Claus M. Schneider

Synthesis and properties of filled carbon nanotubes

Abstract Single- and multi-walled carbon nanotubes are very interesting nanoscaled materials with many possible applications in nanoelectronics. Especially, nanotubes filled with ferromagnetic materials (Fe, Co, Ni) may have significant potential in data storage. Such structures may help to exceed the best available storage densities (>65 Gb/inch2) and show in the case of Fe-filled nanotubes higher coercivities compared to bulk Fe. In addition, metal-filled carbon nanotubes are promising nanowires with excellent oxidation protection. In this paper we describe the synthesis of Fe-, Ni- and Co-filled carbon nanotubes by using the chemical vapor deposition method. Varying the deposition conditions we have obtained filled nanotubes with relatively uniform core diameters and different thicknesses of the carbon walls. The core diameters vary between 15 and 30 nm and the thickness of the carbon shells between 2 and 60 nm. The length of the tubes amounts up to 30 μm. The filled carbon nanotubes are characterised by scanning and transmission electron microscopy and energy dispersive X-ray analysis. The magnetic behaviour of the aligned Fe-filled tubes is investigated using alternating gradient magnetometry measurements and electron holography. The hysteresis loops indicate a magnetic anisotropy. The coercivity depends on the direction of the applied magnetic field. The observed enhanced coercivities are significantly higher than in bulk Fe.

Magnetic properties of aligned Fe-filled carbon nanotubes

We report on the magnetic properties of Fe-filled multiwalled carbon nanotubes(MWNTs) grown by chemical vapor deposition(CVD) on Si substrates with ferrocene as precursor. The MWNTs are aligned perpendicularly to the substrate plane. X-ray diffraction analyses indicate the presence of both bcc and fcc iron with a relatively strong texture. Magnetometry measurements show a pronounced magnetic anisotropy with the easy axis perpendicular to the substrate plane and parallel to the axis of the aligned MWNTs, respectively. The low-temperature behavior suggests a negligible coupling between the two iron phases. We accessed the magnetic properties of individual Fe-filled MWNTs by electron holography using a transmission electron microscope(TEM).

Mössbauer transmission and back scattered conversion electron study of Fe nanowires encapsulated in multiwalled carbon nanotubes

Fe57 transmission Mossbauer spectroscopy (TMS) and back scattered conversion electron Mossbauer spectroscopy (CEMS) measurements were carried out on Fe-filled multiwalled carbon nanotubes (Fe-MWCNTs) grown by chemical vapor deposition with ferrocene as precursor. Samples of Fe-MWCNTs material deposited on the inner wall of the quartz tube reactor and samples of aligned Fe-MWCNTs grown perpendicularly to the oxidized Si substrate were characterised by the TMS method. The data show that Fe phases encapsulated within the carbon nanotubes comprise α-Fe, γ-Fe, and Fe3C in different percentage ratio depending on the sample preparation. These results are in a good accordance with the previously measured magnetic characteristics and with the structural data found by x-ray diffraction as well by selected area electron diffraction methods and allow a new complementary characterization of the Fe(Fe-alloy)-MWCNT systems. The CEMS method applied for the characterization of metal containing MWCNTs reveals that close to...

Synthesis and characterization of aligned Fe-filled carbon nanotubes on silicon substrates

We describe the preparation and the properties of Fe-filled multi-walled carbon nanotubes on Co-coated oxidized silicon substrates. The material was grown by pyrolysis of ferrocene, using a chemical vapor deposition process. Scanning and transmission electron microscopy studies indicate that the material consists of filled and aligned MWNTs. They have outer diameters of 40–100 nm and diameters of the metal core of 20–40 nm. Energy dispersive X-ray analysis of individual tubes reveals that their filling consists of pure Fe. Alternating gradient magnetometry investigations demonstrate the ferromagnetic behavior of the filled tubes. We observe unique magnetic properties differing from those of bulk Fe.

Stress development in sputtered NiO thin films during heat treatment

Nickel oxide thin films with a thickness of 100 nm were deposited on oxidized silicon wafers by rf magnetron sputtering from a NiO target in an Ar (nonreactive case) and an Ar+O2 atmosphere with various oxygen contents (reactive cases). The as-deposited films possess high compressive stresses (up to 3700 MPa) which decrease irreversibly during annealing between 150 and 500u200a°C. Compositional and microstructural analyses were performed on as-deposited and annealed films by means of electron probe microanalysis, transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, thermal-desorption spectrometry, and magnetization measurements. All as-deposited thin films consist of NiOx with x ranging between about 1.15 and 1.27. These oxygen-excess films are thermally unstable. They decompose during heat treatment into thermally more stable, oxygen-poorer NiO and/or metallic Ni. This decomposition is the reason for the observed irreversible stress changes.

Magnetic and structural characteristics of exchange biasing systems based on NiMn antiferromagnetic films

Abstract We have investigated the magnetic and the structural characteristics of bi-layer exchange biasing (EB) systems NiMn–Co as function of the antiferromagnetic (AFM) film deposition parameters and of the post-deposition annealing and field-cooling procedures. The effects of sputtering pressure, growth rate, seed layer and of the annealing parameters were studied by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Magneto-Optical Kerr (MOKE) effect measurements. The results yielded optimised deposition conditions and permitted us to establish efficient annealing and field-cooling procedures. The best obtained EB systems with 5xa0nm Co film reveal coercive field Hc⩾35xa0Oe and exchange bias field HEB⩾95xa0Oe and have good stability in air up to at least 200°C. The obtained magnetic characteristics are closely correlated with the crystalline structure of the as-deposited samples and with the phase transformation effects. According to our results, the as-deposited samples are a mixture of a non-equilibrium FCC NiMn phase (quenched by the sputtering deposition process) and a variable amount of the equilibrium face centred tetragonal (FCT) phase. The ratio of these two phases depends on the deposition conditions—the amount of the FCT NiMn phase is larger if deposition conditions closer to equilibrium have been used, especially at higher sputtering gas pressure and higher deposition rates. The annealed samples contain dominantly the FCT AFM phase as confirmed by XRD and TEM-diffraction analyses. The transition to this AFM state depends on the initial structure of the as-grown samples and, respectively, on their deposition conditions.

Interdiffusion, stress, and microstructure evolution during annealing in Co/Cu/Co trilayers

In order to find interdiffusion and microstructural changes on sputtered 90 nm Co/180 nm Cu/90 nm Co trilayers during annealing, the evolution of the stress and electrical resistance was studied in situ during temperature ramps and isothermal annealing as well as concentration-depth profiles, the grain morphology, and the phase and texture formation were investigated after heat treatment at various temperatures. Up to 450u200a°C, no distinct lattice interdiffusion was observed. Grain-boundary diffusion of Cu through the Co top layer to the surface as well as a distinct growth of Cu grains into the Co top layer start to occur at about 450u200a°C. The conclusions corroborate findings on Cu/Co nanoscale multilayers, which show a deterioration of the giant magnetoresistance effect at high temperatures due to similar microscopic mechanisms.

Growth and magnetic properties of epitaxial ultrathin Ni films on Cu(111) using Sb as a surfactant

We studied the growth and magnetism of ultrathin Ni films on the (111) surface of Cu using Sb as a surfactant. For this purpose we deposited Sb under UHV conditions at room temperature onto the Cu surface prior to the Ni film growth. When the Sb precoverage exceeded a certain threshold [0.7 monolayers (ML)], pronounced intensity oscillations of the medium energy electron diffraction signal indicated a layer-by-layer growth of the deposited Ni films. Low energy electron diffraction patterns of the Ni films revealed a hexagonal structure with a threefold symmetry. Using this approach we prepared high quality epitaxial Ni(111) films up to a thickness of 20 ML and performed in situ magneto-optical Kerr measurements. At a thickness of 7–8 ML an inverse spin reorientation transition occurs from an in-plane magnetization at lower thicknesses to an out-of-plane orientation for higher thicknesses.