http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

National Cell Bank, Pasteur Institute of Iran, Tehran 1316943551, Iran, Cardiovascular Research Center, Mount Sinai School of Medicine, New York, New York 10029, United States, Department of General, Organic, and Biomedical Chemistry, NMR and Molecular Imaging Laboratory, University of Mons, Avenue Maistriau, 19, B-7000 Mons, Belgium, Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588, Iran, Department of Biomedical Engineering, National Yang Ming University, No 155, Sec. 2, LiNong Street, Taipei 112, Taiwan, Nanotechnology Toxicology Consulting & Training, Inc., Nova Scotia, Canada, Faculty of Medicine, Dalhousie Medical School, Dalhousie University, Halifax, Nova Scotia, Canada, and Institute for Nanoscale Science and Technology (INSAT), University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7ER, U.K., Nanotechnology Research Centre, Faculty of Pharmacy, Tehran University of Medical Sciences, Tehran, Iran

Magnetic resonance imaging tracking of stem cells in vivo using iron oxide nanoparticles as a tool for the advancement of clinical regenerative medicine.

Owing to the fast development of wireless information technologies at the high-frequency range, the electromagnetic interference problem has been of increasing significance and attracting global attention. One key solution for this problem is to develop materials that are able to attenuate the unwanted electromagnetic waves. The desired properties of these materials include low reflection loss value, wide attenuation band, light weight, and low cost. This review gives a brief introduction to graphene-based composites and their electromagnetic absorption properties. The ultimate goal of these graphene absorbers is to achieve a broader effective absorption frequency bandwidth (fE) at a thin coating thickness (d). Representative and popular composite designs, synthesis methods, and electromagnetic energy attenuation mechanisms are summarized in detail. The two key factors, impedance matching behavior and attenuation ability, that determine the electromagnetic behavior of graphene-based materials are given particular attention in this article.

A brief introduction to the fabrication and synthesis of graphene based composites for the realization of electromagnetic absorbing materials

The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies in which sample preparation artifacts have been the cause of some uncertainties. Microstructure of iron nanoparticles passivated with iron oxide shell was studied using high-resolution transmission electron microscopy and high-angle annular dark-field imaging in aberration-corrected scanning transmission electron microscopy. Voids were readily observed on both small single-crystal α-Fe nanoparticles formed in a sputtering process and the more complex particles created by reduction of an oxide by hydrogen. Although the formation of hollow spheres of nanoparticles has been engineered for Co at higher temperatures [Y. Yin, R. M. Riou, C. K. Erdonmez, S. Hughes, G. A. Somorjari, and A. P. Alivisatos, Science 304, 711 (2004)], they occur for iron at room temperature and provide insight into the initial oxidation processes of iron. There exists a critical size of ∼8n...

Void formation during early stages of passivation: Initial oxidation of iron nanoparticles at room temperature

An iron (Fe) nanoparticle exposed to air at room temperature will be instantly covered by an oxide shell that is typically approximately 3 nm thick. The nature of this native oxide shell, in combination with the underlying Fe(0) core, determines the physical and chemical behavior of the core-shell nanoparticle. One of the challenges of characterizing core-shell nanoparticles is determining the structure of the oxide shell, that is, whether it is FeO, Fe(3)O(4), gamma-Fe(2)O(3), alpha-Fe(2)O(3), or something else. The results of prior characterization efforts, which have mostly used X-ray diffraction and spectroscopy, electron diffraction, and transmission electron microscopic imaging, have been framed in terms of one of the known Fe-oxide structures, although it is not necessarily true that the thin layer of Fe oxide is a known Fe oxide. In this Article, we probe the structure of the oxide shell on Fe nanoparticles using electron energy loss spectroscopy (EELS) at the oxygen (O) K-edge with a spatial resolution of several nanometers (i.e., less than that of an individual particle). We studied two types of representative particles: small particles that are fully oxidized (no Fe(0) core) and larger core-shell particles that possess an Fe core. We found that O K-edge spectra collected for the oxide shell in nanoparticles show distinct differences from those of known Fe oxides. Typically, the prepeak of the spectra collected on both the core-shell and the fully oxidized particles is weaker than that collected on standard Fe(3)O(4). Given the fact that the origin of this prepeak corresponds to the transition of the O 1s electron to the unoccupied state of O 2p hybridized with Fe 3d, a weak pre-edge peak indicates a combination of the following four factors: a higher degree of occupancy of the Fe 3d orbital; a longer Fe-O bond length; a decreased covalency of the Fe-O bond; and a measure of cation vacancies. These results suggest that the coordination configuration in the oxide shell on Fe nanoparticles is defective as compared to that of their bulk counterparts. Implications of these defective structural characteristics on the properties of core-shell structured iron nanoparticles are discussed.

Morphology and Electronic Structure of the Oxide Shell on the Surface of Iron Nanoparticles

Structure, state, transformations and interactions of iron oxide shell with the iron metallic core in aerosol Fe nano-particles has been studied by X-ray and electron diffraction, TEM, Mossbauer spectroscopy and magnetization measurements. A strong influence of the state of nanoparticles oxide shell has been revealed on their magnetization, coercive force and hysteresis loop shift. A long-term passivation of the particles has been shown to be caused by the primary amorphous oxide. The passivation ability of the oxide shell becomes essentially worse after heat treatment of powder, resulting in its crystallization. Basing on the obtained results, a qualitative mechanism of passivation for nanoparticles covered with amorphous oxide shell has been suggested.

Aerosol Fe Nanoparticles with the Passivating Oxide Shell

The structure and hyperfine fields of Fe1−xCrx (x=0.0236–0.803) nanoparticles (average size of 27±2 nm) are studied at room temperature by combined x-ray diffraction and Mossbauer spectroscopy techniques. They are produced by fast evaporation of bulk alloys at 3 Torr Ar pressure. The bulk alloys of any composition are shown to exhibit a bcc structure, whereas the nanoparticles demonstrate a mixture of bcc and tetragonal σ phases in the Cr range from 24.4 to 83.03 at. %. At the Cr content of 2.36 at. % the lattice constant for nanoparticles is larger than that of the bulk alloy, though the values of hyperfine fields on Fe nuclei do not differ. The Mossbauer spectrum of nanoparticles contains an oxide doublet in addition to the sextet specific to that of the bulk alloy. In both cases the width of the sextet lines is rather narrow. However, even at ∼8 at. % Cr the lines of the sextet are broadened so much that it can be decomposed by two-three components. This is explained by freezing the high-temperature ferromagnetic fcc phase regions in the bcc lattice. As the Cr content increases, the Mossbauer spectra become more complex, transforming finally into a paramagnetic singlet. A complete ferromagnetic→paramagnetic transition is observed for the bulk alloy at 68 at. % Cr and for nanoparticles at 35 at. % Cr. The results are discussed under the assumption that at high temperatures the alloys are not homogeneous and exhibit fluctuations of the composition. With decrease of temperature these fluctuations result in decomposition of the alloy into two phases for nanoparticles whereas they are frozen at the cluster level in the bulk alloys holding a macroscopic homogeneity.

Structure and Mössbauer spectra for the Fe–Cr system: From bulk alloy to nanoparticles

The production and structural characterization of gas-evaporated nanoparticles in the Fe1−xCrx system, with 0<x<0.83 are reported. The results show that for x∼0.5 the metastable σ-FeCr can be stabilized and it constitutes up 60wt% of the material. The sample with the highest σ-FeCr content is further analyzed to study the structural and the magnetic properties of this phase and its thermal stability. The σ-FeCr phase is weakly magnetic with an average magnetic moment of 0.1μB per Fe atom and a Curie temperature below ∼60K. It is stable up to 550K where it starts to transform to bcc-FeCr. Annealing at 700K yields Cr2O3 due to Cr surface segregation and affects the magnetic behavior of the system, which is dominated by interparticle interactions.

/pdf/aerosol-nanoparticles-in-the-fe1-xcrx-system-room-4gtc8vm6uy.pdf

Aerosol nanoparticles in the Fe1−xCrx system: Room-temperature stabilization of the σ phase and σ→α-phase transformation

By comparing results of Mossbauer spectroscopy and x‐ray diffraction investigations of ultrafine Fe and Fe‐Ni particles, a formerly stated supposition on the existence of two spin states of the fcc phase in pure iron and Fe‐Ni alloys has been verified. Some structural peculiarities of particles under study have been observed also. For the pure bcc‐Fe particles with an average diameter of 30 and 50 nm the existence of two hyperfine fields (H1=330 kOe, H2=360 kOe) at room temperature has been found. It is supposed that H2 in the bcc phase could be considered as a remnant of a high spin state of the fcc‐Fe phase at high temperature. Particles of Fe‐Ni(30.3 wt %), Fe‐Ni(35 wt %), and Fe‐Ni(52 wt %) alloys with an average diameter ranging from 5 to 15 nm were studied also. Particles of Fe‐Ni (30.3 wt %) and Fe‐Ni(35 wt %) alloys with diameter of 5–8 nm had the bcc structure. A mixture of the bcc and fcc phases appeared with an increase of the particle size. At the same time only the fcc structure remained for ...

Some specific features of fine Fe and Fe‐Ni particles

The results of combined X-ray and Mossbauer studies of structure and local magnetic ordering in massive substances Fe, Fe–Ni, Fe–Mn, Fe–Ni–Mn, Fe–Pt, Fe–Co and aerosol nanoparticles produced by their evaporation in rare Ar atmosphere are discussed. This technique provides a stochiometric composition of alloys in nanoparticles. The smallest (5–8 nm) particles for all alloys containing Fe 60–65% are shown to have a bcc structure whereas with doubling a size the particles acquire a fcc structure. This is explained by the fact that by cooling the particles in the course of preparation they quickly reach a state close to the equilibrium and, according to the constitution diagram, must decompose into two phases. Such decomposition in massive alloys was never observed at temperatures below 300°C because of diffusive difficulties. It is found that as-fresh aerosol particles are covered with an X-ray amorphous oxide shell, which is displayed in the room temperature Mossbauer spectra as a superparamagnetic doublet and is transformed into sextet at lower temperatures. An availability of the oxide shell has no practical influence on the particles’ structure. The obtained Mossbauer spectra are considered with the model suggested by R.J. Weiss in 1963, on existence of two-spin states in the high-temperature fcc modification of Fe and its alloys. Both states coexist, moreover, in the Mossbauer spectra the ferromagnetic state dominates at high temperature and anti-ferromagnetic one at low temperature. The ferromagnetic state manifests itself as a remnant of the frozen magnetic ordering of the high-temperature fcc modification in the resulting bcc structure, whereas the anti-ferromagnetic state is related to some fcc fraction retained under the particles’ quenching.

E. A. Shafranovsky

Papers

Aerosol Fe Nanoparticles with the Passivating Oxide Shell

Structure and Mössbauer spectra for the Fe–Cr system: From bulk alloy to nanoparticles

Aerosol nanoparticles in the Fe1−xCrx system: Room-temperature stabilization of the σ phase and σ→α-phase transformation

Some specific features of fine Fe and Fe‐Ni particles

Exhibition of High- and Low-spin States of the High-temperature Fcc Phase in Nanoparticles of Fe, Fe-rich and Co-rich Alloys