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Reeson, K. J. ; Hemment, P. L. F. ; Stoemenos, J. ; [et al.] Davis, J. ; Celler, G. E.
Notes: It is demonstrated that well-defined buried layers of β-SiC can be grown epitaxially within a silicon substrate. This structure is formed by implanting high doses of carbon ions (>3×1017 C+ cm−2) at 200 keV into a (100) single-crystal silicon which is maintained at a temperature of approximately 550 °C. During the subsequent anneal at 1405 °C for 90 min redistribution of the implanted species occurs, enabling the formation of a buried layer of β-SiC overlain by high-quality single-crystal silicon (χmin=4.1%).
Olivo, P. ; Ricco, B. ; Nguyen, Thao N. ; [et al.] Kuan, T. S. ; Jeng, S. J.
Notes: Experimental results on oxide breakdown in thin insulator metal-oxide-semiconductor structures are presented to show that at a microscopic level breakdown is related to defects located near the injecting interface. In addition, breakdown is found to be almost independent of electron fluence.
Radulescu, D. C. ; Schaff, W. J. ; Wicks, G. W. ; [et al.] Calawa, A. R. ; Eastman, L. F.
Notes: Deep level transient capacitance spectroscopy has been used to investigate deep level electron traps in thick silicon-doped AlGaAs grown by molecular beam epitaxy (MBE) on GaAs substrates intentionally misoriented (tilted) a few degrees from a nominally (001) surface. Of the three dominant traps observed in AlGaAs, the concentrations of two of these are observed to be a direct function of the substrate tilt angle and tilt direction. The concentration of the third dominant trap, which is related to the DX center, is independent of substrate misorientation during MBE. These observations will help in identifying which impurities and/or defects are affected by substrate misorientation during MBE growth in addition to identifying the origin of deep levels in AlGaAs.
Nouhi, Akbar ; Stirn, Richard J.
Notes: In this letter we report on preliminary results of heteroepitaxial growth of the dilute magnetic semiconductor alloy Cd1−xMnxTe on GaAs by metalorganic chemical vapor deposition. Dimethylcadmium (DMCd), diethyltellurium (DETe), and tricarbonyl (methylcyclopentadienyl) manganese (TCPMn) were used as source materials. The TCPMn had to be heated to as high as 140 °C to provide the required vapor pressure. Films with Mn atomic fractions up to 30% have been grown over the temperature range 410–450 °C. Results of optical absorption/transmission, photoluminescence, and x-ray diffraction measurements are presented along with a scanning electron micrograph showing good surface morphology of the grown layers.
Gupta, A. ; Jagannathan, R.
Notes: Direct writing of high-purity copper lines has been achieved by photothermal decomposition of copper formate films under ambient conditions using a focused argon ion laser (514 nm) beam on a scanning quartz or silicon substrate. The fast decomposition kinetics of the precursor allows use of writing speeds as high as 1 cm/s for deposition of micron-thick copper films. There is negligible oxidation of the copper after decomposition in the presence of air due to the rapid heating and cooling of the deposit during laser writing.
Svensson, B. G. ; Svensson, J. ; Lindström, J. L. ; [et al.] Davies, G. ; Corbett, J. W.
Notes: Czochralski-grown tin-doped silicon samples have been irradiated by 2 MeV electrons at room temperature. The concentration of divacancies is studied as a function of bombardment dose, and as a function of temperature during a subsequent isochronal annealing. The results are compared with that for a control sample and the role of direct generation versus vacancy-vacancy pairing for divacancy formation is discussed.
Miller, B. I. ; Koren, U. ; Capik, R. J. ; [et al.] Su, Y. K.
Notes: High-power semi-insulating blocked planar buried-heterostructure (SIPBH) lasers were grown entirely by atmospheric organometallic vapor phase epitaxy (OMVPE) by using a novel dilution scheme for the trimethylgallium and AsH3. Current thresholds as low as 20 mA and differential quantum efficiencies ≥20% per facet at 1.3 and 1.5 μm were obtained with power outputs of about 25 mW/facet. These results are similar to SIPBH lasers where liquid phase epitaxy was used to grow the active layer, but because of the better planarity and uniformity of OMVPE-grown material, it appears possible to grow material that can give a high yield of distributed feedback lasers.
Notes: We have established the low-temperature process for the preparation of high Tc superconducting films by rf magnetron sputtering. The films were deposited at a temperature (650 °C) lower than the tetragonal-orthorhombic transition point with sufficient crystallizing and oxidizing conditions. The as-deposited Er-Ba-Cu-O film on MgO exhibited a sharp superconductive transition with zero resistance at 86 K. This process prevented diffusion at the film and substrate interface and reduced the porous structure in the films.
Notes: The vertical energy flux density for the electromagnetic fields near the surface of a silver strip on a glass substrate is computed for an incident plane wave to aid in the measurement of the width of the strip. The dimensions of the strip cross section, e.g., 300 nm by 100 nm, are a fraction of the wavelength of the incident light , 632.8 nm. The flux 1 nm above the surface shows sharp spikes at the edges of the strip. The features of the fields near the surface could be used for accurate determination of the width of the strip by measurements up to about 30 nm above the strip. The effects of other variables are also shown in the figures.
Long, Frederick H. ; Anderson, R. Rox ; Deutsch, Thomas F.
Notes: Pulsed photothermal radiometry (PPTR) has been used to analyze depth profile of layered materials, including biological tissue. An analytic theory for the time dependence of the PPTR signal has been developed and compared with experimental results on inert materials and living human skin.
Pinto, R. P. ; Amado, M. M. ; Braga, M. E. ; [et al.] Sousa, J. B. ; Chevalier, B. ; Laffargue, D. ; Étourneau, J.
Notes: Accurate measurements of the electrical resistivity (ρ,dρ/dT) and of the thermoelectric power (S,dS/dT) were performed in the ternary compounds U2Ni2In and Nd2Ni2Sn from 4 K to 250 K, and the results are compared with those previously obtained in U2Ni2Sn. The pair U2Ni2In/U2Ni2Sn enables us to study the influence of the p-electron character (and other band effects related to the elements In/Sn) on the transport properties, whereas the pair Nd2Ni2Sn/U2Ni2Sn provides direct information on the role played by the 4f/5f electrons. All these compounds order initially in the antiferromagnetic state, exhibiting a characteristic minimum in dρ/dT at TN. In the U2Ni2In compound a drastic reduction is observed in ρ(T) slightly below TN, suggesting the coexistence of the antiferromagnetic state with a coherent Kondo effect when T≤0.8 TN. In Nd2Ni2Sn we observe two distinct phase transitions below TN, of first-order character and likely associated with order-order magnetic transitions. The anomalous behavior of ρ(T) in the paramagnetic phase of the Nd2Ni2Sn compound can be satisfactorily described in terms of crystal field effects associated with the 4f-electron levels. © 1996 American Institute of Physics.
Prokeš, K. ; de Visser, A. ; Menovsky, A. A. ; [et al.] Brück, E. ; de Boer, F. R. ; Sechovský, V. ; Gortenmulder, T. J.
Notes: Magnetization, magnetic susceptibility, electrical resistivity, and specific heat measurements of UNiAl point to antiferromagnetic ordering below 19.3 K, which is confirmed by neutron-diffraction experiments. The enhanced γ value of 167 mJ/mol K2 is reflecting pronounced presence of magnetic fluctuations, which influence also the other electronic properties down to low temperatures. The thermal expansion of UNiAl was measured in the temperature range 1.5–210 K on a single crystal along the a and c axis. Similar to other bulk properties, also the thermal expansion of UNiAl is highly anisotropic. The a axis is monotonously expanding with increasing temperature in the whole temperature range. Along the c axis, the lattice first collapses with increasing temperature up to 35 K. Around this temperature, the thermal expansion coefficient αc changes sign and continuous expansion with further increasing temperature is then observed. The sharp anomaly of α near 19.3 K present in both directions is consistent with the magnetic phase transition. The results are discussed in terms of the presence of anisotropic magnetic fluctuations. © 1996 American Institute of Physics.
Prokeš, K. ; de Boer, F. R. ; Nakotte, H. ; [et al.] Havela, L. ; Sechovský, V. ; Svoboda, P. ; Winand, J. M. ; Rebizant, J. ; Spirlet, J. C. ; Hu, X. ; Gortenmulder, T. J.
Notes: U2Pt2Sn is crystallizing in an ordered version (space group P42/mnm) of the tetragonal U3Si2 type of structure. Clear anomalies in the magnetic susceptibility, specific heat, and electrical resistivity around 15 K indicate that U2Pt2Sn orders antiferromagnetically below this temperature. As expected for an antiferromagnet, the susceptibility and specific heat anomalies are shifted to lower temperatures upon application of external magnetic field. The specific-heat coefficient γ=327 mJ/mol f.u. K2 remains unchanged in fields up to 5 T. The antiferromagnetic ground state of U2Pt2Sn can be concluded also from the metamagnetic transition around 22 T in the magnetization at 4.2 K and from magnetic reflections in the neutron-diffraction pattern at low temperatures. © 1996 American Institute of Physics.
Lopez de la Torre, M. A. ; Fernandez, J. Rodriguez ; McEwen, K. A.
Notes: We have performed a detailed study of the magnetization of Y0.6U0.4Pd3 (Tf∼10 K), which shows thermal, transport, and magnetic properties typical of a spin-glass system. Strong irreversibility is observed among our zero-field-cooled and field-cooled measurements in fields as large as 30 kOe. Thus, the freezing temperature displays an unusual weak dependence with field. We observe a very slow temporal dependence of the magnetization below the freezing temperature. The nonlinear susceptibility coefficients extracted from our data show an increase by a factor of approximately 20 when approaching the freezing temperature from above, but do not show a true divergence at Tf, and a scaling analysis does not seem to be possible. We compare these results with those expected for classical spin glasses, and propose a possible mechanism for the onset of frustration below x=0.45 in Y1−xUxPd3. © 1996 American Institute of Physics.
Menon, Latika ; Dhar, S. K. ; Malik, S. K. ; [et al.] Yelon, W. B.
Notes: The compounds CeRhSb and CeNiSn show hybridization gap in the electronic density of states. It is of interest to look for a similar gap in a related compound, CeNiSb. We present here the results of the structural, magnetic, and transport studies on CeNiSb. Rietveld analysis of the room-temperature neutron diffraction data reveals that this compound crystallizes in the hexagonal structure (space group P63/mmc). Magnetization measurements show that this compound is ferromagnetically ordered with a transition temperature of ∼4 K. The magnetic susceptibility of CeNiSb follows Curie–Weiss behavior between 10 and 300 K with effective paramagnetic moment close to that of Ce3+ ion. The heat capacity of CeNiSb shows a rise at about 4 K due to the ferromagnetic ordering and a moderately large value of γ is obtained. Its electrical resistivity shows a weak shallow minima at 15 K which is a characteristic feature of Kondo lattice systems. However, no rise in the resistivity at low temperatures, as seen in CeRhSb and CeNiSn, is observed in CeNiSb. © 1996 American Institute of Physics.
Dieny, B. ; Cowache, C. ; Nossov, A. ; [et al.] Dauguet, P. ; Chaussy, J. ; Gandit, P.
Notes: The giant magnetoresistance (GMR) of magnetic multilayers is usually considered as isotropic, i.e., independent of the direction of the sensing current with respect to the applied field. In spin-valve samples of the form NiFe/Cu/NiFe/FeMn it is possible to accurately determine the amplitude of the GMR (without any contribution from the usual anisotropic magnetoresistance) for various direction of the current with respect to the direction of the magnetization of the two ferromagnetic layers, both in the parallel and antiparallel magnetic configurations. In three series of spin-valve samples of the composition F tF/Cu tCu/NiFe/FeMn, we have observed that the GMR amplitude is larger when the current is perpendicular to the magnetizations than when it is parallel to it. This intrinsic anisotropy in the GMR shows a pronounced maximum (relative amplitude of the anisotropy of the order of 10% at the maximum) for a thickness of the ferromagnetic layer of the order of 150 A(ring). In contrast, this anisotropy depends very weakly on the nonmagnetic spacer layer thickness. The results are compared with semiclassical calculations of Rijks et al. [Phys. Rev. B 51, 283 (1995)]. On another respect, we have measured the in-plane (CIP) and perpendicular to the plane (CPP) giant magnetoresistance of antiferromagnetically coupled (NiFe/Ag) multilayers. Particular attention has been paid on the variation of resistivity with the angle Δθ between the magnetization in the successive magnetic layers. While the CIP GMR varies almost linearly with cos(Δθ), the CPP GMR shows strong deviations from linearity especially at large NiFe thicknesses. The results are discussed in terms of relative role of s-like and d-like electrons in CIP and CPP transport. © 1996 American Institute of Physics.
Notes: The temperature dependence of the giant magnetoresistance (GMR) for currents parallel and perpendicular to the multilayer plane is discussed by taking account of the random exchange potentials, phonon scatterings, and spin fluctuations. The effect of spin fluctuations, which plays an important role at finite temperatures, is included by means of the static functional-integral method developed previously by the present author. Our model calculations well explain the observed features of the parallel and perpendicular GMR of Fe/Cr and Co/Cu multilayers recently reported by Gijs et al. [Phys. Rev. Lett. 70, 3343 (1993); Phys. Rev. B 50, 16 733 (1994)]. © 1996 American Institute of Physics.
Notes: The nonlocal real-space Kubo approach to electron transport in magnetic multilayers is applied to a new geometry, "oblique transport,'' whose complexity is traced back to a combination of the layering, of the nonlocal character of the linear response, and of the oblique direction transport in this new geometry. The problem is dealt with by applying a condition on the average current density vector or on the average electric-field vector, depending upon the external driving conditions. Its solution exhibits a characteristic anisotropy and it yields the global oblique conductance and magnetoresistance as simple trigonometric expressions in terms of the in-plane and vertical conductances and magnetoresistances, for arbitrary noncollinear-magnetization configurations. © 1996 American Institute of Physics.
Levy, P. M. ; Zhang, S. ; Ono, T. ; [et al.] Shinjo, T.
Notes: A new class of corrugated multilayers has been grown on silicon substrates which have (111) faceted grooves etched on their surface. These structures can be probed by conventional means with current at an angle to the plane of layers (CAP) as well as CIP. This angle is fixed by: the depth to width ratio of the grooves (which determines the angle θ), and the angle φ of the current probes with respect to the grooves. We have prepared multilayers of [Co(12 A(ring)) Cu(t)NiFe(12 A(ring)) Cu(t)]y with t=58 and 116 A(ring), and y=167 and 91 repeats, respectively; and have varied φ from 0 and 90° while θ is held fixed at 54.7°. We find the data is very well fit to the theoretical expression for CAP resistivity in terms of the more conventional CIP and CPP resistivities. From measurements of the CIP and CAP–MR's on these corrugated multilayers we are able to predict the CPP–MR for these structures. © 1996 American Institute of Physics.
Oti, J. O. ; Cross, R. W. ; Russek, S. E. ; [et al.] Kim, Y. K.
Notes: The effects of magnetostatic interactions on the giant magnetoresistive (GMR) response of NiFe/Cu/NiFe spin valves are studied using an analytical model. The model is applicable to devices small enough for the magnetic layers to exhibit single-domain behavior. Devices having lengths in the track-width direction of 10 μm and interlayer separations of 4.5 nm are studied. Stripe heights are varied from 0.5 to 2 μm. The magnetization of one magnetic layer is pinned by a transverse pinning field that is varied from 0 to 24 kA/m (300 Oe). GMR curves for transverse fields are calculated. At zero external field the magnetization of the layers shows a tendency to align themselves antiparallel in the transverse direction. This results in an offset from the ideal biasing of the device. Broadening of the curves due to shape anisotropy occurs with decreasing stripe height and increasing magnetic layer thickness, and the magnetization in the pinned layer becomes less stable. © 1996 American Institute of Physics.
Chowdhury, Ataur R. ; Freitag, Andrea E.
Notes: Structural and magnetic ordering of the interfaces determines the physical properties of multilayered structures. Interfaces of Fe/Al multilayers were studied using Mössbauer effect spectroscopy. The samples were fabricated by dc planar magnetron sputtering at room temperature on polyester substrates. The observed spectra indicate that the interfaces have a common structural composition and for thin Fe layers, the whole Fe layer forms mixed phases at the interface. For larger Fe layer thickness, the interface is formed using an about 12-A(ring)-thick Fe layer. Besides the hyperfine field component of bcc Fe, six different magnetic components were identified in all samples (with larger Fe layer thickness). The intensities of the components were determined from the area under the absorption peaks of the Mössbauer spectra of the corresponding phases. The average canting angle of the Fe magnetic moments, as obtained from the spectra, indicate parallel magnetic anisotropy for all phases. © 1996 American Institute of Physics.
Thomson, T. ; Riedi, P. C. ; Wang, Q. ; [et al.] Zabe, H.
Notes: 59Co and 55Mn NMR measurements have been made on a range of CoMn materials: dilute powder alloy, thin-film alloys, and multilayers. Our results suggest that in dilute alloys isolated atoms of Mn couple both ferro and antiferromagnetically to the Co host, with the latter producing a reduction in the magnitude of the Co hyperfine field of 17%. This reduction of hyperfine field appears similar for both the fcc and hcp phases of Co. NMR on Co/Mn sputtered multilayers shows large changes in the hyperfine field distribution between a film with Mn layers of 10 A(ring) and Mn layers of 30 A(ring). © 1996 American Institute of Physics.
Kwon, S. J. ; Choi, S. J.
Notes: Films of 12 Fe/(Ag–Cu) bilayers were prepared to have either granular or multilayer structures on glass substrates with a 50 A(ring) Cu buffer layer. A magnetic Fe layer of 15 A(ring) was deposited by e-beam evaporation, and a nonmagnetic Ag–Cu layer of 20 A(ring) by thermal coevaporation with various Ag/Cu ratio. As the nonmagnetic layer composition changed from Ag to Cu, roughness of the film surface varied from more than 100 A(ring) to less than 10 A(ring), measured with atomic force microscopy. Regularity of multilayer structure was evident in the Fe/Cu film from a superlattice peak at around 2theta=2.5 deg by Cu Kα x-ray diffraction. The peak weakened as the Ag content increased in the nonmagnetic layer, and completely disappeared at Ag/Cu(approximately-greater-than)1, which indicated that granular structure became dominant. Pendelloesung peaks at 2theta=1 to 2 deg had a similar composition dependency. The structural change was confirmed with cross-sectional transmission microscopy, too. The deposited structure is explained in terms of surface tension and wettability of each element. Increasing Cu content decreases the surface tension and increases the wettability of the nonmagnetic layer. Heat treated film structures are explained with identical terms. Change of magnetoresistance is discussed in part by connectivity of the nonmagnetic layer. From the discussion, a reasoning is given to the effect of annealing on the magnetoresistance of the films. © 1996 American Institute of Physics.
Choe, G. ; Walser, R. M.
Notes: This work investigated the effect of ion beam mixing on the magnetic and structural characteristics of Gd-Co multilayer films. Multilayer films with bilayer periods (BP) from 10 to 300 A(ring) were mixed by 150 to 300 keV Ar+ ion doses of 1016 cm−2. The magnetizations of the as-deposited multilayers were dominated by oxidation, and by the asymmetric redistribution of Co atoms. Ion beam mixing eliminated much of these effects, but the degree was strongly dependent on the layer thickness. The small BP films were completely mixed, with amorphous nanoparticles, but the large BP films contained small, microcrystals in an amorphous matrix due to incomplete mixing. The magnetic moment of ion beam mixed films were strongly dependent on the ion beam energy and the layer thickness. © 1996 American Institute of Physics.
Rezende, S. M. ; de Aguiar, F. M.
Notes: Conventional microwave ferromagnetic resonance (FMR) techniques have proven very useful in the investigation of coupled magnetic multilayer systems. Here we show theoretically that with sufficiently high powers, a microwave magnetic field applied either parallel or perpendicular to the dc field, can drive nonlinear effects. The Landau–Lifschitz equation is used to obtain the nonlinear equations of motion for the two normal modes of a coupled two-layer system. In the usual FMR configuration, only the acoustic mode is coupled to the driving field. However, for driving fields above a critical value, energy can be transferred to the optic mode, in a process similar to the subsidiary resonance observed in spin-wave systems. At higher driving the system may display well-known transitions to chaos. © 1996 American Institute of Physics.
Cornejo, D. R. ; Missell, F. P. ; González, J. M.
Notes: Several samples of enhanced-remanence nanocrystalline Sm15Fe19Co66 were prepared by mechanical alloying of the elementary powders followed by heat treatments. Sample A had a remanence ratio η=σr/σs=0.68 and Hc=7.1 kOe, while, for B, η=0.58 and Hc=10.4 kOe, and, for C, η=0.57 and Hc=12.6 kOe. The isothermal remanent magnetization Mr(H), obtained for different initial demagnetized states (ac, dc+, dc−), and the demagnetization remanence Md(−H), obtained for an initially saturated state, were determined. The ac-demagnetization Henkel plot showed both magnetizing and demagnetizing interactions as observed in other nanocrystalline magnets. A comparison with the moving Preisach model indicates the importance of both mean-field and random interactions. © 1996 American Institute of Physics.
Hu, Z. ; Yelon, W. B. ; James, W. J.
Notes: Several NdnFem−x−yVxAly [(n,m)=(1,12), (2,17), (3,29)] samples were prepared and analyzed using neutron powder diffraction. Rietveld analysis of the neutron diffraction data indicates that the V and Al substituents take those sites with similar environments in all three phases, as observed in our previous study of Ti-substituted compounds. It was confirmed that the diffraction data of the 3:29 compound can be better refined using the A2/m space group than using the P21/c space group. The SQUID measurements show that all samples have Curie temperatures well above room temperature. The neutron diffraction results show that the easy direction is along the a axis for the 3:29 compounds, along the c axis for the 1:12 compounds and in the a-b plane for 2:17 compounds, respectively. The average site magnetic moments, the metal–metal bond lengths and the unit cell parameters of these compounds are compared with those of the Ti-substituted compounds. © 1996 American Institute of Physics.
Luo, H. ; Hu, Z. ; Yelon, W. B. ; [et al.] Mishra, S. R. ; Long, G. J. ; Pringle, O. A. ; Middleton, D. P. ; Buschow, K. H. J.
Notes: Six samples of Ce2Fe17−xGax with nominal Ga content x equal to 0, 0.3, 0.5, 0.7, 1.0, 2.0 have been studied by powder neutron diffraction at room temperature. Both crystalline and magnetic refinements have been carried out. All six samples adopt the Th2Zn17-type rhombohedral structure. The only additional phase found is α-iron. Gallium atoms are found to have high affinity for the iron 18h site, and are absent from the 9d and 18f sites. The Ga substitution for Fe leads to an expansion of both the a and c axes. The Curie temperature increases from 238 K for Ce2Fe17 to 406 K for Ce2Fe15Ga2. Magnetic refinements on the samples with x=0.3, 0.5, 0.7, 1.0, and 2.0 reveal that the magnetic moments of the four Fe sites are in the basal plane and that their values increase with increasing Ga content. © 1996 American Institute of Physics.
Geröcs, I. ; Molnár, G. ; Jároli, E. ; [et al.] Zsoldos, E. ; Petö, G. ; Gyulai, J. ; Bugiel, E.
Notes: Epitaxial orthorhombic GdSi2 was grown by in situ vacuum annealing of a 50-nm Gd layer on 〈100〉 silicon. The epitaxy was proved by x-ray diffraction, electron diffraction, and ion channeling measurements. The lattice mismatch between the orthorhombic GdSi2 and 〈100〉 silicon substrate was found to be 4%.
Shah, S. I. ; Carcia, P. F.
Notes: Superconducting stoichiometric YBa2Cu3O7−x films have been grown by rf sputtering of a single bulk material target in an Ar+10% O2 sputtering atmosphere on a variety of substrates. Films on SrTiO3 substrates exhibited the best post-annealing superconducting properties with the critical onset temperature of 90 K and zero resistance at 67 K. The maximum current density for these samples was 1×105 A/cm2 at 4.2 K. An extensive loss of film underneath the target was observed during the growth of these films which can be attributed to resputtering due to oxygen anion or energetic neutral particle bombardment of the substrate.
Nakotte, H. ; Purwanto, A. ; Robinson, R. A. ; [et al.] Prokeš, K. ; de Boer, F. R. ; Havela, L. ; Sechovský, V. ; Swainson, I. P.
Notes: Crystallographic analysis shows that UCuSn does not form in the hexagonal CaIn2 structure as reported previously, but is an ordered ternary compound and forms in an orthorhombic structure (space group: P21cn). Bulk and neutron-diffraction measurements reveal that UCuSn orders antiferromagnetically below 62 K. At 4.2 K, high-field magnetization reveals a complex magnetization process with two metamagnetic transitions. Furthermore, bulk investigations show an additional anomaly at 25 K, but a smooth temperature dependence of various magnetic peaks down to the lowest temperature gives no evidence for a second magnetic transition. Possible scenarios responsible for the drastic changes in the electronic properties around 25 K are discussed. © 1996 American Institute of Physics.
Purwanto, A. ; Robinson, R. A. ; Nakotte, H. ; [et al.] Swainson, I. P. ; Torikachvili, M. S.
Notes: We report on the crystallographic and magnetic structures of the antiferromagnet UCu1.5Sn2, as determined by x-ray and neutron powder diffraction. It crystallizes in the tetragonal CaBe2Ge2 structure type, with space group P/4nmm, and we find no site disorder between two different Sn 2c sites, in contrast with a previous report. UCu1.5Sn2 orders antiferromagnetically with a Néel temperature of about 110 K. This is unusually high among uranium intermetallics. The uranium moments align along the c axis in a collinear arrangement but alternating along the c axis. The low-temperature uranium moment is 2.01μB. © 1996 American Institute of Physics.
Schlottmann, P. ; Hellberg, C. S.
Notes: We consider arbitrary clusters of Kondo holes in a Kondo insulator described by the nondegenerate symmetric Anderson lattice with a nearest-neighbor tight-binding conduction band on a simple cubic lattice. The f-electron self energy is considered within the local approximation. Each Kondo hole introduces a boundstate in the gap. The quantum interference in the scattering off the impurities gives rise to interactions among the Kondo holes. The spectral weight of the bound states is predominantly localized on the sites neighboring the Kondo holes. Clusters of impurities separated by more than one lattice site are disconnected for boundstates at the Fermi level. On a simple cubic lattice the metal-insulator transition in the impurity band then reduces to the site percolation of Kondo holes with first, second and fourth nearest neighbors. We use the low density mean cluster size expansion and a small cell renormalization to estimate the critical concentration. Hopping in the conduction band beyond nearest neighbors reduces the percolation threshold. Hence, 9.9% of Kondo holes is an upper bound for the insulator to become a metal. © 1996 American Institute of Physics.
Sereni, J. G. ; Schlottmann, P.
Notes: Dilute TbxTh1−x alloys have been thoroughly studied in the light of a possible valence instability and Kondo effect in Tb3+ and Tb4+ ions. We review the experimental data for the specific heat, magnetic susceptibility, magnetization, magnetoresistivity, and the change of the superconducting properties of Th. In analogy to Tm (Tm2+ and Tm3+) and Pr (Pr3+ and Pr4+) impurities in metals, Tb involves two electronic configurations each with a magnetic Hund's rule ground multiplet. Two classes of models for valence admixtures of two magnetic configurations have been investigated in the past. Depending on the form of the hybridization and the jj coupling within the 4f shell, the models either have a singlet or a magnetic ground state. As for Tm impurities the latter class of models, related to the undercompensated Kondo effect, appears to describe the system TbTh more appropriately. A qualitative explanation of the data is attempted within a scheme that combines Kondo screening with crystalline field splitting. © 1996 American Institute of Physics.
Kioussis, Nicholas ; Thevenot, J. ; Cooper, Bernard R. ; [et al.] Sheng, Q. G.
Notes: 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. © 1996 American Institute of Physics.
Continenza, A. ; Monachesi, P. ; Galli, M. ; [et al.] Marabelli, F. ; Bauer, E.
Notes: We focus on YbCu4Ag and YbCu4Au and interpret the optical conductivity, measured from 2 meV to 6 eV at 10 and 300 K, in terms of self-consistent electronic structure calculations. Most of the relevant spectral structures are interpreted as interband transitions from hybridized f–d states below and at the Fermi level at high symmetry points. In particular, the large shoulder at about 2.4 and 2 eV in the spectrum of YbCu4Au and YbCu4Ag, respectively, can be interpreted in terms of a transition Γ4→Γ3 involving mostly d states. We also compare the electronic structure of YbCu4Ag with that of LuCu4Ag, to ascertain the influence of the f states on the conduction bands and carrier density at EF. This allows us to draw some qualitative considerations on the f-band hybridization in these compounds. © 1996 American Institute of Physics.
Kuch, W. ; Zharnikov, M. ; Dittschar, A. ; [et al.] Meinel, K. ; Schneider, C. M. ; Kirschner, J. ; Henk, J. ; Feder, R.
Notes: 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. © 1996 American Institute of Physics.
Guenzburger, Diana ; Ellis, D. E.
Notes: 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. © 1996 American Institute of Physics.
Osborn, R. ; Aronson, M. C. ; Goremychkin, E. A. ; [et al.] Greedan, J. E.
Notes: We report on recent inelastic neutron scattering measurements of the magnetic response close to the metal–insulator transition in La1−xSrxTiO3. Specific heat and susceptibility data from Tokura et al. give evidence for a divergent effective mass at the critical concentration xc=0.05 in agreement with recent mean-field theories of the transition. The mass enhancement is believed to arise from the formation of a d-electron resonance at the Fermi energy close to the transition. The aim of this investigation is to look for evidence of this resonance in the dynamic magnetic susceptibility. We studied samples with x=0, 0.05, and 0.2 using incident energies between 25 and 200 meV. After correction for the phonon scattering, we observe a broad response above a threshold of 20–30 meV extending to over 100 meV. In addition, the Mott insulating antiferromagnet (x=0) has a peak at 40 meV, consistent with the estimated activation energy derived from resistivity measurements. This feature becomes washed out with temperature and doping. Possible origins for this peak are discussed. © 1996 American Institute of Physics.
Pekarek, T. M. ; Miotkowski, I. ; Crooker, B. C.
Notes: 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. © 1996 American Institute of Physics.
Kaplan, T. A. ; Mahanti, S. D. ; Su, Yen-Sheng ; [et al.] Kubo, K.
Notes: We raise and discuss the following question. Why does the spectrum for the three-band model of Hybertson, Stechel, Schluter, and Jennison, claimed not to be approachable by perturbation theory because of rather large hopping integrals compared to site energy differences, follow precisely what would be expected by low-order perturbation theory? The latter is, for the insulating case, that the low-lying levels are describable by a Heisenberg Hamiltonian with nearest-neighbor interactions plus much smaller next-nearest-neighbor interactions and n-spin terms, n≥4. We first check whether perturbation theory actually does not converge, treating the hopping and p-d exchange terms as perturbations. For the crystal, we find that the first three terms contributing to the nearest-neighbor exchange coupling J (which are of third, fourth, and fifth order) increase in magnitude, and are not of the same sign, i.e., there is no sign of convergence to this order. We also consider the small cluster, Cu2O7, for which we have carried out the perturbation series to 14th order; there is still no sign of convergence. Thus the nonconvergence of this straightforward perturbation theory is convincingly established. Yet the apparent perturbative nature of the spectrum suggests the existence of some perturbation theory that does converge. The possibility of a particular transformation of the Hamiltonian leading to a convergent perturbation series, thereby answering the above question, is discussed. © 1996 American Institute of Physics.
Umehara, I. ; Nakano, K. ; Lu, Q. F. ; [et al.] Sato, K.
Notes: It was reported that La3Co is an interesting superconductor which has a critical temperature Tc of 4.5 K. Pr3Co and Nd3Co are ferromagnetic materials, which have Curie temperatures of 10 and 25 K, respectively. It was claimed that their magnetic structures are associated with the canting moment reflected to the symmetry of their crystal structure. In this paper we present the results of the specific heat measurement in these compounds in the temperature range from 2 to 30 K by a standard adiabatic heat pulse method. Two phase transitions are observed at 8 and 25 K in Nd3Co. One of them at 25 K corresponds to the Curie temperature mentioned above. Another is the transition corresponds to the moment reorientation, which was observed in the thermomagnetic curves and electrical resistivity measurement. This results claim that the moments are canted in this compounds. Magnetic entropy reaches 90% of R ln 2 at 25 K. However, it increases above the Curie temperature because the crystalline electric field effects plays an important role in this compounds. There are two peaks of 10 and 12 K in Pr3Co. The above temperature corresponds the Curie temperature but another was unknown. Thus we performed the specific heat measurement for Pr7Co3, which is very close to the Pr3Co in the phase diagram. We could observe the phase transition at 12 K for Pr7Co3 in the specific heat measurement and electric resistivity measurement. It is clear that the phase transition at 12 K comes from impurity of Pr7Co3 in our Pr3Co compound. We prepare the good quality sample of Pr3Co and try to examine the specific heat measurement. © 1996 American Institute of Physics.
Babcock, K. ; Manalis, S. ; Elings, V. ; [et al.] Dugas, M. ; Challener, W.
Notes: Magnetic force microscopy is finding widespread use in the analysis of magnetic structure at length scales relevant to modern storage densities. We present a new technique which uses the extremely localized stray fields of MFM tips to probe submicron media characteristics. The tip is brought into contact with the magnetized media, and a uniform external field Hext is momentarily added to the tip's stray field. If the net field exceeds the local media coercivity, the magnetization is reversed locally, and a "bit'' is written. High-resolution MFM imaging of the results is done immediately using the same probe. Bit arrays of several Gbit/in2 can be produced with the lithography software of a commercial MFM. Using the external field Hext as a parameter, the threshold for bit writing can be determined, giving a measure of the local, or "point'', coercivity of the media on a 100 nm scale. Results for perpendicular Co–Cr and magneto-optical media can deviate significantly from conventional bulk hysteresis measurements, in some cases reversing the bulk coercivity ranking. Intermediate values of Hext reveal spatial variations in the point coercivity due to fluctuations in composition or microstructure. In square media, sufficiently strong fields Hext cause existing bits to grow, leading to bulk reversal via front propagation, and allowing a direct measurement of wall motion coercivity. Possible extensions to longitudinal media will be discussed. In general, combining the imaging and writing capabilities of MFM probes gives a direct assessment of media response to very localized fields, and leads to a powerful method for relating microstructure to bulk hysteresis properties. © 1996 American Institute of Physics.
Gomez, Romel D. ; Burke, Edward R. ; Mayergoyz, Isaak D.
Notes: Magnetic force microscopy (MFM) in the presence of an external magnetic field has been developed. This has led to further understanding of image formation in MFM as well as new insights concerning the interaction of magnetic recording media with an external field. Our results confirm that, at low applied fields, image formation results from the interaction of the component by the local surface field along the direction of the probe's magnetization. By reorienting the probe's magnetization by an appropriate application of an external field, it is possible to selectively image specific components of the local field. At higher applied fields, the probe becomes saturated and the changes in the images may be attributed to magnetization reversal of the sample. We have observed the transformations that occur at various stages of the dc erasure of thin-film recording media. This technique has also been applied to conventional magneto-optical media to study domain collapse caused by increasing temperature with an external bias field. The methods, results, and their analysis are presented. © 1996 American Institute of Physics.
Notes: We report on the fabrication and the characteristics of Al0.4Ga0.6As/GaAs tandem solar cells. The annealing characteristics of GaAs tunnel diodes are studied. It is found that the degradation of the tunnel peak current density by the annealing is suppressed for the diodes composed of a GaAs tunnel junction sandwiched between AlGaAs layers. The tunnel junction is applied to the interconnect between the Al0.4Ga0.6As top cell and the GaAs bottom cell for the tandem solar cell. The cell has a short-circuit current density of 13.8 mA/cm2, an open-circuit voltage of 2.10 V, a fill factor of 70.0%, and a conversion efficiency of 20.2% at 1 sun, AM1.5. This efficiency is the highest ever reported at 1 sun for tandem solar cells.
Mimila-Arroyo, J. ; Castanedo, R. ; Chávez, F. ; [et al.] González, R. ; Navarro, G. ; Reynoso, A.
Notes: For the first time gallium arsenide field-effect transistors have been made using epitaxial layers grown by the close spaced vapor transport technique (CSVT). The layers were unintentionally doped and their free-carrier concentration was adjusted through the growth parameters to around 1017 cm−3. For the growth of the layers, water vapor was used as transporting gas. This transistor confirms the capability of the simplest and most inexpensive epitaxial technique (CSVT) for growing device quality gallium arsenide epitaxial layers.
Notes: A new dry etching technique is described which allows for decoupling of the chemical etch component from the ionic (physical) component which cannot be achieved in other conventional plasma processing methods. A magnetically confined rf plasma of SF6 was used to contribute a reactive chemical flux with ion bombardment energies of less than 50 eV. In conjunction with this reactive plasma, a broad beam ion source was used to independently deliver an Ar+ ionic flux to the substrate. It was observed that the ion enhanced etch rates of silicon were greater than the individual magnetron reactive ion etch and the argon ion milling rates combined. However, the structure profiles were observed to be directly related to the separately controllable etch components.
Notes: Slow relaxation in photoconductivity and high transverse field effects have been investigated in GaAs metal-semiconductor field-effect transistors with undoped GaAs buffers and AlGaAs/GaAs superlattice buffers. Persistent photoconductivity (PPC) has been observed in both types of devices at 77 K. Complete electric field quenching of the PPC is possible in devices with a GaAs buffer. A high electric field resulted in more profound effects on the channel conductance of the superlattice buffered devices. The barrier established by the superlattice precludes complete elimination of the PPC and thermal quenching is necessary.
Ihm, Yeong-Eon ; Otsuka, N. ; Klem, J. ; [et al.] Morkoç, H.
Notes: Ordering in (100) GaAs1–x Sbx epilayers grown by molecular beam epitaxy has been studied by transmission electron microscopy. Diffraction patterns taken from plan-view and cross-sectional samples of epilayers reveal a unique evolution of the ordering with the growth temperature, showing the formation of a short range order at lower temperatures and a long range order at higher temperatures. A distinct anisotropy of the formation of ordered structures between  and [01¯1] axes is observed from all epilayers investigated. Based on the analysis of the growth conditions, it is suggested that the anisotropy of the ordering is caused by the surface atomic structure of the growing epilayers.
Notes: The use of a plasma during the deposition of epitaxial silicon from 750 to 800 °C is explored. Emphasis is placed on enhancement of the deposition process as opposed to the predeposition surface clean. Plasma enhancement of the deposition process is observed without a change in the apparent activation energy, and the mild ion bombardment (plasma) exposure during deposition introduced no additional defects observable by cross-sectional transmission electron microscopy. Plasma enhancement is also shown to facilitate deposition of high-quality epitaxial silicon films with low levels of unintentional impurity incorporation.
Kshirsagar, S. T. ; Rajarshi, S. V. ; Dusane, R. O. ; [et al.] Vaidya, Jayashri ; Bhide, V. G.
Notes: Thin layers of amorphous germanium vacuum deposited onto a 〈111〉 surface of crystalline silicon were irradiated with a Kr+ ion beam to produce an amorphous Si-Ge alloy at the interface. Raman scattering measurements were performed on these films both before and after the ion irradiation. The vanishing of the strong Si-Si lattice mode near 521 cm−1 and an appearance of the localized Si-Ge vibrational mode near 375 cm−1 in the Raman spectra of ion-irradiated films are correlated to the formation of an amorphous alloy of probable composition of Si0.2Ge0.8 at the interface.
Skromme, B. J. ; Sandroff, C. J. ; Yablonovitch, E. ; [et al.] Gmitter, T.
Notes: The passivating effects of spin-coated films of Na2S⋅9H2O on GaAs surfaces have been studied using room-temperature photoluminescence (PL) and low-temperature PL spectroscopy. After passivation, the 300 K PL efficiency is increased on both n- and p-type material; improvements of up to 2800× are observed. The surface field and surface recombination-related notch features in the free and bound exciton emission spectra at low temperature are eliminated, implying that the residual band bending under illumination is less than 0.15 V.
Wijewarnasuriya, P. S. ; Sou, I. K. ; Kim, Y. J. ; [et al.] Mahavadi, K. K. ; Sivananthan, S. ; Boukerche, M. ; Faurie, J. P.
Notes: p-type doping of HgCdTe(100) layers with lithium during growth by molecular beam epitaxy is reported. Hall measurements have been performed on these layers between 300 and 30 K. The Li concentration is found to increase with the Li cell temperature. Li-doped HgCdTe layers are estimated to have very shallow acceptor levels. Acceptor concentrations as high as 8×1018 cm−3 have been achieved. At low doping levels, due to residual donors, layers show compensation. Incorporation coefficient of Li close to 1 and almost 100% electrical efficiency for Li in molecular beam epitaxy HgCdTe layers were observed. However, Li is found to diffuse rapidly in HgCdTe layers grown by molecular beam epitaxy.
Palmour, J. W. ; Kong, H. S. ; Davis, R. F.
Notes: Depletion-mode n-channel metal-oxide-semiconductor field-effect transistors were fabricated on n-type β-SiC (111) thin films epitaxially grown by chemical vapor deposition on the Si (0001) face of 6H α-SiC single crystals. The gate oxide was thermally grown on the SiC; the source and drain were doped n+ by N+ ion implantation at 823 K. Stable saturation and low subthreshold current were achieved at drain voltages exceeding 25 V. Transconductances as high as 11.9 mS/mm were achieved. Stable transistor action was observed at temperatures as high as 923 K, the highest temperature reported to date for a transistor in any material.
Notes: Rapid thermal annealing (RTA) and furnace annealing of Ga-implanted Si were studied. Ga atoms implanted into Si are located on substitutional lattice sites in concentration above the solid solubility limit after short-time and low-temperature annealing. Low-resistivity shallow p+ junctions can be fabricated using this metastable layer. However, precipitation and redistribution of the Ga atoms were observed after high-temperature or longer time annealing. Shallow p+ junctions fabricated by Ga implantation and RTA are suitable for submicron complementary-metal-oxide-semiconductor devices.
Marek, H. S. ; Serreze, H. B.
Notes: We present results of open tube Zn diffusion into undoped and S-doped n-type InP over the temperature range 550–675 °C. The process yields reproducible results which are consistent with an interstitial-substitutional diffusion model. For the undoped samples, an activation energy of 1.52 eV and a diffusion constant of 4.9×10−2 cm2/s are obtained. For heavily S-doped samples, values of 2.34 eV and 1.4×103 cm2/s, respectively, result. The difference in activation energy which is comparable to the Fermi level difference in the two substrate types is consistent with the different diffusion mechanisms which occur in these two types of InP.
Shen, Yu-Tang ; Myles, Charles W.
Notes: A theory of the deep energy levels produced by triplet vacancy-impurity complexes in GaAs is described. The major chemical trends in the deep levels of a1 and b1 symmetry are predicted for 56 such complexes.

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