Preparation and characterization of magnetic nanoparticles with controlled magnetization

The effect of molar ratio of two hydrated iron salts used as precursors into a (co)precipitation-based synthesis method, on the composition, size and specific saturation magnetization of mixed iron oxides and oxyhydroxides magnetic nanoparticles as reaction products, was studied. The preparation procedure is based on a salt-assisted solid-state chemical reaction. The obtained products are magnetic multiphase components with the mean size ranging from 3 to 10 nm and specific saturation magnetization between 25 and 95.5 emu/g. The specific saturation magnetization modifies in a non-linear manner as the molar ratio of the iron salts varies. Excepting one sample, for which Fe²⁺/Fe³⁺ molar ratio was zero, all magnetic nanoparticles show a ferrofluid-like behaviour in the colloidal form. The small size, ferrofluid-like behaviour, and controlled specific saturation magnetization allow the use of new synthesized nanoparticles in specific biomedical or industrial applications.     

Magnetism of mass-filtered nanoparticles on ferromagnetic supports

The magnetic properties of 3d-metal clusters significantly differ from bulk behavior and, for small clusters, strongly depend on the number of atoms within each cluster. Such phenomena are caused by a narrowing of electronic states and the high ratio of surface to volume atoms giving rise to enhanced magnetic orbital moments. However, even large Fe nanoparticles (6–12 nm) deposited onto ferromagnetic surfaces show enhanced orbital moments. At a low coverage large iron clusters on a cobalt film exhibit a nearly doubled value for the orbital moments when compared to bulk behaviour. With increasing coverage, the orbital moment is clearly reduced. Additionally, the spin and orbital moments of iron and cobalt in Fe₅₀Co₅₀ alloy clusters with a size of 7.5 nm on a nickel substrate have been investigated. FeCo alloys are known to exhibit very high magnetic moments for soft magnetic materials. PACS 73.22.-f; 75.75.+a; 81.07.-b     

Nanoscale magnetic structure of ferromagnet/antiferromagnet manganite multilayers

We use polarized neutron reflectometry and dc magnetometry to obtain a comprehensive picture of the magnetic structure of a series of La(2/3)Sr(1/3)MnO3/Pr(2/3)Ca(1/3)MnO3 (LSMO/PCMO) superlattices, with varying thickness of the antiferromagnetic (AFM) PCMO layers (0相似文献   

The Electronic and Magnetic Properties of FCC Iron Clusters in FCC 4D Metals

The electronic and magnetic structures of small FCC iron clusters in FCC Rh, Pd and Ag were calculated using the discrete variational method as a function of cluster size and lattice relaxation. It was found that unrelaxed iron clusters, remain ferromagnetic as the cluster sizes increase, while for relaxed clusters antiferromagnetism develops as the size increases depending on the host metal. For iron in Rh the magnetic structure changes from ferromagnetic to antiferromagnetic for clusters as small as 13 Fe atoms, whereas for Fe in Ag antiferromagnetism is exhibited for clusters of 24 Fe atoms. On the hand, for Fe in Pd the transition from ferromagnetism to antiferromagnetism occurs for clusters as large as 42 Fe atoms. The difference in the magnetic trends of these Fe clusters is related to the electronic properties of the underlying metallic matrix. The local d densities of states, the magnetic moments and hyperfine parameters are calculated in the ferromagnetic and the antiferromagnetic regions. In addition, the average local moment in iron-palladium alloys is calculated and compared to experimental results.     

Large specific absorption rates in the magnetic hyperthermia properties of metallic iron nanocubes

We report on the magnetic hyperthermia properties of chemically synthesized ferromagnetic 11 and 16 nm Fe(0) nanoparticles of cubic shape displaying the saturation magnetization of bulk iron. The specific absorption rate measured on 16 nm nanocubes is 1690±160 W/g at 300 kHz and 66 mT. This corresponds to specific losses-per-cycle of 5.6 mJ/g, largely exceeding the ones reported in other systems. A way to quantify the degree of optimization of any system with respect to hyperthermia applications is proposed. Applied here, this method shows that our nanoparticles are not fully optimized, probably due to the strong influence of magnetic interactions on their magnetic response. Once protected from oxidation and further optimized, such nano-objects could constitute efficient magnetic cores for biomedical applications requiring very large heating power.     

Particle size effect on magnetotransport properties of nanocrystalline Nd<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

Nanocrystalline samples with an average particle size of 40 and 52 nm have been synthesized by citrate-complex auto-ignition method. Magnetic properties of the samples show para- to ferromagnetic transition at around 135 K. The electron magnetic resonance (EMR) study on these samples indicates the presence of coexistence of two magnetic phases below 290 K. Electrical resistivity follows variable range hopping (VRH) mechanism in the paramagnetic regime. The magnetoresistance (MR) data has been analysed by spin dependent hopping between the localized spin clusters together with the phase-separation phenomenon. These clusters are assumed to be formed by distribution of canted spins and defects all over the nanoparticle. In addition, the hopping barrier depends on the magnetic moment orientation of the clusters. The magnetic moments of the clusters are narrowly oriented in ferro- and are randomly oriented in paramagnetic phase. The ferromagnetic phase contributes to the total MR at low applied magnetic fields whereas the paramagnetic phase contributes at relatively high fields in both the samples. The average cluster size in ferromagnetic phase is bigger than that in paramagnetic phase. It is also observed that the cluster size, in ferromagnetic phase, in 52 nm sample is bigger than that in the 40 nm sample. However, the average cluster size in paramagnetic phase is almost same in both the samples.     

Starch vermicelli template for synthesis of magnetic iron oxide nanoclusters

A novel method for fabricating magnetic iron oxide nanoparticles was achieved by using transparent vermicelli template as a new stabilizing material. The morphology of the as-prepared magnetic iron oxide deposited on the surface of vermicelli was observed as nanoclusters. The magnetization of the magnetic iron oxide nanoparticles at room temperature was decreased after carbonization at 200 °C. Therefore the thermal decomposition of iron oxide nanoparticles stabilized by starch vermicelli template yielded iron oxide/carbon nanocomposites with the soft magnetic behavior which are useful for biomedical applications.     

Comparative analysis of the 1H NMR relaxation enhancement produced by iron oxide and core-shell iron-iron oxide nanoparticles

Physicochemical and magnetorelaxometric characterization of the colloidal suspensions consisting of Fe-based nanoparticles coated with dextran have been carried out. Iron oxide and iron core/iron oxide shell nanoparticles were obtained by laser-induced pyrolysis of Fe(CO)₅ vapours. Under different magnetic field strengths, the colloidal suspension formed by iron oxide nanoparticles showed longitudinal (R₁) and transverse (R₂) nuclear magnetic relaxation suspension (NMRD) profiles, similar to those previously reported for other commercial magnetic resonance imaging (MRI) contrast agents. However, colloidal suspension formed by ferromagnetic iron-core nanoparticles showed a strong increase of the R₁ values at low applied magnetic fields and a strong increase of the R₂ measured at high applied magnetic field. This behaviour was explained considering the larger magnetic aggregate size and saturation magnetization values measured for this sample, 92 nm and 31 emu/g Fe, respectively, with respect to those measured for the colloidal suspensions of iron oxide nanoparticles (61 nm and 23 emu/g Fe). This suspension can be used both as T₁ and T₂ contrast agent.     

Magnetic Phase Transitions in Nanosystems: Role of Size Effects,Intercluster Interactions and Defects

Shear stress under high pressure loaded nanoclusters of α, γ-ferric oxides and metallic europium showed first- and second-order magnetic phase transitions. For nanoclusters of α, γ-ferric oxides and metallic europium 57Fe and 151Eu, Mossbauer spectroscopy revealed a considerable change in the Curie (Neel) points. The thermodynamic model predicts first-to-second-order (Eu) and second-to-first-order (Fe oxides) changes in the character of magnetic phase transitions and a decrease or increase of critical Curie (Neel) points. The model defines the critical cluster size and concentration of defects in nanosystems responsible for the change in magnetic properties. For nanoclusters of Fe oxides and metallic Eu, the critical (and maximum) concentration of defects corresponds to a cluster size 20–50nm.     

Preparation of functionalized and gold-coated cobalt nanocrystals for biomedical applications

We present the synthesis, structural and magnetic characterization of cobalt-based nanocrystals with controlled size (5–25 nm; ±1 nm) and tailored morphologies (spheres, discs with specific aspect ratio of 5×20 nm and core-shell structures). The reproducible synthesis route for Au-coated, high moment, cobalt nanocrystals presented here opens up possibilities for a number of biomedical applications and surface functionalities.     

BCC Fe surface and cluster magnetism using a magnetic potential

A semi-empirical potential for ferromagnetic bcc iron that also models the co-linear magnetic moment of each atom, has been applied to clusters and surfaces. Surface energies of low index facets, surface relaxation and magnetic moment are calculated and compared to various other empirical potentials, recent density functional theory (DFT) results and experimental data. Quantitatively the resulting surface magnetic moments enhancements are underestimated, however qualitatively the general behavior of the magnetic moment as a function of surface or cluster layer is comparable to that predicted by DFT calculations.     

One-step hydrothermal synthesis of highly water-soluble secondary structural Fe3O4 nanoparticles

Magnetite nanoparticles (MNPs) were prepared using the ferric acetylacetonate as the sole iron source in a facile hydrothermal route, while poly(acrylic acid) (PAA) was chosen as the stabilizer via one-step functionalized MNPs for better hydrophilic properties. The orthogonal was used in the paper for the experimental parameters optimization, including the solvent, the reaction time, the amount of stabilizer and the presynthesis. The obtained highly water dispersible MNPs with uniform size from about 50 to about 100 nm was individually composed of many monodisperse magnetite crystallites approximately 6 nm in size. And the MNPs show high magnetic properties, whose magnetite content was up to 76.76% and the saturation magnetization was 39.0 emu/g. Later the formation mechanism of MNPs was also discussed. Thus the MNPs proved to be very promising for biomedical applications.     

Structure and magnetic properties of Fe/Fe oxide clusters

Fe clusters have been synthesised in ultra-high-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data.     

Role of the chemical ordering on the magnetic properties of Fe–Ni cluster alloys

The spin and orbital moments of fcc Fe-Ni cluster alloys are determined within the framework of a d-band Hamiltonian including the spin-orbit coupling non perturbatively. Different sizes (up to 321 atoms), compositions, and chemical configurations (random alloys as well as core-shell arrays of iron and nickel atoms) are considered in order to reveal the crucial role played by local order and stoichiometry on the magnetic moments of the clusters. Interestingly, we have found considerably reduced average magnetizations for Fe-Ni clusters with Fe cores compared to that of the bulk alloy with the same composition. Indeed, in these configurations not only antiparallel arrangements between the local moments of some Fe atoms within the iron core are found, but also the total magnetization of the surface Ni atoms is significantly quenched. On the opposite, the disordered and Ni-core cluster alloys are characterized by high magnetizations resulting from saturated-like contributions from both Ni and Fe atoms, in agreement with recent ab-initio calculations. In general, the local orbital magnetic moments are strongly enhanced with respect to their bulk values. Finally, the variation of the orbital-to-spin moment ratio with the chemical order is discussed.     

Spin coupling and orbital angular momentum quenching in free iron clusters

Magnetic spin and orbital moments of size-selected free iron cluster ions Fe{n}{+} (n=3-20) have been determined via x-ray magnetic circular dichroism spectroscopy. Iron atoms within the clusters exhibit ferromagnetic coupling except for Fe{13}{+}, where the central atom is coupled antiferromagnetically to the atoms in the surrounding shell. Even in very small clusters, the orbital magnetic moment is strongly quenched and reduced to 5%-25% of its atomic value while the spin magnetic moment remains at 60%-90%. This demonstrates that the formation of bonds quenches orbital angular momenta in homonuclear iron clusters already for coordination numbers much smaller than those of the bulk.     

Synchrotron radiation studies of mass-selected Fe nanoclusters deposited in situ

A portable UHV-compatible gas aggregation cluster source, capable of depositing clean mass-selected nanoclusters in situ, has been used at synchrotron radiation facilities to study the magnetic behaviour of exposed and Co-coated Fe clusters in the size range 250 to 540 atoms. X-ray magnetic circular dichroism (XMCD) studies of isolated and exposed 250-atom clusters show a 10% enhancement in the spin magnetic moment and a 75% enhancement in the orbital magnetic moment relative to bulk Fe. The spin moment monotonically approaches the bulk value with increasing cluster size but the orbital moment does not measurably decay till the cluster size is above ∼ 400 atoms. The total magnetic moments for the supported particles though higher than the bulk value are less than those measured in free clusters. Coating the deposited particles with Co in situ increases the spin moment by a further 10% producing a total moment per atom close to the free cluster value. At low coverages the deposited clusters are super-paramagnetic at temperatures above 10 K but a magnetic remanence at higher temperature emerges as the cluster density increases and for cluster films with a thickness greater than 50 ?(i.e. 2-3 layers of clusters) the remanence becomes greater than that of an Fe film of the same thickness produced by a conventional deposition source. Thick cluster-assembled film show a strong in-plane anisotropy. Received 14 December 2000     

Flame synthesis of superparamagnetic Fe/Nb nanocomposites for biomedical applications

Iron/niobium nanocomposite particles are produced using the sodium flame and encapsulation (SFE) process. Ferrocene is added to the vapor-phase metal halide/sodium reaction to produce metallic iron particles encapsulated in niobium. To accomplish this, the ferrocene is combined with niobium chloride vapor and this mixture is injected as a turbulent jet into a stream of sodium vapor. The ferrocene is expected to decompose upstream of the flame to form iron particles, which pass through the niobium chloride-sodium reaction zone wherein they are encapsulated in niobium. The salt byproduct then encapsulates these particles, preventing oxidation. The as-produced Fe/Nb particles were found to contain Fe particles that are less than 15 nm in diameter and are superparamagnetic with a coercivity of 50 Oe and a saturation magnetization of over 200 emu/g of Fe. In addition to possessing a strong magnetic response and small remnant magnetization, the iron/niobium composite particles are expected to be biocompatible and X-ray opaque. Consequently, these materials hold promise for magnetic navigation in biomedical applications.     

High-magnetic-moment multifunctional nanoparticles for nanomedicine applications

Multifunctional FeCo nanoparticles with narrow size distribution (less than 8% standard deviation) were fabricated by a novel physical vapor nanoparticle-deposition technique. The size of magnetic nanoparticles was controlled in the range from 3 to 100 nm. The shape of nanoparticles was controlled to be either spherical or cubic. The particles had a high specific magnetization of 226 emu/g at low saturation field, which is much higher than the currently commercialized iron oxide nanoparticles. Core–shell-type Co(Fe)–Au nanoparticles were produced by the same technique. They combined the high moment of the Co(Fe) core with the plasmonic feature of a Au shell.     

Polymer stabilized Ni-Ag and Ni-Fe alloy nanoclusters: Structural and magnetic properties

We report here the structural and magnetic behaviors of nickel-silver (Ni-Ag) and nickel-iron (Ni-Fe) nanoclusters stabilized with polymer (polypyrrole). High resolution transmission electron microscopy (HRTEM) indicates Ni-Ag nanoclusters to stabilize in core-shell configuration while that of Ni-Fe nanoclusters in a mixed type of geometry. Structural characterizations by X-ray diffraction (XRD) reveal the possibility of alloying in such bimetallic nanoclusters to some extent even at temperatures much lower than that of bulk alloying. Electron paramagnetic resonance (EPR) spectra clearly reveal two different absorption behaviors: one is ascribed to non-isolated Ni²⁺ clusters surrounded by either silver or iron giving rise to a broad signal, other (very narrow signal) being due to the isolated superparamagnetic Ni²⁺ clusters or bimetallic alloy nanoclusters. Results obtained for Ni-Ag and Ni-Fe nanoclusters have been further compared with the behavior exhibited by pure Ni nanoclusters in polypyrrole host. Temperature dependent studies (at 300 and 77 K) of EPR parameters, e.g. linewidth, g-value, line shape and signal intensity indicating the significant influence of surrounding paramagnetic silver or ferromagnetic iron within polymer host on the EPR spectra have been presented.     

Magnetically directed self-assembly of submicron spheres with a Fe3O4 nanoparticle shell

A grid of micrometer-sized core-shell particles was fabricated by magnetophoretic deposition using a water-based colloidal solution. The core-shell particles consist of a 640 nm diameter polystyrene spherical core covered with a shell of five layers of 12 nm diameter Fe₃O₄ nanocrystals. The separation and the length of the individual chains can be tuned by the magnetic field strength and the concentration of the particle solution. The magnetic properties were characterized by angular-dependent ferromagnetic resonance and SQUID magnetometry.