合成反铁磁Ni81Fe19/Co90Fe10/Ru/Co90Fe10/Ni81Fe19的研究

采用直流磁控溅射方法制备了一系列的合成反铁磁及以其为自由层的自旋阀.研究发现,在Ni81,Fe19与Ru层之间插入适当厚度的Co90Fe10层后,可有效地提高合成反铁磁两磁性层间的反铁磁耦合强度,得到具有饱和场日.更高、饱和磁化强度M.更低、热稳定性更好的合成反铁磁.另外,以这种合成反铁磁作自旋阀的自由层时,可有效提高自旋阀的稳定性.     

Magnetism of Fe,Co, and Ni nanowires encapsulated in carbon nanotubes

We have investigated the electronic and magnetic properties of Fe, Co, and Ni nanowires encapsulated in carbon nanotubes (CNTs) using spin polarized ab initio calculation. The incorporated systems with hollow region between the nanowire and the C shell have the enhanced magnetic moments compared to the ferromagnetic nanowires tightly wrapped by CNTs. The Co nanowire encapsulated in CNTs is a strong ferromagnet and has high spin polarization regardless of the distance between the nanowire and the C shell. The results show that the Co-filled CNTs are useful for spin polarized transport nanodevice.     

Magnetic Properties of Fe/Ni and Fe/Co Multilayer Thin Films

In this work, the magnetic and transport properties of Fe/SiO₂/Ni and Fe/SiO₂/Co multilayers grown on Si/SiO₂ substrates have been studied. The samples have been prepared by two-stage deposition process. In the first stage, Fe layer and SiO₂ interlayer of both samples are grown by ion beam deposition technique at room temperature. Then the samples are taken out to ambient atmosphere and loaded into a pulse laser deposition (PLD) chamber. Prior to the deposition of top layer, the samples are cleaned by annealing at 150 °C. In the second stage, Ni (or Co) layer is prepared by PLD technique at room temperature. The thickness of deposited layers has been measured by Rutherford back scattering (RBS). Magnetic properties of ferromagnetic bilayers have been investigated by room-temperature ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. Standard four-point magneto-transport measurements at various temperatures have been performed. Two-step switching in the in-plane hysteresis loops of Fe/SiO₂/Ni and Fe/SiO₂/Co samples is observed. A crossing in the middle of hysteresis loops of both samples points to a weak antiferromagnetic interaction between the magnetic layers of the stacks. Saturation magnetization values have been obtained from the VSM measurements of samples with DC magnetic field perpendicular to the films surface. Magneto-transport measurements have shown the predominant contribution of anisotropic magnetic resistance both at room and low temperatures. FMR studies of Fe/SiO₂/Ni and Fe/SiO₂/Co samples have revealed additional non-uniform (surface and bulk SWR) modes, which behavior has been explained in the framework of the surface inhomogeneity model. An origin of the antiferromagnetic interaction has been discussed.     

The effect of Fe and Ni catalysts on the growth of multiwalled carbon nanotubes using chemical vapor deposition

The effect of Fe and Ni catalysts on the synthesis of carbon nanotubes (CNTs) using atmospheric pressure chemical vapor deposition (APCVD) was investigated. Field emission scanning electron microscopy (FESEM) analysis suggests that the samples grow through a tip growth mechanism. High-resolution transmission electron microscopy (HRTEM) measurements show multiwalled carbon nanotubes (MWCNTs) with bamboo structure for Ni catalyst while iron filled straight tubes were obtained with the Fe catalyst. The X-ray diffraction (XRD) pattern indicates that nanotubes are graphitic in nature and there is no trace of carbide phases in both the cases. Low frequency Raman analysis of the bamboo-like and filled CNTs confirms the presence of radial breathing modes (RBM). The degree of graphitization of CNTs synthesized from Fe catalyst is higher than that from Ni catalyst as demonstrated by the high frequency Raman analysis. Simple models for the growth of bamboo-like and tubular catalyst filled nanotubes are proposed.     

Electron spectroscopic studies of the oxidation of Fe/Ni alloys

The oxidation of polycrystalline samples of Fe/Ni alloys at 500° C has been studied by means of Auger electron spectroscopy (AES) and soft X-ray appearance potential spectroscopy (APS). Whereas the clean surfaces are always enriched with nickel, oxidation of samples with less than about 70% Ni causes a complete segregation of iron oxide (Fe₃O₄). Alloys containing more than 70% Ni are covered after oxidation with a mixed oxide layer which is identified (in accordance with general experience) with a spinel structure. In the latter case the “chemical shift” of the binding energies of the Ni 2p-electrons decreases from 1.5 to 0.7 eV with decreasing Ni content whereas the corresponding quantities of Fe remain essentially constant over the whole range of concentrations.     

Spin-Seebeck effects in Ni81Fe19/Pt films

The spin-Seebeck effect (SSE) converts a heat current into a spin current, a flow of spin angular momentum, and spin voltage, the driving force for nonequilibrium spin currents, in a ferromagnetic metal. In this study, the SSE in a ferromagnetic Ni₈₁Fe₁₉ film has been investigated by means of the inverse spin-Hall effect (ISHE) in a Pt film at room temperature. The experimental results measured in the Ni₈₁Fe₁₉/Pt system show that the sign of the thermally induced spin voltage is reversed between the higher- and lower-temperature ends of the Ni₈₁Fe₁₉ film. The ISHE in the Pt film allows us to detect the SSE signal with high sensitivity and to separate it from extrinsic thermoelectric effects.     

Studies on the valence electronic structure of Fe and Ni in Fe x Ni1−x alloys

K _β-to-K _α X-ray intensity ratios of Fe and Ni in pure metals and in Fe _x Ni_1−x alloys (x=0.20, 0.50, 0.58) exhibiting similar crystalline structure have been measured following excitation by 59.54 keV γ-rays from a ²⁴¹Am point source, to understand as to why the properties of permalloy Fe_0.2Ni_0.8 is distinct from other alloy compositions. It is observed that the valence electronic structure of Fe_0.2Ni_0.8 alloy is totally different from other alloys which may be attributed to its special magnetic properties.     

Theoretical study of the environmental effects on the hyperfine fields of Ni and Fe in Ni0.75Fe0.25

The dependence of the hyperfine fields B_hf of Ni and Fe in Ni_0.75Fe_0.25 on the surrounding atomic configuration has been studied by performing charge selfconsistent 6 shell-cluster Korringa-Kohn-Rostoker Coherent Potential Approximation (KKR-CPA) bandstructure calculations. By replacing the CPA- scatterers in the various shells around the central atom by Ni - or Fe-atoms, respectively, it could be shown that the hyperfine fields vary linearly with the number of Fe-atoms within a given shell and that the changes of B_hf due to simultaneous changes of the atomic configurations of different shells are additive. The changes of the hyperfine fields upon ordering of Ni_0.75Fe_0.25 solid solution as deduced from our calculations agrees reasonably well with experiments.     

Direct-current induced dynamics in Co90 Fe10/Ni80 Fe20 point contacts

We have directly measured coherent high-frequency magnetization dynamics in ferromagnetic films induced by a spin-polarized dc current. The precession frequency can be tuned over a range of several gigahertz by varying the applied current. The frequencies of excitation also vary with applied field, resulting in a microwave oscillator that can be tuned from below 5 to above 40 GHz. This novel method of inducing high-frequency dynamics yields oscillations having quality factors from 200 to 800. We compare our results with those from single-domain simulations of current-induced dynamics.     

Preparation of single-walled nanotubes with the help of a Ni/Cr-based catalyst

The high-yield synthesis of single-walled carbon nanotubes (SWNTs) is carried out in an electric-arc discharge using the Ni-Cr alloy as a catalyst. A new method of introducing the catalyst into the plasma hot region is used in the synthesis. In this method, the anode with a sandwich structure consists of two longitudinal graphite rods of a rectangular cross section, between which the Ni-Cr alloy in the form of a foil having a thickness approximately equal to 0.2 mm is placed. The obtained samples are investigated using transmission electron microscopy (TEM), Raman spectroscopy, and thermogravimetry. According to the results of TEM observations, SWNTs are tied into bundles with a length of several micrometers and a diameter of about 10 nm. The Raman spectra indicate that the diameter distribution of SWNTs lies between 1.2 and 1.5 nm with a peak at approximately 1.24 nm. The SWNT content in the obtained samples is approximately 20%. Heat treatment at various temperatures with a dosed air supply leads to a noticeable mass loss of the sample and to a change in its composition. For example, thus heating to 600 K causes a mass loss of about 40%, leading to an increase in the content of SWNTs up to 35% without their noticeable destruction. Further heating above 600 K leads to a virtually complete thermal decomposition of SWNTs.     

Study of interdiffusion of Ni/Fe layers by Auger Sputter profiling

Bulk and grain boundary diffusion of Fe into Ni films has been studied under UHV in the temperature range of ?150 to 500°C using AES and sputter profiling methods. The concentration profiles at the interface are corrected for the various broadening and damage effects inherent in ion bombardment. Grain boundary diffusion coefficients are derived on the basis of the Whipple model. The measured activation energies are 46 $\text{kcal}\text{mole}$ for bulk diffusion and 34 $\text{kcal}\text{mole}$ for grain boundary diffusion. An additional migration phenomenon not previously resolved is observed for very thin films annealed at relatively low temperatures (150–250°C). A possible mechanism involved in this initial “interface healing” is discussed.     

Perpendicular interlayer coupling in Ni80Fe20/NiO/Co trilayers

An in-plane perpendicular magnetic coupling between Ni80Fe20 and Co has been found in NiFe/NiO/Co trilayers for a NiO thickness ranging from 4 to 25 nm by magneto-optical Kerr effect and x-ray magnetic circular dichroism measurements. In the easy magnetization direction of the Co layer, the Co coercive field H(C) increases when the thickness of the NiO layer t(NiO) increases. Because of the coupling, H(C) is always larger than for NiO/Co bilayers with the same thicknesses. The saturation field of the NiFe layer H(S) decreases when t(NiO) increases, indicating a weakening of the coupling. Numerical simulations show that the presence of interface roughness combined with a small value of the NiO anisotropy can explain the observed 90 degrees coupling.     

A study of the behavior of Fe and Ni on MoS2

A correlation of AES and work function measurements of Fe and Ni on MoS₂ indicated that, near room temperature of MoS₂ and low coverage, Fe and Ni formed islands. With increasing coverage the Fe-islands were developed to 3-D small particles, while Ni-islands needed higher temperature for this development. The average size and density of the particles depended on the substrate temperature. Beyond a certain size the Fe-particles began to act catalytically on an interaction of the subsequently deposited Fe with S atoms of the top layer of MoS₂. The Ni particles remained clean up to 800 K and did not interact with S.     

Study of the magnetism and the magnetic anisotropy of Fe layers in Ni/Fe/Ni(1 1 1) by Mössbauer spectroscopy

The hyperfine field and the magnetic anisotropy of a Fe layer as a function of thickness have been investigated in several Ni/⁵⁷Fe_x/Ni(1 1 1) trilayers with relatively thick Ni layers by Mössbauer spectroscopy. For Fe layers with thickness below 16 Å, the Mössbauer spectra show always the presence of two ferromagnetic phases with high-spin state. In the range between 6 and 8 Å, also a ferromagnetic phase with low-spin state and a paramagnetic phase have been found. The evolution of the mean hyperfine field of the ⁵⁷Fe nuclei is used to study the Fe growth. A structural FCCBCC phase transition is found to begin with an iron thickness of 8 Å. The easy direction of the magnetization is found out-of-plane for Fe interlayer with FCC structure, and perfectly in plane for Fe interlayer with BCC structure.     

Structural,electronic and magnetic properties of hcp Fe,Co and Ni nanowires encapsulated in zigzag carbon nanotubes

The structural, electronic and magnetic properties of hcp transition metal (TM = Fe, Co or Ni) nanowires TM₄ encapsulated inside zigzag nanotubes C(m, 0) (m = 7, 8, 9, 10, 11 or 12), along with TM _n (n = 4, 10 or 13) encapsulated inside C(12, 0), have been systematically investigated using the first-principle calculations. The results show that the TM nanowires can be inserted inside a variety of zigzag carbon nanotubes (CNTs) exothermically, except from the systems TM₄@(7, 0) and TM₁₃@(12, 0) which are endothermic. The charge is transferred from TM nanowires to CNTs, and the transferred charge increases with decreasing CNT diameter or increasing nanowire thickness. The magnetic moments of hybrid systems are smaller than those of the freestanding TM nanowires, especially for the atoms on the outermost shell of the nanowires. The magnetic moment per TM atom of TM/CNT system increases with increasing CNT diameter or decreasing nanowire thickness. Both the density of states and spin charge density analysis show that the spin polarization and the magnetic moments of all hybrid systems mainly originate from the TM nanowires, implying these systems can be applied in magnetic data storage devices.     

Atom-probe study of hydrogen chemisorption on Fe and Ni

Iron and nickel, which are chemisorbing hydrogen, were mass-analyzed by the atom-probe mass spectrometer. Specimen temperature was varied from 20 to 200 K while maintaining the hydrogen pressure at 4 × 10^?6 Pa. The ratio of the number of hydrogen ions to that of Ni ions decreased sharply at the critical temperature which is low for the closely packed Fe(011) and Ni(111) planes, ~60 K, and high for other planes, ~100 K. The occurrence of hydrides was noticed only in the narrow temperature range around the critical temperature at which the largest number of hydride ions were detected. The observed results were explained as the result of the transition of the adsorption site of hydrogen atoms from the top of surface metal atoms to the sites between or under the surface atoms.     

Adsorption of nonpolar benzene derivatives on single-walled carbon nanotubes

The adsorption of benzene, toluene, and chlorobenzene on single-walled carbon nanotubes (SWCNTs) with and without acid oxidation was conducted to investigate the influences of derivative groups on benzene rings and functional groups from SWCNTs on adsorption by SWCNTs. The SWCNTs of high purity were chosen and moderate acid oxidation was performed so that the surface physical properties remained unchanged after acid oxidation and the influences of acid oxidation on adsorption were only contributed from the modification of the surface chemistry of SWCNTs. The oxygen-containing surface groups introduced by acid oxidation obstructed the interactions between functional groups of nonpolar benzene derivatives and C-rings of SWCNTs significantly. The dispersive interaction between the partially positive H⁺ of the methyl group and the oxygen-containing surface groups slightly increased the adsorption of toluene on oxidized SWCNTs at high solution pH. The thermodynamic of adsorption was also studied at different temperatures.