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Results for “SUPERPARAMAGNETISM”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

A chemical approach toward low temperature alloying of immiscible iron and molybdenum metals

Graphical abstract: - Highlights: • Low temperature pyrolysis of [Fe(bipy){sub 3}]Cl{sub 2} and [Mo(bipy)Cl{sub 4}] homogeneous powder. • Easy low temperature alloying of immiscible metals like Fe and Mo. • Uniform sized Fe–Mo nanoalloy with particle size of 48–68 nm. • Characterization by EDXRF, AFM, XRPD, magnetometery, {sup 57}Fe Mössbauer and impedance. • Alloy behaves as almost superparamagnetic obeying simple –R(CPE)– circuit. - Abstract: The present research is based on a low temperature operated feasible method for the synthesis of immiscible iron and molybdenum metals’ nanoalloy for technological applications. The nanoalloy has been synthesized by pyrolysis of homogeneous powder precipitated, from a common solvent, of the two complexes, trisbipyridineiron(II)chloride, [Fe(bipy){sub 3}]Cl{sub 2}, and bipyridinemolybedenum(IV) chloride, [Mo(bipy)Cl{sub 4}], followed by heating at 500 °C in an inert atmosphere of flowing argon gas. The resulting nanoalloy has been characterized by using EDXRF, AFM, XRD, magnetometery, {sup 57}Fe Mössbauer and impedance spectroscopies. These results showed that under provided experimental conditions iron and molybdenum metals, with known miscibility barrier, alloy together to give (1:1) single phase material having particle size in the range of 48–66 nm. The magnetism of iron is considerably reduced after alloy formation and shows its trend toward superparamagnetism. The designed chemical synthetic procedure is equally feasible for the fabrication of other immiscible metals.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Microwave-polyol controlled synthesis and magnetic properties of monodisperse Co{sub x}Ni{sub 1−x}Fe{sub 2}O{sub 4}/MWCNT nanocomposites

Graphical abstract: - Highlights: • Microwave-polyol preparation of monodisperse Co{sub x}Ni{sub 1−x}Fe{sub 2}O{sub 4}/MWCNT nanocomposites. • The composition is controlled through adjusting atomic ratios of Co and Ni salts. • Ms increases with increasing Co content. Hc is zero exhibiting superparamagnetic. - Abstract: Monodisperse Co{sub x}Ni{sub 1−x}Fe{sub 2}O{sub 4} nanoparticles (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) with controllable composition attached on the multi-walled carbon nanotubes (MWCNTs) were prepared by microwave-polyol method. The composition of Co{sub x}Ni{sub 1−x}Fe{sub 2}O{sub 4} nanoparticles can be controlled through adjusting the atomic ratios of cobalt and nickel nitrate in the mixed solution. The influence of the microwave power and microwave irradiation time on the monodispersion of nanoparticles was also investigated. The results show quasi-spherical Co{sub x}Ni{sub 1−x}Fe{sub 2}O{sub 4} nanoparticles with the face-centered cubic structure and average crystallite size (6 nm) are uniformly dispersed on MWCNTs. The saturation magnetization of Co{sub x}Ni{sub 1−x}Fe{sub 2}O{sub 4}/MWCNT nanocomposites increases gradually from 12.90 to 20.03 emu/g with increasing Co{sup 2+} concentration. The coercivity is almost zero at room temperature, which indicates the superparamagnetic behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Electron spin resonance study of the La{sub 0.8}Sr{sub 0.2}MnO{sub 3} nanoparticle-decorated carbon nanotubes

Graphical abstract: Inverse spin susceptibilities obtained by integration of the distinct ESR line components of the LSMO-CNTs system as a function of temperature. - Highlights: • Spin/charge dynamics in La{sub 0.8}Sr{sub 0.2}MnO{sub 3}-decorated CNTs studied by EPR. • One spin species revealed only paramagnetic–superparamagnetic phase transition. • Another spin species manifested reflected weak localization of spin/charge carriers. • Spins participating in the magnetic phase transition and the itinerant spins well separated by EPR. - Abstract: We have studied La{sub 0.8}Sr{sub 0.2}MnO{sub 3} (LSMO) nanoparticle-decorated carbon nanotubes (CNTs) by means of the electron spin resonance (ESR) spectroscopy in view of our previous work on the magnetic and electrical properties of the system. One of the line components of the ESR spectrum reflected a paramagnetic–superparamagnetic phase transition at T{sub SP}∼200 K, which is accompanied by a concomitant metal–insulator transition (MIT) associated with charge transport taking place through the CNTs network triggered by the LSMO nanoparticles. On the other hand, another ESR line component revealed anomalies at T{sub WL}∼170 K as well, attributable to a 2D weak localization effect of the spin/charge carriers. Thus, magnetic interactions and dynamics of the distinct spin species were sensitively reflected in the LSMO-CNTs system.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Ferrous sulfate based low temperature synthesis and magnetic properties of nickel ferrite nanostructures

Highlights: • Novel low temperature synthesis of nickel ferrite nanoparticles. • Comparison with two conventional synthesis techniques including hydrothermal method. • XRD results confirm the formation of crystalline nickel ferrites at 110 °C. • Superparamagnetic particles with applications in drug delivery and hyperthermia. • Magnetic properties superior to conventional methods found in new process. - Abstract: We report a simple, low temperature and surfactant free co-precipitation method for the preparation of nickel ferrite nanostructures using ferrous sulfate as the iron precursor. The products obtained from this method were compared for their physical properties with nickel ferrites produced through conventional co-precipitation and hydrothermal methods which used ferric nitrate as the iron precursor. X-ray diffraction analysis confirmed the synthesis of single phase inverse spinel nanocrystalline nickel ferrites at temperature as low as 110 °C in the low temperature method. Electron microscopy analysis on the samples revealed the formation of nearly spherical nanostructures in the size range of 20–30 nm which are comparable to other conventional methods. Vibrating sample magnetometer measurements showed the formation of superparamagnetic particles with high magnetic saturation 41.3 emu/g which corresponds well with conventional synthesis methods. The spontaneous synthesis of the nickel ferrite nanoparticles by the low temperature synthesis method was attributed to the presence of 0.808 kJ mol{sup −1} of excess Gibbs free energy due to ferrous sulfate precursor.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Amorphous iron–chromium oxide nanoparticles with long-term stability

Highlights: • Fe–Cr oxide nanoparticles with pre-established metals ratio were obtained. • The amorphous state and its long-term stability were highlighted by X-ray diffraction. • The average diameter of dried nanoparticles was 3.5 nm, as was estimated by TEM, AFM. • In hexane dispersion, nanoparticles with diameter in the range 2.33–4.85 nm were found. • Superparamagnetic state of NPs co-exists with diamagnetism of the organic layer. - Abstract: Iron–chromium nanoparticles (NPs) were obtained through the thermal decomposition of μ{sub 3}-oxo heterotrinuclear (FeCr{sub 2}O) acetate in the presence of sunflower oil and dodecylamine (DA) as surfactants. The average diameter of the NPs was 3.5 nm, as estimated on the basis of transmission electron microscopy and atomic force microscopy images. Both techniques revealed the formation of roughly approximated spheres with some irregularities and agglomerations in larger spherical assemblies of 50–100 nm. In hexane, NPs with diameters in the 2.33–4.85 nm range are individually dispersed, as emphasized by dynamic light scattering measurements. The amorphous nature of the product was emphasized by X-ray powder diffraction. The study of the magnetic properties shows the presence of superparamagnetic state of iron–chromium oxide NPs and the diamagnetic contribution from DA layer forming a shell of NPs.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Synthesis and magnetic properties of bundled and dispersed Co{sub 3}O{sub 4} nanowires

Highlights: • Co{sub 3}O{sub 4} nanowires possessed the same diameter and the different interwires distance. • All samples possessed antiferromagnetism and superparamagnetism at high temperature. • The exchange bias effect was observed at low temperature. • The surface spin coupling restrained the surface effect of magnetic nanostructures. - Abstract: The magnetic Co{sub 3}O{sub 4} nanowires were synthesized using the templates of SBA-15, and then the well-dispersed nanowires (D-wires) were separated from the bundled ordered nanowires (B-wires) with the centrifugal technique. TEM images indicated that D-wires were highly dispersed Co{sub 3}O{sub 4} nanowires and B-wires existed in bundles. All samples possessed the antiferromagnetism and superparamagnetism at high temperature. After revealing the intrinsic magnetic properties of Co{sub 3}O{sub 4} nanowires with D-wires, the magnetic behavior of B-wires was discussed in detail, and then the magnetic interaction between neighboring nanowires could be deduced. The exchange bias effect from the body Co{sub 3}O{sub 4} antiferromagnetism and surface ferromagnetism was observed at low temperature. The magnetization of B-wires was higher than that of D-wires, which was attributed to the constraint of the surface spin coupling between the neighboring nanowires to the surface affect of nanostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Quaternary M{sub 0.25}Cu{sub 0.25}Mg{sub 0.5}Fe{sub 2}O{sub 4} (M = Ni, Zn, Co, Mn) ferrite oxides: Synthesis, characterization and magnetic properties

Highlights: • Superparamagnetic quaternary nanoferrite (M{sub 0.25}Cu{sub 0.25}Mg{sub 0.5}Fe{sub 2}O{sub 4,} where M = Mn, Zn, Co, Ni) were obtained. • C, O, H and metals were observed by XPS analysis. • Phases purity were confirmed by XRD diffraction and crystallite size (3–10 nm) were determind. - Abstract: We report the synthesis of M{sub 0.25}Cu{sub 0.25}Mg{sub 0.5}Fe{sub 2}O{sub 4} (where M = Mn, Zn, Co, Ni) nanoparticles using the coprecipitation method in the presence of carboxymethyl cellulose (CMC) as the in-situ surfactant. The crystalline structure and surface morphology were examined by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) and it was established that the average diameter of the magnetic nanoparticles (MNPs) is in the range of 3–10 nm. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) show that the MNPs are activated by the hydrophilic groups of the surfactant, which coat them and enhance their stability. The vibrating sample magnetometry measurements show the superparamagnetic behavior of the nanoparticles. Due to their small crystallite size, which implies large surface area, and their functionalization with organic groups, the obtained nanoparticles could have medical and catalytic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Controlled synthesis and magnetic properties of Ni nanotubes and nanowires

Graphical abstract: TEM images of the as-prepared Ni nanotubes and nanowires. - Highlights: • Ni nanotubes and nanowires are synthesized by a convenient sol-gel autocombustion method with the assistance of AAO templates. • The products having tubular or wire-like morphologies can be perfectly controlled by adjusting the experimental conditions. • The synthetic mechanism is discussed. • Magnetic measurements indicate that both the Ni nanotubes and the Ni nanowires possess superparamagnetism at room temperature. - Abstract: Ni nanotubes and nanowires with an average diameter of about 140 nm are synthesized by a template-assisted sol-gel autocombustion method using nickel nitrate, citric acid and ammonia as the starting materials. The anodic aluminum oxide templates are prepared by a facile anodizing method. By adjusting the experimental conditions in the sol-gel process, the final products having tubular or wire-like morphologies can be perfectly controlled. Transmission electron microscope characterization indicates that the Ni nanotubes are very friable and can be easily broken into pieces of short nanotubes, but the Ni nanowires are compact and have very high aspect ratios. Magnetic measurements indicate that both the Ni nanowires and the Ni nanotubes possess superparamagnetism at room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Magnetic nanoparticles formed in glasses co-doped with iron and larger radius elements

A new type of nanoparticle-containing glasses based on borate glasses co-doped with low contents of iron and larger radius elements, Dy, Tb, Gd, Ho, Er, Y, and Bi, is studied. Heat treatment of these glasses results in formation of magnetic nanoparticles, radically changing their physical properties. Transmission electron microscopy and synchrotron radiation-based techniques: x-ray diffraction, extended x-ray absorption fine structure, x-ray absorption near-edge structure, and small-angle x-ray scattering, show a broad distribution of nanoparticle sizes with characteristics depending on the treatment regime; a crystalline structure of these nanoparticles is detected in heat treated samples. Magnetic circular dichroism (MCD) studies of samples subjected to heat treatment as well as of maghemite, magnetite, and iron garnet allow to unambiguously assign the nanoparticle structure to maghemite, independently of co-dopant nature and of heat treatment regime used. Different features observed in the MCD spectra are related to different electron transitions in Fe{sup 3+} ions gathered in the nanoparticles. The static magnetization in heat treated samples has non-linear dependence on the magnetizing field with hysteresis. Zero-field cooled magnetization curves show that at higher temperatures the nanoparticles occur in superparamagnetic state with blocking temperatures above 100 K. Below ca. 20 K, a considerable contribution to both zero field-cooled and field-cooled magnetizations occurs from diluted paramagnetic ions. Variable-temperature electron magnetic resonance (EMR) studies unambiguously show that in as-prepared glasses paramagnetic ions are in diluted state and confirm the formation of magnetic nanoparticles already at earlier stages of heat treatment. Computer simulations of the EMR spectra corroborate the broad distribution of nanoparticle sizes found by 'direct' techniques as well as superparamagnetic nanoparticle behaviour demonstrated in the magnetization studies.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

MnBi particles with high energy density made by spark erosion

We report on the properties of low-temperature phase (LTP)-MnBi particles produced by the rapid-quenching technique of spark-erosion. The as-prepared powder consists of amorphous, crystalline, and superparamagnetic particles, mostly as porous aggregates. The major fraction of the powder consists of 20–30 nm particles. A short anneal crystallizes the amorphous particles producing a high moment, >90% of theoretical M{sub S}, albeit with H{sub C} of a few kOe. If lightly milled, the agglomerates are broken up to yield H{sub C} of 1 T. These findings are supported by the x-ray diffraction pattern showing broadened peaks of the predominant LTP-MnBi phase. The combination of spark erosion, milling, and annealing has produced randomly oriented particles with (BH){sub MAX} ∼ 3.0 MGOe. The particles are expected to show record energy product when aligned along their crystallographic easy axes.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Evidence for spin glass state of NdCo{sub 1−x}Ni{sub x}O{sub 3} (x = 0.3−0.5)

Low-temperature magnetic properties of single phase NdCo{sub 1−x}Ni{sub x}O{sub 3}(x = 0.3−0.5) have been studied using ac and dc magnetic susceptibility measurements. Nickel substituted samples have been found to exhibit a different magnetic state at low temperature as compared to pristine NdCoO{sub 3}. The temperature dependent dc magnetization M (T) revealed the presence of a sharp cusp occurring at characteristic temperatures T{sub P}, for x = 0.3, 0.4, 0.5. Below T{sub P}, clear effect of magnetic field can be seen in M (T) curves and T{sub P} decreases with increasing magnetic field as well as Ni substitution content. The isothermal magnetization measurements at low temperatures shows small unsaturated hysteresis loop at lowest temperature (10 K). The ac susceptibility results show a clear frequency dependent feature. These results are analyzed to distinguish superparamagnetic and spin glass behavior by using Néel-Arrhennius, Vogel-Fulcher law, and power law fitting. This analysis ruled out the superparamagnet like state and suggests the presence of significant inter-cluster interactions, giving rise to spin-glass like cooperative freezing.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Surface driven effects on magnetic properties of antiferromagnetic LaFeO{sub 3} nanocrystalline ferrite

LaFeO{sub 3} nanocrystalline ferrites were synthesized through sol-gel method in different size distributions and the effect of finite size on magnetic properties is investigated. Results of magnetization and Mössbauer measurements show that superparamagnetism and weak ferromagnetic behavior in some of the size distributions. The origin of the superparamagnetism is from fine particles similar to ferromagnetic single domains and the weak ferromagnetism comes from surface spin disorder caused by Dzyaloshinskii-Moriya interaction. The magnetic ground state of LaFeO{sub 3} nanoparticles differs from that of bulk, and the ground state is dictated by the finite size effect because density of states depends on the dimensionality of the sample.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Tuning of structural and magnetic properties of Mn-doped CoFe{sub 2}O{sub 4} nanoparticles

In the present manuscript, Mn{sub x}Co{sub 1-x}Fe{sub 2}O{sub 4} (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoparticles have been synthesized with co-precipitation method by annealing at 900 °C and studied the structural and magnetic properties. X ray diffraction pattern reveals the formation of single phase cubic spinel structure with the space group Fd-3m. An impurity peak of α-Fe{sub 2}O{sub 3} has been observed in the XRD patterns of samples with Mn composition, x ≥ 0.4. However, this impurity phase of α-Fe{sub 2}O{sub 3} is not observed in the MnFe{sub 2}O{sub 4} sample synthesized by annealing at 500 °C. FESEM results indicate the formation of nano-sized particles. Magnetic measurements show ferrimagnetic interaction in Mn{sub x}Co{sub 1-x}Fe{sub 2}O{sub 4} (x≥0) nanoparticles at room temperature. However, MnFe{sub 2}O{sub 4} sample synthesized by annealing at 500 °C shows superparamagnetic behavior. We have fitted the M-H curve of MnFe{sub 2}O{sub 4} synthesized by annealing at 500 °C with modified Langvenin function to confirm superparamagnetism. The correlation between the structural and observed magnetic properties of as synthesized samples nanoparticles will be described and discussed in this paper.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Experimental evidence for the formation of CoFe{sub 2}C phase with colossal magnetocrystalline-anisotropy

Attainment of magnetic order in nanoparticles at room temperature is an issue of critical importance for many different technologies. For ordinary ferromagnetic materials, a reduction in size leads to decreased magnetic anisotropy and results in superparamagnetic relaxations. If, instead, anisotropy could be enhanced at reduced particle sizes, then it would be possible to attain stable magnetic order at room temperature. Herein, we provide experimental evidence substantiating the synthesis of a cobalt iron carbide phase (CoFe{sub 2}C) of nanoparticles. Structural characterization of the CoFe{sub 2}C carbide phase was performed by transmission electron microscopy, electron diffraction and energy electron spectroscopy. X-ray diffraction was also performed as a complimentary analysis. Magnetic characterization of the carbide phase revealed a blocking temperature, T{sub B}, of 790 K for particles with a domain size as small as 5 ± 1 nm. The particles have magnetocrystalline anisotropy of 4.6 ± 2 × 10{sup 6 }J/m{sup 3}, which is ten times larger than that of Co nanoparticles. Such colossal anisotropy leads to thermally stable long range magnetic order. Moreover, the thermal stability constant is much larger than that of the commonly used FePt nanoparticles. With thermal stability and colossal anisotropy, the CoFe{sub 2}C nanoparticles have huge potential for enhanced magnetic data storage devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Enhancement in magnetic properties of magnesium substituted bismuth ferrite nanoparticles

We report a potential way to effectively improve the magnetic properties of BiFeO{sub 3} (BFO) nanoparticles through Mg{sup 2+} ion substitution at the Fe-sites of BFO lattice. The high purity and structural changes induced by Mg doping are confirmed by X-ray powder diffractometer and Raman spectra. Enhanced magnetic properties are observed in Mg substituted samples, which simultaneously exhibit ferromagnetic and superparamagnetic properties at room temperature. A physical model is proposed to support the observed ferromagnetism of Mg doped samples, and the superparamagnetic properties are revealed by the temperature dependent magnetization measurements. The improved magnetic properties and soft nature obtained by Mg doping in BFO nanoparticles demonstrate the possibility of BFO nanoparticles to practical applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Alternate current magnetic property characterization of nonstoichiometric zinc ferrite nanocrystals for inductor fabrication via a solution based process

We investigate the ac magnetic behavior of solution processable, non-stoichiometric zinc ferrite nanocrystals with a series of sizes and zinc concentrations. Nearly monodisperse Zn{sub x}Fe{sub 3−x}O{sub 4} nanocrystals (x = 0–0.25) with an average size ranging from 7.4 nm to 13.8 nm are synthesized by using a solvothermal method. All the nanocrystals are in a superparamagnetic state at 300 K, which is confirmed by Superconductive Quantum Interference Device magnetometry. Due to the doping of non-magnetic Zn{sup 2+} into A site of ferrite, the saturation magnetization of nanocrystals increases as the size and Zn concentration increases. The ac magnetic permeability measurements at radio frequencies reveal that the real part of the magnetic permeability of similarly sized ferrite nanocrystals can be enhanced by almost twofold as the Zn{sup 2+} doping level increases from 0 to 0.25. The integration of 12.3 nm Zn{sub 0.25}Fe{sub 2.75}O{sub 4} nanocrystals into a toroidal inductor and a solenoid inductor prepared via a simple solution cast process yields a higher quality factors than air core inductors with the same geometries up to 5 MHz and 9 MHz, respectively, which is in the regime of the switching frequencies for the advanced integrated power converters.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Field induced polarization and magnetization behaviour of Gd-doped lead magnesium niobate ceramics

Both superparaelectric and superparamagnetic behaviour has been observed in rare earth magnetic ion Gd{sup 3+} doped Lead Magnesium Niobate (Gd-PMN). Field induced polarization and magnetization studies reveal hystresis loss free P-E and M-H loop at 300 K and 5 K, respectively. Temperature dependence of inverse susceptibility plot shows deviation at a temperature “t{sub d}” when fitted with the Curie-Weiss law. This deviation has been attributed to transition from paramagnetic to superparamagnetic behaviour as reported in amorphous Pd-Ni-Fe-P alloys.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Role of spin-transfer torques on synchronization and resonance phenomena in stochastic magnetic oscillators

A theoretical study on how synchronization and resonance-like phenomena in superparamagnetic tunnel junctions can be driven by spin-transfer torques is presented. We examine the magnetization of a superparamagnetic free layer that reverses randomly between two well-defined orientations due to thermal fluctuations, acting as a stochastic oscillator. When subject to an external ac forcing, this system can present stochastic resonance and noise-enhanced synchronization. We focus on the roles of the mutually perpendicular damping-like and field-like torques, showing that the response of the system is very different at low and high frequencies. We also demonstrate that the field-like torque can increase the efficiency of the current-driven forcing, especially at sub-threshold electric currents. These results can be useful for possible low-power, more energy efficient applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗