers and the compatibility issues for the growth of manganate/cuprate superconductor multilayers will be discussed.
Mössbauer study of permanent-magnet R2Fe17−xMxCy compounds
Authors: Al-Omari I. A. , Singleton E. W. , Zheng Y. , Hadjipanayis G. C. , Sellmyer D. J.
Published: July 2002

We report Fe57 Mössbauer spectroscopy studies of the effect of carbon doping on the magnetic properties of the Fe sites of R2Fe17−xMxCy, and of the presence of multiple phases in these systems. In this study two systems, Sm2Fe14Al3Cy, where y=0, 1.0, 2.0, and 3.0, and Nd2Fe14.54Si2.46Cy, where y=0, 0.432, and 2.544, were measured at room temperature and analyzed. The ternary compounds R2Fe17−xMxCy studied here have the rhombohedral Th2Zn17 structure with c-axis anisotropy. The Al and Si site occupation used in the fitting were taken from previous results. Analysis of the Mössbauer spectra showed that the hyperfine fields for the different sites decreased in the order 6c, 9d, 18f, and 18h. The average hyperfine field for Sm2Fe14Al3Cy was found to increase with increasing carbon concentration, which is in qualitative agreement with magnetic measurements. For Nd2Fe14.54Si2.46Cy the average hyperfine field was found to increase for low C concentration and decrease for high C concentration; this behavior is in agreement with our magnetization measurements. The measured average isomer shift relative to α-iron was found to increase with y. In these compounds alloying with C and/or M can improve the magnetic properties such as Curie temperature and magnetization which make them potential candidates for permanent magnet development.
Postgrowth annealing effects of TiO2 thin films grown on InP substrate at low-temperature by metal-organic chemical-vapor deposition
Authors: Kim Eun Kyu , Son Maeng Ho , Min Suk-Ki , Han Y. K. , Wang C. H. , Yom S. S.
Published: July 2002

The structural and electrical properties of titanium dioxide (TiO2) thin films grown on n-type InP(100) substrate by low-pressure metal-organic chemical-vapor deposition have been studied with postannealing. The thin films of TiO2 were deposited at a low temperature of 350 °C using titanium isopropoxide and oxygen. After a postgrowth annealing by the rapid thermal annealing method at a temperature of 850 °C for 15 s, the TiO2/InP structure of only the anatase phase with (101) and (200) peaks was observed by x-ray diffraction analysis. No interface reaction between TiO2 and InP was detected by Auger electron spectroscopy depth profiling. From capacitance–voltage measurement of the Al/TiO2/n-InP structure, the interface density of states at midgap energy and the dielectric constant were approximately low 1012 eV−1 cm−2 at midgap energy and about 50, respectively.
Head/disk tribology: Toward 10 Gb/in.2 (invited) (abstract)
Authors: Marchon B.
Published: July 2002

As the race to ever increasing areal densities and lower flying heights becomes more ferocious year after year, the need for an improved understanding of the various physical and chemical processes taking place at a sliding head/disk interface becomes more urgent. This talk will review the various aspects of thin film media technology that will become essential components of a 10 Gb/in.2 interface. Modeling of surface topography and its relationship with glide and stiction properties will be addressed. Recent advances in overcoat and lubrication technology will be assessed, and attempts to decrease magnetic spacing and increase durability and flyability will be discussed. Finally, the concepts of near- and/or full-contact recording and their feasibility will be reviewed.
Calculated spacer-layer Friedel oscillations in Co/Cu(001)
Authors: Nordström Lars , Singh David J.
Published: July 2002

The induced complete position-dependent magnetization density of the copper spacer layer is calculated for Co/Cu(001), as modeled by a Co/Cu9/Co slab. This density is then analyzed in terms of Friedel oscillations. It is found that the variation of the density is well described by two periods, 1.2 and 1.6 monolayers, which are the nonaliased versions of the RKKY periods found in the interlayer coupling. The induced Cu magnetization is not well described in terms of local Cu moments, arising from local exchange interactions, but rather has significant free electron character, in the way that it is mainly induced by magnetic interface scattering.
Heat capacity measurements of NiO/CoO superlattices (invited) (abstract)
Authors: Abarra E. N. , Takano K. , Hellman F. , Berkowitz A. E.
Published: July 2002

We present heat capacity measurements of NiO/CoO superlattices grown by reactive sputtering. Neutron diffraction studies of similar superlattices have shown antiferromagnetic ordering through several bilayers despite the short-range nature of the spin interaction in the constituent materials. Specific heat measurements were made using a unique thin film microcalorimeter capable of measuring the heat capacity of thin films from 1.5 K to well above room temperature. We examine the effect of exchange coupling at the interfaces by varying the thickness of the bilayers. For thin bilayers (26 Å), we observe a single broad heat capacity peak similar to a Ni0.5Co0.5O alloy. This peak is at a temperature which corresponds to the superlattice magnetic blocking temperature. For thicker bilayers (≳50 Å), two broad maxima occur which approach the individual Néel temperatures of CoO and NiO with increasing bilayer thickness. The ordering temperature of the NiO layers is more suppressed than expected, indicating a more pronounced size effect compared to the CoO layers. The magnetic entropy S=R(ln 3+ln 2)/2 for the superlattices, within the uncertainties of the measurement, is conserved. We compare the temperature dependences of the specific heats to various models.
Giant magnetoresistance multilayers of high thermal stability with thicker magnetic layers
Authors: Hossain S. A. , Pirkle B. H. , Yang J. , Parker M. R.
Published: July 2002

The problem of poor thermal stability in NiFeCo/Cu multilayers can be resolved by utilizing thicker magnetic layers than those giving optimum giant magnetoresistance (GMR), in the as-deposited state. This type of structure exhibits very little GMR in the as-deposited state, but upon appropriate post-deposition anneal, produces huge improvements in GMR magnitude and sensitivity. The described technique provides an improvement necessary for retaining the magnetic properties of the multilayers at temperatures that would be encountered in the fabrication and operation of GMR field sensors.
Anomalous perpendicular magnetism in Ni/Cu(001) films and the effects of capping layers
Authors: O’Brien W. L. , Tonner B. P.
Published: July 2002

X-ray magnetic circular dichroism (XMCD) measurements on wedges of Ni grown on Cu(001) are used to investigate magnetic properties in Ni/Cu(001) ultrathin films. A sharp transition from in-plane to perpendicular magnetization is found near 7 ML, and a gradual transition back to in-plane magnetization begins near 37 ML. The critical thickness for epitaxial growth, 13 ML, is determined from a rapid rise in the coercive field versus film thickness. Both transitions in the direction of easy axis are well explained by considering the effects of the surface, shape, and the strain-induced magnetoelastic anisotropies. The critical layer thickness of 13 ML plays a critical role in understanding the transition near 37 ML. Capping the Ni wedge with 2 ML of Co increases the magnitude of the surface anisotropy, forcing the magnetization to remain in plane for thickness up to at least 18 ML. Addition of an Fe capping layer has no effect on the direction of magnetization, suggesting the importance of interface anisotropies or intermixing.
Magnetic losses in stripline/microstrip circulators
Authors: How H. , Fang T. M. , Vittoria C. , Schmidt R.
Published: July 2002

We have included losses in the analysis of a 3N-port stripline/microstrip circulator and have reformulated the circulation conditions previously postulated for the lossless case. Our calculations have been compared to three published data on circulator designs biased below and above ferrimagnetic resonance. Scattering parameters at each port have been calculated as a function of assumed material losses and coupling capacitance of a multiport circulator. Wide transmission band or wide stop bands may be possible for a six port circulator biased above ferrimagnetic resonance.
Power losses in thick steel laminations with hysteresis
Authors: Appino C. , Bertotti G. , Bottauscio O. , Fiorillo F. , Tiberto P. , Binesti D. , Ducreux J. P. , Chiampi M. , Repetto M.
Published: July 2002

Magnetic power losses have been experimentally investigated and theoretically predicted over a range of frequencies (direct current—1.5 kHz) and peak inductions (0.5–1.5 T) in 1-mm-thick FeSi 2 wt. % laminations. The direct current hysteresis properties of the system are described by the Preisach model, with the Preisach distribution function reconstructed from the measurement of the recoil magnetization curve (Bp=1.7 T). On this basis, the time behavior of the magnetic induction vs frequency at different lamination depths is calculated by a finite element method numerical solution of Maxwell equations, which takes explicitly into account the Preisach model hysteretic B(H) relationship. The computed loop shapes are, in general, in good agreement with the measured ones. The power loss dependence on frequency is predicted and experimentally found to change from a ∼f3/2 to a ∼f2 law with increasing peak induction.
Low-temperature thermal conduction in particle-dispersed polyethylene
Authors: Nakamura H. , Matsuura M. , Kawasaki K. , Hiki Y. , Kogure Y.
Published: July 2002

The thermal diffusivity, and specific heat, of polyethylene (76% crystallinity) containing dispersed particles (1%–15% volume fraction) of SiO2 (diameter d=7,16,40 nm; 1,5,10 μm), Fe (d=27 nm), Cu (d=53 nm), and Ag (d=140 nm) have been measured using the laser flash method, and the thermal relaxation method, in the temperature range of 4.2–300 K, and the thermal conductivity was derived from the two quantities. The thermal conductivity was decreased by the addition of particles, and the decrease was more remarkable at lower temperatures for smaller particles. The boundary thermal resistance between the particles and the medium was derived from the change of conductivity due to the nm-diameter dispersed particles. The boundary resistance was independent of the size and the volume fraction of the particles, and was shown to be inversely proportional to the phonon specific heat of particle-dispersed medium in over a wide range of temperatures. The phonon Debye temperature and the strength of the boundary thermal resistance were determined by analyzing the data, and obtained results were quite reasonable.
7Li nuclear magnetic resonance spectra and relaxation in Ni1−xLixO
Authors: Corti M. , Marini S. , Rigamonti A. , Massarotti V. , Capsoni D.
Published: July 2002

7Li nuclear magnetic resonance (NMR) spectra and relaxation rates in Ni1−xLixO, for 0.01≤x≤0.46 and for 20≤T≤550 K are presented. For x≤0.2 the antiferromagnetic (AF) phase typical of NiO is still observed. The Neel temperatures TN(x), the temperature dependence of the field at the Li site, and the role of the extra holes in driving the correlated spin dynamics of Ni2+ are derived. In particular, from 7Li relaxation rates W the hopping frequencies of the holes on the AF background and the energy gaps between itinerant and localized states are extracted. In the mixed regime (x≥0.3, LiNiO2 structure-type) in which partial ordering on alternate Li+ and Ni3+ layers occurs, 7Li NMR data display the features typical of disordered paramagnets, consistent with a picture of random distribution of Ni3+ (S=1/2), Ni2+ (S=1) and of spin vacancies Li+ (S=0). In particular, for x=0.46, on cooling, a paramagnetic-like divergence of the width and of the shift of the NMR line is observed while 7Li W’s indicate slowing down of the spin fluctuations. The fluctuation frequency decreases in the MHz range below a freezing temperature Tf≂40 K.
Order-parameter fluctuations in the frustrated Heisenberg model on the square lattice
Authors: Zhang Shu , Müller Gerhard
Published: July 2002

The T=0 dynamics of the two-dimensional s=1/2 Heisenberg model with competing nearest-neighbor (J1) and next-nearest-neighbor (J2) interactions is explored via the recursion method, specifically the frequency-dependent fluctuations of the order parameters associated with some of the known or suspected ordering tendencies in this system, i.e., Néel, collinear, dimer, and chiral order. The results for the dynamic structure factors of the respective fluctuation operators show a strong indication of collinear order at J2/J1≳0.6 and a potential for dimer order at 0.5≲J2/J1≲0.6, whereas the chiral ordering tendency is observed to be considerably weaker.
Domain collapse in grooved magnetic garnet material (abstract)
Authors: Peredo J. C. , Patterson G. N. , Fedyunin Y. N.
Published: July 2002

Material grooving has been proposed as a possible technique for stabilizing magnetic domains in a thin film. Grooving changes the local properties of the material, and can change the stability range of domains in the vicinity of a groove. In this work, the effects of material grooving on the collapse fields of magnetic domains were simulated, measured, and interpreted magnetostatically for thin, epitaxial (YBiGdHoCa)3(FeGeSi)5O12 and (YBiSmLuCa)3(FeGaSi)5O12 garnet films which were 2 μm thick. The domain collapse field (Hco) was simulated with a recently developed computer code based on a simplified model of domain structure. Simulation results indicated that Hco varied linearly with groove depth (δ) increasing for groove widths (ω) <20 μm, and decreasing for ω≳20 μm. As groove widths increased, Hco is a maximum when ω is near the domain diameter. These trends correspond to the effect induced by the demagnetizing fields from the groove bottom and edges, respectively. The grooved film is modeled as an ungrooved film of reduced film thickness upon which were placed two semi-infinite magnetized film layers separated by ω. Experimental measurement of Hco was performed by magneto–optic observation on an array of grooves at pitches of 4–8 μm. δ from 0.2–0.6 μm, and ω from 1.2 to 2.6 μm. The experimental data were in good agreement with the simulated values and magnetostatic interpretation.
Analytical model in a weakly coupled sandwich for memory purposes
Authors: Wang Zhigang , Nakamura Yoshihisa
Published: July 2002

A simple analytical model explains giant magnetoresistive properties in a weakly coupled sandwich with two magnetic components. The model describes in detail a new storage mechanism in which the minor loop’s slope depends on its past magnetic histories. A new type of solid-state giant magnetoresistance memory based upon this mechanism has been realized by experiments. Furthermore, some important factors included in nondestructive readout capability are also analyzed by this model.
Remanent magnetization in the linear chain antiferromagnet (CH3NH3)Mn1–xMxCl3⋅2H2O, M=Cd or Cu
Authors: Paduan-Filho A. , Becerra C. C. , Palacio F.
Published: July 2002

The substitution of small amounts of Mn2+ by Cu2+ or Cd2+ ions in the linear chain antiferromagnet (CH3NH3)MnCl3⋅2H2O (TN=4.12 K) leads to the appearance of a remanent magnetic moment in the substituted compound. Magnetization measurements carried out in single crystals revealed that the remanent magnetization (Mr) appears along the direction of the easy axis when the sample is cooled through the ordering temperature in the presence of an axial magnetic field as low as few millioersteds. Samples with x∼0.01 of Cu and x=0.008 of Cd were investigated. The measured magnetization M=Mr+χ∥Haxial observed at applied axial fields Haxial=0.8 Oe in both crystals is almost entirely due to the contribution of Mr since χ∥Haxial is very small. Hysteresis curves were also obtained for the Cd diluted system and the results shows that at T=1.5 K a reversal in the direction of Mr is obtained for Haxial around 200 Oe. A comparison with previous recent observations in some three-dimensional diluted antiferromagnets is made.
Co–Cr films prepared by sputtering using electron cyclotron resonance microwave plasma
Authors: Yamamoto S. , Sato K. , Kurisu H. , Matsuura M.
Published: July 2002

The sputtering deposition using an electron cyclotron resonance (ECR) microwave plasma was tried to use in the fabrication of the Co–Cr perpendicular magnetic recording media. As the Ar sputtering gas pressure increased from 4×10−2 to 8×10−2 Pa, the Co(002) x-ray diffraction peak intensity increased and the half-value width of the rocking curve Δθ50 decreased. This result implies that Co–Cr films with high perpendicular orientation and good crystallinity are achieved at high Ar gas pressure. The Co–Cr films deposited at a target to substrate distance of 230 mm had a good preferred crystal orientation (Δθ50 less than 4°), high perpendicular magnetic anisotropy (Hk higher than 4 kOe), and high perpendicular coercivity over 1400 Oe even though the Co–Cr thickness is as small as about 50 nm, and no underlayers were introduced. Thus, the ECR sputtering has high potential in the deposition of the Co–Cr films for ultrahigh density recording media.
Magnetic properties of epitaxial and polycrystalline Fe/Si multilayers
Authors: Chaiken A. , Michel R. P. , Wang C. T.
Published: July 2002

Fe/Si multilayers with antiferromagnetic interlayer coupling have been grown via ion-beam sputtering on both glass and single-crystal substrates. High-angle x-ray diffraction measurements show that both sets of films have narrow Fe peaks, implying a large crystallite size and crystalline iron silicide spacer layers. Low-angle x-ray diffraction measurements show that films grown on glass have rougher interfaces than those grown on single-crystal substrates. The multilayers grown on glass have a larger remanent magnetization than the multilayers grown on single-crystal substrates. The observation of magnetocrystalline anisotropy in hysteresis loops and (hkl) peaks in x-ray diffraction demonstrates that the films grown on MgO and Ge are epitaxial. The smaller remanent magnetization in Fe/Si multilayers with better layering suggests that the remanence is not an intrinsic property.
An ultrasonic study of spin-density-wave effects in a Cr+0.2 at. % Ir alloy single crystal
Authors: Martynova J. , Alberts H. L. , Smit P.
Published: July 2002

Measurements of the temperature dependences of the second-order elastic constants and attenuation of ultrasonic waves through the magnetic phase transition temperatures of a Cr+0.2 at. % Ir alloy single crystal are reported. The incommensurate–commensurate spin-density-wave transition is marked by steplike decreases, accompanied by hysteresis of width about 16 K, in all the elastic constants on heating through the transition temperature. The attenuation coefficient shows a spikelike peak at this transition. A deep minimum in c11 and small peaks in c44 and 1/2 (c11−c12) are observed at the Néel transition.
Magnetovolume and magnetoelastic effects in ternary Cr–Ru–Mo alloys
Authors: Mochele J. T. , Smit P. , Alberts H. L.
Published: July 2002

The influence of Mo additions on the unusual magnetoelastic properties and Invar-type behavior of a Cr0.997Ru0.003 antiferromagnetic alloy was investigated. Measurements were done of the temperature dependence of the thermal-expansion coefficient and bulk modulus of (Cr1−xMox)0.997Ru0.003 alloys with 0≤x≤0.07. Well-defined magnetic anomalies were observed at the Néel temperature implying large magnetic contributions to the thermal expansion and the bulk modulus, making these alloys suitable to be analyzed and parameterized in terms of existing theories. The addition of Mo to Cr0.997Ru0.003 completely suppresses the presence of the commensurate spin density wave phase for x≥0.015 and does not improve the Invar-type behavior to a desired level that may be useful for practical applications.
Superconducting properties of Fe-doped YNi2B2C
Authors: Zhou X. Z. , Kunkel H. P. , Stampe P. A. , Williams Gwyn
Published: July 2002

The effects of Fe substitution for Ni in the intermetallic superconductor YNi2B2C are reported. Samples of nominal composition Y(Ni1−xFex)2B2C with x=0.01, 0.02, 0.05, and 0.1 have been prepared and subsequently characterized by x-ray diffraction, ac susceptibility, and resistivity/magnetoresistance measurements. A summary of these data is presented, and these are compared and contrasted with both the behavior previously reported for and the conclusions drawn from Fe and Co doping in this system.
Gd concentration dependence of the spin reorientation critical field in Eu2−xGdxCuO4
Authors: Butera A. , Fainstein A. , Tovar M. , Fisk Z. , Oseroff S. B.
Published: July 2002

ESR measurements of the microwave absorption signal associated with weak ferromagnetism in single crystals of Eu2−xGdxCuO4 are presented for X band (9.5 GHz) and L band (1.2 GHz) as a function of the Gd concentration. The strong absorption observed at low magnetic fields was interpreted, for samples with low Gd concentration, as due to a field-induced spin reorientation transition occurring at a critical field Hc, coincident with the in-plane magnetic anisotropy effective field Heffy. For larger x the Cu-Gd magnetic interaction needs to be considered leading to smaller Hc values. Our measurements show that for Gd concentrations in the range 0≤x≤1 the experimental data can be very well fitted with parameters derived from previous measurements. This fact indicates that these compounds have nearly the same in-plane anisotropy effective field, in spite of the small changes in lattice parameters. For x=2 lattice distortions increase causing an Hc larger than the expected one.
Hybridization-induced magnetism in correlated cerium systems
Authors: Kioussis Nicholas , Thevenot J. , Cooper Bernard R. , Sheng Q. G.
Published: July 2002

There is a great change in the nature of the magnetic ordering on going from CeIn3, a local moment antiferromagnetic system, to CePb3, a heavy fermion itinerant antiferromagnetic system, both of which have Cu3Au crystal structure. We have applied ab initio electronic structure calculations, based on the linear-muffin-tin-orbital method, and a phenomenological theory of orbitally driven magnetic ordering, to study the effects of the band-f hybridization-induced interactions and the band-f exchange-induced interactions, pertinent to the magnetic behavior of these systems. The position of the Ce 4f energy level relative to the Fermi energy and the intra-atomic Coulomb interaction are obtained from a sequence of three total-energy supercell calculations with two, one and zero f electrons in the Ce 4f core. The calculations elucidate the origins in the electronic structure of the variation of the f-state resonance width characterizing the strength of the hybridization and the density of states at the Fermi energy characterizing the number and character of band states available for hybridization. We present results for the hybridization potential and the hybridization-induced exchange interactions on going from CeIn3 to CePb3, where the only obvious change is the addition of an anion p electron.
Magnetic dichroism study of the relativistic electronic structure of perpendicularly magnetized Ni/Cu(001)
Authors: Kuch W. , Zharnikov M. , Dittschar A. , Meinel K. , Schneider C. M. , Kirschner J. , Henk J. , Feder R.
Published: July 2002

An experimental and theoretical study of magnetic circular dichroism in valence band photoemission from 15 monolayer thick fcc Ni films on Cu(001) is presented. A highly symmetric configuration (light incidence, electron emission, magnetization direction, photon helicity, and surface normal all parallel) allows the illustrative interpretation of the dichroism in terms of the relativistic band structure. Photoemission experiments in the photon energy range of 11–27 eV are compared to fully relativistic one-step photoemission calculations. From this comparison, the dichroic features can be directly related to the double group symmetry of the initial states, which is demonstrated by two examples.
Density functional study of fcc iron and iron particles in copper
Authors: Guenzburger Diana , Ellis D. E.
Published: July 2002

The first-principles spin-polarized discrete variational method in the framework of density functional theory was employed to investigate the electronic and magnetic structure of fcc (γ) Fe and of γ-Fe particles in copper, represented by 62-atom embedded clusters of cubic geometry. The influence of Al substitutional impurities in γ-Fe and in the Fe particle in Cu was also investigated. Magnetic moments and hyperfine fields were obtained.
Magnetic measurements on Cd1−xCrxTe and Zn1−xCrxTe
Authors: Pekarek T. M. , Miotkowski I. , Crooker B. C.
Published: July 2002

We have investigated the magnetic properties of single-crystal Cd1−xCrxTe and Zn1−xCrxTe samples. The magnetization is anisotropic (∼7% for Zn1−xCrxTe and ∼0.4% for Cd1−xCrxTe) with the (111) direction having the greatest value. We have found that the magnetic behavior in fields up to 6 T and temperatures between 1.5 and 324 K for both systems is well described by a theoretical model with crystal field, Jahn–Teller, spin-orbit, and spin-spin terms including ligand field corrections to the spin-orbit and spin-spin parameters. Good agreement was found between the experimental results and the theoretical calculations for a Jahn–Teller splitting of 370 and 320 cm−1 and effective spin-orbit parameters of −49.9 and −59.4 cm−1 for Cd1−xCrxTe and Zn1−xCrxTe, respectively. The key feature is an orbital singlet, S=2 ground state with a small splitting (1.5 K for Cd1−xCrxTe and 12.4 K for Zn1−xCrxTe) of the ms states.
On the relaxation of simple magnetic systems
Authors: González J. M. , Ramírez R. , Smirnov-Rueda R. , González J.
Published: July 2002

The relaxation at constant applied field and temperature of a simple magnetic system is evaluated in the framework of the micromagnetic approximation and using a Monte Carlo algorithm. Our results, and particularly, those corresponding to the time evolution of the probability of magnetic moment reversal, evidence remarkable differences with the classical Arrehnius–Néel predictions. These differences are linked to the fact that relaxation proceeds through the formation of structures involving a large number of degrees of freedom.
Magnetic circular dichroism in EELS (abstract)
Authors: Harp G. R. , Farrow R. F. C. , Marks R. F.
Published: July 2002

We evaluate the possibility of using dichroic electron energy loss spectroscopy (DEELS) as an alternative to x-ray magnetic circular dichroism (XMCD). It is well known that electron energy loss spectroscopy and x-ray absorption spectroscopy are highly analogous, providing similar information. A simple semiclassical model suggests that DEELS might have a magnetic sensitivity similar to that of XMCD. This sensitivity will be reduced, however, by multiple scattering of the probe electron before and after the energy loss event. Thus it is difficult to predict the magnitude of the DEELS effect. Experiments were performed at the L-edge of polycrystalline Fe, Co, and Ni thin film samples with uniaxial in-plane magnetic anisotropy, prepared in situ with a magnetic bias field. Even in those most favorable cases, the DEELS effect is seen to be at least 10 times smaller than similar effects in XMCD.