Showing papers on "Superparamagnetism published in 1991"
TL;DR: Results of Stern-Gerlach experiments for all the systems studied to date are reported, finding that the rare earths gadolinium and terbium exhibit superparamagnetic behavior for certain clusters, while other clusters exhibit behavior consistent with their moment being locked to the crystal lattice.
Abstract: We report in this paper results of Stern-Gerlach experiments for all the systems we have studied to date. Three distinct behaviors are observed. Cobalt exhibits superparamagnetic behavior, with a magnetic moment per atom of 2.29 ± 0.08µ B for N = 115. The rare earths gadolinium and terbium exhibit superparamagnetic behavior for certain clusters, while other clusters exhibit behavior consistent with their moment being locked to the crystal lattice. In both cases, the moments are considerably reduced from the bulk values. The third behavior is no measurable deflection at all, as demonstrated by vanadium, palladium, chromium, and aluminum.
379 citations
TL;DR: In this paper, it was shown that the size of the particle rather than the presence of nonstoichiometry or impurities of reduced nickel determines the magnetic moment of a particle.
Abstract: It has been known for some time that particles of nickel oxide of less than about 100 nm in size show superparamagnetism that increases as the particle size decreases. The origin of the particle magnetic moment responsible for this behavior has never been fully explained. This research indicated that the size of the particle rather than the presence of nonstoichiometry or impurities of reduced nickel determines the moment. The critical experiment was the measurement of magnetization versus magnetic field for a sample of nickel oxide prepared under conditions that preclude metallic nickel. Almost identical results were found for the original sample, which was black in color and thus nonstoichiometric, and after mild reduction in hydrogen at 400 K, which produced stoichiometry and changed the color to green. The magnetic susceptibility was inversely proportional to the particle size for a given method of preparation. This is consistent with a simple model of incomplete edges on the bounding planes of the crystallite and provides a possible basis for a practical method for measuring particle size in nickel oxide‐containing samples.
226 citations
TL;DR: In this article, the size dependence of magnetic properties were studied and the critical size for superparamagnetism at room temperature is about 15 nm, where the specific magnetic moment drastically decreases when reducing the diameter d of particles < 15 nm.
Abstract: Ultrafine nickel particles have intriguing physical and chemical properties, which are interesting both in fundamental and applied research. The size of the particle was controlled by gas pressure. X‐ray diffraction studies showed that fine Ni particles have fcc crystal structure and are coated with thin Ni oxide on the surface. Electron micrographs showed a spherical particle shape, forming a long chain. Size dependence of magnetic properties were studied. The specific magnetic moment drastically decreases when reducing the diameter d of particles <15 nm. The coercivity Hc also approaches zero when d is about 15 nm. Therefore, we can suppose that the critical size for superparamagnetism at room temperature is about 15 nm. According to the superparamagnetic formula KV=25 kT, the value of the magnetic anisotropy constant can be determined, K = −5.8 × 105 (erg/cc). It is found to be larger than bulk Ni [K=−3.4–5.1 × 104 (erg/cc)], the same as Fe particles. The maximum of coercivity at room temperature is ab...
94 citations
01 Feb 1991
TL;DR: In this paper, microfine magnetic particles of magnetite with a mean diameter of 10-15 nm, which is in the range of superparamagnetism, were prepared by coprecipitation from ferrous and ferric electrolyte solution and applied to the enzyme immobilization support.
Abstract: Microfine magnetic particles of magnetite with a mean diameter of 10–15 nm, which is in the range of superparamagnetism, were prepared by coprecipitation from ferrous and ferric electrolyte solution and applied to the enzyme immobilization support. The surface of the particles was treated by a series of amino-functional silane coupling agents capable of covalently bonding with enzyme. A protease, thermolysin, was immobilized on the particles and transferred to ethyl acetate solution containing N-Cbz- l -aspartic acid and l -phenylalanine methyl ester. The dipeptide, aspartame precursor synthetic reaction was then carried out. Higher enzyme activity was found when the silane coupling agents with long spacer arms between the silane functionality and amino functionality were used such as N-(6-aminohexyl)-3-aminopropyltrimethyoxysilane, compared to conventional 3-aminopropyltrichoxysilane. Magnetic recovery of the immobilized enzyme was also demonstrated upon the application of an external magnetic field.
48 citations
TL;DR: The magnetization and susceptibility of superparamagnetic AMI‐25 are far larger than that of ferritin or low molecular weight, paramagnetic chelates, and are a uniquely powerful class of magnetic materials.
Abstract: We have compared the magnetic properties of various types of materials known to affect MR images. The materials compared were: (i) MR contrast agents based on chelates of paramagnetic metals (Gd-DTPA, Dy-DTPA); (ii) biological forms of iron (horse spleen ferritin and deoxyhemoglobin); and (iii) a superparamagnetic iron oxide (AMI-25). The properties compared were the magnetic susceptibility and the magnetization. The magnetization and susceptibility of superparamagnetic AMI-25 are far larger than that of ferritin or low molecular weight, paramagnetic chelates. Superparamagnetic iron oxide colloids, like AMI-25, are a uniquely powerful class of magnetic materials. © 1991 Academic Press, Inc.
38 citations
TL;DR: In this article, the average blocking temperatures of ferritin molecules containing differing amounts of iron were determined by Mossbauer spectroscopy, and it was shown that the magnetic anisotropy of the ferric core particles is a function of particle volume.
Abstract: The average blocking temperatures of ferritin molecules containing differing amounts of iron were determined by Mossbauer spectroscopy. The results imply that the magnetic anisotropy of the ferritin core particles is a function of particle volume. By addition of57Fe to ferritin core particles it was determined that, at a given temperature within the superpara-magnetic temperature region, the “last-in” ferric ions have average relaxation times that are shorter than those of the bulk ferric ions.
31 citations
Patent•
20 Feb 1991
TL;DR: In this paper, a superparamagnetic fine particles of iron oxide, suitable for use in a magnetic recording medium, have an unsaturated fatty acid adsorbed on the surface thereof, a magnetization of not less than 50 Amµ/kg (50 emu/g), and an Feµ³ content (calculated as a Fe³:Fe³µ molar ratio) of from 0.16 to 0.5:1.
Abstract: Superparamagnetic fine particles of iron oxide, suitable for use in a magnetic recording medium, have an unsaturated fatty acid adsorbed on the surface thereof, a magnetization of not less than 50 Am²/kg (50 emu/g), a rate of change of magnetization of not more than 10%, and an Fe²⁺ content (calculated as a Fe²⁺:Fe³⁺ molar ratio) of from 0.16 to 0.5:1.
26 citations
TL;DR: In this paper, the magnetic properties of small neutral transition-metal clusters like Fen and Con are discussed, and the authors claim that the asymmetric Stern-Gerlach deflection profiles are due to a transfer from electronical angular momentum to the cluster rotation, allowing for a depopulation of high energy magnetic levels.
Abstract: We discuss the magnetic properties of small neutral transition-metal clusters like Fen and Con deduced from Stern-Gerlach deflection experiments. We claim that the asymmetric Stern-Gerlach deflection profiles are due to a transfer from electronical angular momentum to the cluster rotation, allowing for a depopulation of the high energy magnetic levels. For finite temperatures we consider two limiting cases. First, the cluster magnetization is assumed to be tied to the random orientation of the cluster easy axes due to the lattice anisotropy. This causes a surprisingly small magnetization for small external magnetic fields. For larger fields and also for increasing temperatures the magnetization is released from the cluster geometry and allowed to align itself parallel to the field. In the second case the clusters are treated as an ensemble of superparamagnetic particles. Here, the effect of the anisotropy is less visible. The cluster lattice anisotropy per atom is expected to decrease for increasing cluster size. Preliminary results support this.
20 citations
TL;DR: In this paper, aqueous ferrofluids based on Mn and Co ferrites have been synthesized by a novel method, and the Mossbauer spectra of dried samples (average particle diameter ≈ 10 nm) were measured in the 77-340 K temperature range.
Abstract: Aqueous ferrofluids based on Mn and Co ferrites have been synthesized by a novel method. Mossbauer spectra of dried samples (average particle diameter ≈ 10 nm) were measured in the 77–340 K temperature range. CoFe2O4 spectra show no superparamagnetic (SP) relaxation, in accordance with the high magnetic anisotropy of this compound. MnFe2O4 spectra exhibit SP relaxation, from which an effectiveK=(8±3)×104 J/m3 is estimated. This value represents a 20× enhancement over intrinsic magnetocrystalline anisotropy.
16 citations
TL;DR: In this paper, the ferromagnetic properties of amorphous oxides with superparamagnetic properties were determined by measuring magnetization measurements at room temperature, and it was found that the magnetization tended to saturate at relatively low external fields.
Abstract: New amorphous oxides with ferromagnetic character have been found in the Sr‐Fe‐O and Bi‐Fe‐O systems. The compositions were 35SrO⋅65Fe2O3 (mol%) and Bi3Fe5O12, and the specimens were prepared by using the twin‐roller quenching method. The ferromagnetic character was ascertained by means of magnetization measurements at room temperature. Although the magnetization tended to saturate at relatively low external fields for both systems, hyperfine structure in the Mossbauer spectra was barely observed at room temperature for Bi3Fe5O12 only. It is thought that this phenomenon is due to the superparamagnetic behavior of microcrystal‐like clusters which may exist in these amorphous oxides.
16 citations
TL;DR: In this paper, the dependence of specific saturation magnetization on the particle size of pure and Co-Ti-substituted Ba-ferrites was interpreted in terms of a nonmagnetic surface layer parallel to thec-planes of 1.3 nm and 0.7 nm, respectively, in thickness.
Abstract: The dependence of the specific saturation magnetization on the particle size of pure and Co-Ti-substituted Ba-ferrites was interpreted in terms of a nonmagnetic surface layer parallel to thec-planes of 1.3 nm and 0.7 nm, respectively, in thickness. Electron microscope investigations of the particles demonstrate that this effect arises neither from a sufficient fraction of superparamagnetic particles in the ensemble nor from the Co-Ti substitution or from surface defects such as steps and stacking disorder of the building blocks. Extrapolating this surface effect to small sizes yields a critical size, with the particles losing their ferrimagnetic behaviour. This critical size agrees surprisingly well with the volume calculated for the transition of the single domain state to the superparamagnetic one.
TL;DR: Structure activity relationship (SAR) of superparamagnetic MR contrast agents is discussed based on physicochemical properties and relaxivity data of 16 different particles, while nonmagnetic monodisperse particles show no effect on the relaxivity.
TL;DR: In this paper, the temperature dependence of the magnetization of a series of Fe(110)/Ag(111) multilayer films grown by molecular beam epitaxy (MBE) was determined.
01 Jan 1991
TL;DR: In this article, the verification of superparamagnetism in liquid crystals is discussed, and it is shown that if they exist, ferromagnetic microdomains can be oriented.
Abstract: The proposed verification of superparamagnetism in liquid crystals is discussed. Due to long range ordering effects, if they exist, ferromagnetic microdomains can be oriented.
TL;DR: In this article, a number of magnetite (Fe3O4) ferrofluids with volume concentrations of e=0.01, 0.02, 0,03,0.04, 0.,06 and 0.08 have been prepared by diluting a stock fluid of Ms=43 kA/m.
Abstract: A number of magnetite (Fe3O4) ferrofluids with volume concentrations of e=0.01, 0.02, 0.03, 0.04, 0.06 and 0.08 have been prepared by diluting a stock fluid of Ms=43 kA/m. These samples were frozen in zero magnetic field, and the variation of the initial susceptibility with temperature was measured for each sample. We find, experimentally, that a maximum occurs at Tp in the initial susceptibility-versus-temperature curve. Tp moves to higher values as the concentration of the ferrofluid increases. A theoretical model derived for a noninteracting fine particle system in a solid matrix along with a semiempirical hypothesis for the effect of the interparticle interaction on the anisotropy constant seems to correctly predict the peak in the susceptibility as well as the effect of the concentration on the temperature of the peak.
TL;DR: In this paper, high-resolution electron microscopy has been carried out on rapidly quenched 10CaO30Bi2O360Fe2O3 and Bi3Fe5O12 which exhibit ferromagnetic character at room temperature.
Abstract: Mossbauer measurements and high-resolution electron microscopy have been carried out on rapidly quenched 10CaO30Bi2O360Fe2O3 and Bi3Fe5O12 which exhibit ferromagnetic character at room temperature. The intensity of the peaks due to internal fields in Mossbauer spectra are very weak at room temperature and increases with a decrease of temperature, indicating that the specimens are superparamagnetic. The electron micrograph clearly shows that the specimens contain microcrystals of 5 to 10 nm. The ferromagnetic character of the present oxides is brought about by the ferrimagnetic microcrystals precipitated.
TL;DR: In this paper, the superparamagnetism of the polymeric material of formula {Fe(C 13 H 17 N 3 ) 2 ]SO 4 ·6H 2 O} n has been investigated.
TL;DR: In this article, the magnetic phase diagram and behavior of Fe1−xAlx (x = 29 − 34) alloys with the (Fe3Al) and (FeAl) type atomic order at low temperature have been investigated by alternating low-field susceptibility and steady-field magnetization measurements.
Abstract: In a certain concentration range the Fe‐Al alloys undergo a complex magnetic transition.1,2 In this paper the magnetic phase diagram and behavior of Fe1−xAlx (x=29–34) alloys with the (Fe3Al)‐ and (FeAl)‐type atomic order at low temperature have been investigated by alternating low‐field susceptibility and steady‐field magnetization measurements. The results have shown that the superparamagnetic transition of the alloys with (FeAl)‐ and (Fe3Al)‐type crystal structure extend from 30.0 to slightly above 30.5 at. % Al and between 30.3 and 30.9 at. % Al, respectively. The type of ordered structure has an obvious influence on the freezing temperature Tf and reversed Curie temperature T0. The superparamagnetic range in the double transition with (FeAl)‐type order is wider than those with (Fe3Al)‐type order so that the magnetic phase diagram is moved in Fe‐rich direction. The alloys with different type order have the different ranges of the multicritical point. It can be understood on the basis of the calculatio...
TL;DR: In this article, several areas of fine particle magnetism are discussed, including microstructure and morphology of particles, estimates of HK distributions in particle assemblies, intrinsic surface magnetization of γ-Fe2O3 particles, anisotropy enhancement by surface treatment, and the long range magnetization order in assemblies of nominally superparamagnetic Fe particles in granular films.
Abstract: Recent progress in several areas of fine particle magnetism are discussed. These are: microstructure and morphology of particles, estimates of HK distributions in particle assemblies, intrinsic surface magnetization of γ-Fe2O3 particles, anisotropy enhancement by surface treatment, and the long range magnetization order in assemblies of nominally superparamagnetic Fe particles in granular films.
TL;DR: In this article, it was shown that the magnetic anisotropy relative to the film normal, the ground state magnetic moment per Fe atom and thermal spin excitations are affected by the structure of the films.
Abstract: Ultrathin epitaxial films grown in UHV – Fe(110) on Au(111) and Ag(111), Co(0001) on Au(111) – , sputtered Fe films between Ag and sputtered Fe/Au multilayers were studied by SQUID magnetometry and Mossbauer spectroscopy (CEMS). It could be shown that the magnetic anisotropy relative to the film normal, the, ground state magnetic moment per Fe atom and thermal spin excitations are affected by the structure of the films. In particular, a more 3-dimensional growth mode in the early state of film formation which is observed except in a certain temperature range around 300 K reduces the apparent magnetic interface anisotropy and the ferromagnetic ground state moment, and it enhances the thermal spin fluctuations and the tendency for superparamagnetic relaxation in the thinnest films.
TL;DR: In this article, the authors describe principle of particulate films for magneto-optical (MO) recording which have magnetic fine particles dispersed in binder and obtain a high optical quality of the particulate film by suppressing Rayleigh scattering of light.
Abstract: This paper describes principle of particulate films for magneto-optical (MO) recording which have magnetic fine particles dispersed in binder. In order to obtain a high optical quality of the particulate film by suppressing Rayleigh scattering of light, it is essential to make particle size much smaller than light wavelength and also to make the refractive index of the binder close to that of the particle. The latter is also important to obtain large Faraday rotation of the particles. Perpendicular magnetic anisotropy is attained by inverse magnetostriction effect (which is caused by stress induced by curing of binder) and/or magnetic-field orientation. Even when the particle has quite high Curie temperature Tc, we can perform thermomagnetic writing on the particulate film utilizing superparamagnetic critical temperature which is lower than Tc.
TL;DR: In this article, the stability of magnetite particles in novel medicinal magnetic ointments of multipurpose application was studied using Mossbauer spectroscopy, and the results obtained by model fitting of 57Fe nuclei spectra with those known for the system Fe3O4-γ-Fe2O3 allowed to identify the phase composition of the particles.
Abstract: Stability of the properties of magnetite particles in novel medicinal magnetic ointments of multipurpose application was studied using Mossbauer spectroscopy. Comparative analysis of the results obtained by model fitting of57Fe nuclei spectra with those known for the system Fe3O4-γ-Fe2O3 allowed to identify the phase composition of the particles. This composition, as well as that of the initial pure component in the form of a highly dispersed fraction (∼100 A), differs noticeably from the stoichiometric one. Despite their small sizes, the particles exhibit no superparamagnetism (in the temperature range from 95 to 300 K). Radiative sterilization of the ointments has no effect on the magnetic component composition.
TL;DR: In this article, the β-phase of Zircaloy-4 has been reported to be a supersaturated solid solution with additives, such as iron, chromium, nickel and so forth.
TL;DR: Magnetic resonance spectra of Fe3+ and Mn2+ ions in natural volcanic glasses, black obsidians from Lipari (Island of Southern Italy), have been investigated at 9 and 35 GHz.
Abstract: Magnetic resonance spectra of Fe3+ and Mn2+ ions in natural volcanic glasses, black obsidians from Lipari (Island of Southern Italy), have been investigated at 9 (X band) and 35 GHz (Q band). The spectrum in the X band is found to exhibit at g=4.3 the classical signal of isolated Fe3+ ions.1 At g=2 it is found the superposition of a superparamagnetic signal and a ferromagnetic resonance arising from the small and not so small magnetite particles previously found.2 It was also observed at X band a granular dependence of the g=2 line shape, similar to that observed in polycrystalline powders. At Q band there were two signals, one due to iron clusters at g=2 and a weak hyperfine sextet attributed to Mn2+ free ions. The magnetization for a given temperature (from 1.75 to 20 K) and very high magnetic fields (up to 150 kOe) shows the lack of hysteresis loops and the absence of remanent magnetization, both indicative of superparamagnetism. However, the magnetization curves do not superpose to a Langevin function...
01 Jan 1991
TL;DR: The magnetic properties of fine metallic particles have been extensively investigated during the last decade [1, 2, 3]. The interest in this subject has both fundamental and technological aspects as mentioned in this paper, which is very interesting.
Abstract: The magnetic properties of fine metallic particles have been extensively investigated during the last decade [1, 2, 3]. The interest in this subject has both fundamental and technological aspects. Magnetic metallic particles with dimensions of 5 to 50 nm are predominantly single domain ferromagnetic particles with characteristic magnetic properties as e.g. saturation magnetization and magnetic anisotropy which depend on the method of preparation and may be considerably different from that of bulk materials. Moreover, the large fraction of surface to volume atoms leads to modified magnetic interactions while the comparable values of magnetic anisotropy and thermal energy per particle cause superparamagnetic behavior. From the point of view of applications fine magnetic particles are used in ferrofluids, magnetic recording and catalysis.
TL;DR: In this article, a mixture of two compounds with the same iron to manganese ratio was characterized as a mixture with two cubic spinels with different crystallographic parameters, one being a defect spinel and the other being a non-defect spinel.
Abstract: Iron-manganese oxide catalysts are known to influence the production of short chain hydrocarbons in the Fischer-Tropsch-synthesis process. XRD, XRF, gravity measurements and Mossbauer-effect spectroscopy has been used to study a catalytic precursor made by coprecipitation of iron and manganese oxides. The sample is characterized as a mixture of two compounds with the same iron to manganese ratio. The two compounds are cubic spinels with the same crystallographic parameters, however, one compound is a defect spinel while the other is not. In the Mossbauer studies the defect spinel shows superparamagnetic relaxation behavior—fluctuations of the magnetization vector along two opposite easy directions, while the non-defect spinel shows evidence of collective excitations—fluctuations of the magnetization vector in directions close to the easy direction. These properties are related to the particle sizes.
TL;DR: In this article, the morphology and magnetic phase transitions of monolayer-range Fe films on a reconstructed Au(001) surface using the combination of High-Resolution Low-Energy Electron Diffraction (HRLEED) and Surface Magneto-Optic Kerr Effect (SMOKE) techniques were studied.
Abstract: We have studied the morphology and magnetic phase transitions of monolayer-range Fe films on a reconstructed Au(001) surface using the combination of High-Resolution Low-Energy Electron Diffraction (HRLEED) and Surface Magneto-Optic Kerr Effect (SMOKE) techniques. Through the measurement of energy dependent angular profiles of the specularly diffracted beam, we discovered a simple method to determine not only the interfacial spacings (Fe-Au and Fe-Fe) but also to detect the build up of the second layer. For a one-level (2-D) Fe films with coverage of ∼ 0.6 monolayer (ML) perpendicular magnetization was observed. For a 1 ML equivalent 3-D film both perpendicular and in-plane magnetization were observed. The magnetic phase transitions of these magnetic islands showed finite size broadening effect. The perpendicular saturation magnetization decreases with temperature above 100r.C but never vanishes and the magnetization curve has no hysteresis loop above ∼ 120r.C. The implication of superparamagnetism is discussed.
TL;DR: In this article, the lattice constant of reentrant spin glass has a nature different from that of spin glass and the time dependence of dc magnetization was observed for these samples, which may be produced by the distribution of superparamagnetic clusters.