Nanocomposite Magnetic Materials Research Articles

Pulse-Laser-Crystallization of Amorphous Nd<sub>3.7</sub>Pr<sub>3.4</sub>Dy<sub>0.9</sub>Fe<sub>86</sub>B<sub>5</sub>Nb<sub>1</sub>

The pulse-laser-crystallization (PLC) technique was applied to Nd3.7Pr3.4Dy0.9Fe86B5Nb1 amorphous ribbons. After irradiation with a 248nm KrF pulse excimer laser at a fequency of 15Hz for 1 min, the Nd3.7Pr3.4Dy0.9Fe86B5Nb1 amorphous ribbons crystallized into the homogeneous Nd2Fe14B/α-Fe nanocomposite permanent magnetic materials with an average grain size around 26nm. The obtained coercivity, remanence, and (BH)max values were 6.6 K, 1.23 T, and 18.5 MGOe, respectively.

Size-Dependence of Effective Anisotropy and Coercivity in Nanocomposite Permanent Magnetic Materials

Take into account different degree of exchange-coupling interaction between soft and hard grains, the effective anisotropy and coercivity in nanocomposite permanent magnetic materials has been investigated by adopting a statistical average physical model. The calculated results show a strong dependence of effective anisotropy and coercivity on the grain size in these materials. Using this model, the serious deterioration of coercivity in experiments especially when the grain size is less then 15nm could be explained in terms of the dramatic drop of the effective anisotropy in nanocomposites.

The Measurement for the Effective Anisotropy Constant of the Nanocomposite Permanent Magnetic Material Nd<sub>2</sub>Fe<sub>14</sub>B/α-Fe

The effective anisotropy constant and the saturation magnetization in nano composite Nd2Fe14B/α-Fe have been calculated by the law of approach to saturation (LATS) method. The applicability of the method has been analyzed. The calculated result shows that the LATS method is suitable for the calculation of the effective anisotropy constant and the saturation magnetization in nano composite permanent magnetic materials.

Preparation of functional magnetic nanocomposites and hybrid materials: recent progress and future directions

The aim of this article is to provide an overview of current research activities on functional, magnetic nanocomposite materials. After a brief introduction to general strategies for the synthesis of superparamagnetic nanoparticles (NPs), different concepts and state-of-the-art solution chemical methods for their integration into various types of functional, magnetic nanocomposite materials will be reviewed. The focus is on functional materials which are based on discrete magnetic NPs, including multicomponent nanostructures, colloidal nanocrystals, matrix-dispersed composite materials and mesoscaled particles. The review further outlines the magnetic, structural, and surface properties of the materials with regard to application.

Magnetic hardening of soft phase in nanocomposite permanent magnetic materials by exchange coupling

In this work, the issue of magentic hardening of soft phase in nanocomposite permanent magnetic materials has been investigated using one-and three-dimensional models. For the same microstructure, it is found that the coercivity is decreased and the low-field demagnetization curve keeps unchanged when the anisotropy constant of magnetic hard phase is decreased in anisotropic one-or three-dimensional samples. Therefore, the drop in anisotropy of magnetic hard phase will not lead to the increase of remanence and maximum energy product (BH)max. According to the simulation results of isotropic three-dimension samples, both the remanence and (BH)max will be obviously decreased by the drop in anisotropy. As a result, enhancing the anisotropy and/or enlarging the grain size of magnetic hard phase is one of the means to improve the hard magnetic properties of nanocomposite permanent magnetic materials.

Magnetic nanocomposite materials obtained using magnetic nano fluids and resins

The paper presents the possibility of creating a new category of magnetic nanocomposite materials, using magnetic nanofluids (MNF) and resins (Crainic et al., 2004). Polymer-embedded nanostructures are potentially useful for a number of technological applications, especially as advanced functional materials (e.g., high-energy radiation shielding materials, microwave absorbers, optical limiters, polarisers, sensors, hydrogen storage systems, etc.) (Carotenuto and Nicolais, 2003; Caseri, 2000; Nicolais and Carotenuto, 2005; Barnet and Peuker, 2006). In addition to the intrinsic nanoscopic material properties and the possibility to make transparent metal-polymer combinations, these materials are interesting also because the presence of a very large filler-matrix interface area can significantly affect the polymer characteristics (e.g., glass transition temperature, crystallinity, free volume content, etc.), allowing the appearance of further technologically exploitable mechanical and physical properties (e.g., fire resistance, low gas diffusivity, etc.) (Nicolais and Carotenuto, 2005).

Magnetic properties of Ni nanoparticles on microporous silica spheres

Ni nanoparticles (~32 nm particle diameter) have been synthesized on the walls of microporous (~1 nm pore diameter) silica spheres (~2.6 μm sphere diameter) and characterised magnetically to potentially produce a new class of core (silica micro-spheres)-shell (nanometallic)-type nanocomposite material. These magnetic nanocomposite materials display a characteristic increase in coercivity with reducing temperature. The average particle size has been used to calculate the anisotropy constant for the system, K. The discussion postulates the potential mechanisms contributing to the difference between the calculated K value and the magnetocrystalline anisotropy constant of bulk Ni. Various factors such as surface anisotropy and interparticle interactions are discussed as possible contributing factors to the anisotropy values calculated in the paper.

Magnetic nanocomposite materials: structure and mechanical properties

Composite materials reinforced by Nd-Fe-B (hard magnetic material) or FINEMET (soft magnetic material) particles with polymer matrix were manufactured by one-sided uniaxial pressing. The complex relationships among the manufacturing technology of these materials, their microstructure, as well as their mechanical and physical properties were evaluated. The main purpose of obtaining this kind of composite materials is broadening possibilities of nanocrystalline magnetic materials application that influence on the miniaturisation, simplification and lowering the costs of devices. The manufacturing of composite materials greatly expand the applicable possibilities of nanocrystalline powders of magnetically hard and soft materials. However, further examination to obtain improved properties of magnetic composite materials and investigations of constructions of new machines and devices with these materials elements are still needed. This paper shows the base of the technological solution which makes it possible in obtaining magnetic composite materials and mechanical properties of these composite materials. [Received on 15 April 2006; Accepted on 10 February 2007]

From Nanocrystals to Nanorods: New Iron Oxide−Silica Nanocomposites from Metallorganic Precursors

Sol−gel one-step processing of the Fe(II) alkoxide [Fe(OBut)2(THF)]2 and tetraethyl orthosilicate (TEOS) at various ratios afforded new silica−iron oxide magnetic nanocomposite materials, ranging in shape from nanocrystals to nanorods depending on the percentage of TEOS added. The nanocomposites have been investigated by Fourier transform infrared and Raman spectroscopy, X-ray diffraction, 57Fe Mossbauer spectroscopy, magnetization measurements, transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive X-ray analysis. These silica-coated magnetic materials might have a range of potential applications in biotechnology, medicine (e.g., as magnetic fluids for MRI), catalysis, and magnetic storage.

Crystallographic Texture and Domain Structure of Nd3.8Dy0.7Pr3.5Fe86Nb1B5 Nanocomposite Prepared by Direct Rapid Solidification

Strong crystallographic texture and high performance of Nd3.8Dy0.7Pr3.5Fe86Nb1B5 (containing 30% -Fe) nanocomposite permanent magnetic material was prepared by direct rapid solidification. X-ray diffraction analysis and magnetic measurement indicated that the ribbons had preferential orientation. The easy magnetization direction switched from perpendicular to the ribbon plane to parallel to the ribbon plane direction as the wheel speed increased from 10 m/s to 30 m/s. The multigrain domains were observed by scan probe microscope (SPM) in the ribbons prepared at wheel speed of 1030 m/s. The Henkel plots were employed to investigate the interactions of the grains in the samples. A very fine and uniform microstructure with the average grain size about 16 nm was obtained in the sample prepared at wheel speed of 30 m/s. The sample consisted of highly oriented hard magnetic phase (Nd,Dy,Pr)2(Fe,Nb)14B and soft magnetic phase -Fe. High performance of Br=1.29 T, Mr/Ms= 0.76 and (BH)max=158.4 kJ/m3 was achieved due to the strong crystallographic texture, fine and homogeneous microstructure and enhancement of the exchange coupling between the soft and hard magnetic phases in this sample. The mechanism of the formation of the crystallographic texture and the multigrain domains was also discussed.

Microstructure and Morphology of Alumina-Iron Nanocomposite Powders Produced by High Energy Mechanical Milling

Alumina-iron nanocomposite powders containing 5vol.% of iron were fabricated by high-energy ball milling with different ball-to-powder weight ratios (BPRs) as part of the study of ceramic-metal nanocomposite magnetic materials. The microstructure and morphology of the composite powders were characterized using the X-ray diffraction, optical microscopy and scanning electron microscopy. XRD analysis and SEM examination in combination with energy dispersive X-ray spectrometry confirmed that the nanocomposite structure of the powder particles formed only after 8 hours milling for both BPRs used. With a higher BPR of 16:1, Fe-Cr alloy material was broken from the stainless steel balls and incorporated into the nanocomposite powder. However, such a problem did not occur with a lower BPR of 5:1. The mechanism for formation of the alumina matrix nanocomposite powder is found to be dependent on BPR and milling time.

Anisotropy and pinning field in nanocomposite magnetic materials

The pinning fields of domain wall at different grain boundary H 0 have been calculated based on the given expression of anisotropy at grain boundary. The coercivity of nanocomposite magnetic materials is thought to be determined by H 0 . Compared with the experiential formula of coercivity, the expression of exchange–coupling coefficient α ex corresponding to pinning fields on grain size D was given too. The results showed that H 0 and α ex increase rapidly first, then increase slowly with increasing D . Our results are consistent with the relative theoretical and experimental results given by others.

The one-pot synthesis of dextran-based nanoparticles and their application in in-situ fabrication of dextran-magnetite nanocomposites

The dextran-based nanoparticles containing carboxyl groups were synthesized by a one-pot approach, without using any organic solvents and surfactants. The resultant dextran-based nanoparticles was used as a host for the growing and organization of Fe(3)O(4) nanoparticles. The approach consists of the mixture of ferrous/ferric ions aqueous solution and host nanoparticles and subsequent coprecipitation of ferrous/ferric ions in basic medium. The magnetic nanocomposite material obtained was characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS), thermogravimetric analysis (TGA), X-ray diffraction techniques (XRD) and vibrating sample magnetometry (VSM). The data demonstrate that the carboxyls which can capture cationic ferrous/ferric by electronic interaction in the dextran-based hosts plays a crucial role in fabricating nanocomposites with a homogeneous spatial distribution of magnetite nanoparticles. The magnetic nanocomposites exhibit comparable saturation magnetizations to that of reported Fe(3)O(4) nanoparticles, and therefore display great potential in a large scope of biomedical fields.

Carbon film deposition by powerful ion beams

Carbonaceous thin films can be used in microelectronics, superconductors, solar batteries, logic and memory devices, and to increase processing tool wear resistance, and as magnetic nanocomposite materials for information storage. This paper presents a study of carbonaceous thin films deposited on silicon substrates using ablation plasma generated by pulsed power ion beams (H +—60%, C +—40%, E = 500 keV, τ = 100 ns, density = 8 J/cm 2) on graphitic targets. The concentrations of sp 3-bonded crystalline diamond, and amorphous and crystalline phases of carbon were determined by X-ray diffraction analysis (XRD). It was observed that the concentration of the crystalline diamond phase in films deposited under various conditions did not exceed 5%. A substantial concentration (30–95%) of the carbon crystalline phase is in the form of C 60 and C 70 fullerenes. It is shown that the concentration of fullerenes and the ratio between the relative amounts of C 60 and C 70 greatly depends on the graphitic target density, carbon film deposition conditions and above all on the distance from the graphitic target to the silicon substrate. This distance determines the film deposition rate and the degree of cooling of the plasma generated on the substrate, which can cause changes in film crystallization conditions.

Preparation and Characterization of Nd2Fe14B/α-Fe Nanocomposite Magnetic Material by Reduction Diffusion Process

Thermal decomposition process was applied to synthesize Fe3O4 and Nd2Fe14B nanoparticles which were prepared by reduction diffusion process for preparing exchange-coupled nanoparticles. The magnetic Nd2Fe14B nanoparticles used in this study as the starting materials were synthesized via metal oleate complex method. Also mechanical ball mill technique was applied to the mixture of magnetic Nd2Fe14B and α-Fe nanoparticles to build an exchange-coupled nanoparticle. The mixture of Nd2Fe14B/α-Fe of nanoparticle was confirmed by transmission electron microscopy (TEM). The crystal structure of nanoparticle was characterized by X-ray diffraction pattern (XRD). The magnetic properties were characterized with saturation magnetization from hysteresis loop by vibrating sample magnetometer (VSM). Thermogravimetry using a microbalance with magnetic field gradient positioned below the sample was used for the measurement of a thermomagnetic analysis (TMA) curve showing the downward magnetic force versus temperature.

Methods of characterization of multiphase Nd-Fe-B melt-spun alloys

Nanocomposite permanent magnetic materials based on Nd-Fe-B alloys with a low Nd content are a new type of permanent magnetic material. The microstructure of these nanocomposite permanent magnets is composed of a mixture of magnetically soft and hard phases providing the so called exchange coupling effect. Beside the optimization process parameters, methods of characterization have a very important role in the design of an optimal magnetic matrix of multiphase melt-spun Nd-Fe-B alloys. Different methods and techniques of characterization were used for observation and study of the microstructure evolution during crystallization. A summary results of measurements using different methods of characterization are presented to enable a better insight into relations between the microstructure and magnetic properties of the investigated melt-spun Nd-Fe-B alloys. .

Crystalline evolution and large coercivity in Dy-doped (Nd,Dy)2Fe14B/α-Fe nanocomposite magnets

Nanocomposite hard magnetic materials (Nd,Dy)4.5Fe77.5B18 (No. 1) and (Nd,Dy)4.5Fe76B18Nb1.2Cu0.3 (No. 2) have been prepared by crystallizing amorphous ribbons, fabricated by single roll melt-spinning. The evolution of a multiphase structure was monitored by an x-ray diffractometer and by thermomagnetic measurement. We observed that, at annealing temperatures below 670 °C, there is crystallization of soft phase Fe3B and a small amount of hard phase Nd2Fe14B. At annealing temperatures above 670 °C, crystallization of α-Fe and probably Dy2Fe14B phases with large magnetocrystalline anisotropy led to a drastic enhancement in the hard magnetic properties of the materials. The maximum value of HC is found to be 4.2 kOe for sample No. 1. For sample No. 2, with co-doping of Nb and Cu, nanostructure refinement yields a strong enhancement in exchange coupling between the component phases. Thereby, we obtained high reduced-remanence of 0.78, high remanence of 1.15 and a high (BH)max value up to 16.2 MGOe.

Fe3O4–SiO2 nanocomposites obtained via alkoxide and colloidal route

The magnetic nanocomposite materials represent an important class of nanomaterials extensively studied nowadays due to their varied applications from medical diagnostic to storage information. The iron oxides in silica matrix systems are highly investigated. The sol-gel method is a suitable way of preparation of Fe3O4-SiO2 nanocomposite materials, since this method allowed the preparation of nanocomposite materials with narrow size distribution of magnetite in silica matrix. In the present work, nanocomposite materials in the Fe3O4-SiO2 system were prepared by sol-gel method via alkoxide and aqueous route. As SiO2 sources, tetraethoxysilan (TEOS) for the alkoxide route, as well as silica sol Ludox (30%) for the aqueous route, were used. This study shows the influence of the type of silica matrix on the structure, size, and distribution of the Fe3O4 nanoparticles in the Fe3O4-SiO2 systems. The gels were annealed at 550°C in order to consolidate the matrices. The structural characterization of the obtained materials via the two preparation routes was performed by DTA/TGA analysis, X-ray diffraction, IR and Mossbauer spectroscopy, Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED).

Ordered mesoporous silicas as host for the incorporation and aggregation of octanuclear nickel(ii) single-molecule magnets: a bottom-up approach to new magnetic nanocomposite materials

Silica-based mesoporous materials have been employed as the support host for a suitably designed small octanuclear nickel(II) guest complex with a moderately anisotropic S = 4 ground spin state (D = −0.23 cm−1), which behaves as a single-molecule magnet at low temperature (TB = 3.0 K). Both unimodal MCM-41 and bimodal UVM-7 porous silica provide appropriate template conditions for the incorporation and aggregation of the Ni8 complex precursor into larger complex aggregates, showing slow relaxation of the magnetization at higher blocking temperatures than the crystalline material. By playing with the initial complex vs. silica concentration, two series of samples with varying complex loading amounts have been obtained. The degree of aggregation varies, largely depending on the silica used, being higher for the bimodal UVM-7 silica series. The mesophasic and porous nature of the Ni8 adsorbed silica samples has been verified from XRD and TEM images. N2 adsorption–desorption isotherms show that incorporation initiates inside the small intra-particle mesopores while subsequent aggregation occurs at the external particle surface (close to the mesopore entrances). Both DC and AC magnetic susceptibility measurements have demonstrated the occurrence of such a unique silica-mediated surface aggregation process of cationic Ni8 molecules into oligomeric [Ni8]x aggregates of large spin values (S = 4x) and high blocking temperatures (TB = 4.5–10.5 K). The existence of a wide distribution of aggregates with different conformation and association degree (size distribution) and the presence of weak interactions between the aggregates leads to an exotic spin glass magnetic behavior for this family of host–guest hybrid nanocomposite materials.

Magnetic properties of sintered high energy sm-co and nd-fe-b magnets

Magnetic properties of permanent magnetic materials based on intermetallic compounds of Sm-Co and Nd-Fe-B are in direct dependence on the microstructure. In the first part of this paper, having in mind the importance of the regime of sintering and heat treatment to obtain the optimal magnetic structure, yet another approach in defining the most adequate technological parameters of the sintering process for applied heat treatment conditions was made. The goal of these investigations was to use the correlation that exists between sintering conditions (temperature and time) and intensity of the diffraction peak of the (111) plane of the SmCo5 phase to optimize. In the second part a brief overview of high energy magnetic materials based on Nd-Fe-B is presented with special emphasis to the current research and development of high remanent nanocomposite magnetic materials based on Nd-Fe-B alloys with a reduced Nd content. Part of experimental results gained during research of the sintering process of SmCo5 magnetic materials were realized and published earlier. The scientific meeting devoted to the 60th anniversary of Frankel?s theory of sintering was an opportunity to show once more the importance and role of sintering in optimization of the magnetic microstructure of sintered Sm Co5 magnetic materials.