bluuebunny/crossref_metadata_2025 huggingface dataset:
Irreversible alterations of ferroelectric domain structure in paramagnetic rare earth molybdates induced by a magnetic field
Authors: Ponomarev B. K. , Ivanov S. A. , Red’kin B. S. , Kurlov V. N.
Published: July 2002

The effect of a magnetic field on the ferroelectric domain structure of single crystal samples of the metastable orthorhombic ferroelectric ferroelastic β′-phase of Sm2(MoO4)3, Gd2(MoO4)3, Tb2(MoO4)3, and TbGd(MoO4)3 was investigated at 78 K in pulsed magnetic fields up to 100 kOe. It was shown that a magnetic field of certain orientation may change a ferroelectric domain structure in Tb2(MoO4)3 and TbGd(MoO4)3 that was accompanied by large changes of the electric polarization of the sample under investigation. The effect is explained by large magnetostrictive deformations. The prediction is made that there must be a positive longitudinal magnetostriction in a magnetic field applied along the [100] crystallographic axis and a negative transverse magnetostriction in the (001) plane in a magnetic field applied along the [100] crystallographic axis in Tb2(MoO4)3 and in TbGd(MoO4)3 β′-phases at 78 K and at a magnetic field strength of about 100 kOe.
Photoluminescence studies of sidewall properties of dry-etched InGaAs/InP quantum wires
Authors: Gu S. Q. , Liu X. , Covington M. , Reuter E. , Chang H. , Panepucci R. , Adesida I. , Bishop S. G. , Caneau C. , Bhat R.
Published: July 2002

InGaAs/InP quantum wires with widths ranging from 200 to 40 nm have been fabricated using high-resolution electron-beam lithography and CH4/H2 reactive-ion etching. The excitation intensity dependence of the photoluminescence (PL) energies and line shapes for relatively wide wires (∼100 nm) exhibits the effects of band filling in k space and band-gap renormalization due to many-body effects in dense electron-hole plasmas (EHP). In the narrowest wires studied (∼40 nm), the effects of sidewall surface recombination limit the attainable EHP density. In addition, the results show a blue shift of PL energies when wire width decreases below 100 nm.
Remote oxygen-containing hydrogen plasma treatment of porous silicon
Authors: Grüning U. , Gujrathi S. C. , Poulin S. , Diawara Y. , Yelon A.
Published: July 2002

The effects of treatment with remote oxygen-containing hydrogen plasma on electrochemically etched porous silicon have been studied. X-ray photoelectron spectroscopy, infrared, and elastic recoil detection measurements showed the nonuniform incorporation of oxygen in the porous silicon layer. The amount of nitrogen increased while the carbon concentration dropped in the oxidized layer. The photoluminiscence (PL) intensity of plasma-treated films increased by up to a factor of approximately 70 compared to as-prepared samples, while the peak position was red shifted. The PL enhancement seems to be correlated with an O/Si ratio near 1.5. After treatment, bright PL was observed from a partially oxidized layer, covered by a SiO2 layer which could be several nm thick.
Collisional versus electronic sputtering of SiO2
Authors: Jacobsson Harald , Holmén G.
Published: July 2002

The SiO2 sputtering yield was determined for 170–300 keV He+ ion bombardment. The low sputtering efficiency and blistering at high ion doses make high-energy He+ ion sputtering yields difficult to determine, but by modifying a measurement method previously used for heavy ions, the sputtering yield could be quite accurately determined after sputtering only 20 Å of SiO2. The sputtering yield was found not to be proportional to the energy deposited by the ion in elastic collisions at the surface of SiO2. Comparison with SiO2 sputtering yields found in literature shows that the sputtering yield increases with increasing energy deposited in electronic excitations for similar energy deposited in elastic collisions, indicating that electronic effects probably have to be included in the description of the sputtering process. Since the electronic effects do not seem to be independent of the sputtering by elastic collisions, it is suggested that SiO2 sputtering be controlled by a mixed collisional-electronic mechanism. Sputtering yield measurements were also performed for varying angles of ion incidence and, here also, good agreement could be achieved with predictions based on a mixed sputtering mechanism.
Oxygen incorporation and oxygen-induced defect formation in thin Si and Si1−xGex layers on silicon grown by chemical vapor deposition at atmospheric pressure
Authors: Krüger D. , Morgenstern Th. , Kurps R. , Bugiel E. , Quick Ch. , Kühne H.
Published: July 2002

The incorporation of oxygen into thin epitaxial Si and heteroepitaxial Si1−xGex layers deposited, applying a conventional atmospheric pressure process, from silane, germane, hydrogen chloride, and hydrogen gas mixtures in a temperature range from 1070 to 720 °C is analyzed. The role of oxygen for defect formation has been shown by means of a correlation between high resolution defect analysis using transmission electron microscopy and quantitative oxygen depth profiling using Auger electron spectroscopy and secondary ion mass spectrometry. In the low-temperature region traces of residual H2O vapor lead to oxygen precipitation. These precipitates are the origin of extended lattice defects such as stacking faults and microtwins and can result in highly defective films with polycrystalline inclusions and increased surface roughness. It was found that, in order to prevent the observed defects, it is necessary to keep the oxygen concentration below 3×1019 cm−3. However, by carefully controlling the experimental parameters it is also possible to realize nearly defect-free structures with high oxygen concentrations up to 1020 cm−3.
Limitations in the application of hook spectroscopy for density measurement in high-density-gradient media
Authors: Zerkle David K. , Sappey Andrew D.
Published: July 2002

Hook spectroscopy has been successfully implemented to measure ground-state Cu atom density in laser-ablated copper vapor plumes in a background gas. In 1–25 Torr of background gas, the technique has been used to measure Cu atom densities up to 4×1015 cm−3. With higher background gas pressures, increasing distortion of the hook spectra is observed until they are rendered nearly uninterpretable at background gas pressures as low as 100 Torr. This effect is attributed to wavelength-dependent beam deflection caused by large gradients in copper atom density present at high background gas pressures.
Interaction of α-radiation induced defects with Pd-related deep levels in silicon
Authors: Gill Asghar A. , Zafar N. , Zafar Iqbal M. , Baber N.
Published: July 2002

Results of a detailed deep level transient spectroscopy study of Pd-doped p+n Si diodes irradiated with 5.48 MeV α particles are presented, which also include investigations of isochronal annealing behavior of the deep level spectra up to a temperature of 350–400 °C. An extended comparison with results obtained on reference samples as well as with previously published results of deep level studies on Pd-doped samples (unirradiated) and on undoped α-irradiated samples provides valuable information, since the same starting material is used in all these studies. It is observed that, in general, the Pd-related deep levels increase in concentration upon irradiation at the expense of α-radiation-induced levels. In particular, the A center sharply decreases in concentration upon post-irradiation annealing with a corresponding increase in the concentration of the dominant Pd-related level. The results also show that, contrary to the previously held belief, two well-known Pd-related levels at Ec−0.37 eV and Ec−0.59 eV are not states of the same defect and an off-center/on-center substitutional Pd-vacancy model for the defects corresponding to Ec−0.18 eV and Ec−0.22 eV Pd-related levels cannot explain new data presented here. In the reference (undoped, heat treated) samples, a few levels are observed after α-irradiation or post-irradiation annealing which were not detected in the untreated diodes, pointing to their relationship to quenched-in defects. A new annealed-in hole level H(α-Pd) at Ev+0.27 eV is observed which seems to be a complex of Pd with some α-irradiation-induced defect.
Observation of Cr3+ electron paramagnetic resonance center in GaAs co-doped with Cr and In
Authors: Park Young Ju , Yeom Tae Ho , Min Suk-Ki , Park Il-Woo , Choh Sung Ho
Published: July 2002

We have observed a well-defined Cr3+(3d3) electron paramagnetic resonance signal at 4 K in vertical gradient freeze semi-insulating GaAs single crystal through a co-doping with Cr and In. On the basis of analyzing the rotation pattern of the Cr3+ center, the z-axis of the D-tensor for Cr3+ center in the principal axis system was found to be parallel to the [11̄0] direction in the crystal. The g tensor and zero field splitting parameters are determined with monoclinic spin Hamiltonian. The local site symmetry of the Cr3+ center in GaAs:Cr, In is different from that reported in GaAs:Cr. The lowering of the substitutional Cr3+ at Ga site in symmetry from cubic (Td) to monoclinic field (C2) seems to be attributed to the change of the local field due to an In atom existed in the 2nd nearest neighbor.
Effects of impurity concentration on dielectric loss in Zn-doped InP at microwave X-band frequencies
Authors: Ding X. Z. , Taijing Lu , Ong C. K. , Tan B. T. G.
Published: July 2002

Microwave dielectric measurements have been performed on various Zn-doped InP crystals using a vector network analyzer. There are two kinds of dielectric response in Zn-doped InP crystals depending on their Zn concentration. The general dielectric response in InP in the microwave frequency region is dielectric relaxation, which is related to the dipolar species formed from the ionized substitutional ZnIn−. The other dielectric response of InP crystals doped with a higher Zn concentration is dielectric loss. The crystal doped with Zn to a concentration of 2.14×1018 cm−3 shows a strong dielectric loss at 11 GHz, but no dielectric loss peaks are found in crystals doped with a lower Zn concentration of 4.36×1017 cm−3. The dipolar species, which gives rise to the dielectric loss in Zn-doped InP crystals, is believed to be a result of vacancy complex defects of neutral substitutional ZnIn and two P vacancies.
Structural properties of CdMgTe/CdTe superlattices
Authors: Gerthsen D. , Meertens D. , Heinke H. , Waag A. , Litz T. , Landwehr G.
Published: July 2002

CdMgTe/CdTe superlattices with a high degree of structural perfection were grown on CdTe and Cd0.975Zn0.025Te substrates by molecular-beam epitaxy. The structural properties of the superlattices, i.e., the morphology of the layers and the state of strain relaxation, were examined by transmission electron microscopy and x-ray diffractometry. High-resolution transmission electron microscopy (HRTEM) reveals the structural quality of the superlattices on an atomic scale. The width of the chemical transition between the CdTe and CdMgTe layers was determined by HRTEM using chemically sensitive imaging conditions. Two different mechanisms of misfit dislocation generation could be observed in situ in the electron microscope studying a cross-section specimen of a superlattice which was originally fully strained.
Optimized poling of nonlinear optical polymers based on dipole-orientation and dipole-relaxation studies
Authors: Ren W. , Bauer S. , Yilmaz Ş. , Wirges W. , Gerhard-Multhaupt R.
Published: July 2002

Nonlinear optical polymers contain molecular dipoles with very large hyperpolarizabilities in a glassy polymer matrix. Two typical examples—a guest-host system with dispersed polar dye molecules and a side-chain material with chemically attached molecular dipoles—were investigated by means of poling experiments, dielectric spectroscopy, thermally stimulated depolarization, and electro-optical thermal analysis. The dielectric behavior of both polymers can be described by the phenomenological Havriliak–Negami equation, and the existence of master curves for both materials demonstrates the validity of the time-temperature superposition principle above the respective glass transitions. Temperature-dependent mean relaxation times and relaxation-time distributions calculated from the dielectric data allow for an optimization of poling times. The dielectric relaxation strengths obtained from poling current and field, from dielectric measurements, and from thermally stimulated depolarization are in very good agreement and thus represent a useful measure of the polarization in poled polymers. From the temperature dependence of the polarization, optimal poling temperatures may be derived. Electro-optical thermal analysis yields the same temperature-stability curves as thermally stimulated depolarization and is therefore a valuable tool for investigating the stability of poled polymers, especially since it is not sensitive to charge effects. Optimal poling fields and currents must be selected as a compromise between high dipole mobilities (short relaxation times) and low bulk conductivities.
The magnetic transformation of iron hydroxides under the action of micro-organism (Azotobacter vinelandii) (abstract)
Authors: Verkhovtseva Nadezda V. , Shpilkina Irina V. , Babanin Vyacheslav F.
Published: July 2002

To analyze the action of Azotobacter vinellandii intact cells on synthetic high-dispersed iron hydroxides, Mössbauer spectroscopy and magnetic measurements were used for different conditions: for the nitrogen-free medium and for the NH-containing medium. The preparations of Azotobacter vinelandii obtained on the nitrogren-free medium with decreased acidity is characterized by relatively high contents of paramagnetic combinations (χpar=(6.6...31.4)×10−6 sm3/g, Isat=(0.12...0.73)×10−3 at 12 day cultivations, χpar=(8.1...23.2)×10−6 sm3/g, Isat =(0.07−0.93)×10−3 at 35 day cultivation). The quantity of magnetic ordered combinations in these samples is greater than 1.5–7 times that in bacteria at pH=7. The paramagnetic attributes of samples in the medium with NH is decreased more in 12 day culture (up to 50% compare to nitrogen-free variation) then in 35 day culture (up to 20%). The presence of doublets in Mössbauer spectra at different conditions (room temperature in damp, air-dry states, and frozen at the 100 K state) and strong differences in probability of sorption of Mössbauer radiation of damp preparations allow us to conclude that magnetic ordered combinations in the type of crystallines with particle size <10 nm are present in the composition of preparations. It is found from magnetic measurements that at pH<7 the paramagnetic component of susceptibility and magnetic saturation is high. At pH=7 the most intensive decrease of susceptibility and magnetic saturation is medium with nitrogen and at large aeration.
Amorphous to polycrystalline transformation of (BiDy)IG films (abstract)
Authors: Zhang H. Y. , Gu B. X. , Zhai H. R. , Miao Y. Z. , Lu M. , Tang T. , Huang H. B.
Published: July 2002

(BiDy)Fe5O12 films in the amorphous state were made by rf magnetron sputtering from a sintered Bi2DyFe5O12 ceramic target onto quartz substrates and confirmed by x-ray diffraction. The films were annealed at different temperatures ranging from 500 to 800 °C for 1 h in air. The crystallization temperature was about 650 °C, as measured by DTA method. Magnetization M versus temperature relation of well crystallized films showed both a compensation temperature Tcomp (80 K) and Curie temperature TC (563 K). The M-T curve from 1.5 to 600 K of amorphous film showed no definite TC or Tcomp; M measured in a field of 20 kOe decreased monotonously. The Faraday rotation (500–700 nm) of amorphous films was small. The M and Faraday rotation (633 nm) increased rapidly at the crystallization temperature. The Bi ion induced peak (550 nm) in the Faraday spectra got higher with the increasing annealing temperature and was up to 9.14 degree/μm. The Faraday spectra of films annealed at 650 °C for different times showed that the films were well crystallized after annealing for 10 min. The optical absorption of the films in the visible region varied little during the transformation from amorphous to polycrystalline.
Formation and magnetic properties of Y2(Fe0.8Ga0.2)17Cx compounds
Authors: Wang Fang-Wei , Kong Lin-Shu , Cao Lei , Hu Ming , Shen Bao-Gen , Zhao Jian-Gao
Published: July 2002

The alloys Y2(Fe0.8Ga0.2)17Cx (x=0–2.5) with the rhombohedral Th2Zn17 phase were prepared by arc melting. The lattice parameters and the unit-cell volumes derived from x-ray-diffraction measurements increase linearly with the carbon content x. The Curie temperature Tc increases at first with increasing carbon concentration x, shows a maximum value of 558 K at about x=1.0, then decreases with x. The saturation magnetization at 1.5 K and the Fe magnetic moment have a slight change with the interstitial carbon content. The dilution of Fe by nonmagnetic Ga decreases the saturation magnetization and the Fe magnetic moment.
Time evolution of magnetization in the FeRh system near antiferromagnetic-ferromagnetic transition temperature (abstract)
Authors: Yuasa Shinji , Otani Yoshichika , Miyajima Hideki
Published: July 2002

The body-centered cubic FeRh is known to exhibit a first-order phase transition from antiferromagnetic to ferromagnetic at about 400 K, accompanied by a volume expansion of about 1%. The time evolution of the magnetization M of ordered Fe0.5Rh0.5 and FeRh0.958Pt0.042 just below and above the transition temperature (T0) was measured as functions of temperature and magnetic field. Around T0, the magnetization increased logarithmically with time up to M*(=41 emu/g), the value of which indicates that the ferromagnetic grains occupy one third of the antiferromagnetic matrix. When M=M*, there appeared a jump in M. After that, M varies as M(t) =M∞[1−exp(−t/τ)], where τ is a relaxation time. It should be noted that the value of M* is independent of both temperatures and external magnetic fields. Microscopic observation and x-ray diffraction measurement showed that the ferromagnetic grains nucleated in the antiferromagnetic matrix began to grow with time just below T0. In this work, the time evolution of the phase transition was analyzed by considering the magnetic free energy, the elastic energy, and the strain energy at the surface of the grains. The above mentioned magnetic relaxation process is deeply related to the time evolution of the grain size.
Magnetization reversal dynamics in CoPt alloys and Co/Pt multilayers (abstract)
Authors: Valentin J. , Kleinefeld Th. , Weller D.
Published: July 2002

We have studied the dynamics of magnetization reversal processes in magneto-optic recording CoPt samples with perpendicular anisotropy. Thin films of subatomic layered Co25Pt75 alloys as well as Co/Pt multilayers with total thicknesses up to 300 Å were investigated. By means of polar Kerr microscopy we were able to obtain the magnetic domain pattern. High temporal resolution was achieved by using a rapid scanning CCD camera system. Depending on film parameters, we found wall motion dominated and nucleation dominated magnetic reversal processes. Detailed analysis of the domain topography shows evidence for a correlation of the fractal dimension of the domain shape to the macroscopic coercivity measured with a VSM and conventional Kerr hysteresis loops. In particular, we investigated the dynamics of magnetization reversal and observed magnetic aftereffect phenomena. The time constant of the relaxation of the magnetization is strongly affected by the film structure and by the film thickness. We discuss our results in terms of the theoretical relaxation model of Fatuzzo and recent computer simulations.
Prediction of pressure-induced changes in magnetic ordering of correlated-electron uranium systems (invited)
Authors: Sheng Q. G. , Cooper Bernard R.
Published: July 2002

Experimentally, hydrostatic pressure experiments provide a very sensitive way to probe the development of magnetic ordering in correlated-electron systems. We have now developed and applied theory allowing us to understand and quantitatively predict the variation of ordering temperature with pressure in uranium-based correlated-electron materials on a wholly predictive calculated basis, i.e., without using any experimental data as input in the calculation. The theory physically captures the changes in the f spectral density distribution in space and time that are driven by pressure-induced increased band-f hybridization. As a test case, we have predicted behavior in good agreement with experiment for UTe where experimentally (Link et al.) Tc increases from 104 K to a maximum of 181 K at 7.5 GPa and then decreases to 156 K at 17.5 GPa. Our calculations: (1) using full-potential total-energy calculations match the experimental lattice parameter change with pressure within 2% and provide the decrease in average 5f-electron number; (2) extract information from the electronic structure calculation on the hybridization-induced changes and insert this into many-body theory to calculate the increase in two-ion coupling with pressure (from increased f spectral admixture into the bands) giving the initial increase in Tc; (3) calculate the decrease in ordering temperature (in terms of coupling) with decreased 5f number (localized spectral density) which gives the ultimate decrease in Tc.
Magnetic properties of the lattice Anderson model
Authors: Lin H. Q. , Chen H. , Callaway J.
Published: July 2002

We perform exact diagonalization studies of the one-dimensional lattice Anderson model for various clusters. The ground state energy and spin–spin correlation functions are calculated as functions of Hubbard U, hybridization V, and f level occupancies. For the symmetric case, we compare our results with weak and strong coupling perturbation theory.
Structural and magnetic ordering in the cerium hydride (abstract)
Authors: Arons R. R. , Cockcroft J. K. , Ressouche E.
Published: July 2002

Depending on the H concentration at the octahedral (O) sites, x(O), various tetragonal phases, due to H ordering, are observed in the Ce hydride at low temperatures. While our single crystal work showed that the tetragonal phase for H/Ce≊2.7, corresponding to x(O)≊3/4, is only stable in a region of x(O)=±0.05, the other one was believed to be caused by a single H ordering process for 0.15≤x(O)≤0.5 up to now. However, strong discrepancies in the structure were reported from different neutron diffraction studies. We have studied both the structural H ordering and the magnetic ordering by neutron diffraction work for 0.2≤x(O)≤0.5. It will be shown that the H ordering for x(O)=1/4 and 1/2 are completely different (Space groups: I4/mmm and I41/amd, respectively). Accordingly, for x(O)=1/2 the Ce atoms are crystallographically identical, while for x(O)=1/4 two different Ce atoms are present in the lattice. Also the magnetic order is completely different for the two phases. For x(O)=0.5, a commensurate magnetic phase appears below TN≊3.5 K. The structure is described by a canted ferromagnet with μF=1.3 μB/Ce atom along the c axis and μAF=1.1 μB/Ce atom within the plane normal to c. On the other hand, for 0.2≤x(O)≤0.25, an incommensurate AF phase with a long period, described by the wave vector τ=(0.12, 0.12, 0), is obtained at 1.4 K. Above 2.6 K, this phase is progressively replaced by the commensurate lines discussed before. For x(O)=0.265, the two phases coexist at 1.4 K, while for x(O)=0.28 only the commensurate phase is retained. Apparently, the long period phase is characteristic of the ideal H ordering around x(O)=1/4, whereas it is strongly affected by the H occupation of the new positions for higher concentrations.
Structure and giant magnetoresistance in sputtered and MBE grown Fe/Cr superlattices (invited) (abstract)
Authors: Schuller Ivan K. , Kelly David M. , Schad R. , Potter M. , Bruynseraede Y.
Published: July 2002

We have performed a comparative study of Fe/Cr superlattices prepared by sputtering and molecular beam epitaxy. Detailed structural and magnetic measurements show that structural parameters are important in controlling the magnitude of the magnetoresistance. X-ray diffraction results show that films grown by molecular beam epitaxy (MBE) and sputtering are structurally different, and corresponding to these differences are large changes in the values of the magnetoresistance and its dependence on Cr thickness. The results indicate a strong dependence of the magnetoresistance on the structure of the superlattices.
Secondary ion bombardment effects on the magnetic properties and microstructures of ion-beam-deposited TbFe thin films
Authors: Yatsuya S. , Hintz M. B.
Published: July 2002

TbFe films were produced by ion beam deposition both with and without 300 eV secondary ion bombardment (SIB). The magnetic properties and microstructures of the films were investigated. SIB was found to increase the perpendicular anisotropy coefficient, but decrease the magnetization-coercivity product (MHc=energy product) of the films. The microstructure of the films changed dramatically; the film made in the absence of SIB is amorphous while the film made with SIB during deposition consists of nanocrystals in an amorphous matrix.
Uniaxial anisotropy of double-layered garnet films and magneto-optical recording characteristics
Authors: Nakagawa Katsuji , Kurashina Seiji , Itoh Akiyoshi
Published: July 2002

For reducing the critical diameter of the written domain in magnetic garnet films for magneto-optical (MO) recording, double-layered garnet films were investigated. The coercivity of the double-layered garnet film was increased, when the upper layer was crystallized. This increase of the coercivity is caused by the increase of the uniaxial anisotropy energy. The increase of the uniaxial anisotropy energy also leads to a decrease in the noise level of the MO readout signal. As a consequence, the critical diameter of written domains was reduced to 0.2 μm. This critical diameter is smaller than the reading out beam spot for the case with a green laser and a high numerical aperture objective lens. The garnet films are sufficient for high density next generation MO recording.
Giant magnetoresistance of dilute Cu(Co) granular films
Authors: Gambino R. J. , McGuire T. R. , Harper J. M. E. , Cabral Cyril
Published: July 2002

Granular films of Cu (Co) with Co concentrations from 5.4% to 12% in the as-deposited form had magnetization showing about 60% of the Co as ferromagnetic particles and exhibited giant magnetoresistance (MR) of about 20% at 4.2 K. Annealing at ∼310 °C increased the MR to 40% and also increased the saturation magnetization indicating additional Co precipitation. At higher annealing temperatures MR decreased to 1%. The Zhang theoretical model involving polarized conduction electron scattering at particle interfaces seems to give a reasonable description of our results including the quadratic behavior of MR vs [M(H)]2 and MR vs [Ms(T)]2.
Magnetoresistance in a granular Fe–Mg system
Authors: Pettit Kevin , Kita E. , Araga K. , Tasaki A. , Salamon M. B.
Published: July 2002

We have prepared granular films of Fe (36 at. %)–Mg (64 at. %) and measured the magnetization, resistance, and thermopower as a function of the applied magnetic field at temperatures between 300 and 6 K. The as-prepared sample has a magnetoresistance ratio (ΔR/Rmax) of −6.3% in a field of 5.5 T at 6 K. The samples were annealed at temperatures between 150 °C and 228 °C for one hour. The MR does not vary as the square of the magnetization, as predicted by theory. We attribute this to a particle size distribution effect.
High spatial resolution spin-polarized scanning electron microscopy (abstract)
Authors: Matsuyama H. , Koike K. , Tomiyama F. , Aoi H. , Shiroishi Y. , Ishikawa A.
Published: July 2002

Spin-polarized scanning electron microscopy (spin SEM) to observe magnetic domains has the potential for spatial resolution as high as 10 nm. The best resolution from spin SEMs attained so far, however, is around 50 nm. In this talk, we report a newly developed spin SEM with 20 nm resolution. The major factor determining the resolution of a spin SEM is probe diameter. A smaller probe diameter, however, results in lower probe current. This is a serious issue in spin SEM because the efficiency of a spin detector is very low. Thus, to get high resolution with a reasonable signal/noise (S/N) image, higher efficiency in the spin detector and/or higher probe current are needed in addition to get a smaller probe diameter. The higher probe current has been attained by using a thermal assisted field emission cathode in an electron gun. The higher efficiency in the spin detector was achieved by increasing the acceptance angle of electron detectors inside the spin detector and optimizing both a secondary collector and transport optics. Obtaining a smaller probe diameter required a low aberration objective lens and a short working distance. Those were achieved by computer calculation so that there was no interference between the objective lens and the secondary collector. This achieved both a short working distance of 20 mm and high secondary collection efficiency. As a result, magnetic images of 20 nm resolution with a 25 keV and 1 nA probe beam have been attained. We have applied this spin SEM to observing recorded bits of longitudinal thin film media at high recording densities.
Double compensation point media for direct overwrite
Authors: Hatwar T. K. , Genova D. J. , Victora R. H.
Published: July 2002

Two magneto-optic media configurations are discussed that are suitable for direct overwrite by light intensity modulation using only one conventional bias magnet. One configuration consists of two magnetic layers (memory and reference having perpendicular anisotropy) exchange coupled through an intermediate soft magnetic layer. The second media configuration consists of only a memory layer and an intermediate layer. This scheme produced a respectable direct overwrite performance of 48 dB using a 3.5 μm mark size, 30 kHz bandwidth, and 7 m/s disk velocity. The major advantages of this scheme are simpler disk structures relative to a previously proposed quadrilayer scheme, and a wide range for Curie temperatures, thicknesses, and coercivities of the individual layers.
Growth and subsequent relaxation of the anisotropic structure of amorphous Tb-Fe (abstract)
Authors: Hellman F. , Robson M. C. , Messer M. T.
Published: July 2002

The large, growth-induced magnetic anisotropy in amorphous rare earth-transition metal alloys such as Tb-Fe are shown to depend strongly on the deposition temperature and only weakly on deposition rate or deposition technique (e.g., sputtering versus electron beam co-evaporation). These dependencies can be well fit with a thermally activated form involving minimization of surface energy during the growth by a re-orienting of adatom configurations over potential energy barriers. In this model, the growing film lowers its surface energy by a partial alignment of local clusters, presumably such as to maximize the number of in-plane bonds, although chemical effects undoubtedly also play an important role. These effects are somewhat analogous to a surface reconstruction which becomes trapped into the growing film by low bulk diffusion rates. In particular, a two-level model with a flat distribution of energy barriers is here shown to provide an excellent fit to the observations. Such a model leads to a ln(t) dependence on deposition rate and an exponential dependence on deposition temperature. We have also studied the subsequent irreversible relaxation of the anisotropy upon annealing. This relaxation is strongly influenced by the original growth temperature. In particular, the higher the original growth temperature, the more resistant the film is to subsequent relaxation. This result has important technological implications. As is commonly observed, the relaxation is well fit by a two-level model, again with a flat distribution of energy barriers over a range of energies, producing a ln(t) dependence on annealing time and a thermally activated dependence on annealing temperature. In annealing, of course, the lower energy state is isotropic, unlike the surface-induced anisotropic state produced during growth. The influence of the growth temperature on this relaxation implies that the actual process of creating the anisotropic state during the growth has the consequence of eliminating free volume in the sample, thereby raising the energy barriers to subsequent relaxation.
Element specific magnetic microscopy with x rays (invited) (abstract)
Authors: Hermsmeier B. , Stöhr J. , Wu Y. , Samant M. , Harp G. , Koranda S. , Dunham D. , Tonner B. P.
Published: July 2002

We have used the magnetic circular x-ray dichroism (MCXD) technique to image magnetic domains with a spatial resolution of 1 μm. The experiments employed circularly polarized soft x rays near the edges of the three-dimensional (3d) transition metals Co (≂780 eV) and Ni (≂850 eV) and a photoemission microscope based on an electrostatically imaging lens system. Element specific contrast is obtained by tuning to a specific L edge resonance (‘‘white line’’) and imaging the number of secondary electrons created locally near the surface of the sample. The magnetic contrast arises from the fact that the white line intensity depends on the relative orientation of the photon spin and the local magnetization direction. The obtained contrast is remarkably large (≂20%) such that images can be recorded without background subtraction. Images taken at the L3 and L2 resonance energies give reversed contrast, as expected. Because of the long x-ray and secondary electron mean free paths, images can be recorded for buried magnetic layers which makes this new technique especially valuable for technological applications.
MO polar Kerr studies of Co rich molecular beam epitaxy grown Au/Co multilayers
Authors: Višňovský S. , Nývlt M. , Prosser V. , Atkinson R. , Hendren W. R. , Salter I. W. , Walker M. J.
Published: July 2002

Magneto-optical (MO) polar Kerr rotation and ellipticity (PKR and PKE, respectively) of Co rich Au(111)/Co(0001) multilayers grown by molecular beam epitaxy on GaAs(110) substrates are reported. The individual layer thicknesses, tAu and tCo were tAu/tCo=2.6/3.2, 1.0/1.5, and 0.65/0.8 nm. The samples displayed perpendicular magnetic anisotropy smaller than demagnetization energy. PKR and PKE were measured at photon energies between 1.5 and 5.2 eV. In the multilayer with tCo=1.5 nm, PKR at 3.1 eV reached a value of −0.34°, remarkably higher than that reported on Au rich multilayers. The spectra were compared with theory. A satisfactory agreement was obtained only for the sample with the largest bilayer thickness, tAu+tCo=5.8 nm. This indicates that when tAu and tCo are reduced to a few atomic monolayers the role of interfaces is increased. The effects of strain, defects, and atomic mixing at Au-Co interfaces become more important. Both the magnetic and optical properties are modified.
Magneto-optical properties and magnetization processes in superlattices
Authors: Atkinson Ron , Kubrakov Nikolai F. , Utochkin Sergey N. , Zvezdin Anatoley K.
Published: July 2002

Spin-reorientation processes in anisotropic magnetic superlattices (SLs) with antiferromagnetic interfacial coupling are theoretically analyzed. Results provide magnetic phase diagrams of the kind ‘‘external field–relative magnetic moment,’’ as well as equations for the lines of the phase transitions. The influence of possible inhomogeneous magnetization in SLs is considered phenomenologically for the normal-incidence polar Kerr magneto-optical effect assuming the Q parameter to be a function of distance along the normal to the interface. A simple approach to the solution of this problem is outlined.
Measurement of the Faraday effect of garnet film in alternating magnetic fields
Authors: Ouyang Jia , Zhang Ying , He Huahui
Published: July 2002

A method of measuring the peak value of the Faraday rotation ΦF (hereafter referred to as the dynamic Faraday rotation) induced by an alternating magnetic field is described. It requires accurate measurement of the fundamental and the second harmonics of the photodetector currents. The sample is a (BiPrGdYb)3(FeAl)5O12 magneto-optic garnet film prepared by liquid phase epitaxy. Using the presented method, we successfully measured the dynamic Faraday rotation in the garnet film at the near infrared wavelength 1.55 μm of the light beam. The result shows that Faraday rotation induced by an alternating magnetic field is proportional to ac current and decreases with increasing ac frequency. The method is simple, general, accurate, and can be used at any ac frequency. Experimental errors in our measurement are analyzed and an accuracy better than 2% can be obtained. Finally, some of the factors which may influence the measurement results are discussed.
Soft x-ray synchrotron radiation facilities for the study of magnetic materials (invited) (abstract)
Authors: Tonner Brian , O’Brien W. , Green M. A. , Höchst H. , Reininger R.
Published: July 2002

Synchrotron radiation sources are used for a variety of measurements of magnetic materials, including photoemission spectroscopy of band-structure (both with and without spin polarization), photoabsorption spectroscopy, and x-ray scattering. Recently, considerable attention has been paid to applications of circularly polarized monochromatic synchrotron radiation to spectroscopy and scattering of magnetic materials. Magnetic x-ray circular dichroism (MXCD), present in both x-ray absorption and x-ray photoemission spectra, can be used as a site- and element-specific probe of local magnetic moment. With suitable instrumentation, the MXCD effect can be used to image magnetic domain structure of surfaces and buried layers, with element (and potentially chemical-state) specificity. Examples of studies using MXCD with an existing bending-magnet light source will be shown, and used to introduce the performance of a new, undulator-based variable polarization beamline under construction at SRC.
ac susceptibility of PbxBi2−xSr2Ca2Cu3O10 high T
c superconductors sintered for different durations (abstract)
Authors: Mathur S. C. , Dube D. C. , Sinha Urvija , Raghavan P. K. N.
Published: July 2002

ac magnetic susceptibility measurements is one of the processes by which the superconducting properties of mixed oxide superconductors can be characterized. The study of the superconductors at an externally applied magnetic field predicts the existence of two superconducting transitions due to: (i) the one reflecting the intragrain properties and (ii) the other describing intergrain interactions which might be assumed as due to Josephson tunneling. In this abstract, we report our work on the ac susceptibility measurements in PbxBi2−xSr2Ca2Cu3O10 superconductors to study the real part of χ′ as a function of temperature, Pb concentration, and sintering time. The results have been compared with the x-ray diffraction (XRD) results. The material preparation and the characterization techniques used would be discussed. The XRD and the susceptibility measurements indicate that too high Pb concentrations and too low sintering time does not seem to improve the high Tc phase. The weak link coupling phase determined from the χ′-T dependence is strongly dependent on both the Pb concentration and the sintering time. The transition temperature for the low Tc phase decreases with sintering time and increases with the increasing Pb concentration. The results would be discussed in the light of the best superconducting properties achieved by variation of the material preparation.
Nonepitaxial sputter synthesis of aligned strontium hexaferrite, SrO⋅6(Fe2O3), films
Authors: Hegde H. , Samarasekara P. , Cadieu F. J.
Published: July 2002

Films of strontium hexaferrite were synthesized through rf diode sputter deposition, nonepitaxially onto polycrystalline Al2O3 substrates. So far such films for the analogous Ba phase have been synthesized mostly through epitaxial growth onto suitably chosen matching substrates. Crystallization of the strontium hexaferrite phase was achieved for deposition temperatures as low as 500 °C. For nonepitaxially synthesized 2–3-μm-thick polycrystalline films, a fairly high degree of variable and controlled c-axis orientation of the crystallites has been obtained. The x-ray-diffraction traces of films with a strong c-axis in-plane alignment showed predominantly (110)-type reflections. The film coercivities were increased by postdeposition annealing in O2. The remnant to saturation flux density ratio of such in-plane anisotropy films for in-plane measurements was 0.62, with an in-plane coercivity ≊2.3 kOe.
Magnetic properties of sintered Alnico 5 magnet via rapid solidification technology
Authors: Yang C. J. , Lee W. Y. , Choi S. D.
Published: July 2002

A process for making Alnico magnets (grade 5–8) via rapid solidification technology has been developed. The process includes extractive melt spinning of alloy powders, press forming, heat treatment under a magnetic field, and aging. The Alnico 5 magnets made by this process showed superior magnetic properties to those of cast magnet of the corresponding composition. The superior magnetic properties of the sintered magnets with Br=13.2 kG, Hc=680 Oe, and (BH)max=6.02 MGOe are due to the spinodal decomposition which is completely free of γ phase.
Coercivity in hard magnets based on Sm2Co17 (abstract)
Authors: Lectard Eric , Maury Claire , Allibert Colette H. , Rabenberg Lew
Published: July 2002

Despite the research devoted to new compounds, the Sm-Co-Cu-Fe-Zr alloys with the 2:17 type are, as yet, the only hard magnets to be used above 200 °C. However, these materials are not optimized because their coercivity mechanism is not clearly understood. Their typical microstructure consists of a network of 2:17 cells separated by 1:5 boundaries. Their coercivity is ascribed to the domain wall pinning by the 1:5 cell boundaries. The compositions of the 1:5, 2:17 phases during the cell microstructure genesis were recently evaluated for the alloy Sm(Co0.66Fe0.25Cu0.06Zr0.03)7.6. In the present work, the coercivity behavior of this alloy is analyzed from the comparison of the domain wall energies γ 1:5, γ 2:17. Sintered samples, heat-treated in various conditions, are characterized by transmission electron microscopy and hysteresis curves. γ 2:17 and γ 1:5 are calculated from measured and published data of magnetization and anisotropy. The different treatments generate similar cell microstructures but two ranges of coercivity (about 600 and 2200 K A m−1). In the two cases, γ 1:5 is larger than γ 2:17: the cell boundaries appear as repelling energy barriers rather than pinning sites. The existence of a disturbed anisotropy layer along the 1:5/2:17 interface is proposed to explain the higher coercivity state. The evaluation of Hc using the usual models is attempted.
Magnetic properties of cobalt clusters deposited on MgO substrates by molecular beam epitaxy (abstract)
Authors: Morel R. , Barthélémy A. , Charrière F. , Childress J. R. , Fert A. , Bellamy B. , Masson A.
Published: July 2002

We report on the preparation and characterization of clusters of Co deposited on MgO(001) and MgO(110) substrates. The preparation is by cell evaporation in ultrahigh vacuum. The size of the clusters is controlled by the substrate temperature and deposition rate or, after deposition, by annealing. This results in clusters with diameters ≊100–1500 Å and heights ≊30–200 Å. The characterization is by Rutherford backscattering, photoemission spectroscopy, and atomic force microscopy. The magnetic properties have been investigated by SQUID magnetometry. Depending on the size of the clusters, we have observed superparamagnetic or ferromagnetic behaviors. When the cluster diameter is much smaller than its height, the magnetization is much easier for in-plane fields, due to the demagnetization energy. In some of the samples, we have combined clusters of Co and nonmagnetic Ag, with sufficient density for metallic percolation, and we present their magnetoresistance properties.
Magnetic structure of NdCu2 (abstract)
Authors: Arons R. R. , Loewenhaupt M. , Gratz E.
Published: July 2002

RCu2 compounds (R=rare earth) with orthorhombic crystal structures exhibit a rich variety of magnetic structures at low temperatures, originating from a subtle interplay between RKKY-type exchange and crystal field anisotropy. Though most of the magnetic properties of NdCu2 are well understood,1 there are controversial proposals for the magnetic phases, as deduced from different experiments.1,2 Therefore we have investigated the magnetic structure of NdCu2 by neutron diffraction as a function of temperature between 1.4 and 8 K in the zero external field. The diffraction patterns of a powdered sample were obtained on the multidetector DN5 at the SILOE reactor of CEN-G. Only two different magnetic phases were observed, in agreement with Ref. 1. For temperatures between 5.2 K and TN=6.5 K, the magnetic structure can be described by a sinusoidal oscillating component along b with wave vector τ2=(0.62, 0.042, 0). Accordingly, the structure is incommensurate with the lattice. Below 4.0 K the structure remains an oscillating component along b. However, the wave vector becomes commensurate with the lattice and is given by τ1=(0.60, 0, 0). Additionally, a progressive squaring up from the appearance of the third harmonic 3τ in the range from 4.0 K down to 1.4 K is observed. At 1.4 K the amplitudes of the fundamental and the third harmonic are 2.4μB and 0.9μB, respectively. Around 4.4 K, the spectra seem to be determined by a superposition of the high- and low-temperature phases. The low-temperature structure of NdCu2 is comparable with that for TmCu2, observed earlier in Ref. 3. Though in that work a different interpretation was chosen, the wave vector of the fundamental oscillation can be described by (0.625, 0, 0), while the amplitudes for the fundamental and third harmonic are given by 8.0μB and 2.6μB, respectively.
Magnetic properties of metallic Co- and Fe-based granular alloys
Authors: Xiao Gang , Wang Jian-Qing
Published: July 2002

We have studied the magnetic properties of Co-Ag and Fe-Ag granular alloys made using vapor-quenching techniques and thermal annealing. Magnetic coercivity (Hc) and remanence can be controlled over a large range by varying annealing temperature and particle volume fraction. A large Hc on the order of 2 kG has been obtained in the Co-Ag system. We have investigated magnetic anisotropy, the effect of particle size, and coalescence in these nanostructured materials.
Paramagnetic/ferromagnetic transition of Co/Cu(001) films during growth (abstract)
Authors: Schumann F. O. , Buckley M. E. , Bland J. A. C.
Published: July 2002

We have investigated paramagnetism in epitaxial Co/Cu(001) films grown at room temperature in ultrahigh vacuum (UHV) using the transverse magneto-optical Kerr effect. These measurements are made possible by the very strong magnetic response of a two-dimensional (2D) paramagnet, and in this way we have extended our previous study of the thickness dependent coercivity close to the magnetic phase transition. The optical reflectivity was used to determine the thickness in reduced units d/dc to better than 10% accuracy where the critical thickness dc is determined by the onset of long-range order. In the paramagnetic region we observe a linear M-H curve with a sensitively thickness dependent saturation field Hs, which decreases to zero as the critical thickness dc is approached. At the critical point, the onset of long range order is manifested by a strongly thickness-dependent coercive field Hc, which is found to increase from zero with thickness as a power law.
Spatial modulation of the magnetic moment in Co/Pd superlattices observed by polarized neutron reflectivity
Authors: Borchers J. A. , Ankner J. F. , Majkrzak C. F. , Engel B. N. , Wiedmann M. H. , Van Leeuwen R. A. , Falco C. M.
Published: July 2002

We have measured the specular reflectivity of spin-polarized neutrons from two (111) superlattices, [Pd 80 Å‖Co 60 Å]×20/Pd 300 Å/Co 10 Å/GaAs and [Pd 53 Å‖Co 32 Å]×10/Pd 30 Å/Cu 354 Å/Si, grown by molecular-beam epitaxy. In a saturating field of 2300 Oe, we extract for the first superlattice a Co moment value of corresponding to 78% of the bulk Co moment. At temperatures ranging from 38 to 295 K, no excess magnetic moment from polarization of the Pd atoms is evident in the nominal Pd layer. Comparable reflectivity data from the second sample support these conclusions.