Ferromagnetism of Nanographite Structures in Carbon Microspheres (original) (raw)

Magnetic properties of dense carbon nanospheres prepared by chemical vapor deposition

Carbon nanospheres (B $ 50 nm) prepared by chemical vapor deposition exhibit Curie-type paramagnetism, with a localized spin density N S = 4.01 · 10 19 spins/g. The spheres are solid, as shown by their cross-sections. Their low surface area excludes the presence of exposed graphene layers at the surface being the origin of paramagnetic centers. Inside the carbon spheres, HRTEM reveals a concentric wavy pattern of graphitic (sp 2 -bonded) clusters (size < 2 nm) embedded in disordered carbon. The high density of carbon atoms with sp 2 -sp 3 hybrid orbitals acting as paramagnetic centers, at interfaces between graphitic (sp 2 -bonded carbon) clusters and amorphous carbon, can explain the magnetic measurements.

Magnetic Properties of Carbon Nanospheres at low temperatures

2011

The magnetic and electrical properties of Nitrogen doped carbon nanospheres can be probed using Electron Spin Resonance (ESR) techniques. Previous ESR measurements showed large peak on nitrogen doped spheres, implying that N forms paramagnetic impurities within the graphitic lattice. Low temperature measurements with continuous wave ESR, revealed different interactions of conduction electrons with nitrogen impurities and acoustic phonons hence two different spin relaxation mechanisms, Elliot-Yafet and D’yakonov-Perel’ compete in the whole temperature range[5-300K].

Magnetic properties of graphene nanodisk and nanocone powders at low temperatures

Physical Review B, 2015

Our aim is the study of magnetization phenomena of carbon powders consisting of graphene nanocones and nanodisks. Magnetization measurements were carried out using a superconducting quantum interference devices in magnetic fields −5 < B < 5 T for temperatures in the range 0.5 ≤ T < 300 K. We have observed that magnetization M depends on temperature T as a power law M ∝ T −α where the scaling exponent α = 0.79 ± 0.04 was determined. We found that the magnetization consists of a diamagnetic background and a paramagnetic contribution of localized spins. Considering saturation magnetization M S in the free spin magnetization model and diamagnetic susceptibility χ D for independent of temperature T we found that the effective spin value S increases from S = 1/2 at temperature T > 20 K to S = 3 at low temperatures 0.5 ≤ T ≤ 20 K.

Magnetic properties of carbon-coated, ferromagnetic nanoparticles produced by a carbon-arc method

Journal of Applied Physics, 1994

The Kratschmer-Huffman carbon-arc method of preparing fullercnes has been used to generate carbon-coated transition metal (TM) and TM-carbide nanocrystallites. The magnetic nanocrystallites were extracted from the. soot with a magnetic gradient field technique. For TM=Co the majority of nanocrystals exist as nominally spherical particles, 0.5-S nm in radius. Hysteretic and temperature-dependent magnetic response, in randomly and magnetically aligned powder samples frozen in epoxy, correspond to fine particle magnetism associated with monodomain TM particles. The magnetization exhibits a unique functional dependence on H/T, and hysteresis below a blocking temperature T, . Below TR, the temperature dependence of the coercivity can be expressed as Hc=Hcu[ 1 -(T/TB)1'2], where HcO is the (1 K coercivity. 5882

Magnetoresistance of composites based on graphitic discs and cones

Journal of Physics D: Applied Physics, 2014

We have studied the magnetotransport of conical and disk-shaped nanocarbon particles in magnetic fields |B| ≤ 9 T at temperatures 2 ≤ T ≤ 300 K to characterize electron scattering in a three dimensional disordered material of multilayered quasi 2D and 3D carbon nanoparticles. The microstructure of the particles was modified by graphitization at temperatures 1600 • C and 2700 • C. We find clear correlations between the microstructure as seen in transmission electron microscopy and the magnetotransport properties of the particles. The magnetoresistance measurements showed a metallic nature of samples and positive magnetoconductance which is a signature of weak localization in disordered systems. We find that the magnetoconductance at low temperatures resembles quantum transport in single-layer graphene despite the fact that the samples are macroscopic and three dimensional, consisting of stacked and layered particles, which are randomly oriented in the bulk sample. This graphene-like behaviour is attributed to the very weak interlayer coupling between the graphene layers.

Magnetism in nanoscale graphite flakes as seen via electron spin resonance

Physical Review B, 2012

Magnetic properties of a large assembly of ultrathin graphitic particles obtained by heavy sonication of graphite powder dispersed in N-methylpyrrolidone were measured by electron-spin resonance (ESR). The ESR signal was decomposed into one narrow and one broad component. The narrow component was associated with localized Curie-type defects. The temperature dependence of the predominant broad component points to a transition to a superparamagnetic-like state at 25 K. By performing the density-functional-theory calculations for graphene with selected extended defects (the sheet edges, zigzag chains of chemisorbed H atoms, and pentagon-octagon rows), we found considerable magnetic moments at C atoms in their vicinities. We attribute the magnetism in the graphitic particles to the localized electronic states near the defects in the network of the π electrons of graphene. The ferromagnetic (FM) correlations among magnetic moments at carbon atoms near the edges are not able to give rise to a long-range FM order.

Magnetic properties of carbon nanoparticles

IOP Conference Series: Materials Science and Engineering, 2012

Magnetization M (T, B) of powder and glassy samples containing carbon nanoparticles is investigated in the interval of temperatures T between ~ 3  300 K and magnetic fields B up to 5 T. Low-field magnetization, M (T), exhibits a strong magnetic irreversibility, which is suppressed above the field of ~ 1 T. The dependence of M (B) saturates at high temperatures above B ~ 2 T, magnetic hysteresis is observed already at 300 K. The values of the saturation magnetization, the coercivity field and the maximum blocking temperature are obtained. Analysis of the experimental data gives evidence for concentration of the magnetization close to the surface of the particles, which is consistent with the origin of magnetism in nanocarbon presumably due to intrinsic disorder and surface defects.

Preparation and properties of carbon-coated magnetic nanocrystallites

Physical Review B, 1993

Carbon-coated gadolinium carbide nanocrystallites are generated by a process based on the Kratschmer-HufFman carbon-arc method of preparing fullerenes, and a novel magnetic-field-gradient separation technique is used to separate them. This separation of nanocrystalline byproducts of the carbon arc process is a prerequisite for many of the proposed applications of these materials. While the data presented pertain to Gd2C3, this method is generally applicable to any paramagnetic or ferromagnetic compound. Structural characterization by x-ray and electron difFraction and high-resolution transmission electron microscopy reveal the presence of a single gadolinium-containing (Gd2C3) phase and excess carbon. The carbide phase exists as 10-50 nm spherical particles. SQUID magnetometry shows paramagnetic response attributed to Gd + ions. Carbon-coated metal carbide nanocrystallites such as LaC2 nanoparticles are an intriguing byproduct of endohedral fullerene production. ' Here we describe the preparation of similar magnetic nanocrystallites, along with a novel method for separating them. The data discussed are for paramagnetic gadolinium carbide, but we have also produced and separated paramagnetic holmium and ferromagnetic iron, cobalt, nickel, and manganese bismuth by this method. Results concerning these materials are discussed elsewhere.

Unusual scenario of the temperature evolution of magnetic state in novel carbon-based nanomaterials

Two porous carbon-based samples doped with Au and Co, respectively, are investigated. The neutron diffraction study reveals an amorphous structure of both samples. The Co-doped sample contains structural clusters with larger size and exhibits a long-range ferromagnetic (FM) ordering at 2.6 K. The NMR investigations demonstrate, that the samples are obtained with a partial carbonization of initial aromatic compounds and do not reach a state of glassy carbon. The magnetization