Inst of Applied & Technical Physics
Vienna Univ of Technology
Wien, A-1040 AUSTRIA
Div of Materials Science
Naval Research Laboratory
Washington, DC 20375-5000
Amorphous Metals Division
Allied Signal Corp
Morristown, NJ 07962-1057
Institute for Materials Research
Sendai, Miyagi 980-8577 JAPAN
Materials Science and Engineering
Carnegie Mellon Univ
Pittsburgh, PA 15213-3890
*Quantum Design, Inc.
Proceedings published as Volume 577
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
SESSION I1: FINE PARTICLE MAGNETS 8:30 AM *I1.1
Chair: Michael E. McHenry
Monday Morning, April 5, 1999
Salon 5 (M)
CHEMICAL CHARACTERIZATION OF MAGNETIC MATERIALS WITH HIGH SPATIAL RESOLUTION. John Henry J. Scott , National Institute of Standards and Technology (NIST), Gaithersburg, MD.
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 nanostructured 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 as well as recent advances in new methods such as backscattered electron Kikuchi patterns (BEKP). 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.
9:00 AM *I1.2
MAGNETIC THIN FILMS PREPARED FROM MONODISPERSE COBALT-BASED NANOCRYSTALS. Shouheng Sun , C.B. Murray, IBM T.J. Watson Research Center, Yorktown Heights, NY.
Controlling particle dimensions and making monodisperse nanocrystal thin films are attractive goals in technological applications of nanomaterials. A regular array of monodisperse magnetic nanocrystals separated from each other by nonmagnetic barriers has great potential in ultra-high density recording and nanoelectronic devices. In this talk, we will present our progress in size and crystallinity control of monodisperse cobalt-based nanocrystals and their assembly into magnetic thin films. The superlattices of these monodisperse nanocrystals can be readily generated on a solid surface via self-assembly. We further demonstrate that the distance between two particles can be adjusted by either thermal treatment or chemical ligand exchange.
9:30 AM I1.3
THERMAL PLASMA SYNTHESIS OF -FeNx NANOPARTICLES AS PRECURSORS FOR THE Fe16N2 SYNTHESIS BY ANNEALING. Z. Turgut , M.E. McHenry, Carnegie Mellon University, Dept of Materials Science & Engineering, Pittsburgh, PA; D.E. Ferguson, Virginia Polytechnic Institute and State University, Dept of Physics, Blacksburg, VA; M.Q. Huang, W.E. Wallace, Carnegie Mellon Research Institute, Pittsburgh, PA.
An ordered Fe16N2 phase has been reported with iron moments as high as 3.2 . 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 FeNx nanoparticles quenched into a body centered tetragonal (bct) structure as precursors for further annealing studies to form 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 m) was injected into the plasma stream using Ar flowing 15 slpm as a carrier gas. Nitrogen and Ammonia were used as 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 -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 -phase is 27 nm, whereas this value is found to be 55 nm. for -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 -FeNx phase.
9:45 AM I1.4
A NOVEL APPARATUS FOR THE SYNTHESIS OF GRAPHITE ENCAPSULATED METALLIC NANOCRYSTALS. Kevin L. Klug , D. Lynn Johnson, Vinayak P. Dravid, Northwestern Univ, Dept of Materials Science and Engineering, Evanston, IL; Kiumars Parvin, San Jose State Univ, Dept of Physics, San Jose, CA.
Graphite encapsulated metallic nanocrystals (GEM nanocrystals) are a promising new class of materials with both scientific and industrial significance. Several encapsulating graphite layers protect the metallic core, 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. Iron, nickel, and cobalt GEMs have been successfully prepared at Northwestern University. The high temperature of an electric arc struck between a tungsten cathode and a graphite/metal composite anode is sufficient to co-evaporate both the carbon and metal of interest. Encapsulation is accomplished in-situ. The powder collected in this process consists of both encapsulated and bare nanoparticles, as well as amorphous carbon (a feature prohibitive to industrial GEM application). Extensive magnetic measurements have been conducted on nickel GEM nanocrystals. The Curie temperature and saturation magnetization agree quite well with similar tests on bulk nickel samples, suggesting that these properties are insensitive to the encapsulating graphite layers. In contrast, the room temperature coercivity of nickel nanoparticles differs significantly from that of bulk nickel; near-superparamagnetic behavior has been observed in nickel nanoparticles at 300 K. Based on past synthesis runs and observations, an improved apparatus for GEM production has been designed. Two major objectives of this work are to decrease the amount of amorphous soot produced and to increase GEM yield. The three segment system consists of 1) 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, and 3) an increased-efficiency powder collection unit. The resistive evaporation configuration will allow for alloy and compound GEM synthesis. The focus of this presentation will be the design and optimization of the new synthesis apparatus, as well as some salient results from past and future generations of GEM nanocrystals.
10:30 AM I1.5
THE KINETICS OF FORMATION OF NANOCRYSTALLINE -NI-FE POWDERS DURING MECHANO-CHEMICAL SYNTHESIS. Peter Knorr , Beom-Song Kim, Jai-Sung Lee, Dept of Metallurgy and Materials Science, Hanyang University, Ansan, KOREA.
One of the promising methods for the production of low coercitivity alloys on a commercial basis may be the mechano-chemical process, since it appears a relatively simple and cost effective method that may easily be employed for the production of large quantities. Moreover, unlike mechanical alloying it largely avoids the introduction of defects and internal strain which is crucial for the fabrication of functional engineering materials. A potential application of the mechano-chemical process for a large-scale production of soft-magnetic alloys requires an understanding of the underlying formation kinetics of the alloyed phase. The mechano-chemical production method consists of high energy ball-milling of the oxide powders and their reduction in a hydrogen gas atmosphere. The formation of the nanocrystalline alloy during mechano-chemical treatment is preceded by a heterogeneous chemical reaction of the solid oxide phases. This chemical reaction might significantly affect the nature of the subsequent diffusive processes, since it may modify the microstructural environment, in which the following atomic transport occurs, and it may result in different onsets for the diffusion of the different metal atoms depending on which of the oxide phases is transformed more rapidly. This study presents a comprehensive experimental investigation of the kinetics of all partial processes involved in the mechano-chemical process. The collected data on the kinetics of the heterogeneous chemical reactions and the alloying process are used for the construction of a TTT-diagram for the overall process.
10:45 AM I1.6
PREPARATION AND MAGNETIC PROPERTIES OF ULTRAFINE IRON POWDER. L. Takacs , University of Maryland, Baltimore County, Dept. Physics, Baltimore, MD; L.K. Varga, L. Pogàny, Research Institute for Solid State Physics, Budapest, HUNGARY; K. Làzàr, Institute for Isotopes and Surface Chemistry, Budapest, HUNGARY.
There is significant interest in ultrafine iron powders, as their large magnetization is beneficial in most applications. Finding the most suitable method to prepare large quantities of pure, uniform powder with low level of aggregation is still an open problem. Two possible methods have been investigated in this work. (1) Nanosized hematite powder has been reduced by hydrogen gas at moderately high temperatures for between 1 and 9 hours. It is found that fully reduced ultrafine iron powder can be obtained at 450 degrees, much below the equilibrium reducing temperature. Agglomeration is avoided by performing the reduction in a fluidizing chamber. (2) Ball milling is used to reduce ferric chloride with sodium or calcium metal. Some extra sodium chloride is added to avoid high local temperatures due to thermal run-away reactions. The product of this mechanochemical process is an ultrafine iron powder, its agglomeration is prevented by the chloride by-product. The chloride phase is removed by subsequent washing with deoxygenated water and methanol. The washing process is optimized to obtain a clean, chloride and oxide free, nonagglomerated powder, with the smallest possible loss of iron. The phase composition of the powders is investigated by X-ray diffraction and Mössbauer spectroscopy, the morphology is studied by electron microscopy. The magnetic hysteresis is measured at different stages of the preparation process.
11:00 AM I1.7
MAGNETIC IRON CLUSTERS IN SODALITES. Henning Trill , Helmut Eckert, University of Muenster, Department of Physical Chemistry, Muenster, GERMANY; Vojislav Srdanov, Galen D. Stucky, University of California-Santa Barbara, CA.
Kaolin is a common precursor for synthetic sodalites. It will be shown, that paramagnetic FeIII atoms which result from an iron impurity of kaolin and occupy aluminum sites in the sodalite framework can be reduced by reacting the sodalite at elevated temperatures with sodium vapor. Reduced iron atoms diffuse through the host lattice to eventually coalesce into single domain iron clusters. The clusters are trapped within the sodalite matrix and show no sensitivity to air. The mean seize of these clusters can be controlled by the properties of the halide sodalite as well as by the temperature applied during the reduction. The field dependent magnetic susceptibility of the iron/sodalite composite shows superparamagnetic behavior at room temperature and hysteresis with coercive fields between 20 and 450 Gauss at 5K. Temperature dependent AC and DC susceptibility has been measured to determine the mean particle seize which varies between a few atoms and about 5nm. Moessbauer and ESR spectroscopy have been applied to characterize the iron states. Inductive coupled plasma elemental analysis, thermogravimetric analysis and x-ray diffraction have been used to determine the properties of the sodalite matrix which will be related to the magnetic properties of the material.
11:15 AM I1.8
GMR EFFECT IN COBALT-SILVER GRANULAR FILMS FORMED BY IMPLANTATION WITH A METAL VAPOR VACUUM ARC ION SOURCE. S.P. Wong , M.F. Chiah, W.Y. Cheung, N. Ke, J.B. Xu, The Chinese University of Hong Kong, Dept of Electronic Engineering, Shatin, N.T., HONG KONG, CHINA; X.X. Zhang, Hong Kong Univ of Science and Technology, Dept of Physics, Kowloon, HONG KONG, CHINA.
Cobalt-silver granular thin films exhibiting giant magnetoresistance (GMR) effect 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 GMR effect and their relation with the processing conditions were studied and discussed in conjunction with results of Rutherford backscattering spectrometry, atomic force microscopy and magnetic force microscopy (MFM). The magnetization curves were measured using a SQUID magnetometer. For one sample obtained in this study, the magnitude of the GMR effect measured at a magnetic field strength of 7.6 kOe increases from about 1% at room temperature to over 7% at 20K. The coercive field Hc in the perpendicular-to-film direction determined from GMR measurements showed an anomalous temperature variation that 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 are more than one magnetic phases 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. This work is supported in part by the Research Grants Council of Hong Kong (RGC ref. no.: CUHK374/96E).
11:30 AM I1.9
INTERFEROMETRY OF OPTICAL SECOND HARMONIC GENERATION FROM Gd-CONTAINING LANGMUIR-BLODGETT SUPERSTRUCTURES: MAGNETO-INDUCED EFFECTS. E.I. Vishnevskaya , A.A. Fedyanin, N.V. Didenko, T.V. Murzina, A.A. Nikulin, G.B. Khomutov, O.A. Aktsipetrov, Department of Physics, Moscow State University, Moscow, RUSSIA.
The magnetization induced second harmonic generation (MSHG) from multilayer Gd-containing Langmuir-Blodgett (LB) films is studied by second harmonic (SH) interferometry. The films are superstructures deposited on fused quartz substrates and containing 40 structural units (periods). The structural unit consists of a monolayer of Gd ions surrounded by two monolayers of stearic acid molecules. According to the X-ray diffraction data, the films have nearly perfect periodic structure in the normal direction. We observe the MSHG interferometry pattern for the longitudinal configuration of the film magnetization with the use of the thin SnO2 film as a source of the reference SH wave. The relative phase shift between two MSHG interferometry patterns measured at opposite directions of the DC magnetic field of 1 kOe is about 2.0 rad for the s-in, s-out wave polarization combination and less than 0.1 rad for the p-in, p-out combination. The observed effect is explained within the framework of the model that treats the film as a current sheet. The magnetization induced contributions to both linear and quadratic optical susceptibility tensors of the film are taken into account. Thus the resulting magnetization induced phase shift is interpreted as due to superposition of the linear and nonlinear magneto-optical Kerr effects. Their relative strength is estimated from the experimental data. An analogous estimation of the relative magnitude of the terms that are odd and even in the magnetic polarization is made for the quadratic susceptibility tensor of the film.
11:45 AM I1.10
A METHOD FOR CONTROLLED SYNTHESIS OF ANISOTROPIC NANOPARTICLES AND NANOSYSTEMS. Gennady B. Khomutov , TETRA Consult Ltd, Moscow, RUSSIA; Sergey P. Gubin, Institute of General and Inorganic Chemistry, RAS, Moscow, RUSSIA; Alexander Yu. Obydenov, Faculty of Physics, Moscow State University, Moscow, RUSSIA; Eugeny S. Soldatov, Faculty of Physics, Moscow State University, Moscow, RUSSIA; Artem S. Trifonov, Faculty of Physics, Moscow State University, Moscow, RUSSIA.
Development of methods to control the content, morphology, size and shape of metallic nano-particles are of principal importance for production of nanostructures for a wide range of applications. In particular, for magnetic recording applications, the media is required with ultrasmall particles to increase the storage density and with nanoparticulated patterned magnetic structures to overcome the super-paramagnetic limit. A novel, internationally patented method for synthesis of metallic ultrafine particles (including magnetic ones) and nanostructures with controlled particle size, shape and orientation is presented. The method is based on the control of anisotropic diffusion-limited nucleation and growth processes of nanoparticles. The nanoparticles were formed by the ultraviolet decomposition of initial metal-containing substance (in particular, iron pentacarbonyl) placed in surfactant monolayer (stearic acid) at the gas/liquid interface. Thus the properties of fatty acids to form Langmuir monolayer and to stabilize nanoparticles and to prevent aggregation in nanoparticulate structures were combined successfully. STM analysis showed that the shape of discrete particles strongly depends on reaction conditions and media state and can be varied from isotropic plate-like to ellipsoidal and needle-like. The discrete anisotropic ultrafine particles can be space oriented. We can also form nanoparticulated oriented linear chain structures. The extended metallic string-like nanostructures (nano-wires) with cross section about 10 nm and less were also formed. The method allows: preparation of mixed (intermetallic) particles of different metals; formation of ordered mono- and multilayer systems of synthesized nanoparticles and nanostructures oriented relatively to solid substrate and characterized by high planarization performance. The composition, shape and orientation of particles and/or nanostructures can be varied from layer to layer. EPR study of nanoparticulate samples indicated the ferromagnetic-like ordering in material.
SESSION I2: NANOCRYSTALLINE ANTIFERRO- AND FERRIMAGNETS 1:30 PM *I2.1
Chair: Vincent G. Harris
Monday Afternoon, April 5, 1999
Salon 5 (M)
SYNTHESIS AND CHARACTERIZATION OF ANTIFERROMAGNETIC KMNF3 NANOPARTICLES. Claudio Sangregorio, Everett E. Carpenter, and Charles J. O'Connor , Advanced Materials Research Institute, University of New Orleans, New Orleans, LA.
Despite the large amount of work carried out on ferro- or ferrimagnetic materials, there are very few reports on the magnetic properties of nanosized antiferromagnetic particles. Here we report on the synthesis and on the superparamagnetic behavior of potassium fluoromanganate nanometric particles. KMnF3 is well known to be an antiferromagnet with a Néel temperature of 88 K and is therefore a well-suited material for the investigation of the magnetic properties of antiferromagnetic nanoparticles. The nanoparticles were synthesized by the microemulsion technique using the aqueous core of reverse micelles as a constrained microreactor for the precipitation of the particles. The structural characterization was accomplished by XRD and TEM. Our results reveal uniform size and cubic shaped crystalline nanoparticles of KMnF3. Two different samples were prepared with average size, as evaluated by both the techniques, of 13 and 22 nm, respectively. Below the ordering temperature, both the samples display superparamagnetic behavior with blocking temperatures of 26 K and 12 K for the 22 nm and 13 nm samples, respectively. Below the blocking temperature both the samples display a hysteretic behavior with coercivity at 5 K of 180 Oe for the 22 nm sample and 70 Oe for the 13 nm one. Hysteresis loops measured at the same temperature by cooling the sample through the Néel temperature with an applied field of 10 kOe were shifted by 30 Oe along the applied field direction.
2:00 PM I2.2
MAGNETIC PROPERTIES OF -Fe203 and CoFe204 NANOCLUSTERS WITHIN BLOCK COPOLYMER TEMPLATES. Sufi R. Ahmed , Peter Kofinas, University of Maryland, College Park, MD.
The overall goal of this research is to explore techniques for the development of novel frequency agile materials by utilization of the magnetic field dependence of the permeability of ferrite materials. The self-assembled nanoscale morphologies exhibited by block copolymers are used as templates to control the nucleation, growth and distribution of metal oxide nanoclusters. Metal nanoclusters are subject of current interest because of their unusual optical, electronic and magnetic properties, which often differ from their bulk properties. Although nanoclusters have received attention from both theoretical and experimental standpoints, the greatest challenge at present is to find out an effective synthesis procedure. We have synthesized diblock copolymers by ring opening polymer metathesis (ROMP) of norbornene (NOR) and norbornene trimethylsilane (NORCOOTMS). The Grubbs catalyst was used to initiate the polymerization. This catalyst has high tolerance towards impurities, and hence enabled us to use commercially available norbornene with out any further purification. The diblock copolymers undergo microphase separation, forming spherical, cylindrical or lamellar morphologies depending on the volume fraction of each block. Using a 400:50 and 4000:500 ratio between the NOR and NORCOOTMS blocks we synthesized a diblock polymer exhibiting a spherical nanomorphology. Nanoclusters of Iron Oxide-Polymer and Iron Cobalt Oxide were formed within these polymeric nanospheres. The microwave absorption pattern of composites containing iron oxide exhibits an absorption peak at 3680 Oe, which is indication of the presence of a spherical morphology. TEM of these nanocomposites showed that the clusters have dimensions of 30 nm. The nanoclusters exhibit very narrow hysteresis. The nanocomposite with higher MW shows narrower hysteresis and stronger magnetic moment than the that of the nanocomposite with lower MW. The polymer-iron oxide nanocomposite shows a wider hysteresis and weaker magnetic moment that the that of the polymer-cobalt oxide nanocomposite. These results indicate that we can control the magnetic properties of the nanocomposite by changing the molecular weight and composition of the blocks in the diblock copolymer. So, insertion of metal or metal oxide into a polymer matrix could be an efficient and economical way to fabricate novel thin film devices.
2:15 PM I2.3
POLYMER COATED NANOPARTICULATE FERRITE POWDERS, APPLICATION AND PROPERTIES. Dieter Vollath , Dorothoe V. Szaboe, Joachim Fuchs, Forschungszentrum Karlsruhe, Institut for Materialforschung III, Karlsruhe, GERMANY.
Superparamagnetic behaviour is characterised by a thermally fluctuating vector of magnetisation, leading to magnetisation 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 synthesised using the microwave plasma process. This process is well suited to produce of nanoparticulate powders of oxides, nitrides, sulphides or selenides with mean particle sizes of less than 10 nm. 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. The coating material may be a ceramic or a polymer one. The thickness of the coating can be adjusted in the range from 1 to 5 nm. The composition of the ferrite kernel is selected to minimise the energy of unisotropy. They may consist either of maghemite, manganese-, manganese-zinc-, or magnesium iron spinelle. Polymer coated ferrite nanoparticles can be consolidated by hot pressing with temperatures around 100C. Parts made of polymer coated ferrite nanoparticles are superparamagnetic. This can be shown by static magnetic measurements and Moessbauer spectrometry. Dynamic measurements of the complex suszeptibility show interesting properties up to frequencies in the gigahertz range.
2:30 PM I2.4
MICRO-RAMAN STUDY OF BARIUM FERRITE POWDER FROM WATER-IN-OIL MICROEMULSION. Mingsong Chen , Zexiang Shen, National Univ of Singapore, Dept of Physics, SINGAPORE; Xiangyuan Liu, John Wang, National Univ of Singapore, Dept of Materials Science, SINGAPORE.
Barium ferrite (BaM) BaFe12O19 powder is a suitable material for high-density perpendicular magnetic recording applications. Ultrafine BaM powder of high coercivity and saturation magnetization was successfully prepared using water-in-oil microemulsion synthesis route, and the powders calcined between 500 and 1000C were characterized using DTA, XRD, M"ossbauer spectroscopy, SEM and TEM. Here we report the study by micro-Raman spectroscopy. The samples calcined at 500C are amorphous, and the crystalline phase was formed above 600C. The trace of the phase -Fe2O3 was observed in samples calcined between 600 and 700C, which was not detectable by XRD and FTIR. The presence of -Fe2O3 phase showed that the reaction was not complete. With further increase in the calcining temperature, the -Fe2O3 phase decreased due to reaction with BaCO3 to form BaM.