Magnetic properties of self-assembled nanostructure films on the base of amorphous Ni granules

We report on structural and magnetic properties of granular films consisting of 2.5 nm Ni nanoparticles. The films are fabricated by the original laser electrodispersion technique, which allows producing nearly monodisperse and amorphous particles. Atomic force microscopy (AFM) study shows that in 8 nm thickness films the particles are self-assembled in clusters with the lateral size 100-150 nm and the height of about 8 nm. Performed by SQUID, the films magnetization measurements reveal superparamagnetic behaviour, characteristic for an ensemble of non-interacting single domain magnetic particulates. It is found that the magnetic moment of the particulate is equal to that of about 3000 individual Ni nanoparticles and the blocking temperature is close to room temperature. Defined from magnetic measurements, the size of single domain particulates correlates well with the size of the clusters determined from AFM images. We propose that exchange interaction plays an important role in the formation of the particulates by aligning the magnetic moments of the individual Ni nanoparticles inside the clusters. Presence of magnetic clusters with high blocking temperature makes the fabricated films potentially useful for high-density magnetic data storage applications.     

Effect of Ni doping on the structural and magnetic properties of FePt nanoparticles

A serial of FePtNi nanoparticles were investigated on their crystal structure and magnetic properties. The FePtNi nanoparticles were synthesized simultaneously by the reduction of iron (III) acetylacetonate, platinum (II) acetylacetonate and nickel (II) acetylacetonate with 1,2-hexadecanediol as the reducing agent. The X-ray diffraction patterns indicate that the addition of 8, 12, 17 at% Ni in FePt nanoparticles suppressed the transformation of the particles from disorder face-centered cubic to order face-centered tetragonal L1₀-phase under annealing treatment. However, further increasing Ni contents to 21 at%, the nanoparticle transformed to L1₂ phase. Doping of Ni into the FePt compound system may decrease coercivity and crystal anisotropy energy. A maximum coercivity of 7 KOe at room temperature was obtained for (Fe₅₂Pt₄₈)₉₂Ni₈ nanoparticles after annealing at 600 °C for 30 min.     

Phase transition and magnetic properties of Mg-doped hexagonal close-packed Ni nanoparticles

Mg-doped Ni nanoparticles with the hexagonal close-packed (hcp) and face-centered cubic (fcc) structure have been synthesized by sol-gel method sintered at different temperatures in argon atmosphere. The sintering temperature played an important role in the control of the crystalline phase and the particle size. The pure hcp Mg-doped Ni nanoparticles with average particle size of 6.0 nm were obtained at 320 °C. The results indicated that the transition from the hcp to the fcc phase occurred in the temperature range between 320 °C and 450 °C. Moreover, the VSM results showed that the hcp Mg-doped Ni nanoparticles had unique ferromagnetic and superparamagnetic behavior. The unsaturation even at 5000 Oe is one of the superparamagnetic characteristics due to the small particle size. From the ZFC and FC curves, the blocking temperature T_B of the hcp sample (6.0 nm) was estimated to be 10 K. The blocking temperature was related to the size of the magnetic particles and the magnetocrystalline anisotropy constant. By theoretical calculation, the deduced particle size was 6.59 nm for hcp Mg-doped Ni nanoparticles which was in agreement with the results of XRD and TEM.     

Antibody and DNA dual-labeled gold nanoparticles: Stability and reactivity

Gold nanoparticles labeled by both antibody (IgG) and single stranded DNA (ss-DNA) have been synthesized and characterized. The stability and reactivity of the dual-labeled nanoparticles were compared with the conventional IgG or ss-DNA modified nanoparticles. It was found that the IgG adsorption significantly improved the stability of the nanoparticles in aqueous solution, which is beneficial for attaching ss-DNA. The presence of IgG also effectively prohibits the desorption of ss-DNA against dithiothreitol (DTT) displacement. The coverage on dual-labeled nanoparticles was found to be 50 ± 15 ss-DNA/nanoparticle and 10 ± 2 IgG/nanoparticle, respectively, compared to the value of 70 ± 15 ss-DNA/nanoparticle of only ss-DNA-labeled gold nanoparticles. Dot-immuno and cross-linking experiments confirmed that both the IgG and ss-DNA retained their bioactivity on the nanoparticle surface. The dual-labeled nanoparticles have potential to be used as novel bio-probes for ultrasensitive detection.     

A Highly Tunable Silicone-Based Magnetic Elastomer with Nanoscale Homogeneity

Magnetic elastomers have been widely pursued for sensing and actuation applications. Silicone-based magnetic elastomers have a number of advantages over other materials such as hydrogels, but aggregation of magnetic nanoparticles within silicones is difficult to prevent. Aggregation inherently limits the minimum size of fabricated structures and leads to non-uniform response from structure to structure. We have developed a novel material that is a complex of a silicone polymer (polydimethylsiloxane-co-aminopropylmethylsiloxane) adsorbed onto the surface of magnetite (γ-Fe₂O₃) nanoparticles 7-10 nm in diameter. The material is homogenous at very small length scales (<100 nm) and can be crosslinked to form a flexible magnetic material, which is ideally suited for the fabrication of micro- to nanoscale magnetic actuators. The loading fraction of magnetic nanoparticles in the composite can be varied smoothly from 0 to 50 wt% without loss of homogeneity, providing a simple mechanism for tuning actuator response. We evaluate the material properties of the composite across a range of nanoparticle loading, and demonstrate a magnetic-field-induced increase in compressive modulus as high as 300%. Furthermore, we implement a strategy for predicting the optimal nanoparticle loading for magnetic actuation applications, and show that our predictions correlate well with experimental findings.     

Optical properties of sol-gel fabricated Ni/SiO2 glass nanocomposites

Nickel nanoparticles were grown in silica glass by annealing of the sol-gel prepared silicate matrices doped with nickel nitrate. TEM characterization of Ni/SiO₂ glass proves the formation of isolated spherical nickel nanoparticles with mean sizes 6.7 and 20 nm depending on annealing conditions. The absorption and photoluminescence spectra of Ni/SiO₂ glasses were measured. In the absorption spectra, we observed the band related to the surface plasmon resonance (SPR) in Ni nanoparticles. The broadening of SPR was observed with decrease of Ni nanoparticle size. The width of the surface plasmon band decreases 1.5 times at the lowering of temperature from 293 to 2 K because of strong electron-phonon interaction. The spectra proved the creation of nickel oxide NiO clusters and Ni²⁺ ions in silica glass as well.     

Characterization of magnetite nanoparticles for SQUID-relaxometry and magnetic needle biopsy

Magnetite nanoparticles (Chemicell SiMAG-TCL) were characterized by SQUID-relaxometry, susceptometry, and TEM. The magnetization detected by SQUID-relaxometry was 0.33% of that detected by susceptometry, indicating that the sensitivity of SQUID-relaxometry could be significantly increased through improved control of nanoparticle size. The relaxometry data were analyzed by the moment superposition model (MSM) to determine the distribution of nanoparticle moments. Analysis of the binding of CD34-conjugated nanoparticles to U937 leukemia cells revealed 60,000 nanoparticles per cell, which were collected from whole blood using a prototype magnetic biopsy needle, with a capture efficiency of >65% from a 750 μl sample volume in 1 min.     

FORC analysis of Ni(SiO2) nanogranular film in the blocked regime

In this work the magnetic and structural properties of granular Ni(SiO₂) films are studied by means of FORCs diagrams and microscopy. Transmission electron microscopy images show that the sample is composed of a fine dispersion of Ni nanoparticles with 3.7 nm in average sizes. Magnetic measurements as function of temperature show that the nanoparticles are superparamagnetic at room temperature and are blocked at 5 K. The FORCs diagrams obtained below the blocking temperature allow us to determine the average size of the nanoparticles and the distribution of sizes in a very good agreement with TEM images.     

Magnetic nanoparticles as building blocks for high-performance magnetic materials

We report on the magnetic behaviour of films of Fe nanoparticles deposited from the gas phase with sizes in the range 2–3 nm embedded in Ag and Co matrices and Co nanoparticles of the same size embedded in Ag matrices. Magnetometry measurements, using a VSM, of very low volume fraction (1–2%) assemblies of Fe and Co in Ag show perfect superparamagnetism and enable us to confirm that the size distribution of the particles in the matrix is the same as that of the free particles prior to deposition. The hysteresis loops at 2 K, which is below the blocking temperature, show that the particles have a uniaxial anisotropy that is randomly oriented in three dimensions with the Co nanoparticles having a much higher anisotropy than the Fe particles. The soft magnetic behaviour of pure Fe and Co nanoparticle films with no matrix is well described by a random anisotropy model and is consistent with the formation of a correlated super-spin glass (CSSG) characteristic of amorphous materials. The Co nanoparticle films appear to have a lower random anisotropy than the Fe ones in contrast to the behaviour observed for the isolated particles. Films of Fe nanoparticles embedded in Co matrices, whose saturation magnetization exceeds the Slater–Pauling curve, also show magnetic behaviour consistent with a CSSG. At high volume fractions, the films of Fe nanoparticles embedded in Co matrices behave almost identically to films of pure Co nanoparticles.     

Effect of nanoparticles on the magnetic properties of Mn–Zn soft ferrite

In this paper, the effect of nanostructures on the magnetic properties like the specific saturation magnetization (σ_S) and the coercivity (H_C) for Mn_0.4Zn_0.6Fe₂O₄ ferrite prepared by the co-precipitation method has been presented. We have shown by means of X-ray diffraction that the resulting ferrite is made up of nanoparticles, and that the average size of these nanoparticles calculated with the Scherrer formula depends upon the sintering temperature. When the sintering temperature is increased from 500 to 900 °C, the average nanoparticle diameter varies from 19.3 to 36.4 nm. The nanoparticle phase is further confirmed by scanning electron microscopy (SEM). Both results are found to be in good agreement. The magnetic properties are explained on the basis of the single-domain and multi-domain theory.     

Magnetic, resonance and transport properties of nanopowder of La0.7Sr0.3MnO3 manganites

X-ray powder diffraction, magnetization, transport and magnetic resonance measurements of nanosize La_0.7Sr_0.3MnO₃ (LCMO) manganites have been performed. The nanosize manganites were synthesized with a co-precipitation method at different (600, 700, 800 and 1000 °C) temperatures. The crystal structure of the nanopowders obtained was determined to be perovskite-like with a rhombohedral distortion (the space group R3¯c). The average size of synthesized nanoparticles (from 17 to 88 nm) was estimated using the X-ray diffraction and low temperature adsorption of argon methods. All the nanosize manganites show ferromagnetic-like ordering. Both the Curie temperature and magnetization decrease with reducing the particle size. The decrease of magnetization is due to the disordered surface shell of particles. The disordered surface layer is a source of the surface anisotropy and is responsible for the increase of coercivity. Temperature dependences of the magnetic resonance spectra parameters have allowed obtaining information on dynamics of magnetic properties in the nanoparticle systems. The resistivity was established to become higher by reducing the particles’ size and increases to a great extent in nanoparticles with the smallest average size at low temperatures. The magnetic entropy was shown to be smaller for the small particles. Using the temperature dependence of magnetic entropy the relative cooling power of the nanosize samples studied was evaluated.     

Heat generation of aqueously dispersed CoFe2O4 nanoparticles as heating agents for magnetically activated drug delivery and hyperthermia

Self-heating from magnetic nanoparticles under AC magnetic field can be used either for hyperthermia or to trigger the release of an anti-cancer drug, using thermo-responsive polymers. The heat generated by applying an AC magnetic field depends on the properties of magnetic nanoparticles (composition, size, crystal structure) as well as the frequency and amplitude of the magnetic field. Before these systems can be efficiently applied for in vitro or in vivo studies, a thorough analysis of the magnetically induced heating is required. In this study, CoFe₂O₄ nanoparticles were synthesized, dispersed in water, and investigated as heating agents for magnetic thermo-drug delivery and hyperthermia. The temperature profiles and infrared (IR) camera images of heat generation of CoFe₂O₄ nanoparticles under various AC magnetic fields of 127–700 Oe at 195, 231, and 266 kHz were measured using an IR thermacam, excluding the external AC magnetic field interruption. The CoFe₂O₄ nanoparticles were successfully dispersed in water using an 11-mercaptoundecanoic acid ligand exchange method to exchange the solvent used for synthesis of hexane for water. During the heating experiments, each of CoFe₂O₄ nanoparticle solutions reached a steady state where the temperature rose between 0.1 and 42.9 °C above ambient conditions when a magnetic field of 127–634 Oe was applied at 231 or 266 kHz. The heat generation was found to be dependent on the intensity of AC magnetic field and applied frequency. Therefore, the desired heating for magnetically triggered drug delivery or hyperthermia could be achieved in water-dispersed CoFe₂O₄ nanoparticles by adjusting the AC magnetic field and frequency.     

Effect of the number of iron oxide nanoparticle layers on the magnetic properties of nanocomposite LbL assemblies

Aqueous colloidal suspension of iron oxide nanoparticles has been synthesized. Z-potential of iron oxide nanoparticles stabilized by citric acid was −35±3 mV. Iron oxide nanoparticles have been characterized by the light scattering method and transmission electron microscopy. The polyelectrolyte/iron oxide nanoparticle thin films with different numbers of iron oxide nanoparticle layers have been prepared on the surface of silicon substrates via the layer-by-layer assembly technique. The physical properties and chemical composition of nanocomposite thin films have been studied by atomic force microscopy, magnetic force microscopy, magnetization measurements, Raman spectroscopy. Using the analysis of experimental data it was established, that the magnetic properties of nanocomposite films depended on the number of iron oxide nanoparticle layers, the size of iron oxide nanoparticle aggregates, the distance between aggregates, and the chemical composition of iron oxide nanoparticles embedded into the nanocomposite films. The magnetic permeability of nanocomposite coatings has been calculated. The magnetic permeability values depend on the number of iron oxide nanoparticle layers in nanocomposite film.     

Morphologies and magnetic properties of FeCo nanoparticles modulated by changing the types of ligands of Co

A new way of preparing FeCo nanoparticles has been developed using the co-precipitation technique based on the appropriate Co precursors of different types of ligands. These new precursors have been prepared by the reaction of cobalt acetate with three various coordination compositions for forming cobalt complex types. This technique provides proper control on nanoparticle size distribution after annealing to 500 °C. The variation of the magnetic properties with the type of ligands of Co can arise from the changes of the microstructures and crystalline anisotropies. Maximum coercivity values of 480 Oe were obtained. It is found that the precursors with special structures can prevent from agglomeration without presence of any surfactants.     

Multiferroic TbMnO3 nanoparticles

We report on the synthesis of TbMnO₃ nanoparticles by chemical co-precipitation route and their structural, chemical bonding, magnetic and dielectric properties. It is shown that the interesting multiferroic properties of this system as reflected by the concurrent occurrence of magnetic and dielectric transitions are retained in the nanoparticles (size∼40 nm). However, the nanoparticle constitution and properties are seen to depend significantly on the calcination temperature. While the nanoparticles obtained by calcination at 800 °C correspond very well with the reported properties of single phase TbMnO₃ (all the key magnetic and dielectric features near 7, 27 and 41 K, albeit with reduced dielectric constant) the nanoparticles obtained by calcination at 900 °C develop a Tb deficient skin which softens the transitions, reducing the dielectric constant further.     

Desorption of surfactant and sintering of surface-modified PdxNi1 − x nanoparticles

A series of Pd_xNi_1 − x nanoparticles in a diameter of 6-7 nm were prepared by wet chemical reduction. They were then modified with two surfactants, stearic acid (SA) and polyethylene glycol (PEG). Desorption of the surfactant was studied using a temperature programmed desorption technique, and the sintering behavior of surface-modified Pd_xNi_1 − x nanoparticles was examined. Since surface energy of the nanoparticles depends on the alloy composition, it can be correlated with the desorption temperature of surfactant from the nanoparticle surface. Because Ni has a higher surface energy, the surfactant desorption temperature increases as the Ni content increases. With the same stoichiometry, the desorption temperature of SA is always higher than that of PEG. The SA-modified nanoparticles have higher thermal stability and are less sintered than PEG-modified nanoparticles. The sintering and growth behavior of the nanoparticles can be correlated with variation of surface energy due to different surface modification.     

Particle size effect on phase and magnetic properties of polymer-coated magnetic nanoparticles

Polymer-coated magnetic nanoparticles are hi-tech materials with ample applications in the field of biomedicine for the treatment of cancer and targeted drug delivery. In this study, magnetic nanoparticles were synthesized by chemical reduction of FeCl₂ solution with sodium borohydride and coated with amine-terminated polyethylene glycol (aPEG). By varying the concentration of the reactants, the particle size and the crystallinity of the particles were varied. The particle size was found to increase from 6 to 20 nm and the structure becomes amorphous-like with increase in the molar concentration of the reactant. The magnetization at 1 T field (M_1T) for all samples is > 45 emu/g while the coercivity is in the range of 100-350 Oe. When the ethanol-suspended particles are subjected to an alternating magnetic field of 4 Oe at 500 kHz, the temperature is increased to a maximum normalized temperature (3.8 °C/mg) with decreasing particle size.     

Ni thin films vacuum-evaporated on polyethylene naphthalate substrates with and without the application of magnetic field

We study the structural properties of the surface roughness, the surface mound size and the interfacial structure in Ni thin films vacuum-deposited on polyethylene naphthalate (PEN) organic substrates with and without the application of magnetic field and discuss its feasibility of fabricating quantum cross (QC) devices. For Ni/PEN evaporated without the magnetic field, the surface roughness decreases from 1.3 nm to 0.69 nm and the surface mound size increases from 32 nm to 80 nm with the thickness increased to 41 nm. In contrast, for Ni/PEN evaporated in the magnetic field of 360 Oe, the surface roughness tends to slightly decrease from 1.3 nm to 1.1 nm and the surface mound size shows the almost constant value of 28-30 nm with the thickness increased to 35 nm. It can be also confirmed for each sample that there is no diffusion of Ni into the PEN layer, resulting in clear Ni/PEN interface and smooth Ni surface. Therefore, these experimental results indicate that Ni/PEN films can be expected as metal/insulator hybrid materials in QC devices, leading to novel high-density memory devices.     

Effects of crystalline grain size and packing ratio of self-forming core/shell nanoparticles on magnetic properties at up to GHz bands

Self-forming core/shell nanoparticles of magnetic metal/oxide with crystalline grain size of less than 40 nm were synthesized. The nanoparticles were highly concentrated in an insulating matrix to fabricate a nanocomposite, whose magnetic properties were investigated. The crystalline grain size of the nanoparticles strongly influenced the magnetic anisotropy field, magnetic coercivity, relative permeability, and loss factor (tan δ=μ″/μ′) at high frequency. The packing ratio of the magnetic metallic phase in the nanocomposite also influenced those properties. High permeability with low tan δ of less than 1.5% at up to 1 GHz was obtained in the case of the nanoparticles with crystalline grain size of around 15 nm with large packing ratio of the nanoparticles.     

Details of crystalline growth in co-deposited electroplated nickel films with hard (nano)particles

Electroplated nickel dispersion films with incorporated hard particles, primarily titanium oxide, were studied. A sufficient dispersion of nanometre-scaled particles in Watts solution was reached by application of ultrasonic energy to the galvanic bath. Crystal morphology, mean grain size and formation of textures were examined by electron backscattering diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The typical columnar structure of pure Ni films was refined by means of ultrasound. Incorporation of micron-sized TiO₂ particles generates additional nucleation surfaces in contrast to SiC particles. Textures of the subsequent columnar nickel crystals change from 〈2 1 1〉 (silent condition) or 〈1 1 0〉 (ultrasonic condition) fibre textures in growth direction to 〈1 0 0〉 and 〈1 1 1〉 textures under the influence of nanoparticle incorporation. Moreover, nanoparticles remarkably decrease the grain size and grain aspect ratio. Their incorporation takes place inside the crystals as well as between grains.