bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupling effects in Fe/CoNbZr and Fe/Ag/CoNbZr sandwiches studied by magneto-optical techniques
Authors: Contreras M. C. , F.-Calleja J. , Gutiérrez M. O. , Rivas M. , Krishnan R.
Published: July 2002

Experimental characterization of magnetic anisotropy, coercive field, and anisotropy dispersion of sputtered Fe/CoNbZr and Fe/Ag/CoNbZr sandwiches were carried out by transverse biased initial susceptibility (TBIS) measurements with a magneto-optic Kerr effect at both film/air and glass/film interfaces. Three different behaviors have been observed depending on the range of magnetization fluctuation. The results indicate that Fe grains play a fundamental role in the type of the dispersion (long range or short range). The results obtained from the coercive field and from x-ray diffractograms agree with those obtained by TBIS.
Magnetic properties and transmission electron microscopy microstructures of exchange coupled Nd12−xFe82+xB6 melt spun ribbons
Authors: Chang W. C. , Hsing D. M.
Published: July 2002

The nanocomposite melt spun ribbons of the alloys Nd12−xFe82+xB6 (x=0, 2.5, 3, and 4) have been prepared by rapid quenching and/or post heat treatment. In the alloy with x=0, homogeneous grain morphologies were obtained at the lower wheel speed (18 m/s≤Sw≤25 m/s) to result in a strong exchange coupling effect between Nd2Fe14B and α-Fe grains, the magnetic properties of Br=9.6 kG, iHc=7.7 kOe, and (BH)max=18 MGOe was achieved. In the alloys with x=2.5, 3, and 4, exchange coupling effect is also found and the best magnetic properties of Br=11.4 kG, iHc=6.3 kOe, and (BH)max=19 MGOe are achieved in the alloy ribbons with x=2.5 quenched at a wheel speed of 18 m/s without any post heat treatment. Inasmuch as the crystallization temperature of the ribbons increased with decreasing the Nd content, the microstructures, the size and volume fraction of the α-Fe and Nd2Fe14B phases evaluated by transmission electron microscopy observation, of the above alloy ribbons were dominated by the Nd content, the wheel speed and the heat treatment temperature apparently. As a result, the exchange coupling effect and the magnetic properties of the ribbons were strongly affected.
Magnetic phase transitions in perovskite-type anilinium-based tetrachlorocuprates
Authors: Nelson D. J. , Chan K. , Cervantes-Lee F. , Haar L. W. ter
Published: July 2002

Our research efforts have expanded upon the work previously done on the molecular composites (RNH3)2CuCl4 (R=alkyl-) by utilizing various para-substituted anilines and anilinium derivatives as design tools. The resulting structures have the general formula (RC6H4NH3)xCuCl4 (R=methyl-, fluoro-, O2N-, for x=2; R=H3N-, H3NC6H4- for x=1) and are similar to the aforementioned compounds, consisting of two-dimensional layers of corner-sharing CuCl6 octahedra, resulting in a type of perovskite lattice. Equivalent magnetic sites are eclipsed between layers in the case of a small interleaving organic moiety, such as H3NC6H4NH3, but are otherwise staggered along one or more axes. Powder susceptibility data for representative compounds containing small anilinium derivatives reveal magnetic transitions at temperatures ranging from ∼11.5 to ∼7 K, depending upon the chemical nature of the anilinium substitution and the interlayer spacing.
Exchange coupling in sputtered giant magnetoresistance NiFe/Cu/CoFe/Cu multilayers
Authors: Kief M. T. , Bresowar J. , Leng Q.
Published: July 2002

We will report on the exchange coupling, magnetoresistance (MR) and film structure in the system NiFe/Cu/CoFe/Cu as a function of multilayer number (bilayers=6 to 20) and Cu spacing (5 to 31 Å). The multilayers were prepared by sputter deposition and investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), vibrating sample magnetometry (VSM), ferromagnetic resonance (FMR), and standard MR methods. The exchange coupling, Hex, and MR are shown to vary with Cu spacing. The XRD data indicates the multilayer structure is evolving through at least the first 20 bilayers. AFM measurements suggest subtle variations in interface quality and a rms roughness near 0.4 nm. These variations in structure are manifested in the FMR and MR. Multilayers with Cu spacers ≳11 Å generally show the expected two FMR peaks. Some multilayers show three peaks indicating defects within the multilayer. These defects can be viewed as the elusive ‘‘pinholes’’ which act to ferromagnetically short the magnetic layers together. This demonstrates that FMR provides very valuable information about defects in nonideal multilayers which is critical to our understanding and applications of giant magnetoresistance (GMR) systems.
Structural and magnetic properties of fcc Pt/Fe(111) multilayers
Authors: Wu Ruqian , Chen Lujun , Kioussis Nicholas
Published: July 2002

The structural, electronic, and magnetic properties of the 2Pt/5Fe/2Pt(111) system were investigated using the total energy full-potential linearized augmented-plane-wave method. The interplanar relaxation, determined by the atomic force approach, was found to be large between the nearest Pt–Fe and Fe–Fe distances, thus altering the electronic and magnetic properties of the system. We find an enhancement of the spin and orbital Fe magnetic moment (2.44 μB) at the interface, and a large induced magnetic moment (0.27 μB) at the interfacial Pt site, which is relatively short ranged. The effects of the interface on the contact hyperfine fields is evaluated: The (negative) core-polarization contribution is found to scale with the moment, but the valence contribution changes from negative in the interior to positive at the interface. The trend of the calculated layer-by-layer hyperfine fields agrees well with experiment.
Aligned high magnetization Nd(Fe,Co,Mo)12 films nitrided by N ion implantation
Authors: Qian X. R. , Hegde H. , Cadieu F. J.
Published: July 2002

Ion implantation has been used to nitride highly aligned ThMn12-type structure Nd(Fe,Co,Mo)12 film samples. The initial 1–12-type films were made by sputtering with the crystallite c axes aligned perpendicular to the film plane. The films were then nitrided to produce fully nitrided 1–12 films, but only for a limited range of N ion doses. Much of the 1–12 phase was made amorphous before full nitriding was observed. It was not possible to thermally anneal the N implanted samples to lower the α-Fe concentration resulting from the N ion bombardment. This is only consistent with the Nd(Fe,Co,Mo)12N phase being metastable. Low temperature heat treatments of the N implanted films at temperatures less than 270 °C produced evidence for Fe16N2 formation, but little restoration of the 1–12 phase. For heat treatments at temperatures of 270 °C and above no evidence of Fe16N2 was detected. These results indicate that it is not possible to produce nitrided ThMn12 films with Fe16N2 inclusions by ion implantation.
Exchange and crystal-field interactions in R3(Fe,Ti)29 and R3(Fe,Ti)29Ny (R=Nd,Sm)
Authors: Li Hong-Shuo , Courtois D. , Cadogan J. M.
Published: July 2002

A quantitative analysis of the available magnetization data on the R3(Fe,Ti)29 and R3(Fe,Ti)29Ny (R=Nd,Sm) compounds have been carried out. The two R sites of R3(Fe,Ti)29 compounds have opposite signs of their leading crystal-field coefficient A20; namely, A20<0 for the 4e site (2:17 like) and A20≳0 for the 2a site (1:12 like) (site notation is that of the space group P21/c). A R–Fe exchange coefficient of 285±5μ0 and crystal field coefficients A20(2a)=+16 K a−20 and A20(4e)=−25 K a−20 were obtained. The type-II first-order magnetization processes observed in both Nd3(Fe1−xTix)29 and Sm3(Fe1−xTix)29 have been well reproduced: μ0Hcr=2.4 T for R=Nd at 5 K (observed ∼2 T), and μ0Hcr=3.0 T for R=Sm at 4.2 K (∼3.2 T). Based on the bonding charge model, we estimate that the values of A20(4e) and A20(2a) for the fully nitrided 3:29 compounds are A20(4e)=−187 K a−20 and A20(2a)=−184 K a−20. Our calculations show that R3(Fe1−xTix)29Ny nitrides become uniaxially anitsotropic for y≥2 and a maximum room temperature uniaxial anisotropy field of 11.4 T for fully nitrided Sm3(Fe1−xTix)29N4 is predicted which compares well with the experimental values which lie in the range 10.7–13 T.
Magnetostriction and thermal expansion measurements on FeRh1−xPtx alloys
Authors: Algarabel P. A. , Ibarra M. R. , Marquina C. , Yuasa S. , Miyajima H. , Otani Y.
Published: July 2002

We report high-field magnetostriction and thermal expansion measurements in FeRh1−xPtx (x=0.75, 0.765, and 0.7) compounds. From the thermal expansion measurements we have observed the para-ferromagnetic and ferro-antiferromagnetic transitions for x=0.75 and 0.765, and the para-antiferromagnetic transition in the 0.7 compound. The volume expansion associated with the first-order transitions has been determined. The results of the magnetostriction measurements are complex and are explained here on the basis of an induced transition from an antiferromagnetic to a ferromagnetic state, induced by the applied magnetic field.
Dependence of the interlayer exchange coupling on the constitution of the magnetic layers
Authors: Ounadjela K. , Zhou Li , Stamps R. , Wigen P. , Hehn M. , Gregg J.
Published: July 2002

We describe the consequences on the interlayer exchange coupling by the addition of a small amount of Ag impurities within the Co layers of high crystalline quality Co/Cu/Co(Ag) and Cu/Ru/Co(Ag) trilayers, while maintaining the integrity of the spacer layers. We discuss the consequent changes in the amplitude, period, and phase of the coupling in terms of the modification of interfacial spin-dependent potentials.
Role of lattice distortions in the transport property of R0.7Me0.3MnO3 (R=Pr,Nd,La and Me=Ba,Sr,Ca) (abstract)
Authors: Sharma R. P. , Xiong G. C. , Ogale S. B. , Greene R. L. , Venkatesan T.
Published: July 2002

There are different models which to try to explain the transport properties of the manganese oxides in terms of magnetic polarons, Zener double exchange and Jahn-Teller distortions of the lattice which is temperature dependent. Using Rutherford backscattering techniques (2–3 MeV He ions) we are measuring the presence of lattice distortions by studying the angular scans in the channeling mode. The angular widths (FWHM) can be used to extract both dynamic and static displacements of the atoms from their equilibrium sites. The technique is extremely powerful in the sense that local  uncorrected displacements as small as 0.01 Å from the equilibrium position can be detected. The extraction of the physics from the data is relatively straight forward compared to other techniques such as XRD or neturon scattering. Using a variable temperature backscattering system at our center (one of three or four systems around the world) the angular width (FWHM) is found to exhibit a dramatic change with temperature increasing with decreasing temperature showing a possible correlation with both the transport and magnetic properties. The results suggest a decrease in the lattice disorder with reduced temperature far in excess of what one would predict on the basis of a simple Debye behavior. The reduction in the lattice disorder correlates very well with the resistance vs temperature dependence. A variety of manganese oxide epitaxial thin films grown by pulsed laser deposition have been studied and the data suggests a role for structural distortions in any mechanism to explain the transport properties of these materials.
The effect of disorder and fluctuations on the magnetotransport of a double-exchange ferromagnet (abstract)
Authors: Byers Jeff M.
Published: July 2002

The discovery of colossal magnetoresistance (CMR) in the doped perovskite manganites has reawakened interest in the double-exchange mechanism proposed to Zener. To account for the close relation between ferromagnetism and metallic transport in lanthanum manganites doped with divalent cation (Ca, Sr, Ba) Zener claimed that an electron could delocalize on lattice of spins and still conform to Hund’s Rule if a ferromagnetic coupling between spins were mediated by that same electron. Thus, the onset of metallic behavior (delocalization) is intimately linked to ferromagnetic ordering of the spin lattice. Clearly, the double-exchange mechanism provides some necessary physics but is not sufficient in explaining the key mystery of the CMR materials: What causes the large peak in the resistivity vs. temperature and why is it removed by an applied magnetic field. The effect of disorder and fluctuations on the double-exchange mechanism may provide the answers. Several sources of disorder in these materials act to form a mobility edge via Anderson localization: intrinsic divalent/trivalent cation disorder, off-diagonal disorder caused by the spin lattice and oxygen vacancy disorder. A mean-field calculation reveals that below the Curie temperature those carriers aligned opposite to the magnetization experience a narrowing band as the temperature is reduced. Fermi glass behavior is induced in this minority carrier band by the Fermi level falling below the mobility edge. However, the mean-field result does not contain a peak in resistivity since the majority carrier band does not behave as a Fermi glass and effectively ‘‘shorts out’’ the more resistive minority conduction channel. The formation of the resistivity peak requires the inclusion of ferromagnetic fluctuations above the Curie temperature that tend to ‘‘mix’’ the two conduction channels. The application of a magnetic field unmixes the conduction channels once again allowing the carriers aligned with the magnetization to carry current solely in the low-resistance channel (causing the dramatic negative magnetoresistance). This work supported by a National Research Council postdoctoral fellowship.
Magnetic properties of 3%Si-Fe thin sheets annealed in Ar atmosphere (abstract)
Authors: Yamashiro Y. , Yonesu A. , Hashi S. , Ishiyama K. , Arai K. I.
Published: July 2002

It has been actually required in recent years to decrease iron losses in transformer cores, as increment of electric energy consumption in all industries and homes. Answering to such requirements, Arai et al. developed a new type magnetic core material; it was tertiary recrystallized Si steel with 50 μm thickness, and it was of as low iron loss as an amorphous materials one. The Si steel was made by an annealing in a vacuum of 1×10−3 Pa and at a temperature of 1200 °C. In this study, we prepared samples for measurements from (110)[001] grain-oriented 3%Si steel with 300 μm thickness. We coldrolled them to about 50 μm using a four-high rolling mill, and then annealed in an atmosphere of highly pure Ar gas, from a point of view of saving energy. We investigated effects of annealing time, heating rate and gas flow rate on magnetic properties. The annealing temperature was fixed at 1000 °C. Hysteresis loops were drawn with single-sheet tester. Grain structure and its direction were decided by etch pit method. Coercive force Hc and induction B8 which is the induction at 800 A/m applied field were obtained from the hysteresis loops. The magnetic properties were somewhat changed according to the annealing condition. The coercive force varied from 5 to 30 A/m, and the induction varied from 1.45 to 1.85 T. The best properties, Hc was 5 A/m and B8 was 1.85 T, were obtained under the annealing condition; annealing time was 60 min, heating rate was 3 °C/s, and gas flow rate was 2 ι/min.

The surfaces of the samples were not covered with tertiary recrystalline, but wholly with secondary recrystalline. About 90% grains were (110) plane, the rest was (100) plane. And about 90% [001] axes oriented to the rolling direction within five degree misorientation.
Influence of grain boundary scattering in the infrared response of silicide Schottky barrier diodes
Authors: Roca E. , Larsen K. Kyllesbech , Kolodinski S. , Mertens R.
Published: July 2002

The infrared response of polycrystalline and epitaxial CoSi2/Si Schottky diodes has been studied as a function of the silicide thickness. Although both types of diodes present very similar barrier height, it is observed that, for thick silicide films, the quantum efficiency of the polycrystalline diodes is two times higher than for the epitaxial diodes. This behavior is attributed to grain boundary scattering of the excited carriers. Nevertheless, this effect is not so strong for thin silicide films because the thickness of the film is much smaller than the grain diameter, and the dominant effect in the quantum efficiency is the scattering at the silicide interfaces and not at the grain boundaries.
Hydrogen bonding and void microstructure of a-Ge:H films
Authors: Mulato M. , Chambouleyron I. , Torriani I. L.
Published: July 2002

This article reports on the microvoid structure of hydrogenated amorphous germanium films, as determined from small angle x-ray scattering data and infrared transmission spectroscopy, and its dependence on three deposition parameters, namely, the substrate temperature, the particle bombardment during film growth, and the partial pressure of hydrogen in the deposition chamber. The structure of the alloys depends on the first two deposition parameters and not on the partial pressure of hydrogen. The dependence of the optical gap on hydrogenation and microstructure is established for a-Ge:H films for a wide range of deposition conditions.
Giant energy product of a two-phase magnet with tabletlike inclusions
Authors: Lee Ching-Ming , Chang Ching-Ray
Published: July 2002

Influence of soft-phase to hard-phase ratio on energy product of a nanostructured two-phase magnet has been analyzed with micromagnetics. Below a certain critical size, the inclusion of soft phase not only reduces the consumption of rare-earth elements but also improves the energy product. Calculated energy product significantly depends on the shape of soft phase, and indicates that giant energy product of the tabletlike soft inclusions is feasible.
Effect of annealing on the structure and diamagnetic transition width of high-quality crystal Bi2Sr2CaCu2Oy studied by x-ray-diffraction and ac susceptibility measurements
Authors: Wu Wenbin , Zhu Jingsheng , Sun Xuefeng , Zhao Xiaoru , Wang Liangbin , Zhou Guien , Li Xiao-Guang , Zhang Yuheng
Published: July 2002

A large and highly oriented Bi2Sr2CaCu2Oy crystal with only a single domain was progressively air annealed at 500 and 750 °C for 4 and 8 h. Measurements of ac susceptibility showed that Tc (onset) and ΔTc (10%–90% of the complete transition) of the crystal were 74 and 3 K (as grown), 85 and 0.6 K (annealed at 500 °C for 4 h), and 89 and 2.5 K (annealed at 750 °C for 8 h), respectively. To explain this typical change of ΔTc with annealing temperature, a full structural characterization by x-ray diffraction in the b*c* plane and by Laue photographs of the crystal as grown and after each annealing treatment was carried out. Apart from variations of the lattice parameters, it is noted that the smaller the ΔTc, the more highly oriented and structurally homogeneous the crystal. We explain the annealing effect on ΔTc by variations in crystallinity, structural homogeneity, and surface phase decomposition, and the changes in oxygen stoichiometry caused by the annealing process.
Excited-state absorption of Tm3+-doped single crystals at photon-avalanche wavelengths
Authors: Garnier N. , Moncorgé R. , Manaa H. , Descroix E. , Laporte P. , Guyot Y.
Published: July 2002

Excited-state absorption (ESA) spectra calibrated in units of cross sections are reported in the case of Tm3+-doped YAG, YAP, and Y2O3 single crystals around wavelengths at which photon-avalanche absorptions were observed recently. The peak ESA cross sections are compared with those obtained theoretically by using the Judd–Ofelt approach.
Electron-driven rates in HeXe glow discharges
Authors: Drallos P. J. , Nagorny V. P. , Williamson W.
Published: July 2002

We have performed a series of space-independent, electron Boltzmann calculations for the determination of electron-driven rate coefficients in helium/xenon glows for a variety of He/Xe mixture ratios. The present work was performed in support of research efforts involving plasma display panels. Our Boltzmann calculations do not involve any term-expansions of the electron velocity distribution function (EVDF) and, in principle, incorporate the exact EVDF. Legendre decompositions of our complete EVDFs suggest that the more common two-term approximation methods may be unreliable for the determination of certain rates in gas mixtures. More generally, this data can be used to provide the rate and transport coefficients necessary for fluid simulations involving He/Xe glows under a limited range of discharge conditions. For each mixture ratio considered, polynomial fits to the resulting, steady-state rate coefficients, electron drift velocities and average energies, as functions of E/N are presented. As expected, the data is very sensitive to the mixture ratio of the two gas components.
Epitaxial single crystal Fe16N2 films grown by facing targets sputtering
Authors: Sun D. C. , Jiang E. Y. , Tian M. B. , Lin C. , Zhang X. X.
Published: July 2002

α″-Fe16N2 single crystal films can be prepared successfully by facing targets sputtering directly onto NaCl(100) substrates in a mixture of argon and nitrogen gases. Both x-ray diffractometer and transmission electron microscope are employed to characterize the crystal structure of the films. The perfect electron diffraction patterns of α″-Fe16N2 single crystal in [1̄11], [011], and [001] directions can be distinctly observed by double tilting. These patterns confirm that the crystal structure of the films corresponds to a body-centered tetragonal (bct) lattice with the parameters of a=b=5.72 Å and c=6.29 Å . The x-ray diffraction patterns show that α″-Fe16N2 epitaxially grows on the NaCl(100) substrate with an orientation relationship α″-Fe16N2(001)∥NaCl(001) and α″-Fe16N2[100]∥NaCl[100]. The saturation magnetization of the Fe16N2 films is around 2100−2300 emu/cc, which agrees well with the value reported by Sugita et al..
Onset of C(2×2) ferrimagnetic order in Cr islands deposited on Fe(001) as a function of island size
Authors: Pizzagalli L. , Stoeffler D. , Vega A. , Bouarab S. , Demangeat C. , Dreyssé H. , Gautier F.
Published: July 2002

We present a study of the magnetic properties of compact Cr clusters deposited on Fe(001). A Cr monolayer deposited on Fe(001) is found to be c(2×2) ferrimagnetic (AP) and the moments of a single adatom and very small clusters are antiferromagnetically aligned with the Fe substrate moments (P). Therefore a transition from P to AP order is expected when the cluster size increases. An Ising model is used to model the magnetic energy of two geometric configurations, a square-shaped and a diamond-shaped cluster. Self-consistent tight-binding calculations allow us to determine the P to AP transition which is obtained for a 400 adatoms square-shaped cluster. The pair magnetic energies have been estimated to −105.7 meV for the first neighbors Fe–Cr interaction and to −111.3 meV for the second neighbors Cr–Cr interaction in the Ising model.
Improvement of Co71Cr19Pt10/Ti90Cr10 perpendicular recording media by independent optimization of film nucleation and growth processes
Authors: Nolan T. P. , Hirayama Y. , Futamoto M.
Published: July 2002

CoCrPt/TiCr perpendicular recording media having independently optimized nucleation and growth conditions have been prepared by changing the argon pressure during sputter deposition of each film. A low argon pressure CoCrPt nucleation layer produces strong c-axis perpendicular orientation that can be maintained during continued CoCrPt deposition at high argon pressure. The two-layer media combines increased particle separation from high pressure growth and strong orientation to produce higher signal to noise ratio than either high pressure or low pressure single layer CoCrPt media. TiCr underlayers, despite poor orientation and a noncolumnar structure, improve CoCrPt c- axis perpendicular orientation. Low argon pressure during TiCr deposition maximizes CoCrPt orientation. A TiCr bilayer having a low pressure nucleation layer followed by a high pressure growth layer improves performance of the subsequent CoCrPt layer. A TiCr bilayer having a high pressure growth layer followed by a thin low pressure template produces even greater recording performance enhancement. This media has Hc=2560 Oe, D50=90 kfci and S0/Nd at 240 kfci=4.0. An identical CoCrPt layer deposited on the low pressure TiCr underlayer has Hc=1850 Oe, D50=80 kfci and S0/Nd at 240 kfci=2.4.
Electronic properties of UCuSn
Authors: Nakotte H. , Purwanto A. , Robinson R. A. , Prokeš K. , de Boer F. R. , Havela L. , Sechovský V. , Swainson I. P.
Published: July 2002

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.
Magnetic field and temperature dependence of critical current densities in multilayer YBa2Cu3O7−δ films
Authors: Afonso S. , Chan F. T. , Chen K. Y. , Salamo G. J. , Tang Y. Q. , Wang R. C. , Xu X. L. , Xiong Q. , Florence G. , Scott S. , Ang S. , Brown W. D. , Schaper L. W.
Published: July 2002

In order to build high-temperature superconductor (HTS) multichip modules (MCMs), it is necessary to grow several epitaxial layers of YBCO that are separated by thick dielectric layers without seriously affecting the quality of the YBCO layers. In this work, we have successfully fabricated YBCO/YSZ/SiO2/YSZ/YBCO structures on single-crystal LaAlO3 substrates using a combination of pulsed laser deposition for the YBCO layers and ion-beam-assisted rf sputtering to obtain biaxially aligned YSZ intermediate layers. The bottom YBCO layer had a Tc∼89 K, Jc∼7.2×105 A/cm2 at 77 K, whereas the top YBCO layer had a Tc∼86 K, Jc∼6×105 A/cm2 at 77 K. The magnetic field and temperature dependence of Jc for the YBCO films in the multilayer have been obtained. The results for each of the YBCO layers within the YBCO/YSZ/SiO2/YSZ/YBCO structure are quite similar to those for a good quality single-layer YBCO film.
Magnetic properties of a high T
c superconductivity related system Y1−xPrxBa2Fe3O8
Authors: Yuen T. , Seyedahmadian M. , Salomon R. E. , Myer G. H. , Cao G.
Published: July 2002

We studied Pr substitution for Y in a 123 high Tc superconductor related system, Y1−xPrxBa2Fe3O8 using of x-ray diffraction, magnetization, and 57Fe transmission Mössbauer spectroscopy. The Fe sublattices are magnetically ordered through the whole system. The Pr substitution, however, suppressed the Fe ordering temperature. The low temperature susceptibility across the system increases with increased Pr concentration x, and an additional antiferromagnetic ordering at 24 K is observed for x=1, indicating a possible Pr ordering. The averaged magnetic hyperfine field at Fe sites measured at 15 K is also reduced as x increases. The dramatic effect of Pr substitution for Y in this system, which is not observed in studies of Y substitution by other rare-earth elements, is interpreted as the effect of the hybridization between the Pr 4-f electrons and the electrons in the Fe(2)O2 layers, similar to the Pr hybridization in Y1−xPrxBa2Cu3O7−y system.
Spin resolved resonant Raman scattering
Authors: Caliebe W. A. , Kao C.-C. , Berman L. E. , Hastings J. B. , Krisch M. H. , Sette F. , Hämäläinen K.
Published: July 2002

Spin resolved resonant Raman scattering measurements in Gd metal made by exciting x-ray resonant Raman scattering with circularly polarized x rays near the LIII edge of Gd are presented. The incident photon energy was fixed at the peak of the 2p→4f quadrupolar transition, and the scattered photon was energy analyzed around the 3d→2p fluorescent energy. Asymmetry ratios in the scattered intensity much larger than that of the LIII XMCD effect (x-ray magnetic circular dichroism) were observed upon reversal of the magnetization of the sample or the helicity of the photon. A detailed comparison of these results with XMCD results from both the MIV,V edges and LIII edge of Gd metal will be discussed.
Hall-effect measurements on Cr films deposited on Ge substrates
Authors: Hsieh Chien-Sheng , Schröder Klaus
Published: July 2002

The Hall effect of Cr films on Ge substrates was measured at room temperature in a high-vacuum system. It was found that the Hall coefficient RH depends on the pressure in the vacuum system. RH of films about 1–2 nm thick was about 5×10−11 m3/C for a pressure during Cr evaporation in the 10−7 Torr range. The Hall coefficient decreased to less than 10−11 m3/C if pressure decreased into the 10−8 Torr range. These RH values are much lower than found in bulk chromium. They would give in a one-band model about 1 electron hole/atom for the higher-pressure, and about 12 electron holes/atom for the lower-pressure experiments. A two-band model could give the observed low Hall coefficients if the hole current nearly compensated the electron current. Both models cannot be reconciled with presently accepted electron band models for chromium.
Transport properties of amorphous CoZrNd thin films with very low Nd content (abstract)
Authors: de Francisco C. , Muñoz J. M. , Calleja J. F. , Rivas M. , Contreras M. C.
Published: July 2002

The composition and temperature dependence of electrical resistivity were studied for a series of amorphous (Co93Zr7)100−xNdx for x<3.4 and for a temperature range between 77 and 300 K. The films were deposited by rf sputtering. Measurements of electrical resistivity were performed by the standard four probe technique with ac currents of 10 mA. The rate of heating and cooling was 1 K/min. The electrical resistivity is very sensitive to the presence of Nd ions, as the resistivity clearly increases with a very slight increase of Nd content. The temperature dependence of resistivity does not obey the T2 law predicted by the diffraction model as proposed by Meisel and Cote but follows very nicely the law ρ(T)=aT3/2−bT+c, a result also found for amorphous Gd–Co. The first term corresponds to the electron scattering from the magnetic spin and the second term to the electron scattering from the disordered configuration of atomic potentials.
On perturbation theory for the three-band model of cuprates
Authors: Kaplan T. A. , Mahanti S. D. , Su Yen-Sheng , Kubo K.
Published: July 2002

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.
Magnetic dichroism in angle-resolved UV photoemission from valence bands, using linearly polarized light
Authors: Venus D. , Kuch W. , Dittschar A. , Zharnikov M. , Schneider C. M. , Kirschner J.
Published: July 2002

Magnetic dichroism measurements of the valence bands of films of fcc Co/Cu (001) have been performed using angle-resolved UV photoemission in low-symmetry, off-normal emission geometries, and linearly polarized light. Asymmetries of magnitude 4% are observed upon magnetization reversal. Evidence of both spin-dependent surface transmission and magnetic dichroism in the angular distribution of photoelectrons is seen. The asymmetry spectra indicate sensitivity to both magnetic exchange splitting and to spin–orbit splitting.
Optical and magnetic studies of SmCo and SmFe films (abstract)
Authors: Wu K. T. , Yao Y. D. , Chen T. C.
Published: July 2002

Permanent magnetic materials based on iron, cobalt, and rare earths have been extensively studied during the past years. It is very interesting to study the physical properties of film type permanent magnet materials. In this investigation, we report the optical and magnetic properties of SmCo, SmFe, Sm, Co, and Fe films as well as the comparison of the electrical resistivity and magnetization behaviors between these films. The optical transmittance and reflectance have been measured as functions of the wavelength and the thickness for all the films with thickness less than 2000 Å. The electrical resistivity and magnetization have been measured below room temperature. The slope of electrical resistivity is decreased with decreasing the thickness of the films. The films are transparent with thickness less than 800 Å for magnetic films. For films with thickness at 200 Å, the values of the transmittance are 50, 48, 38, 18, and 14 for Sm, SmCo, SmFe, Co, and Fe, respectively. This is explained due to the difference of the magnetic permeability of all the films. The oscillatory behaviors of reflection for all the films are qualitatively consistent with that of the theoretical predict for an absorbing surface. Up to now, the best magnetic properties for magnetic films at room temperature are with an intrinsic coercivity of 1450 Oe and an energy product of 4.5 MGOe.
Thermal expansion of single-crystalline UNiAl
Authors: Prokeš K. , de Visser A. , Menovsky A. A. , Brück E. , de Boer F. R. , Sechovský V. , Gortenmulder T. J.
Published: July 2002

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.
Superconductivity and structural transformation in HfV2 and Nb-doped HfV2
Authors: Chu F. , Chen Z. W. , Fuller C. J. , Lin C. L. , Mihalisin T.
Published: July 2002

The temperature dependence of the specific heat shows that the strong electron-phonon coupling system HfV2 undergoes a structural transformation at 116 K and becomes superconducting around 9 K. The structural instability can be interpreted in terms of a strong electron-phonon coupling and Fermi surface nesting. The structural transformation shifts to lower temperatures with very dilute Nb substitution in HfV2, and completely disappears with only 5 at. % Nb. The low-temperature specific heat of HfV2 shows a large density of states at the Fermi level. These experimental results are consistent with predictions from band-structure calculations.
Giant thermopower in 3D-magnetic multilayers. Structural and electron band effects (abstract)
Authors: de Azevedo M. M. P. , Almeida B. G. , Amaral V. S. , Braga M. E. , Sousa J. B. , Freitas P. P. , Krishnan R.
Published: July 2002

We report measurements of the thermoelectric power (S) and electrical resistivity (ρ) in sputtered multilayers of M(10 Å)/Au25Cu75(t) (with M=Co or Ni) directly grown on Si (100), and in UHV—evaporated Pt 20 Å/Ni(x) multilayers grown on Si(111). The multilayers have thicknesses t=21 Å, 23 Å, and x=19.5 Å, 39 Å, and the data spans the temperature range 10–300 K. All the samples exhibit an anomalous thermopower at high temperatures, with a huge maximum in S and a sharp peak in the temperature derivative dS/dT at a characteristic temperature T*, e.g., ∼190 K for Ni/Pt and ∼260 K for M/Au25Cu75 multilayers. In all cases the thermopower decreases abruptly at T*, from a giant positive value (S≊80 μV K−1 in Ni/Pt and S≊12–50 μV K−1 in M/Au25Cu75) to values close to zero below T*. The electrical resistivity also displays an anomaly over the same temperature range, characterized by a rapid resistivity enhancement Δρ∼1 μΩ cm (within ΔT ∼60 K) when T decreases below T*.

This produces a sharp minimum in the derivative dρ/dT at T*. The shape of the S(T) anomaly is remarkably similar in all cases (mutatis mutandis) for ρ(T), suggesting a general underlying physical mechanism. We found that a suitable buffer (e.g., 100 Å Pt in Ni/Pt, Fe50 Å in Co/Au25Cu75 and 50 Å in Ni/Au25Cu75) suppresses the anomalies, indicating a structural-related underlying effect. In this case, S values are small and negative (as occurs with Ni and Co), exhibiting the usual smooth variation over the whole temperature range. At this stage, it is not clear what is the precise physical mechanism responsible for the observed anomalies. Interfacial atomic mixing in the nonbuffered multilayers may cause individual magnetic atoms or clusters to have entirely nonmagnetic environments (Au, Cu, or Pt in our samples). This situation could give an anomalous transport contribution, through conduction electron scattering at virtual bound states (VBS) localized in the (3-D) magnetic atoms. A sharp peak occurs in the local density of states, N(E), at a characteristic energy EL. A giant thermopower may result when the Fermi level is close to EL (S∝dN/dE; very large).

The VBS model successfully explains large S values observed in bulk noble metals diluted with 3-D magnetic impurities such as Co and Ni. However, the standard treatment does not lead to a sharp cutoff-temperature, below which S(T) gets negligible. Another contribution is the usual s-d electron transition mediated by phonon scattering. This leads to a thermopower proportional to the energy derivative of the splitted ferromagnetic 3D band density of states at EF, which can be very large when sub-band filling is almost complete.
Structural characterization of epitaxial Co–Ag
Authors: Azizi A. , El Chahal L. , Ounadjela K. , Deville J. P. , Thompson S. M. , Gregg J. F.
Published: July 2002

Co–Ag granular films are prepared by molecular-beam epitaxy to enable a detailed structural study to be conducted in conjunction with magnetotransport properties. It is demonstrated that the shape, size, location, and segregation of the magnetic Co grains and the morphology of the surface depend on the growth temperature. Detailed transmission electron microscopy reveals inherent differences between samples prepared at room temperature and those prepared at high temperature. Small clusters of only a few atoms are found to be present in the matrix of the low-temperature samples significantly affecting the spin diffusion length. Well-crystallized regions of both Co and Ag are found in the high-temperature samples, however, the Co grains remain relatively small in size even at the highest growth temperatures.
Study of interface structure of Fe/Al multilayers
Authors: Chowdhury Ataur R. , Freitag Andrea E.
Published: July 2002

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-Å-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.
The anhydrous alums: A versatile new series of model triangular lattice magnets (abstract)
Authors: Bramwell S. T. , Nixon L. , Parkin I. P. , Kariuki B. M. , Harris K. D. M. , Carling S. G. , Harding C. J.
Published: July 2002

There is considerable theoretical interest in triangular and stacked triangular lattice antiferromagnets, particularly as regards their phase transitions and the nature of the ground state of quantum systems with spin S=1/2. There are, however, no good experimental representatives of the S=1/2 triangular lattice antiferromagnet, and indeed there are few good representatives of those with higher spin. Given the diversity of theoretical problems associated with these systems there is a clear need to find appropriate new model materials. We report the discovery of a versatile new series of triangular lattice magnets, the anhydrous alums of general formula AM(SO4)2, with A=alkali metal, M=Ti,V,Cr,Fe. In these materials the magnetic M ions occupy a regular or very slightly distorted triangular lattice in well-separated layers. All show magnetic ordering effects in the range 2–10 K, the near-neighbor exchange being antiferromagnetic for Ti, V, and Fe and ferromagnetic for Cr. The Ti materials, which we have made for the first time, have S=1/2, and may prove to be the best examples of the S=1/2 triangular lattice antiferromagnet yet discovered. In general, our ability to control the spin value, spin anisotropy, and exchange anisotropy makes the anhydrous alums a potentially useful series for the investigation of a variety of physical problems.
Finite size effects in thin NiMn spin glass layers
Authors: Hoines L. , Cowen J. A. , Bass J.
Published: July 2002

Measurements of the normalized spin freezing temperatures, Tf/Tbf, (Tbf=bulk spin-freezing temperature) are reported for NiMn/Cu multilayers in which the Mn concentrations are in the spin-glass range and the Cu thickness is held constant at a large enough value (tCu=30 nm) to magnetically decouple the NiMn layers. The variation of Tf/Tbf with NiMn layer thickness, tNiMr, is very similar to the variations previously found for CuMn, AgMn, and AuFe. Quasistatic finite size effects in NiMn spin-glass thus appear to be rather similar to those in the more traditional noble-metal spin glasses.
Ferromagnetic critical correlations and dynamics in AuFe reentrant ferromagnets
Authors: Pappas C. , Alba M. , Brulet A. , Viel V. , Mezei F.
Published: July 2002

We determined the paramagnetic critical scattering of the reentrant ferromagnets Au1−xFex, with x=0.2, 0.18, and 0.16, i.e., close to the threshold (xc∼0.155), below which ferromagnetism disappears and a spin-glass state sets in. Following the Ornstein–Zernicke formalism, we derived the inverse critical ferromagnetic correlation length (κ=ξ−1) and the related magnetic susceptibility. The macroscopic susceptibility χq=0 of the samples was also determined in a separate experiment with a Faraday balance. The neutron results show the same behavior of critical type with χq=0 for x=0.18 but not for x=0.16. We interpret this discrepancy as an indication for a non-well-defined ferromagnetic transition for x=0.16. A further comparison of our results with those for Fe shows that the amplitude of the correlation length increase with increasing disorder so that, at the same reduced temperature τ, the correlations are much larger in the disordered samples than in Fe. As a consequence of this, the ferromagnetic correlations in the disordered samples persist well above Tc and lead to a much wider critical region than is the case in Fe and in usual ferromagnets.
Magnetic ordering in pyrochlore Ho2Mn2O7
Authors: Raju N. P. , Greedan J. E. , Pedersen J. S. , Simon Ch. , Maignan A. , Niraimathi A. M. , Gmelin E. , Subramanian M. A.
Published: July 2002

The magnetic susceptibility of Ho2Mn2O7 with a spontaneous rise below about 40 K and a paramagnetic Curie temperature of +39 K suggests a ferromagnetic ordering. Indeed neutron diffraction profiles show strongly enhanced Bragg peaks with a temperature dependence which indicates an apparent Tc≊35 K. Nonetheless, the magnetic diffraction pattern is not consistent with a collinear ferro or ferrimagnetic ordering of the Ho3+ and Mn4+ sublattices. Furthermore, specific heat and small angle neutron scattering (SANS) show features which are also incompatible with conventional long-range order.
Spectroscopic Kerr investigations of CoNi/Pt multilayers
Authors: Van Drent W. P. , Suzuki T. , Meng Q. , Lodder J. C. , Popma Th. J. A.
Published: July 2002

CoxNi1−x/Pt multilayers with x=0.4 and 0.5 were sputtered onto Si substrates. Magnetic and spectroscopic magneto-optic measurements (1.4–5.2 eV) reveal a Kerr rotation up to −0.48° at about 4.5 eV for a sample with rectangular hysteresis loop, about 170 kA/m coercivity, and a Curie temperature of about 300 °C. Pt layers could be made extremely thin (2.9 Å) without loss of perpendicular anisotropy and rectangular hysteresis loop. Simulations show that the Kerr rotation peak shifts back from 4.5 to 3.9 eV with increasing number of bilayers.