MFM and AFM study of thin cobalt films modified by fluorosilane

Polycrystalline cobalt films 100 nm thick were thermally evaporated on oxidized Si(100) substrates. Then 1H, 1H, 2H, 2H perfluorodecyltrichlorosilane (FDTS) films of various thicknesses, in the range of about 2 nm to 30 nm, were grown on cobalt surfaces by vapor phase deposition (VPD). The cobalt films modified by FDTS were investigated using magnetic force microscopy (MFM) and atomic force microscopy (AFM). MFM observation showed that the magnetic structure of the cobalt films modified by FDTS is composed of domains with a considerable component of magnetization perpendicular to the film surface. This in turn indicates that the cobalt films on oxidized Si(100) substrates crystallize in the hexagonal close-packed (HCP) phase and exhibit a texture with the hexagonal axis perpendicular to the film surface. The magnetic domains formed a maze structure. The domain width increased from typically 80–120 nm to 400–500 nm with increasing the thickness of FDTS films from about 2 nm to 30 nm. AFM imaging of the surfaces of FDTS films revealed the presence of an agglomerate morphology. The agglomerates varied in size from typically 30–70 nm to 150–300 nm as the film thickness was increased from about 2 nm to 30 nm.     

Synthesis,structure stability and magnetic properties of nanocrystalline Ag–Ni alloy

Silver–nickel alloy nanoparticles with an average size of 30–40 nm were synthesized by chemically reducing the mixture of silver and nickel salts using sodium borohydride. The structure and the magnetic properties of the alloy samples with different compositions were investigated. The phase stability of the material was analysed after annealing the sample in vacuum at various temperatures. The material exhibits single fcc phase which is stable up to 400 °C and Ni precipitation sets in when the sample is annealed to 500 °C. The thermal analysis using DSC was carried out to confirm the same. The alloy compositions are found to be in close correlation with the metal salt ratios in the precursors. The synthesized samples exhibit weak paramagnetic to ferromagnetic behaviour. The magnetic measurements reveal that by adjusting the precursor ratio, the Ni content in the material can be altered and hence its magnetic properties tailored to suit specific requirements. The formation of Ag–Ni alloy is confirmed by the observed Curie temperature from the magneto thermogram. Annealing the sample helps to produce significant enhancement in the magnetization of the material.     

Influence of induced anisotropy on the processes of magnetization reversal of cobalt circular nanodots

The magnetic structure and the processes of magnetization reversal of individual cobalt nanodots and arrays of cobalt nanodots have been studied using the magneto-optical Kerr effect and magnetic force microscopy. Arrays of nanodots have been prepared by ion etching from a continuous cobalt film. Magnetic anisotropy is induced during deposition of the cobalt films. The nanodots have the diameter d = 600 nm and the period varying from 1.5d to 3.0d. Magnetic force microscopy images have shown that the induced magnetic anisotropy affects the orientation of magnetization of noninteracting nanodots and the direction of displacement of the magnetic vortex center in the nanodots coupled by the dipole-dipole interaction.     

Microstructure and magnetic properties of Co(Cr)/Ti(Cr)/ZnO films prepared by laser ablation on Si substrates

High-quality Co(Cr)(50–66 nm)/Ti(Cr)(25–30 nm)/ZnO(165–225 nm) films were grown on Si substrates by the pulsed-laser deposition technique at 450–500°C. The microstructure of the films and their magnetic properties were investigated by X-ray diffraction, transmission electron microscopy and a vibrating sample magnetometer. It was found that the Co(Cr)/Ti(Cr)/ZnO thin films had a highly preferential c-axis direction normal to the surface of the substrate, and the ZnO sublayer had columnar structures in the growth direction. The interface of the multi-layer films is smooth without interface reaction. The as-produced films had in-plane easy magnetization. It indicates that the enhancements of the magnetic properties of the films are mainly ascribed to the grain structure and multi-domain state of the Co-alloy layer, which can be controlled by the ZnO sublayer.     

Structural and magnetic properties of high magnetic moment electroplated CoNiFe thin films

CoNiFe alloy thin films deposited at various cobalt concentrations were galvanostatically electrodeposited on the pre-cleaned copper substrates. The effects of cobalt concentration on the structural, compositional, morphological, and magnetic properties of the films were investigated. X-ray diffraction patterns revealed that the deposited films possess polycrystalline in nature with mixed (fcc–bcc) cubic structure at optimized cobalt concentration. Microstructural properties of the films were calculated from predominant diffraction lines. The surface morphology and surface roughness were characterized using scanning electron microscopy and atomic force microscopy, respectively. EDAX results were revealed that the cobalt content increases as nickel content decreases whereas ferrous content initially increases and then eventually decreases in the CoNiFe alloy. VSM results show a higher value of saturation magnetization (4πM _s) above 2 T with coercivity 154 A/m for films deposited in the optimized deposition condition.     

FMR and TEM Studies of Co and Ni Nanoparticles Implanted in the SiO<Subscript>2</Subscript> Matrix

Fused silica plates have been implanted with 40 keV Co⁺ or Ni⁺ ions to high doses in the range of (0.25–1.0) × 10¹⁷ ions/cm², and magnetic properties of the implanted samples have been studied with ferromagnetic resonance (FMR) technique supplemented by transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The high-dose implantation with 3d-ions results in the formation of cobalt and nickel metal nanoparticles in the irradiated subsurface layer of the SiO₂ matrix. Co and Ni nanocrystals with hexagonal close packing and face-centered cubic structures have a spherical shape and the sizes of 4–5 nm (for cobalt) and 6–14 nm (for nickel) in diameter. Room-temperature FMR signals from ensembles of Co and Ni nanoparticles implanted in the SiO₂ matrix exhibit an out-of-plane uniaxial magnetic anisotropy that is typical for thin magnetic films. The dose and temperature dependences of FMR spectra have been analyzed using the Kittel formalism, and the effective magnetization and g-factor values have been obtained for Co- and Ni-implanted samples. Nonsymmetric FMR line shapes have been fitted by a sum of two symmetrical curves. The dependences of the magnetic parameters of each curve on the implantation dose and temperature are presented.     

Assembly of γ-Fe2O3/polyaniline nanofilms with tuned dipolar interaction

The internal morphology and magnetic properties of layer-by-layer assembled nanofilms of polyaniline (PANI) and maghemite (γ-Fe₂O₃—7.5-nm diameter) were probed with cross-sectional transmission electron microscopy (TEM) and magnetization measurements (magnetic hysteresis loops, magnetization using zero-field cooled/field-cooled protocols, and ac magnetic susceptibility). Additionally, simulations of the as-produced samples were performed to assess both the nanofilm’s morphology and the corresponding magnetic signatures using the cell dynamic system (CDS) approach and Monte Carlo (MC) through the standard Metropolis algorithm, respectively. Fine control of the film thickness and average maghemite particle–particle within this magnetic structure was accomplished by varying the number of bilayers (PANI/γ-Fe₂O₃) deposited onto silicon substrates or through changing the concentration of the maghemite particles suspended within the colloidal dispersion sample used for film fabrication. PANI/γ-Fe₂O₃ nanofilms comprising 5, 10, 25 and 50 deposited bilayers displayed, respectively, blocking temperatures (T _B) of 30, 35, 39 and 40 K and effective energy barriers (ΔE/k _B) of 1.0 × 10³, 2.3 × 10³, 2.8 × 10³ and 2.9 × 10³ K. Simulation of magnetic nanofilms using the CDS model provided the internal morphology to carry on MC simulation of the magnetic properties of the system taking into account the particle–particle dipolar interaction. The simulated (using CDS) surface–surface particle distance of 0.5, 2.5 and 4.5 nm was obtained for nanofilms with thicknesses of 36.0, 33.9 and 27.1 nm, respectively. The simulated (using MC) T _B values were 33.0, 30.2 and 29.5 K for nanofilms with thicknesses of 36.0, 33.9 and 27.1 nm, respectively. We found the experimental (TEM and magnetic measurements) and the simulated data (CDS and MC) in very good agreement, falling within the same range and displaying the same systematic trend. Our findings open up new perspectives for fabrication of magnetic nanofilms with pre-established (simulated) morphology and magnetic properties.     

Iron-doped titanium dioxide nanotubes: a study of electrical,optical, and magnetic properties

Abstract  

Iron doped titanium oxide nanotubes (TNTs) were synthesized by hydrometallurgical process using a mixture of NaOH and methanol as precipitating and reducing agents, respectively. Nanotubes with a high purity and good aspect ratio are produced as indicated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The characterization data show that high-purity Fe-doped TNTs with diameter around 12–14 nm and length around 400–460 nm are synthesized using this process. The TNTs are found to be in the anatase phase and as the Fe doping is increased, the conductivity increases. UV/VIS data suggest the red shift in the peaks and increased absorption on the account of doping. The studies on microcosmic magnetic properties of the sample with 15% Fe content indicate the component of magnetic moment in the axial direction of nanotubes. Doping of Fe is found to considerably affect the crystallite size, Curie temperature (T _c), DC conductivity (σ), and Raman shifts.     

Investigation of the magnetization reversal of a magnetic dot array of Co/Pt multilayers

The magnetization reversal behavior of a dot array consisting of Co/Pt multilayers with perpendicular magnetic anisotropy was investigated. The size of the dots was varied from 200 nm down to 40 nm, while keeping the filling factor constant at about 0.16. The structural properties were determined by scanning electron microscopy, whereas the magnetic investigation was performed using SQUID and MFM techniques. It was observed that the dot size has a severe impact on the magnetization reversal mechanism where only the smallest dots with a size of 40 nm are found to be in a magnetic single-domain state. Moreover, the patterning process leads to a degradation of the multilayer, leading to a reduction of the switching field and an increase of the switching field distribution with decreasing dot size. In addition, micromagnetic simulations were performed to understand the magnetization reversal mechanism in more detail.     

Fabrication of magnetic gold nanorod particles for immunomagnetic separation and SERS application

The preparation and application of rod-shaped core–shell structured Fe₃O₄–Au nanoparticles for immunomagnetic separation and sensing were described for the first time with this study. To synthesize magnetic gold nanorod particles, the seed-mediated synthetic method was carried out and the resulting nanoparticles were characterized with transmission electron microscopy (TEM), ultraviolet visible spectroscopy (UV–Vis), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD). Magnetic properties of the nanoparticles were also examined. Characterization of the magnetic gold nanorod particles has proven that the resulting nanoparticles were composed of Fe₃O₄ core and the gold shell. The rod-shaped gold-coated iron nanoparticles have an average diameter of 16 ± 2 nm and an average length of about 50 ± 5 nm (corresponding aspect ratio of 3). The saturation magnetization value for the magnetic gold nanorod particles was found to be 37 emu/g at 300 K. Rapid and room temperature reaction synthesis of magnetic gold nanorod particles and subsequent surface modification with E. coli antibodies provide immunomagnetic separation and SERS application. The analytical performance of the SERS-based homogenous sandwich immunoassay system with respect to linear range, detection limit, and response time is also presented.     

Comparative study of core–shell iron/iron oxide gold covered magnetic nanoparticles obtained in different conditions

In this study, we report the synthesis and characterization of the core–shell Fe covered with Au shells nanoparticles with mean diameters between 5 and 8 nm. The inverse micelles method was utilized to produce the samples. X-ray diffraction studies show that both core–shell systems have the expected crystalline structure. High resolution transmission electron microscopy and atomic emission spectroscopy techniques give additional information concerning the structure and composition of nanoparticles. An intermediate shell of amorphous oxidized iron was found between the magnetic Fe core and the external gold shell. The magnetic behavior of different core–shell samples shows no hysteresis loop indicating the superparamagnetic behavior of Fe@Au systems. The superparamagnetic behavior is also evidenced from FC and ZFC dependences of the magnetization versus temperature. By using the temperature dependence of the thermoremanent magnetization combined with magnetization versus applied magnetic field, the effective anisotropy constant was determined. The Fe/Au interface contribution to the effective anisotropy constant was calculated and discussed in relation with the combined shape and stress anisotropies.     

The influence of pH electrolyte on the electrochemical deposition and properties of nickel thin films

Ni thin films were electrodeposited on gold substrate from chloride solution with different pH at room temperature. The effect of electrolyte pH on Ni coatings was studied by using the cyclic voltammetry, the scanning electron microscopy (SEM), x-ray diffraction, and alternating gradient force magnetometer measurements. From electrochemical measurements, the onset potential for reduction of Ni was gradually shifted towards more cathodic scan with increase in pH; this is due to the protons in the case of low pH values and to the hydroxide ions in the case of higher pH values. The SEM study showed that a granular and compact structure of the electrodeposited Ni layers and the variation of film morphology with bath pH are established. The x-ray diffraction spectra revealed the formation of fcc structure Ni thin films with a preferential orientation along the Ni(111). The size of the deposited crystals in both the cases has been found to be in the range of 49–153 nm. Magnetic properties such as coercivity and saturation magnetization showed strong dependence on the electrolyte solution pH and consequently the crystallite size. Coercivity higher than 130–160 Oe was achieved for a pH value of 4 to 5. The differences observed in the magnetic properties were attributed to the structural changes caused by the electrolyte pH.     

Inverse magnetocaloric effect in sol–gel derived nanosized cobalt ferrite

The magnetocaloric properties of cobalt ferrite nanoparticles were investigated to evaluate the potential of these materials as magnetic refrigerants. Nanosized cobalt ferrites were synthesized by the method of sol–gel combustion. The nanoparticles were found to be spherical with an average crystallite size of 14 nm. The magnetic entropy change (ΔS _m) calculated indirectly from magnetization isotherms in the temperature region 170–320 K was found to be negative, signifying an inverse magnetocaloric effect in the nanoparticles. The magnitudes of the ΔS _m values were found to be larger when compared to the reported values in the literature for the corresponding ferrite materials in the nanoregime.     

Investigation of micromagnetism and magnetic reversal of Ni nanoparticles using a magnetic force microscope

Isolated Ni nanoparticles were studied in situ by atomic and magnetic force microscopy in the presence of an additional external field up to 300 Oe. By comparing topographic and magnetic images, and also by computer modeling of magnetic images, it was established that particles smaller than 100 nm are single-domain and easily undergo magnetic reversal in the direction of the applied external magnetic field. For large magnetic particles, the external magnetic field enhances the magnetization uniformity and the direction of total magnetization of these particles is determined by their shape anisotropy. Characteristics of the magnetic images and magnetic reversal of particles larger than 150 nm are attributed to the formation of a vortex magnetization structure in these particles. Fiz. Tverd. Tela (St. Petersburg) 40, 1277–1283 (July 1998)     

Effect of the soft layer thickness on magnetization reversal process of exchange-spring nanomagnet patterns

A systematic study of the magnetization reversal behavior in the regular arrangement of L1₀-FePt based exchange-spring nanomagnets with different thicknesses of the Co soft magnetic layer is presented. The magnetic property of the hard magnet is compared to two tuned exchange-spring magnets: its systems of 20 nm L1₀-FePt/3 nm, and 7 nm Co. In particular, we focus on the switching field distribution. The exchange coupling showed narrower SFD, in spite of the decoupled part switches earlier. The magnetization switching mechanism of exchange-spring nanomagnets patterns has been revealed with a first-order reversal curves technique and the switching field distribution. Further, the microscopic results using magnetic force microscopy show that the spin rotation of the non-interacting part in the thicker soft layered exchange-spring magnet. The part influences the magnetization reversal process. According to the experimental results, exchange coupling strength can be tuned by the thickness of the soft magnetic layer.     

One-pot synthesis and characterization of rhodamine derivative-loaded magnetic core–shell nanoparticles

A new method to produce elaborate nanostructure with magnetic and fluorescent properties in one entity is reported in this article. Magnetite (Fe₃O₄) coated with fluorescent silica (SiO₂) shell was produced through the one-pot reaction, in which one reactor was utilized to realize the synthesis of superparamagnetic core of Fe₃O₄, the formation of SiO₂ coating through the condensation and polymerization of tetraethylorthosilicate (TEOS), and the encapsulation of tetramethyl rhodamine isothiocyanate-dextran (TRITC-dextran) within silica shell. Transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were carried out to investigate the core–shell structure. The magnetic core of the core–shell nanoparticles is 60 ± 10 nm in diameter. The thickness of the fluorescent SiO₂ shell is estimated at 15 ± 5 nm. In addition, the fluorescent signal of the SiO₂ shell has been detected by the laser confocal scanning microscopy (LCSM) with emission wavelength (λ_em) at 566 nm. In addition, the magnetic properties of TRITC-dextran loaded silica-coating iron oxide nanoparticles (Fe₃O₄@SiO₂ NPs) were studied. The hysteresis loop of the core–shell NPs measured at room temperature shows that the saturation magnetization (M _s) is not reached even at the field of 70 kOe (7T). Meanwhile, the very low coercivity (H _c) and remanent magnetization (M _r) are 0.375 kOe and 6.6 emu/g, respectively, at room temperature. It indicates that the core–shell particles have the superparamagnetic properties. The measured blocking temperature (T _B) of the TRITC-dextran loaded Fe₃O₄@SiO₂ NPs is about 122.5 K. It is expected that the multifunctional core–shell nanoparticles can be used in bio-imaging.     

Superparamagnetic Ni:SiO<Subscript>2</Subscript>–C nanocomposites films synthesized by a polymeric precursor method

In this work, we report on the magnetic properties of nickel nanoparticles (NP) in a SiO₂–C thin film matrix, prepared by a polymeric precursor method, with Ni content x in the 0–10 wt% range. Microstructural analyses of the films showed that the Ni NP are homogenously distributed in the SiO₂–C matrix and have spherical shape with average diameter of ~10 nm. The magnetic properties reveal features of superparamagnetism with blocking temperatures T _B ~ 10 K. The average diameter of the Ni NP, estimated from magnetization measurements, was found to be ~4 nm for the x = 3 wt% Ni sample, in excellent agreement with X-ray diffraction data. M versus H hysteresis loops indicated that the Ni NP are free from a surrounding oxide layer. We have also observed that coercivity (H _C) develops appreciably below T _B, and follows the H _C ∝ [1 – (T/T _B)^0.5] relationship, a feature expected for randomly oriented and non-interacting nanoparticles. The extrapolation of H _C to 0 K indicates that coercivity decreases with increasing x, suggesting that dipolar interactions may be relevant in films with x > 3 wt% Ni.     

Carbon encapsulated nickel nanoparticles synthesized by a modified alcohol catalytic chemical vapor deposition method

In this work, a simple and cost-efficient method was demonstrated for the production of carbon encapsulated nickel nanoparticles (CENPs). The CENPs were synthesized from nickel oxide by a modified alcohol catalytic chemical vapor deposition method at atmospheric pressure. Structural characterizations using scanning electron microscopy, transmission electron microscopy and X-ray diffraction indicated that the Ni-core mean diameter and the graphite-layer mean thickness were 98 ± 31 nm and 65 ± 35 nm, respectively. Room temperature magnetization results showed a ferromagnetic behavior of the CENPs, in which the saturation magnetization decreased with decreasing the Ni-to-C ratio whereas the coercivity increased with decreasing the Ni-core diameter.     

Micromagnetic properties and magnetization switching of single domain Co dots studied by magnetic force microscopy

Magnetic force microscopy was applied to study the magnetic properties of Co dot microstructures. The high density magnetic dot arrays were fabricated using nanolithographic techniques on GaAs substrates. The ferromagnetic Co dots were found to be in a single domain state for Co film thicknesses of 7 nm and 17 nm. The magnetization of the as-prepared Co dot array was found to be in a non-uniform state. After applying a magnetic field the Co dots are in a uniform magnetization state. Induced switching of the magnetization of single Co dots by the stray field of the probing tip using an additionally applied in-situ magnetic field has been demonstrated.     

Study of nanoconductive and magnetic properties of nanostructured iron films prepared by sputtering at very low temperatures

A combined study of the surface nanostructure and electrical characteristics of iron thin films prepared on naturally passivated silicon wafers is presented. By means of conductive-scanning force microscopy, the influence of the substrate temperature during film preparation on both surface morphology and conductivity response is investigated. In addition, magnetic properties of these films are reported and correlated with the nanostructural properties. Films prepared at 200 K show granular core–shell magnetic behaviour exhibiting exchange bias. Both conductive and magnetic data indicate that samples prepared at low temperature behave as a percolated network of nanometric metallic iron clusters (with typical sizes of 20 nm) interconnected by oxidized chains (of 10 nm in diameter), showing an excellent system for nanotechnological exchange bias applications.