Ferromagnetically filled carbon nanotubes: Radial structures and tuning of magnetic properties through new synthesis methods.

PhDMultiwall carbon nanotubes filled with continuous single-crystals of the ferromagnetic phase -Fe were produced with two new synthesis methods: the boundary layer chemical vapour synthesis and the perturbed vapour chemical vapour deposition. In the first method, the nanotubes nucleate and grow radially from a central agglomeration of homogeneously nucleated spherical particles in a randomly fluctuating vapour created in the viscous boundary layer between a rough surface and a laminar pyrolyzed-ferrocene/Ar vapour flow. In the second method, the nanotubes nucleate and form in a flower-like arrangement departing from homogeneously nucleated particles. These particles are produced by the creation of a local perturbation in a vapour with a high density of Fe and C species obtained from the pyrolysis of ferrocene in a laminar Ar flow. Electron microscopy investigations revealed that the continuous single crystals obtained with both methods exhibit diameters much lower than the critical diameter for a single magnetic domain of -Fe (~ 66 nm). In the radial structures, the single-crystal diameter is in the range of ~ 17-37 nm, while in the flower-like structures the single crystals show mainly a diameter of ~ 30 nm and ~ 55 nm. The average single crystals length is 7-8 m in the case of the radial structures and 19-21 m in the case of the flower-like structures. DC magnetization measurements at 5 K show different magnetic behaviours. The flower-like structures present a very high saturation magnetization of 189.5 emu/g and a high coercivity of 580 Oe. The radial structures exhibit an exchange-coupled ferromagnetic/antiferromagnetic system despite only 2% of -Fe is present inside the nanotubes. The radial structures obtained at flow-rates of 3.5 ccm and 20 ccm, show saturation-magnetizations of 31emu/g and 13 emu/g, and coercivities of 790 Oe and 843 Oe respectively.South-East Physics Networ

Anomalous low-temperature saturation effects and negative thermal expansion in the c-axis of highly oriented pyrolytic graphite at the magic angle

We report a novel T-XRD and Rietveld-refinement investigation of pyrolytic-graphite samples with high degree of graphene-layer-orientation and misfit-rotational-angle of ~ 0.8o in the T-range from 12K to 298K. An anomalous variation of the graphitic c-axis which involves firstly negative-thermal-expansion (from 12K to ~50K), a saturation-effect (from 50K to ~160K) and then a positive expansion (from ~180K to 298K) is evidenced. The reported trend is significantly different with respect to that expected by considering the standard-thermal-expansion α-parameter where no saturation-effect is present. SQUID-magnetometry revealed further presence of superconducting-like hysteresis which resemble those observed by Scheike et al

Temperature driven structural-memory-effects in carbon nanotubes filled with Fe₃C nano crystals

We report the observation of novel temperature-driven structural-memory-effects in carbon nanotubes (CNTs) filled with Fe3C nano-crystals. These structural-transitions were measured by means of temperature (T) dependent X-ray diffraction (XRD) in the T-range from 298 K to 12 K. A clear reversible 2theta-shift in the 002-peak of the graphitic-CNTs-walls is found with the decrease of the temperature. As determined by Rietveld refinement, such 2theta-shift translates in a not previously reported decrease in the value of the CNT graphitic c-axis with the decrease of the temperature (from 298 K to 12 K). Also, a clear reversible 2theta-shift in the 031 and 131 diffraction-peaks of Fe3C is observed within the same T-range. Rietveld refinements confirm the existence of such memory-effect and also reveal a gradual decrease of the 010-axis of Fe3C with the decrease of the temperature. These observations imply that the observed structural-memory-effect is a characteristic of CNTs when Fe3C is the encapsulated ferromagnet. The generality of such memory-effects was further confirmed by additional measurements performed on other types of CNTs characterized by continuous Fe3C-filling. XRD measurements in the T-range from 298 K to 673 K revealed also an unusual reversible decrease of the Fe3C-peak intensities with the increase of the temperature. These observations can have important implications on the magnetic data recording applications of these nanostructures by helping in better understanding the unusual temperature-dependent magnetic instabilities of iron-based nano-crystals which have been recently reported in literature
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Magnetic vortex and unsaturated magnetization components in highly oriented pyrolytic graphite

Observation of ferromagnetic and granular superconductive features in highly-oriented-pyrolytic-graphite (HOPG) has recently attracted an important attention. We report a novel temperature dependent XRD and SQUID investigation of HOPG in the temperature range from 300.15 to 77.15 K. Unusual hysteresis features indicate the possible presence of vortex states in conditions of magnetic field approximately perpendicular to the HOPG layers. This interpretation is further supported by additional measurements performed on intermediate lamellae extracted by exfoliation. Evidence of a possible structural-transition in the c-axis of HOPG in the temperature range between 77 K and 100K is also provided by using the Rietveld refinement method. ZFC and FC measurements performed at high field values of 5000-10000 Oe, together with mFC-mZFC subtraction, highlight absence of a sharp depletion of the difference between magnetization signals towards zero. These observations may indicate the possible presence of additional unsaturated weak features, which are ascribed to superconductive signals as previously predicted by Scheike et al. [8]

Observation of enhanced magnetic transition in Pbnm SmFeO3

Rare-earth orthoferrite RFeO3 materials have recently attracted great attention for their intriguing technological potential. Among these materials, SmFeO3 holds great promise, not only for its excellent physical properties (fast magnetic switching, spin reorientation, and magnetization reversal), but also for its potential ferroelectric properties, which have been recently under debate. Here, we focus our attention on the T-dependent Zero Field Cooled and Field Cooled magnetization properties of micrometer scale crystals of SmFeO3 obtained by annealing methods. We report the observation of an enhanced magnetic transition at a temperature of approximately 139 K. From literature bulk susceptibility measurements, it has been suggested that below the temperature of about 140 K, Sm3+ moments begin to order antiparallel to the Fe-moments due to antiferromagnetic f-d exchange interactions. We attribute the observed transition to compensation effects induced by the appearance of long range ordering in Sm3+ spins. The magnetic-nature of the observed transition is confirmed by additional temperature dependent XRD analysis, which did not show structural changes in the samples in the same temperature range (from 298 K to 100 K). Due to residual small fractions of ferromagnetic α-Fe from the sample-growth, possible interactions between the magnetic moment of α-Fe and the SmFeO3 crystals at the compensation temperature cannot be excluded and could be at the origin of the enhanced magnetic signal reported in this work

Scanning tunneling microscopy identification of van Hove singularities and negative thermal expansion in highly oriented pyrolytic graphite with hexagonal moiré superlattices

We report a novel investigation on the structural, electronic and magnetic properties of highly-oriented-pyrolytic-graphite (HOPG) by employing STM/S, TEM and SQUID. Van Hove singularities (vHs) were identified by STS-acquisition of local-density-of-states spectra (LDOS) acquired from moiré-superlattice-periodicities D ∼0.5 nm (θ rot ∼28.6°), ∼2 nm (θ rot of ∼7.08°) and ∼ 4 nm (θ rot ∼3.6°). A possible interlayer-coupling- or moiré-thickness-induced variation of the vHs-separation-parameter was identified. Investigation of the spatial-distribution of the moiré-superlattices by exfoliation revealed also D ∼13.87 nm (θ rot ∼1.02°), ∼13.0 nm (θ rot ∼1.09°), ∼12.65 nm (θ rot ∼1.12°) and ∼2.03 nm (θ rot ∼7.0°). These observations were further supported by additional LDOS spectra acquired on HOPG from moiré-superlattice-periodicities D ∼8 nm (θ rot ∼1.8°). In the latter, the unusual presence of four vHs peaks evidenced the existence of multiple rotational effects between internal sublattices. Extended analyses were further performed by T-XRD from 12K to 298K. Unknown peak-features exhibiting unusual T-dependent shifts were analysed at 2θ ∼ 21° (Fig. 6A-C) and ∼ 43 ° (Fig. 6D). ZFC and FC-signals from the exfoliated lamellae further evidenced an anisotropic ferromagnetic behaviour, possibly involving the coexistence of 1) disorder induced percolative ferromagnetism and 2) additional magnetic components arising from the locally twisted sublattices, which were found to exhibit the moiré superlattices

Cl-Assisted Large Scale Synthesis of Cm-Scale Buckypapers of Fe3C-Filled Carbon Nanotubes with Pseudo-Capacitor Properties: The Key Role of SBA-16 Catalyst Support as Synthesis Promoter

We show a novel chemical vapour deposition (CVD) approach, in which the large-scale fabrication of ferromagnetically-filled cm-scale buckypapers is achieved through the deposition of a mesoporous supported catalyst (SBA-16) on a silicon substrate. We demonstrate that SBA-16 has the crucial role of promoting the growth of carbon nanotubes (CNTs) on a horizontal plane with random orientation rather than in a vertical direction, therefore allowing a facile fabrication of cm-scale CNTs buckypapers free from the onion-crust by-product observed on the buckypaper-surface in previous reports. The morphology and composition of the obtained CNTs-buckypapers are analyzed in detail by scanning electron microscopy (SEM), Energy Dispersive X-ray (EDX), transmission electron microscopy (TEM), high resolution TEM (HRTEM), and thermogravimetric analysis (TGA), while structural analysis is performed by Rietveld Refinement of XRD data. The room temperature magnetic properties of the produced buckypapers are also investigated and reveal the presence of a high coercivity of 650 Oe. Additionally, the electrochemical performances of these buckypapers are demonstrated and reveal a behavior that is compatible with that of a pseudo-capacitor (resistive-capacitor) with better performances than those presented in other previously studied layered-buckypapers of Fe-filled CNTs, obtained by pyrolysis of dichlorobenzene-ferrocene mixtures. These measurements indicate that these materials show promise for applications in energy storage systems as flexible electrodes

Unusual butterfly-shaped magnetization signals and spin-glass-like behaviour in highly oriented pyrolytic graphite

We report a novel investigation on the relationship between magnetic-ordering and graphitic-structure in highly-oriented-pyrolytic-graphite (HOPG). By employing orientation-dependent-X-ray-diffraction, Raman-spectroscopy and temperature-dependent-superconductive-quantum-interference-device (T-SQUID) we examined the presence of ferromagnetic- and superconductive-ordering in HOPG systems with 1) disordered (HOPG1, containing carbon-vacancy-rich weak-Bernal-stacking and Moiré-superlattices with θmisfit ∼ 0.5°) and 2) ordered (HOPG2, containing higher-degree of Bernal-stacking and Moiré-superlattices with θmisfit ∼ 0.5°, 0.8°, 11°) graphitic-layer-arrangement. A perfect-HOPG is expected to exhibit a diamagnetic-response to an applied-magnetic-field. Instead, additional 1) ferromagnetic-signals presenting a characteristic width-enhancement with the field increasing in HOPG1 and 2) complex butterfly-shaped ferromagnetic signals in HOPG2, are demonstrated. Temperature-dependent-magnetometry evidenced further the presence of randomly oriented ferromagnetic clusters originating from topological disorder in both HOPG1 and HOPG2. These magnetic signals were explained on the basis of the percolative-type model

Peeling off effects in vertically aligned Fe3C filled carbon nanotubes films grown by pyrolysis of ferrocene

We report the observation of an unusual self-peeling effect which allows the synthesis of free standing vertically aligned carbon nanotube films filled with large quantities of Fe3C and small quantities of ?-Fe crystals. We demonstrate that this effect depends on the interplay of three main factors: (1) the physical interactions between the chosen substrate surface and grown carbon nanotubes (CNTs), which is fixed by the composition of the used substrate (111 SiO2/Si or quartz), (2) the CNT-CNT Van der Waals interactions, and (3) the differential thermal contraction between the grown CNT film and the used substrate, which is fixed by the cooling rate differences between the grown film and the used quartz or Si/SiO2 substrates. The width and stability of these films are then further increased to cm-scale by addition of small quantities of toluene to the ferrocene precursor

Peeling off effects in vertically aligned Fe3C filled carbon nanotubes films grown by pyrolysis of ferrocene

We report the observation of an unusual self-peeling effect which allows the synthesis of free standing vertically aligned carbon nanotube films filled with large quantities of Fe3C and small quantities of ?-Fe crystals. We demonstrate that this effect depends on the interplay of three main factors: (1) the physical interactions between the chosen substrate surface and grown carbon nanotubes (CNTs), which is fixed by the composition of the used substrate (111 SiO2/Si or quartz), (2) the CNT-CNT Van der Waals interactions, and (3) the differential thermal contraction between the grown CNT film and the used substrate, which is fixed by the cooling rate differences between the grown film and the used quartz or Si/SiO2 substrates. The width and stability of these films are then further increased to cm-scale by addition of small quantities of toluene to the ferrocene precursor