Magnetic field heating study of Fe-doped Au nanoparticles

Fe-doped Au nanoparticles are ideal for biological applications over magnetic oxides due to their conjugation chemistry, optical properties, and surface chemistry. We present an AC magnetic field heating study of 8 nm Fe-doped Au nanoparticles which exhibit magnetic behavior. Magnetic heating experiments were performed on stable aqueous solutions of the nanoparticles at room temperature. The nanoparticles exhibit magnetic field heating, with a specific absorption rate (SAR) of 1.84 W/g at 40 MHz and H=100 A/m. The frequency dependence of the heating follows general trends predicted by power loss equations and is similar to traditional materials.     

Magnetic Behaviour and Heating Effect of Fe3O4 Ferrofluids Composed of Monodisperse Nanoparticles

Fe3O4 ferrofluids containing monodisperse Fe3O4 nanoparticles with different diameters of 8, 12, 16 and 18nm are prepared by using high-temperature solution phase reaction. The particles have single crystal structures with narrow size distributions. At room temperature, the 8-nm ferrofluid shows superparamagnetic behaviour, whereas the others display hysteresis properties and the coercivity increases with the increasing particle size. The spin glass-like behaviour and cusps near 190K are observed on all ferrofluids according to the temperature variation of field-cooled （FC） and zero-field-cooled （ZFC） magnetization measurements. The cusps are found to be associated with the freezing point of the solvent. As a comparison, the ferrofluids are dried and the FC and ZFC magnetization curves of powdery samples are also investigated. It is found that the blocking temperatures for the powdery samples are higher than those for their corresponding ferrofluids. Moreover, the size dependent heating effect of the ferrofluids is also investigated in ac magnetic field with a frequency of 55 kHz and amplitude of 200 Oe.     

Silicon nanowire and polyethylene superhydrophobic surfaces for discrete magnetic microfluidics

A microfluidic method to manipulate small drops of water is studied on two different superhydrophobic surfaces. Using this digital magnetofluidic method, water drops containing paramagnetic carbonyl-iron microparticles were displaced on silicon nanowire (Si NW) and low-density polyethylene (LDPE) superhydrophobic surfaces using magnetic fields. Horizontal, vertical, or upside-down drop movement is made possible by the action of capillary forces induced by paramagnetic particles aligning and following a magnetic field, indicating that three-dimensional digital microfluidics is possible. Also, both Si NW and LDPE superhydrophobic surfaces combine surface chemistry with nano and microscale surface roughness to make drop movement possible. Si NW superhydrophobic surfaces were prepared using vapor-liquid-solid growth systems followed by coating with a perfluorinated hydrocarbon. LDPE superhydrophobic surfaces were prepared by growing polyethylene crystals on a polyethylene substrate through careful rate control.     

Femtosecond laser structuring of titanium implants

In this study we perform the first femtosecond laser surface treatment of titanium in order to determine the potential of this technology for surface structuring of titanium implants. We find that the femtosecond laser produces a large variety of nanostructures (nanopores, nanoprotrusions) with a size down to 20 nm, multiple parallel grooved surface patterns with a period on the sub-micron level, microroughness in the range of 1-15 μm with various configurations, smooth surface with smooth micro-inhomogeneities, and smooth surface with sphere-like nanostructures down to 10 nm. Also, we have determined the optimal conditions for producing these surface structural modifications. Femtosecond laser treatment can produce a richer variety of surface structures on titanium for implants and other biomedical applications than long-pulse laser treatments.     

Improving the bioactivity of NiTi shape memory alloy by heat and alkali treatment

TiO₂ films were formed on an NiTi alloy surface by heat treatment in air at 600 °C. Heat treated NiTi shape memory alloys were subsequently alkali treated with 1 M, 3 M and 5 M NaOH solutions respectively, to improve their bioactivity. Then treated NiTi samples were soaked in 1.5SBF to evaluate their in vitro performance. The results showed that the 3 M NaOH treatment is the most appropriate method. A large amount of apatite formed within 1 day's soaking in 1.5SBF, after 7 day's soaking TiO₂/HA composite layer formed on the NiTi surface. SEM, XRD, FT-IR and TEM results showed that the morphology and microstructure are similar to the human bone apatite.     

Detection of magnetic nanoparticles with magnetoencephalography

Superconducting quantum interference devices (SQUIDs) have been widely utilized in biomedical applications due to their extremely high sensitivity to magnetic signals. The present study explores the feasibility of a new type of nanotechnology-based imaging method using standard clinical magnetoencephalographic (MEG) systems equipped with SQUID sensors. Previous studies have shown that biological targets labeled with non-toxic, magnetized nanoparticles can be imaged by measuring the magnetic field generated by these particles. In this work, we demonstrate that (1) the magnetic signals from certain nanoparticles can be detected without magnetization using standard clinical MEG, (2) for some types of nanoparticles, only bound particles produce detectable signals, and (3) the magnetic field of particles several hours after magnetization is significantly stronger than that of un-magnetized particles. These findings hold promise in facilitating the potential application of magnetic nanoparticles to in vivo tumor imaging. The minimum amount of nanoparticles that produce detectable signals is predicted by theoretical modeling and computer simulation.     

An optimal principle in fluid-structure interaction

We study the steady terminal orientation of a fore-aft symmetric body as it settles in a viscous fluid. An optimal principle for the settling behavior is discussed based upon entropy production in the system, both in the Stokes limit and the case of near equilibrium states when inertial effects emerge. We show that in the Stokes limit, the entropy production in the system is zero allowing any possible terminal orientation while in the presence of inertia, the particle assumes a horizontal position which coincides with the state of maximum entropy production. Our results are seen to agree well with experimental observations.     

Magnetic properties of p-doped GaMnN diluted magnetic semiconductors containing clusters

Magnetic properties of p-doped GaMnN diluted magnetic semiconductors, having both randomly distributed Mn ions and Mn_xN_y clusters, are presented under the theory based on the hole-mediated ferromagnetism. The critical temperature of the second order phase transition between ferromagnetic and paramagnetic phases and the magnetization as a function of temperature are obtained from the free energy calculation. The Curie temperature of the p-doped GaMnN containing clusters depends not on the type of clusters but on the composition rate of clusters. The behavior of the spontaneous magnetization as a function of temperature is strongly affected by carrier concentration. The p-doped GaMnN diluted magnetic semiconductors containing clusters have room temperature ferromagnetism regardless of the magnetic type of clusters, as long as hole-mediated spin-spin interactions occur in them.     

Magnetic anisotropy and anisotropic ballistic conductance of thin magnetic wires

The magnetocrystalline anisotropy of thin magnetic wires of iron and cobalt is quite different from the bulk phases. The spin moment of monatomic Fe wire may be as high as 3.4 μ_B, while the orbital moment as high as 0.5 μ_B. The magnetocrystalline anisotropy energy (MAE) was calculated for wires up to 0.6 nm in diameter starting from monatomic wire and adding consecutive shells for thicker wires. I observe that Fe wires exhibit the change sign with the stress applied along the wire. It means that easy axis may change from the direction along the wire to perpendicular to the wire. We find that ballistic conductance of the wire depends on the direction of the applied magnetic field, i.e. shows anisotropic ballistic magnetoresistance. This effect occurs due to the symmetry dependence of the splitting of degenerate bands in the applied field which changes the number of bands crossing the Fermi level. We find that the ballistic conductance changes with applied stress. Even for thicker wires the ballistic conductance changes by factor 2 on moderate tensile stain in our 5×4 model wire. Thus, the ballistic conductance of magnetic wires changes in the applied field due to the magnetostriction. This effect can be observed as large anisotropic BMR in the experiment.     

Electronic and magnetic properties of CeAl2 nanoparticles

We presented the X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) studies of heavy fermion compound CeAl₂ bulk and 8 nm nanoparticles, performed at the Ce M_4,5- and L₃- absorption edges. XMCD and XAS revealed that Ce in bulk CeAl₂ exhibits localized 4f¹ character with magnetic ordering. The Ce in nanoparticles, on the other hand, shows a small amount delocalized 4f⁰ character with non-magnetic Kondo behavior. By applying general sum rules, an estimation of the orbital and spin contribution to those Ce 4f moments can be obtained. Our results also demonstrated that the magnetic behavior in CeAl₂ is very sensitive to the degree of localization of the 4f electrons.     

Automated detection system of single nucleotide polymorphisms using two kinds of functional magnetic nanoparticles

Single nucleotide polymorphisms (SNPs) comprise the most abundant source of genetic variation in the human genome wide codominant SNPs identification. Therefore, large-scale codominant SNPs identification, especially for those associated with complex diseases, has induced the need for completely high-throughput and automated SNP genotyping method. Herein, we present an automated detection system of SNPs based on two kinds of functional magnetic nanoparticles (MNPs) and dual-color hybridization. The amido-modified MNPs (NH₂-MNPs) modified with APTES were used for DNA extraction from whole blood directly by electrostatic reaction, and followed by PCR, was successfully performed. Furthermore, biotinylated PCR products were captured on the streptavidin-coated MNPs (SA-MNPs) and interrogated by hybridization with a pair of dual-color probes to determine SNP, then the genotype of each sample can be simultaneously identified by scanning the microarray printed with the denatured fluorescent probes. This system provided a rapid, sensitive and highly versatile automated procedure that will greatly facilitate the analysis of different known SNPs in human genome.     

A quantitative analysis of magnetic vortices in Permalloy nanoparticles characterized by electron holography

The present work investigates experimentally curling magnetic configurations locally observed in almost dispersed Permalloy nanoparticles in the remanent state. Magnetic analysis is performed in a field emission TEM using off-axis electron holography. Particularly, electron holography is used to characterize the magnetic microstructure of Fe₃₀Ni₇₀ nanoparticles, whose average diameter (50 nm) is expected to be close to the critical size for a curling magnetic structure (vortex) formation. The vortex core diameter D_core and the bulk magnetic profile of the vortex are measured and compared with a “rigid vortex” micromagnetic model. The connection between vortex structure and the characteristic micromagnetic length of the system deduced from magnetization curve measurements is discussed.     

Astroid curves of high-moment antiferromagnetic nanoparticles with tunable magnetic properties

We have determined astroids for high-moment antiferromagnetic nanoparticles (AN), which have been recently discovered and used in numerous biomedical applications. The astroid curves for such a system, which is a stack of two isolated disk-shaped ferromagnetic nanoparticles interacting antiferromagnetically, show the regions in the magnetic field plane where different numbers of minima associated with stable or metastable states may exist. We describe the properties of these ANs and estimate their other characteristic parameters such as magnetic saturation field and exchange antiferrtomagnetic coupling. We argue that the finding of these astroids and the properties of ANs is crucial for the use of ANs in numerous applications and for modeling stable information storage devices.     

Determination of anisotropy constants of protein encapsulated iron oxide nanoparticles by electron magnetic resonance

Angle-dependent electron magnetic resonance was performed on 4.9, 8.0, and 19 nm iron oxide nanoparticles encapsulated within protein capsids and suspended in water. Measurements were taken at liquid nitrogen temperature after cooling in a 1 T field to partially align the particles. The angle dependence of the shifts in the resonance field for the iron oxide nanoparticles (synthesized within Listeria-Dps, horse spleen ferritin, and cowpea chlorotic mottle virus) all show evidence of a uniaxial anisotropy. Using a Boltzmann distribution for the particles’ easy-axis direction, we are able to use the resonance field shifts to extract a value for the anisotropy energy, showing that the anisotropy energy density increases with decreasing particle size. This suggests that surface anisotropy plays a significant role in magnetic nanoparticles of this size.     

High concentration of hematite nanoparticles in a silica matrix: Structural and magnetic properties

The α-Fe₂O₃/SiO₂ nanocomposite containing 45 wt% of hematite was prepared by the sol-gel method followed by heating in air at 200 °C. The so-obtained composite of iron(III) nanoparticles dissolved in glassy silica matrix was investigated by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. XRPD confirms the formation of a single-phase hematite sample, whereas TEM reveals spherical particles in a silica matrix with an average diameter of 10 nm. DC magnetization shows bifurcation of the zero-field-cooled (ZFC) and field-cooled (FC) branches up to the room temperature with a blocking temperature T_B=65 K. Isothermal M(H) dependence displays significant hysteretic behaviour below T_B, whereas the room temperature data were successfully fitted to a weighted Langevin function. The average particle size obtained from this fit is in agreement with the TEM findings. The small shift of the T_B value with the magnetic field strength, narrowing of the hysteresis loop at low applied field, and the frequency dependence of the AC susceptibility data point to the presence of inter-particle interactions. The analysis of the results suggests that the system consists of single-domain nanoparticles with intermediate strength interactions.     

Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles

CoFe₂O₄ ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe₂O₄. The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co²⁺ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe₂O₄ samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state.     

Structural and magnetic features of heterogeneously nucleated Fe-oxide nanoparticles

Iron oxide nanoparticles of diameter 14 nm were synthesized by applying Pt seed-assisted heterogeneous thermal decomposition of Fe(CO)₅ in a two-stage procedure. The intense heating treatment resulted in a remarkable mean volume increment compared to previous studies. This method is able to control the nanoparticle mean diameter, keeping the demand for thermal energy at low levels. High-resolution electron microscopy images and the corresponding electron diffraction patterns revealed the appearance of a FePt₃ core in each nanoparticle, surrounded by highly crystallized inverse spinel Fe₃O₄ formed after atmospheric oxidation, as shown by a combination of X-ray diffraction and chemical analysis. Magnetic measurements indicated that the presence of Pt-rich core does not cause any visible modification to the values of saturation magnetization and anisotropy constant of nanoparticles, compared to homogeneously nucleated iron oxide particles of the same size.     

Magnetic force microscopy applied in magnetic data storage technology

Microstructured thin-film elements with critical dimensions of 1 μm or less play an increasingly important role in magnetic components for information technology applications. Devices that are directly based on such microstructures are key components in magnetoelectronics for storage and sensor applications as well as modern concepts which are likely to substitute today’s hard disk drives. Basic research on magnetic materials as well as industrial applications create an increasing demand for high-resolution magnetic imaging methods. One such method is magnetic force microscopy (MFM). In spite of considerable achievements, MFM also has some serious shortcomings, which have not been overcome to date. Under normal circumstances, the method yields only qualitative information about the magnetic object and it is difficult to improve the resolution to values below 100 nm. In this paper, we will report on advanced MFM probe preparation, based on electron beam methods, and discuss the possibilities for batch fabrication of such advanced MFM tips. We show that the advanced probes allow high-resolution imaging of fine magnetic structures within thin-film permalloy elements without perturbing them. Additionally, we present high-frequency MFM measurements on a hard disk write head. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-681/302-3790, E-mail: m.koblischka@mx.uni-saarland.de     

Equilibrium magnetic properties of dipolar interacting ferromagnetic nanoparticles

We use Monte Carlo simulations to study the influence of dipolar interaction on the equilibrium magnetic properties of monodisperse single-domain ferromagnetic nanoparticles. Low field magnetizations simulated in zero field cooling (ZFC)/field cooling (FC) procedures and field-dependent magnetization curves above the blocking temperatures show strong dependence on the concentration and the spatial arrangement (cubic or random) of the magnetic particles. The field-dependent magnetizations can not be simply described by the T^* model at relative low temperatures due to the interplay between anisotropy and dipolar interactions, as well as the spatial arrangement effect.