The fabrication of nanoscale magnetic materials and novel heterostructures has placed great demands on the analytical tools used to characterize the chemical and structural variations in these samples. Attempts to determine the composition, phase, crystallinity, and interface structure of nanoscale materials often involve tradeoffs between accuracy, sensitivity, and spatial resolution. Traditional techniques such as electron energy-loss spectroscopy (EELS), energy dispersive x-ray spectrometry (EDS), secondary ion mass spectrometry (SIMS), and scanning Auger microscopy (SAM) will be surveyed in this context. Powerful new tools for probing the chemical heterogeneity of materials at ultrafine length scales will also be discussed, including energy-filtered transmission electron microscopy (EFTEM) and spectrum imaging, as well as recent advances in new methods such as backscattered electron Kikuchi patterns (BEKP) and microcalorimetry.

General synthetic routes to monodisperse c-cobalt (β-Mn type phase) nanocrystals (ε-Co) and controlled assembly of these nanocrystals are presented in this paper. The ε-Co particles are obtained by superhydride reduction of cobalt chloride (anhydrous or hexahydrate) in a high temperature solution phase (200°C) in the presence of a combination of long chain diol, oleic acid and trialkylphosphine. Monodisperse nanocrystals are isolated by size selective precipitation. As synthesized cobalt particles are each a single crystal with a complex cubic structure related to the beta phase of elemental manganese (β-Mn). Self-assembly of these uniform cobalt particles on solid substrates is induced by evaporation of the carrier solvent producing 2-D and 3-D magnetic superlattices. Annealing of assembled ε-Co nanocrystal arrays converts them to the hcp cobalt crystal arrays. The inter-particle distance can be adjusted by selected thermal treatments or by chemical ligand exchange. This control over particle dimensions, crystallinity and assembly offers a model system for the study of ultra-high density recording media.

An ordered Fe16N2 phase has been reported with iron moments as high as 3.2 μB. It is precipitated from nitrogen martensite structures ideally containing 10.5 at.% nitrogen. Due to the highly distorted crystal structure and metastability of this phase non-equilibrium processing routes are sought to synthesize this phase. Here we report on radio frequency (RF) plasma torch synthesis which is used to produce FeN. nanoparticles quenched into a body centered tetragonal bct) structure as precursors for further annealing studies to form α“- Fe16N2 phase. We have employed a Tekna PL-50 type 50 kW, RF plasma torch. A plasma gas mixture containing 40 standard liters per minute (slpm) Ar and 8 slpm Hydrogen - 70 slpm Ar gas was used as a sheath gas. Iron powder ( < 10 μm) was injected into the plasma stream using Ar flowing 15 slpm as a carrier gas. Nitrogen and Ammonia were used as a nitrogenization sources. Relatively low injection rates were used in order to achieve smaller particle sizes and thus faster quenching rates. We were able to produce particles containing up to 45 % of the quenched γ-phase. Observations based on x-ray diffraction (XRD) determination of lattice expansion and phase transition temperatures observed by differential thermal analysis (DTA) indicated that the quenched phase contains 6.5 atomic % nitrogen. Scherrer analysis of the fine particle broadening indicated that the average particle size for γ- phase is 27 nm, whereas this value is found to be 55 nm. for α-Fe. Nitrogen is well known for its grain size refinement in Fe thin films. Saturation magnetizations were found to be as low as 123 emu/g due to the presence of the nonmagnetic γ-FeNx phase.

Graphite encapsulated metal nanocrystals (GEM nanocrystals) are a promising new class of materials with both scientific and industrial significance. Several encapsulating graphite layers protect the nanoscale metallic core without compromising the inherent magnetic properties of the metal, thereby enhancing studies of small particle magnetism. The size and single domain nature of these particles immediately suggest a wide variety of potential applications ranging from increased density magnetic data storage to immunoassays. Encapsulated nanoparticles have traditionally been synthesized by evaporating a composite metal/graphite anode via an electric arc. While effective, that method also results in a great deal of unwanted amorphous carbon debris in the product. Based on past synthesis runs and observations, an improved apparatus for GEM production has been designed with the goals of increasing GEM yield and reducing soot production. The four segment system consists of I ) a chamber capable of arc or resistive evaporation of raw material(s), 2) a tube furnace for the chemical vapor deposition of carbon on raw material(s) via hydrocarbon dissociation, 3) an increased-efficiency powder collection unit, and 4) a final powder reservoir. Details of the new apparatus are presented here.

An experimental study of the kinetics of the three partial processes involved in the mechano-chemical synthesis of nanocrystalline (nc) Ni-Fe is presented. These processes are the reduction of nc NiO, the reduction of nc Fe2O3, and the alloying of Ni and Fe to a γ-Ni-Fe solid solution. Fr the reaction rates of the reduction of nc NiO and nc Fe2O3 activation enthalpies of 82 kJ mol−1 and 47 kJ mol− were obtained. The progress of the overall synthesis was found to follow the sequence ‘NiO red. → Fe2O3 red. → γ-Ni-Fe form.’.

Cobalt-silver granular thin films were formed by Co implantation into Ag using a metal vapor vacuum arc (MEVVA) ion source. The magnetic field dependence and the temperature variation of the giant magnetoresistance (GMR) effect and their relation with the processing conditions were studied and discussed in conjunction with results of Rutherford backscattering spectrometry, atomic force microscopy, magnetic force microscopy (MFM), and SQUID measurements. Anomalous temperature dependence of the coercive field Hc in the perpendicular-to-film direction determined from GMR measurements was observed for some samples. For one sample, Hc shows a maximum value at around 240K and decreases with decreasing temperature from 240K to 20K. The temperature variation of the magnetization M of this sample exhibits a minimum. The maximum in the Hc-T curve corresponds well with the minimum in the M-T curve. The M-T curve suggests that there is more than one magnetic phase present in this sample. The domain structures of the implanted granular films as revealed by MFM images exhibit very different features compared with those of sputter deposited CoAg granular films.

Nonlinear magneto-optical Kerr effect in Gd-containing Langmuir-Blodgett superstructures is studied by second harmonic generation (SHG) interferometry. The intensity of the regular magneto-induced second harmonic (SH) is found to be by a factor of about 2 larger than the intensity of the regular SH component of a non-magnetic origin. The phase shift between the magneto-induced and non-magnetic contributions to the nonlinear polarization is about 46° The intensity of the magnetization-independent hyper-Rayleigh scattering (HRS) is by two orders larger than that of mnagneto-induced HRS.

In the present work it has been shown that the shape of ultrafme metallic magnetic particles is controlled by applied magnetic and/or electric fields and the state of reaction media during the nanoparticles formation. The ulraviolet (UV) decomposition of volatile organometallic compound iron pentacarbonyl was used to produce amorphous metallic nanoparticles. The two-dimensional photochemical synthesis of nanoparticles was carried out in a mixed Langmnuir monolayer at the gas/water interface with stearic acid as surfactant matrix and also in a mixed film deposited onto graphite substrate. Scanning tunneling microscopy analysis of deposited nanoparticulate monolayers showed that the size and shape of nanoparticles formed were dependent on the initial mixed monolayer content, compression extent and exposure time to UV, and were changed dramatically from isotropic plate-like to aligned ellipsoidal and needle-like when external magnetic and/or electric fields were applied during the synthesis. Magnetic nanoparticles with diameter exceeding 5 nm formed chain structures alignable by external fields. The one-dimensional aligned metallic string-like nanostructures (nanowires) with cross-section size about 10+20 rm were also formed at elevated temperatures under synergistically applied magnetic and electric fields. Magnetic properties of nanoparticulate multilayer Langmuir-Blodgett films were studied using electron paramagnetic resonance technique. The ferromagnetic resonance and superparamagnetic signals were observed in the material.

The magnetic properties of nanosized antiferromagnetic particles of KMnF3 are presented. The particles were synthesized using the microemulsion technique, i.e. by using the aqueous core of reverse micelles as constrained microreactors for the precipitation of the particles. The structural characterization of the samples, accomplished by TEM and XRD, reveal that the samples consist of cubic-shaped, crystalline KMnF3 nanoparticles of uniform size. Control over the average size of the particles was achieved by changing the reaction time. Four different samples of average size in the range 13-35 nm were prepared. DC magnetic susceptibility measurements revealed superparamagnetic behavior of the particles. Hysteresis loops measured after field cooling the samples through TN were shifted. The shift is ascribed to the exchange coupling between the antiferromagnetic core of the particles and the uncompensated spin shell surrounding it.

Superparamagnetic behavior is characterized by a thermally fluctuating vector of magnetization, leading to magnetization curves free of hysteresis: it is a property of isolated ferrite particles with sizes below ca. 10 nm. These particles fulfil the condition Kv < kT with K … energy of unisotropy, v … volume of the particle, kT has the usual meaning. To produce a superparamagnetic macroscopic part it is necessary to avoid the interaction of the particles. This can be achieved by coating the particles with a second non-magnetic phase. This special material can be synthesized using the microwave plasma process. Because of the specific interaction of charged particles with an oscillating electrical field, microwave plasmas excel in relatively low reaction temperatures. The low reaction temperature and the electrical charging of the particles in the plasma reduce the probability of agglomeration. Therefore, it is possible to pass the gas stream with the as produced particles through a second reaction zone, where the particles will be coated with a polymer. The thickness of the coating can be adjusted in the range from 1 to 5 nm. The composition of the ferrite kernel is selected either for maximum susceptibility or maximum frequency of application. They may consist either of maghemite, manganese-, manganese-zinc-, or magnesium iron spinell for maximum susceptibility or cobalt-iron spinelle type material for maximum frequency. Polymer coated ferrite nanoparticles can be consolidated by hot pressing with temperatures around 100 °C. Superparamagnetism is shown by static magnetic measurements and Moessbauer spectrometry. Dynamic measurements of the complex susceptibility show interesting properties up to frequencies in the gigahertz range.

Giant magnetoresistance (GMR) as large as 25% at 25°C has been observed for multilayer ultrathin films of iron oxide (Fe3O4) nanoclusters and polyimide molecules alternately adsorbed onto single crystal silicon and quartz substrates using a novel self-assembly technique. This process involves the alternate dipping of a substrate into an aqueous solution of anionic polyimide precursor (polyamic salt, PAA) followed by dipping it into an aqueous solution of cationic polydiallyldimethylammonium chloride (PDDA)-coated Fe3O4, nanoparticles. The regular formation of alternating monolayers is verified by UV-vis spectroscopy and contact angle measurements. Vibrating sample magnetometry indicates the formation of ultrasoft films.

New soft magnetic nanocrystalline materials in Fe-M-B (M = Zr, Hf, Nb) and Fe-M-O (M = Zr, Hf, rare earth) systems have been fabricated by use of partial crystallization of melt-spun amorphous phase and sputtering-induced partial crystallization, respectively. These alloys have a mixed structure of nanoscale a-Fe grains and an amorphous phase containing large amount of M, B and M, O, respectively. This structural property should be a dominant factor for achieving good soft magnetic properties in both alloys. The Fe85.5Zr2Nb4B8.5 ribbon exhibits a high mag- netic flux density (Bs) of 1.64 T and a high permeability of 60,000 at 1 kHz as well as a very low core loss of 0.09 W/kg at B = 1.4 T and 50 Hz. The soft magnetic Fe-M-O films with high O content of 10-35 at.% maintain high permeability up to the high frequency above 100 MHz be- cause of higher electrical resistively of 6 to 23 µΩm than those of other conventional soft mag- netic alloy films. The Fe-M-B ribbons "NANOPERM" should be more suitable for low frequen- cy transformers. The Fe-M-O films are useful for high frequency applications such as thin-film inductors and transformers for micro switching converters for portable electric equipment.

Alloys consisting of Fe-Co-M-B-Cu (with M = Zr, Hf, Nb), called HITPERM alloys, have been developed. Synchrotron X-radiation studies have been used to show that the ferromagnetic phase in an equiatomic FeCo-based alloy is the α'-FeCo phase. Since both the α'-FeCo phase and the FeCo-based amorphous phase of the nanocrystalline alloy have high Curie temperatures, a high magnetization persists up to the α -> γ structural phase transformation temperature of 980°C. Room temperature AC permeability measurements have shown that the alloys maintain a high permeability of ∼2000 up to a frequency of 20 kHz. The room temperature core loss has also been shown to be competitive with commercial high temperature magnetic alloys with a value of 1 W/g at Bs = 10 kG andf= 10 kHz. Analysis of extended X-ray absorption fine structure (EXAFS) data is consistent with a two-phase mixture of nanocrystalline body centered cubic derivative FeCo structure and an amorphous Zr-rich phase. A differential scanning calorimetry study of the primary crystallization reaction shows an activation energy of 323.3 kJ/mol. As a preliminary study of phase and grain stability, broadening of X-ray diffraction peaks indicates little grain growth after annealing at 600 °C for 3072 hours.

A national initiative is underway to develop and test a more electric aircraft (MEA) and is being led by the U.S. Air Force Research Laboratory at Wright-Patterson Air Force Base, Ohio. The MEA concept is based on utilizing electric power to drive aircraft subsystems which are currently driven by a combination of hydraulic, pneumatic, electric and mechanical power transfer systems. A major objective of this effort is to increase military aircraft reliability, maintainability and supportability and to drastically reduce the need for ground support equipment. These improvements will be realized through the further advancement of key MEA technologies, including magnetic bearings, aircraft integrated power units (IPU), and starter/generators (IS/G) internal to an aircraft main propulsion engine. These advanced developments, as well as weapon and space power applications, are the driving force for the new emphasis on high temperature and high strength magnetic materials for power applications. In determining the best magnetic material for an application it is typically necessary to conduct an engineering trade-off analysis which takes into consideration mechanical behavior, electrical loss, and magnetic properties under the conditions of actual usage. New materials solutions are required to meet these challenges, as designers often find the magnetic material performance to be the technological limitation.

Formation of the nanocrystalline bcc Fe phase due to thermal treatment of the amorphous Fe80M7B12Cu1 (M: Ti, Ta, Nb, Mo) precursors is studied by the Mössbauer and differential scanning calorimetry techniques. The dependence of the formation of the bcc Fe phase on the alloy composition is discussed. In order to determine the optimal soft magnetic properties of these nanocrystalline alloys the rf-Mössbauer technique is used in which rf collapse effect induced by a radio-frequency (rf) magnetic field is employed. It was found that anisotropy fields in the nanocrystalline phase were smaller in Nb- and Mo-containing alloys as compared to the alloys which contain Ti or Ta. Variations of the anisotropy field vs. alloy composition and annealing temperature are discussed in detail.

The effects of partial substitution of Al for Fe in Fe73.5-xAlxSi13B9Nb3Cu1, nano composite alloys both on the a.c. magnetic properties such as permeability and core loss, saturation magnetostriction and core loss are presented. The observed improvements in material properties are discussed in relation to the nanocrystallite and residual glass component chemistry.

A novel magnetic field sensor, based on the giant magneto-impedance effect is described and analysed. The sensor utilises an amorphous wire, annealed under optimised conditions. The conditions required for the optimisation of the magnetic properties of the wire are shown. The wire can be up to 2m long, and integrates the field over the length of the wire. It operates in open-loop mode. With a bias field of ∼500 A/m it achieves a sensitivity of 0.02 A/m and linearity of better than 5% for external fields between ±300 A/m.

The nanocrystallization processes in Fe-Si-B-Nb-Cu and Fe-Nd-B(-Cu-Nb) amorphous alloys have been studied by transmission electron microscopy (TEM) and a three dimensional atom probe (3DAP). Cu additions are effective in refining the nanocrystalline microstructures of both alloys, because Cu atom clusters formed prior to the crystallization reaction serve as heterogeneous nucleation sites for the primary crystals. However, the clustering behaviors of Cu atoms in these two alloy systems are different, i.e., Cu completely dissolves in the Nd2Fe1 4B phase in the final microstructure of the Nd4.5Fe75.8B18.5Cu0.2Nb1 alloy, whereas CL' clusters grow to fcc-Cu particles in the Fe73.5Si13.5B9Nb3Cu1 alloy. The nanocrystallization processes in these two alloys clarified by the 3DAP results are compared.

This paper employs an analytical model for the phase shifts imparted on a beam electron by a spherical particle with a uniform magnetization. Model image simulations of Fresnel and Foucault type Lorentz microscopy contrast are presented. Estimates of a lower particle size limit for which no magnetic information can be extracted from the images are given and compared to similar estimates for the electron holography observation method. It is concluded that conventional Lorentz methods have a lower particle size limit of around 30 nm (for cobalt particles), while electron holography can be used to even smaller sizes.

New nanocrystalline, multicomponent extremely soft magnetic materials with superior high temperature magnetic properties hold great promise in power applications. Fabricated in ribbon form by rapid solidification methods, the initial material is amorphous. By controlled annealing procedures, the amorphous material was transformed into a nanocrystalline form with the degree of crystallinity determined by the annealing temperature and time. The magnetic structures of ribbons, as-fabricated and annealed at temperatures from 550 to 750 °C were examined by magnetic force microscopy to determine the impact of residual stress and nanocrystallinity on the observed structure. A correlation was seen between the magnetic structures and surface microstructure. The wheel side of the as-processed ribbon was rougher than the top side of the ribbon and a complicated magnetic domain structure was present in the amorphous material. After annealing, nanocrystals formed, increasing in size with increasing temperature. The lowest temperature annealed sample had a bimodal grain size distribution and a combination of stripe and localized domains. After annealing little difference was seen between the two sides of the ribbons. Stripe domains were absent in the ribbons annealed at the highest temperatures.