Showing papers on "Superparamagnetism published in 1995"

Soft magnetic nanocrystalline materials

Abstract: The microstructure property relationship in soft magnetic nanocrystalline Fe-base materials is surveyed: The basic mechanism is that the magneto-crystalline anisotropy of the small, randomly oriented bcc-Fe grains is averaged out by exchange interaction. The resulting magnetic behaviour can be well described with the random anisotropy model. The latter has been extended to multiphase materials including the case that the grain size exceeds the exchange length and, is able now to describe the hardening mechanism in the overannealed nanocrystalline state due to the precipitation of borides. Further, the crucial role of the coupling mechanism between the grains is discussed. Thus, as a function of temperature, a single sample combines a variety of phenomena ranging from soft, over hard to finally superparamagnetic behaviour. Finally it is shown, how the structural phases present lead to low or vanishing magnetostriction; the random anisotropy effect guarantees an essentially isotropic behaviour comparable to the amorphous case. Together with the suppressed magneto-crystalline anisotropy, the low magnetostriction provides the basis for the superior soft magnetic properties observed in particular compositions.

Theory of 1/T1 and 1/T2 NMRD profiles of solutions of magnetic nanoparticles.

Abstract: Organically coated iron oxide crystallites with diameters of 5-50 nm ("nanoparticles") are potential magnetic resonance imaging contrast agents. 1/T1 and 1/T2 of solvent water protons are increased dramatically by magnetic interactions in the "outer sphere" environment of the nanoparticles; subsequent diffusive mixing distributes this relaxation throughout the solvent. Published theory, valid for the solute magnetic energy small compared with thermal energy, is applicable to small magnetic solutes (e.g., gadolinium and manganese diethylenetriaminopentaacetic acid, and nitroxide free radicals) at generally accessible fields (< or = 50 T). It fails for nanoparticles at fields above approximately 0.05 T, i.e., at most imaging fields. The authors have reformulated outer sphere relaxation theory to incorporate progressive magnetic saturation of solute nanoparticles and, in addition, indicate how to use empirical magnetization data for realistic particles when their magnetic properties are not ideal. It is important to handle the effects of rapid thermally induced reorientation of the magnetization of the nanoparticles (their "superparamagnetism") effectively, including their sensitivity to particle size. The theoretical results are presented as the magnetic field dependence (NMRD profiles) of 1/T1 and 1/T2, normalized to Fe content, for three sizes of particles, and then compared with the limited data extant for well-characterized material.

Enhanced magnetization of nanoscale colloidal cobalt particles.

Abstract: We have used a microemulsion technique to synthesize metallic cobalt particles in the size range 18 to 44 \AA{} diameter. The particles are spherical, not aggregated due to their surfactant coating, and free of oxide. Magnetic properties such as total moment per particle, blocking temperature, and hysteresis all show reasonable size dependencies. The effects of small size are seen in: (1) the anisotropy constant increased markedly as particle size decreased, and (2) the total magnetic moment per atom in the Co particles was enhanced with decreasing particle size by as much as 30% over the bulk value. Magnetization versus applied field curves indicate the particles are heterogeneous with two magnetic phases, possibly a core-shell structure. The core phase has a large total moment and the shell phase is superparamagnetic with an effective moment of 7.5\ifmmode\pm\else\textpm\fi{}1${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for all sizes. We propose that the shell phase is responsible for the enhanced anisotropy and magnetization.

Magnetism and Giant Magneto-Transport Properties in Granular Solids

Abstract: Magnetic granular solids consist of nanometer-sized magnetic particles uniformly dispersed and embedded in an immiscible insulating or metallic medium. They display a rich variety of magnetic properties and giant magneto-transport properties due to their unique nanostructure, which can be usefully described by the metal volume fraction and the metal particle diameter of granular solids. Fabrications and characterizations of granular solids, their enhanced magnetic properties, percolation behavior, single-domain characteristics, superparamagnetic relaxation, and superparamagnetism are reviewed. Giant magneto-transport properties, recently observed in granular solids, including giant magnetoresistance, extraordinary Hall resistivity, magneto-thermal conductivity, and Wiedemann-Franz law are discussed.

Magnetic behaviour of γ-Fe 2 O 3 nanoparticles by mössbauer spectroscopy and magnetic measurements

Abstract: γ-Fe2O3 nanoparticles with varying state of dispersion in a polymer have been investigated by Mossbauer spectroscopy, static magnetic measurements at low applied field, and alternative susceptibility measurements over a large range of frequencies (2×10−2–104 Hz). The dynamical behaviour was characterized through the variation of the blocking temperature with the characteristic time of the measurement. The Mossbauer blocking temperature was determined according to a procedure described. For quasi-isolated particles an Arrhenius law is demonstrated. Effects of interparticle interactions in concentrated and aggregated systems are satisfactorily explained by the previous model. Dependence of the superparamagnetic susceptibility on the experimental conditions interpreted using the Lorentz or Onsager fields is mentioned.

Electrochemical growth of superparamagnetic cobalt clusters

Abstract: The magnetization of stabilized cobalt colloids in tetrahydrofuran has been studied by a superconducting quantum interference device (SQUID) and magnetic balance measurements in dependence of applied magnetic field and temperature. The colloids are generated by a newly developed electrochemical method which allows one to generate clusters containing about 1000 atoms with a narrow size distribution. The final size distribution of the clusters is examined by high resolution transmission electron microscopy and small angle x‐ray scattering. The magnetization curves have been determined with special emphasis on changes at the freezing point of the solution. The curves of the liquid phase can be reasonably described by the Langevin function and the magnetic moments of isolated cobalt clusters that have been recently measured by Stern–Gerlach experiments. Deviations that appear at the freezing point can be understood in terms of magnetic anisotropy effects. It is shown that the cluster sizes and the susceptibil...

Constant-magnetic-field effect in Néel relaxation of single-domain ferromagnetic particles.

Abstract: The relaxation behavior of an assembly of noninteracting single-domain ferromagnetic particles in the presence of a constant magnetic field is studied by solving the corresponding Fokker-Planck equation. The analysis is performed by first converting that equation into a hierarchy of differential-recurrence relations by expanding the solution in Legendre polynomials. The spectrum of eigenvalues and their associated amplitudes is then determined by matrix methods where all the desired physical quantities such as the magnetization correlation time and complex magnetic susceptibility may be computed numerically. In order to ensure the accuracy of the results obtained this solution is compared with an exact solution derived in terms of matrix continued fractions. It is shown that the conventional assumption in the theory of superparamagnetism, that except in the very early stages of relaxation to equilibrium the only appreciable time constant is the one associated with the smallest nonvanishing eigenvalue, is no longer true when an applied constant magnetic field exceeds a certain critical value. The breakdown of this assumption manifests itself in (a) a dramatically large deviation of the magnetization correlation time (area under the curve of the decay of the magnetization) from the inverse of the lowest eigenvalue, and (b) in the presence of relatively strong high-frequency modes superimposed on the N\'eel one usually assigned to the lowest eigenvalue. The results are compared with available experimental data.

Superparamagnetism and reduction diagenesis in pelagic sediments: enhancement or depletion?

Abstract: Superparamagnetic grains, ultra-fine (d < 0.05µm) particles unable to retain a remanence at room temperature, have come under increased scrutiny as indicators of the diagenetic and authigenic history of sedimentary rocks. In marine sediments, fine-grained magnetite with large surface area to volume is thought to be preferentially removed during magnetic mineral reduction. On the basis of new magnetic hysteresis, low-temperature, and geochemical studies of pelagic sediments from the western equatorial Pacific Ocean, it is proposed that reduction processes increase rather than deplete the ultra-fine magnetic grain population. At the modern Fe-redox boundary enhanced superparamagnetism coincides with a coarsening of remanence-carrying grains. Superparamagnetism also tracks proposed temporal changes in magnetite reduction caused by climatically-driven fluctuations in organic carbon (Corg) supply. Together with hysteresis characteristics, changes in superparamagnetism may help the identification of paleointensity artifacts resulting from non steady-state magnetic mineral reduction.

Field-Cooled and Zero-Field-Cooled Magnetization of Superparamagnetic Fine Particles in Cu97Co3 Alloy: Comparison with Spin-Glass Au96Fe4 Alloy

Abstract: The field-cooled (FCM) and zero-field-cooled (ZFCM) magnetization of ferromagnetic fine cobalt particles in a Cu 97 Co 3 alloy has been studied. The spin-glass-like temperature dependence of the magnetization has been observed; ZFCM exhibits a spin-glass-like maximum (at T p ) and FCM is larger than ZFCM at low temperatures. However, the difference between FCM and ZFCM obviously exists far above T p . Furthermore, FCM increases monotonically with decreasing temperature even below T p while that of typical spin glasses is nearly independent of temperature. The analysis of the magnetization shows that the temperature dependence of FCM and ZFCM of Cu 97 Co 3 is well described on the basis of the superparamagnetic blocking model with no interaction between the particles, whereas that of a typical spin-glass Au 96 Fe 4 cannot be explained by the blocking model.

Proton Transverse Nuclear Magnetic Relaxation in Oxidized Blood: a Numerical Approach

Abstract: When red blood cells are deoxygenated, hemoglobin, which is then transformed into deoxyhemoglobin or methemoglobin, becomes paramagnetic. The transverse nuclear magnetic relaxation rate of water protons is considerably enhanced by this chemical transformation. A general agreement exists about the origin of the phenomenon--local field inhomogeneities induced by paramagnetic centers randomly distributed within the cell--but the localization of the region that dominates the relaxation is unclear. We addressed this problem with a computer simulation devoted to the determination of transverse magnetic relaxation of water protons in the presence of superparamagnetic MRI contrast agent candidates. The simulation confirms an earlier experimental result that shares equitably the responsibility for the observed relaxation between intracellular and extracellular water.

Superparamagnetic particles, process for their manufacture and usage

Abstract: Superparamagnetic particles consist of superparamagnetic one-domain particles and aggregates of superparamagnetic one-domain particles to whose surfaces are bound organic substances optionally having further binding sites for coupling to tissue-specific binding substances, diagnostic or pharmacologically active substances. The superparamagnetic particles consist of a mixture of small superparamagnetic one-domain particles with a particle size from 3 to 50 nanometers and stable, degradable aggregates of small superparamagnetic one-domain particles with a particle size from 10 to 1000 nanometers. They are made of iron hydroxide, iron oxide hydrate, iron oxides, iron mixed oxides or iron to the surface of which are bound mono- and/or polyhydroxylic group-containing aromatic substances, polyglycerines, amino-acid-containing substances, silicate group-containing substances among the orthosilicic acids and their condensation products and phosphate group-containing substances among the ortho- or metaphosphoriic acids and their condensation products. These substances may have further binding sites. These new particles may be used to destroy tumors, increase immunity, in magnetic drug targeting, for cell fusion, gene transfers, as contrasting agents, for in vitro diagnosis, as magnetic ion exchangers and magnetic adsorbents, if required by exposure to magnetic fields.

AC Susceptibility Relaxation Studies on a Manganese Organic Cluster Compound: Mn12Ac

Abstract: We present static and dynamical magnetic properties of a new organic cluster compound, Mn12Ac. High field magnetization and susceptibility data indicate that the clusters have a S=10 ground state with a high anisotropy and are magnetically well isolated from each other. Low temperature specific heat results are in agreement with this picture. Those properties make this system ideal to study superparamagnetic relaxation and possible quantum effects at the nanoscopic scale at very low temperatures. Above 4K the relaxation times follow an Arrhenius law \( \tau = {\tau _o}{e^{\Delta /k}}{_b^T} \) with Δ/kb~64 K, in agreement with high field magnetization, and τO~10-7 S. AC susceptibility data under dc field show a new and unexpected field dependence of the relaxation time that seems to indicate that thermally activated Quantum Tunneling of the Magnetization through the anisotropy energy barrier occurs.

The influence of particle size and interactions on the magnetization and susceptibility of nanometre-size particles

Abstract: We studied the magnetic properties of frozen magnetic liquids containing amorphous Fe1-xCx particles. By varying the particle size and concentration, we could separate single particle effects from interactions. In samples containing particles with median diameters 5.0 nm and 3.8 nm and with spontaneous saturation magnetizations 7.1 kA m-1 and 9.2 kA m-1, effects of particle interactions are observed. For 3.2 nm no interactions were observed. In samples with negligible interactions the superparamagnetic blocking temperature increases with the particle size. The effective anisotropy constant increases with decreasing particle size, implying that the anisotropy is enhanced by surface effects.

Structure and magnetic properties of magnesium ferrite fine powders

Abstract: Fine powders of magnesium ferrite, MgFe2O4, were produced through the sol-gel supercritical drying method, with two portions then being calcined at 773 K and 1073 K. The powder structural and magnetic properties were determined from transmission electron microscope micrographs, x-ray diffraction, Mossbauer effect spectroscopy and magnetometry measurements. The powder structure matched the MgFe2O4 spinel phase, with small amounts of α-Fe2O3 being observed in heated samples. As-produced powders were superparamagnetic at room temperature, with single magnetic domain particle behavior being observed at low temperatures, and for the 1073 K heated sample. The particle size distribution for the as-produced powder was evaluated separately from the micrographs, by fitting the magnetization data to a weighted Langevin function, and by fitting Mossbauer spectra taken at temperatures from 25 K to 298 K. Very similar particle size distributions were found from all three methods. The average particle diameter was 11 nm for the as-produced powder, and increased for heated samples. The saturation magnetization and magnetocrystalline anisotropy energy density values were both consistent with bulk values, in contrast to the large differences between particle and bulk values described for other fine particle systems.

Zero-field cooled magnetization of amorphous Fe1-xCx particles - field dependence of the maximum

Abstract: We measured the zero-field cooled magnetization, MZPC of a frozen magnetic liquid with amorphous Fe1-xCx particles. MZFC has a maximum at Tp approximately=20 K, which is interpreted as due to blocking of superparamagnetic relaxation in single particles. The maximum shifts towards higher temperatures with increasing measuring field. The shift is explained by the nonlinear field dependence of the magnetization of particles with a size distribution. At temperatures below Tp the magnetic particles have a coercivity and remanence in good agreement with those expected for an ensemble of non-interacting uniaxial particles with a random distribution of easy axes.

Ferrimagnetic zinc ferrite fine powders

Abstract: Partially inverted fine powders of zinc ferrite (ZnFe/sub 2/O/sub 4/) were produced by chemical aerogel synthesis. These powders showed magnetic ordering at temperatures to 250 K. Upon ball milling, a much higher inversion parameter was attained, yielding ferrimagnetic powders having small remanence and coercive fields at room temperature. Due to the small mean particle size, superparamagnetic behavior was seen at temperatures above 25 K. Powder properties were characterized by X-ray diffraction, electron microscopy, calorimetry, magnetometry, and Mossbauer effect spectroscopy techniques.

Superparamagnetic relaxation of Fe deposited on MgO(001).

Abstract: Superparamagnetic behavior is investigated for Fe grown at 700 K onto MgO(001) to a thickness equivalent to that of a ten monolayer film. Two such Fe deposits separated by a 200-A deposit of MgO exhibit a ferromagnetic response with no hysteresis at either 300 or 150 K, but with identical reduced magnetization curves {ital M}({ital H}/{ital T}) which confirms the existence of superparamagnetism. {ital M}({ital H}) data at 300 K were fitted to a Langevin function to yield an average particle size of 100 A diameter. {ital M}({ital T}) for field-cooled and zero-field-cooled samples shows behavior characteristic of superparamagnetic particles with a distribution in particle size. Time-dependent remanent magnetization data measured over a 20 h period at various temperatures show nonexponential decay attributed to the distribution in particle size and interactions among the particles.

Synthesis, structure, properties and magnetic applications of carbon-coated nanocrystals produced by a carbon arc

Abstract: Out of the fullerene field has grown the exciting potential for synthesis of magnetic nanoparticles of both scientific and technical interest. We employ a Kratschmer-Huffman carbon are method to synthesize carbon-coated elemental transition metal and alloy particles with nanocrystalline dimensions. These have included FeCx, Co[C] and Ni[C] ferromagnetic particles. Recently, SmCox[C] particles with nanocrystalline dimensions have been produced by this method starting with a Sm2Co7 metallic precursor. Structural characterization by X-ray diffraction and high resolution transmission electron microscopy illustrate the phases present and the particle size distribution. Magnetic properties, as determined by superconducting quantum interference device magnetometry are also reported. These include hysteretic, temperature and time-dependent magnetic responses and an inferred superparamagnetic response. A unifying theme in this research is the carbon are synthesis of materials with interesting magnetic properties and assessment of their use in xerography, magnetic resonance imaging, ferrofluids, data storage and magnetic inks.

An investigation of the electrodeposited inhomogeneous alloyed film Cu0.94Co0.06 by means of SQUID magnetometry

Abstract: The magnetic behaviour of the inhomogeneous alloyed film, Cu0.94Co0.06, produced by electrodeposition has been investigated in the temperature range 2 to 300 K and in magnetic fields of up to 5 T using SQUID magnetometry. The films exhibit either hysteretic or reversible behaviour depending upon the temperature. This is interpreted as being due to the presence of small, FCC, Co-rich particles of FCC Co in the non-magnetic matrix, giving rise to superparamagnetism at higher temperatures. With the help of low-field susceptibility measurements made on samples cooled in zero field we have been able to determine the distribution of blocking temperatures and thus make an estimate of the size distribution of the particles in the Cu matrix; these have been found to range in diameter from 6 to 12 nm. These results have been compared with values obtained from a measurement of the temperature dependence of the remanence. We have found that, even in such a relatively dilute magnetic system as Cu0.94Co0.06, interactions between magnetic clusters are not negligible. The interaction energy is estimated to be of the order of 1.7*10-3 eV.

Template-Controlled Synthesis of Superparamagnetic Goethite within Macroporous Polymeric Microspheres.

Abstract: A superparamagnetic form of goethite, α-[FeO(OH)], has been obtained within macroporous crosslinked poly(divinylbenzene) microspheres (average diameter 3.6 μm and pore size 50-200 nm) by a chemical process involving (1) sulfonation of the internal and external surfaces of the microspheres, (2) treatment of the sulfonated microspheres with aqueous ferrous chloride solution, and (3) oxidation with hydrogen peroxide, pH 14, 70 °C. Steps 2 and 3 were repeated four times, yielding particles containing up to 19% Fe per total weight of material. TEM micrographs of sectioned microspheres revealed the presence of two forms of goethite : disks 25 nm in diameter and needles 80 nm long and 25 nm wide. Temperature dependent magnetization and susceptibility data indicate that the materials are superparamagnetic above 50 K. The calculated magnetic susceptibility for the particles is 3 orders of magnitude larger than that of bulk goethite. Aqueous solutions of ferrous chloride were oxidized under the same conditions (H 2 O 2 , pH 14, 70 °C) (1) in the absence of microspheres and (2) in the presence of nonporous polystyrene latex particles carrying surface sulfate groups. Neither situation yielded superparamagnetic goethite.