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We show that epitaxial nickel (fcc structure, lattice constant of 0.3528nm) nanocrystals are formed inside magnesium oxide (sodium chloride structure, lattice constant of 0.4201nm) matrix, where the misfit ranges from 3.0% to 31.3% on different interfaces. By controlling the annealing conditions, we obtained two distinct epitaxial morphologies: (1) cube-on-cube with <100>Ni // <100> MgO with a misfit of about 18.0%; and (2) <112> morphology with <112> // <002> MgO (misfit 31.3%); <111> Ni // <200> MgO (misfit 3.0%); and <110> Ni // <020> MgO (misfit 17.0%). These results on epitaxial growth of nickel on MgO with misfit ranging from 3.0% to 31.3% are consistent with the domain matching epitaxy paradigm (DME), where integral multiple of lattice planes match across the film-substrate interface. The lattice planes include all the planes in a crystal structure, not just the diffraction planes involved in the X-ray, electron and neutron scattering. The residual misfit away from the integral multiples is accommodated by the principle of domain variation, where two or three sets of domains alternate with a certain frequency to minimize the misfit close to zero. The epitaxy in the DME paradigm is defined as the film having a fixed orientation which could be the same under certain conditions. From these results on Ni epitaxy, the dominant role of planes and matching of integral multiples of planes to accommodate small to large misfit are clearly established according to the DME paradigm.
Under neutral conditions (pH=7-9) and temperature as low as 4°C we successfully synthesized ferrite nanoparticles with an intermediate structure between Fe3O4 and γ -Fe2O3 from an aqueous solution. These experimental conditions enabled us to immobilize even very unstable bioactive molecules onto the surfaces of the ferrite nanoparticles during their synthesis. The bioactive molecules were strongly fixed onto the ferrite surface intermediated by specific amino-acids or related-structure molecules each having pairs of carboxyl (COOH) groups. The COOH group pair was found to make a chemical bond with the ferrite particles. Utilizing the strong bond between such molecules and ferrites, we successfully prepared bioactive ferrite nanobeads where the ferrite nanoparticles were encapsulated in a polymer that exhibited negligible non-specific absorption of proteins. We also succeeded in fixing bioactive molecules onto the surfaces of the polymer coating of the ferrite nanobeads. We describe promising applications of our novel bioactive ferrite nanobeads such as high performance magnetic carriers for bio-screening, hypothermia, MRI contrast enhancement and magnetic drug delivery.
The formation of organized structures of magnetic nanoparticles have been studied by scanning probe microscopy when depositing Ni nanoclusters on conducting (gold on HOPG) substrates. In this case magnetic Ni nanoparticles formed by laser electrodispersion technique based on laser ablation (size of particles ∼ 2.5 nm) form highly ordered superstructures, including helical (double helix with outer diameter ∼ 10 nm, distance between sequences ∼ 5 nm, length of such helix is typically hundreds of nm), linear (chains of clusters), etc. The structures discovered were investigated also by STS methods, with clear demonstration of Coulomb blockade effect over single nanoclusters. The results of calculations for dipole-dipole and exchange interaction of magnetic nanoparticles support such formation of helix-like structures.
Methods to form magnetic nanoparticle monolayers using non-aqueous Langmuir layers are reported. Following a discussion of the driving forces in various self-assembly techniques, we describe how aqueous Langmuir layers can be modified for use in conjunction with oxidationsensitive nanoparticles. Monolayers are formed using Fe and–Co nanoparticles, and transferred to carbon-coated transmission electron microscopy grids using the Langmuir-Schaefer method.
Amounts of oligonucleotides adsorbed onto the Au/γ-Fe2O3 composite nanoparticles synthesized by gamma-ray irradiation and picked up by a magnet were evaluated using fluorescence technique. The adsorbing capacity of the oligonucleotides on our nanoparticles are larger than a commercial magnetic beads for a separation of biomolecules.
A low-cost method has been developed to produce metallic, perpendicularly oriented ferromagnetic nanowires embedded in crystalline silicon. The mesoporous silicon structure consists of channels with a diameter of about 60 nm and about 30 μm in depth. The electrochemically prepared ferromagnetic nanosystem is a composition of granules and wires which leads to a peculiar magnetic behaviour. The interesting magnetic properties of this bimodal system are investigated by SQUID-magnetometry. The hysteresis loop shows two switching fields at different magnetic fields, one in the low field range and the other one in the high field range. The first switching field at about 500 Oe is due to the magnetization of the granules, the second switching field nearby 5 T (at room temperature) is caused by dipolar coupling of the nanowires which become single domain at high fields due to Bloch Wall motions. This promising ferromagnetic nanocomposite system is not only interesting in basic research but gives raise to a lot of silicon based applications like spin-injection devices and magnetic field sensors.
We present here the growth of Co-ZrO2 granular films by pulsed laser deposition (PLD). Co-ZrO2 prepared with PLD is an ideal system for investigating the properties of magnetic nanoparticle since the Co-ZrO2 interfaces are of high quality with no evidence of intermixing. The average composition of the samples is determined by x-ray photoemission spectroscopy and microprobe exsperiments. High resolution scanning electron microscopy shows existence of a regular distribution of Co nanoparticles embedded in the amorphous ZrO2 matrix. Ferromagnetic correlations among the Co nanoparticules are evident in the field-cooled state. The mean particle size and width of the distribution are determined by fitting the low-field magnetic susceptibility and isothermal magnetization in the paramagnetic regime to a distribution of Langevin functions. Magnetoresistance confirms its origin from the particle magnetization and also validates information about the particle distribution.
Flexible field responsive superparamagnetic substrates were prepared by electrospinning a solution of elastomeric polyurethane containing ferrite nanoparticles (ca. 14 nm) of Mn-Zn-Ni. The flexible mats were characterized in terms of fiber morphology and magnetic properties. Field Emission Scanning Electron Microscopy (FESEM) indicated that the diameter of these composite fibers was ca. 300-500 nm. Furthermore, the back-scattered electron FESEM images indicated agglomeration of the nanoparticles at higher wt% (ca 17-26 wt%) loading in the electrospun fibers. The induced specific magnetic saturation and the relative permeability were found to increase linearly with increasing wt% loading of the ferrite nanoparticles on the submicron electrospun fibers. A specific magnetic saturation of 1.7 – 6.3 emu/g at ambient conditions indicated superparamagnetic behavior of these composite electrospun substrates. Additionally, dielectric constant values of the electrospun fibers were measured to be between 2.3 and 5.8.
Ordering of Co nanoparticles (∼11 nm in diameter) into 2-D and 3-D arrays on Si/Si3N4 substrates in external magnetic field and without field is reported. Arrays of particles were studied by TEM, SEM and GISAXS. The GISAXS measurements were performed at the wavelengths 0.155 nm and 0.336 nm and the spectra were simulated using distorted wave Born approximation approach. From results it follows that 2-D ordered monolayers of particles are composed of hexagonal close-packed mosaic blocks. 3-D arrays – rods are formed along magnetic field direction, being parallel or perpendicular to the substrate surface, when the colloid was more concentrated. Distribution of particles in rods was analyzed only by GISAXS and it was described by close packing of hard spheres. Their effective diameter was 14.7 nm.
We developed a physical vacuum deposition technique combining an on-line sputtering/evaporation process with an integrated nanocluster deposition process to prepare core-shell type nanoparticles. High magnetic moment (Fe60Co40)coreAushell and (Fe60Co40)coreAgshell superparamagnetic nanoparticles with controllable particle size of 10 – 20 nm and Au/Ag shell thickness of 1 – 3 nm were prepared successfully by using method. Au shell is not only functional for the potential biocompatibility but also the key to prevent oxidation of FeCo nanoparticles. Saturation magnetization of (Fe60Co40)coreAushell nanoparticles was found three times higher than that of iron oxide nanoparticles. This novel technique enables us to control independently the dimensions of core and shell and select individually materials for core and shell for other core-shell type nanoparticles.
We present a novel self-assembly nanowire synthesis technique capable of producing nickelrich oxide nanowires of lengths up to 20μm and diameters as small as 5nm. The method was discovered while examining the oxidation of Alloy 600 (nickel- 15.5a/o Cr, 8a/o Fe) in a pressurized water reactor environment. The nanowires have been grown on substrates of Alloy 600 and other nickel-chromium substrates exposed to oxidizing conditions in 1500psi pressurized water with 2ppm lithium and 1200ppm boron at temperatures ranging from 238°C to 288°C. Oxidizing conditions can be controlled in one of two ways: by controlling the aqueous solution's dissolved oxygen concentration, or by use of a potentiostat. Compositional studies performed via energy dispersive spectroscopy (EDS) using a transmission electron microscope (TEM) indicate the content of the nanowires grown on Alloy 600 to be 49a/o oxygen, 47a/o Ni, and 4a/o Fe. Preliminary TEM analysis has revealed the nanowires to be single crystalline with an aspect ratio up to 1000:1. The nickel-rich oxide nanowires are particularly exciting because of their functional properties. The oxide composition of the nanowires gives them an inherent resistance to electrochemically aggressive environments, such as ones found in the body or many other aqueous solutions, in contrast to simple metal nanowires, which are susceptible to corrosion in such environments. Most importantly, analysis with a superconducting quantum interference device (SQUID) magnetometer indicates that the nickel-rich oxide nanowires are ferromagnetic with a coercivity of approximately 850e and a remnant field of 0.032emu/g at 300K.
We report Mn-doped GaN nanowires exhibiting ferromagnetism even at room temperature. The growth of single-crystalline wurtzite structured GaN nanowires doped homogeneously with about 5 atomic % Mn was achieved by chemical vapor deposition using the reaction of Ga/GaN/MnCl2 with NH3. The ferromagnetic hysteresis at 5 and 300 K and the temperature-dependent magnetization curves suggest the Curie temperature around 300 K. Negative magnetoresistance of individual nanowires was observed at the temperatures below 100 K.
We report on the self-assembled formation of iron nanowires from iron nanoparticles. Nanosized iron particles with a diameter of about 35 nm are synthesized by thermal decomposition of iron pentacarbonyl Fe(CO)5 in a hot wall reactor. This particle size is chosen to produce single domain ferromagnetic particles. As a result, the particles are attracted by magnetic forces, leading to iron nanowires of up to 300 μm in length. HRTEM and EELS investigations give detailed morphological, structural and chemical information. They reveal a big metallic iron core surrounded by an iron oxide shell with a thickness of 3-4 nm, originating from self limiting surface oxidation under ambient conditions. For electrical characterization, single iron wires are thermophoretically sampled on interdigital contacts. Impedance spectroscopy on single nanowires indicates both, capacitive and ohmic contributions to the overall conductivity. Magnetic properties are investigated with SQUID magnetometry. Magnetization measurements reveal a saturation magnetization of 160 emu/g at 5 K, which is more than 70% of the iron bulk value.
The relation between particle size and anisotropy in the fcc to L10 phase transformation in FePt nanoparticles is described. After annealing to partially sinter the particles, the volume distribution was found by transmission electron microscopy, and related to the magnetic switching field distribution. With the assumption that larger particles have higher switching fields, we find a consistent size threshold, d*, of about 12 nm for high anisotropy, independent of the annealing conditions. Our analysis shows that the phase transformation is limited by the lack of nucleation sites in small particles rather than by the chemical inhomogeneity.
Carbon nanocapsules with a ferromagnetic core of single-crystalline Fe3O4 are demonstrated to be effectively synthesized and collected separately from the other nano-carbon products of the low-temperature reaction of catalytic disproportionation of carbon monoxide. HRTEM demonstrated a defect-free crystalline structure of the Fe3O4 nanoparticles. The encapsulating carbon shells of the Fe3O4 nanoparticles are stable in air at room temperature, but do not prevent them at high temperatures. Accordingly, these nanoparticles may also act as catalysts for the corresponding production of carbon nanomaterials via carbon monoxide disproportionation. In particular, we demonstrate the corresponding transformation of a Fe3O4 core to an iron carbide nanoparticle with simultaneous formation of additional encapsulating carbon layers. Characterization of the synthesized materials by DC magnetization represents clearly resolved hysteresis loops. However characteristic S-shape of the loops (magnetization is still not saturated at 16 kOe) points out some superparamagnetic effects driven by the nano-size origin of the samples. Analysis of the sample's EPR spectra provides an additional insight to the coexistence of several magnetic phases in the synthesized nanomaterials.
Nanosized Co-doped ZnO samples were synthesized using an ultrasonic spray assisted chemical vapour deposition method. Microstructural and magnetic properties of these samples were studied. The room-temperature ferromagnetism was observed in the Co-doped ZnO. Also, x-ray analysis revealed a wurtzite ZnO structure with a small change of the lattice constants due to the doping of Co in ZnO. Raman spectroscopy of the Co-doped ZnO films indicated direct substitution of Co. Scanning electron microscopy showed nanostructured Co-doped ZnO with a ring or cup shape. Transmission electron microscopy analysis revealed nano grains within the rings of an average diameter of around 10 nm. Both energy dispersive spectroscopy and energy-filtered transmission electron microscopy indicated a uniform distribution of Co.
Highly ordered composite nanowires with multilayer of Ni/Cu, have been fabricated by pulsed electrodeposition into nanoporous alumina membrane. The diameter of wires can be easily controlled by pore size of alumina, ranging from 30 to 100 nm. The applied potential and the duration of each potential square pulse determine the thickness of the metal layers. The nanowires have been characterized by transmission electron microscopy (TEM), magnetic force microscopy (MFM), and vibrating sample magnetometer (VSM) measurements. From the result of MFM analysis, the magnetic multilayer nanowires indicate unique magnetic property. The MFM images indicate that every ferromagnetic layer separated by Cu layer was present as single isolated domain like magnet. This technique has potential for use in the measurement and application of magnetic nanodevices.
Magnetoresistance and.magnetization measurements have been performed on nanogranular, cosputtered Ag100-xFex films (x= 10 to 30) in the 4 K – 300 K temperature interval. The analysis reveals that the films with x ≤ 14 are interacting superparamagnets, characterized by a magnetic correlation length of the order of the electronic mean free path λ. Films with x ≥ 26 behave as concentrated magnets with strong competing interactions among magnetic moments (frustrated ferromagnets) and magnetic correlation length much larger than λ.
A novel method is proposed whereby non-magnetic objects can be moved along a surface at the microscale and nanoscale. It uses a negative magnetophoretic force, explained in the caption for figure one, on the non-magnetic objects which results from stabilized 10nm diameter iron oxide particles (ferrofluid) being attracted to regions of field maxima around magnetic islands on a surface, which pushes the non-magnetic objects to regions of field minima. By varying an external magnetic field we can control where these minima are and thus control how objects will position themselves with static fields and by using rotating time varying fields we can control how they move across the surface. This method does not require the objects to be initially in contact with the surface, as they will be pulled down to the surface from solution. While this paper deals with beads, any arbitrarily shaped object should be manipuable using this method. Additionally, while we address non-magnetic objects in this work similar methods could easily manipulate objects that are magnetic.