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Granular CoPt: B films, consisting of high anisotropy, face centered tetragonal (FCT) CoPt particles embedded in a B matrix have been successfully prepared. Their structural and magnetic properties are investigated as a function of annealing temperature and boron content, to examine their suitability for magnetic recording media. Grain size and isolation can be adjusted by the boron concentration and annealing process: boron is found to lower the required processing temperature. A minimum grain size is obtained for the 20 vol% B samples for all annealing temperatures. The activation volume as estimated by magnetic relaxation and remanence measurements approaches the physical particle size only for the nanocomposites with grain sizes below 20 nm, indicating a coherent rotation mechanism in these dims. (C) 2001 Elsevier Science B.V. All rights reserved. Journal of Magnetism and Magnetic Materials

A comprehensive and systematic study has been made on Sm(CobalFevCuyZrx)zmagnets to completely understand the effects of composition and processing on their magnetic properties. The homogenized Sm(Co,Fe,Cu,Zr)z magnets have a featureless microstructure. A cellular/lamellar microstructure develops after 2-3 hours of aging at 800-850°C, but the coercivity increases only after a subsequent slow cooling to 400°C. During cooling, diffusion takes place and Cu is concentrated in the 1:5 cell boundaries and Fe in the 2:17R cells. This dilutes the magnetic properties of the 1:5 phase and causes domain wall pinning/nucleation at the cell boundaries. Higher ratio z leads to larger cells as expected due to the larger amount of the 2:17 phase. For a fixed Cu content, this translates to a larger amount of Cu in the 1:5 cell boundaries, and therefore, to a higher coercivity. Magnets without Cu but with Zr have a lamellar and a cellular like microstructure. In Zr free samples, however, a larger amount of Cu is needed to form the cellular microstructure. This cellular microstructure is unstable with prolonged isothermal aging. A uniform and stable cellular/lamellar microstructure is only observed in alloys containing both Cu and Zr. A higher aging temperature Tag leads to larger cells and higher coercivity as explained above. The results of all these studies clearly show that the amount of Cu in the 1:5 cell boundaries controls both the coercivity and its temperature dependence leading to positive and negative temperature coefficients of coercivity in low and high Cu content alloys, respectively.

CoPt nanoparticles with the high anisotropy Llo ordered phase have been prepared and embedded in C and BN matrices by sputtering from Co50Pt50 and matrix targets onto Si substrate using the tandem mode. The as-made films showed a disordered face-centered-cubic structure which is magnetically soft. Ordered CoPt particles were obtained by post-annealing the films at 700°C. Typical particle size obtained was in the range from a few nanometers to 20 nm, which was controlled by adjusting the bilayer thickness and annealing conditions. The microstructure of the granular films at the optimumally annealed state showed a uniform size distribution with an average particle size below 10 nm. The coercivity development in the films was investigated in conjunction with the growth and ordering of the particles.

The coercivity of melt-spun Pr-Fe-B magnets has been found to increase with small additions of Al, Cu, and Zn. Systematic high-resolution electron microscope (HREM) observations were carried out to reveal the structural changes that might occur with these substitutions. Cu additions lead to planar imperfections which are found locally along

Sm-Fe-N films have been made by DC magnetron sputtering and heating in-situ in a nitrogen atmosphere. It is observed that the anisotropy changes from perpendicular to in-plane by nitrogenation. The Curie temperature is about 380 K. Aspects of structure, hysteresis loops, initial magnetization curves, and temperature dependence of coercivity are discussed. >

The authors have prepared nanometer-size magnetite particles in Aerosol OT (AOT)/water/iso-octane water-in-oil (w/o) microemulsions. The sizes of the particles in the colloids were inferred from dynamic light scattering (DLS) and were on the order of one nanometer. Those in dried samples were measured using transmission electron microscopy (TEM) and were 5-20 nm. This implies that aggregation occurs during drying. X-ray diffraction showed that the particles' phase was consistent with magnetite. Magnetic properties were studied using a superconducting quantum interference device (SQUID) magnetometer. The colloids showed a superparamagnetic behavior and the dried samples showed a ferromagnetic behavior, supporting the size measurements using DLS and TEM, respectively. >

Fine particles of Co in the size range of 50-350 AA by the vapor deposition technique have been prepared. The dependence of magnetization on size and temperature has been studied. A maximum coercivity of 1500 Oe was obtained for a particle size of 350 AA. Magnetic and structural measurements indicate a shell-type structure with an FCC (face centered cubic) Co core surrounded by a Co-oxide shell. The effect of exchange interactions between the antiferromagnetic Co-oxide shell and the ferromagnetic Co core on the magnetic behavior of the powders has been investigated. The core/shell structure is responsible for the large coercivities present at cryogenic temperatures and the nonsaturation effects, which increase with decreasing particle size and with higher oxidation. The disappearance of the shift at 150 K marks the loss of exchange interaction between the Co-core and its CoO oxide coating. Thus the 150 K temperature could either be the blocking temperature or the Neel temperature of CoO crystallites present in the oxide shell. >

In this paper a systematic study is undertaken of the structure and magnetic properties of RFe 10 V 2 nitrides using X-ray diffraction, thermomagnetic analysis, SQUID magnetometry and neutron diffraction. The RFe 10 V 2 N x compounds were prepared by heat-treating fine powders of single-phase RFe 10 V 2 alloys in N 2 gas. These studies showed that the interstitial nitrogen atoms expand the crystal lattice of ThMn 12 -type crystal structure, resulting in a substantial increase of Curie temperature. The average increase of Curie temperature across the series is about 200 K. The absorption of nitrogen also introduces a significant change in the magnetocrystalline anisotropy from “weak uniaxial”-type to “strong uniaxial”-type for NdFe 10 V 2 N x and from “uniaxial” to “planar” in SmFe 10 V 2 N x .

The effects of surface oxidation on the structural and magnetic properties of fine Fe particles prepared by the evaporation technique have been studied using transmission electron microscopy, x‐ray photoelectron spectroscopy, superconducting quantum interference device magnetometry, and Mossbauer spectroscopy. By varying the argon pressure, particles were obtained with sizes in the range of 60–350 A. The hysteresis behavior was found to be strongly dependent on the variation in the amount of surface oxidation. The differences in the magnetic behavior due to variation in size and oxidation have been explained by considering a shell/core model for the particle morphology with the shell consisting of Fe oxides surrounding the α‐Fe core.

Fe100−xCrx ultrafine particles (UFP) with x=5–55 and a size in the range of 100–500 A have been prepared by the gas evaporation method using Ar gas pressures P=2, 5, and 8 Torr. Transmission electron microscopy with energy dispersive x‐ray analysis showed that the composition and structure of the UFPs was the same as in the original bulk alloy. The coercivity Hc at room temperature for the Fe95Cr5, Fe90Cr10, and Fe80Cr20 UFPs reached values up to 1250, 780, and 625 Oe, respectively. The temperature coefficients of these UFPs α(Hc) = −0.09%/°C were much smaller than those of Fe in the temperature range of 20–450 °C when x≤20. A strong antiferromagnetic coupling between Fe and Cr appeared in the small particles. Both the coercivity and saturation magnetization were found to decrease with Cr addition. For samples with x=50%–60%, Hc = 200–300 Oe, and Ms = 30–60 emu/g.

The magnetic properties of amorphous and crystallized B‐rich R‐Fe‐B alloys with R=Y, Pr, Nd, and Gd have been studied from 4.2 to 300 K using ac susceptibility and SQUID magnetometry. The amorphous samples with R=Y, Pr, and Nd show spin‐glass‐type transitions with ordering temperatures at about 150, 200, and 250 K, respectively. The magnetization of the samples is found to decrease from light to heavy rare earths to yttrium corresponding to μFe values from 1.35 to 0.74 μB. The magnetic properties can be explained by a noncollinear arrangement of the rare‐earth and Fe magnetic moments. Crystallized samples consist of multiple phases with R1.1Fe4B4 as the majority phase; for the 12% Nd sample Fe2B is believed to be one of the minority phases while for 13% Nd the minority phase is R2Fe14B.

The effect of chemical composition and particle size on the magnetic properties of Fe‐Co‐B fine particles has been studied. The powders were produced by chemical reduction of aqueous solutions of ferric chloride and cobalt chloride with sodium borohydride. The structure of the initially reduced powders varied from amorphous to crystalline bcc depending on the boron concentration. Chemical composition analysis showed a boron content in the range 5–30 at. %. The particle size was found to be 25 nm. The magnetization and coercivity of as‐made powders were relatively low. After annealing, the maximum values of magnetization and coercivity obtained were about 160 emu/g and 1940 Oe, respectively. The temperature coefficient of coercivity varied from negative to positive with sample composition and packing fraction.

We have developed synthetic techniques involving coprecipitation of iron(III) and manganese(II) ions from aqueous solutions using NaOH to create single‐crystal, stoichiometric MnFe2O4 particles. The particle diameter could be varied between 5 and 25 nm by varying the metallic‐ion to hydroxide‐ion concentration ratio. Magnetic measurements revealed that the saturation magnetization decreased, the ferrimagnetic to paramagnetic transition broadened, and the apparent transition temperature increased, all with decreasing particle size.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

The previous paper contains no new information but is rather based on some wrong conclusions drawn from a paper entitled “Magnetization reversal in ferrite magnets” by Hadjipanayis et al. published in J. Magn. Magn. Mat. 81 (1989) 318. The authors of the previous paper (Paoluzi and Turilli) missed altogether the objectives of the article by Hadjipanayis et al. which were to explain the differences observed in the temperature dependence of coercivity of bulk and small particle BaFe 12 O 19 magnets and relate them to the magnetization reversal processes. The conclusions were that the bulk sintered magnets behave as “nucleation”-type, the ultrafine particles as “single domain”-type and the aerosol coarser particles show a behavior which is a mixture of the above two. In this paper we dispute in detail the main points made by the authors.

The magnetic properties, structure, microstructure, and chemical composition of new phases found in melt‐spun Nd‐Fe alloys are reported. High coercivities were obtained in ribbons after a heat treatment for 10–15 min at temperatures in the range of 450–550 °C. Thermomagnetic data showed the existence of two new phases with Curie temperatures at around 250 and 340 °C, respectively. Electron microscopy observations confirmed the existence of two different iron‐rich phases. One of them had 70 at. % Fe and a tetragonal structure with a=8.0 A and c=11.9 A, similar to Nd2Fe14B. The other one had 85 at. % Fe and a hexagonal structure with a=20.5 A and c=11.5 A. These phases are probably ternary Nd‐Fe‐O stabilized by oxygen. The hexagonal phase is believed to correspond to the phase with a Curie temperature of 250 °C.

Large coercive fields have been obtained in melt‐spun Co‐Zr(Hf)‐B based alloys. The highest coercivities have been observed in Co76 Zr18 B3 Si3 and Co78 Hf16 B3 Si3 samples with values 6.7 and 6.5 kOe, respectively. The saturation magnetization for both samples is estimated to be around 60 emu/g. Thermomagnetic data showed the presence of two phases with the major phase having a Curie temperature at around 450 °C. TEM and SEM studies showed a microstructure consisting of a mixture of two phases with the grain size in the range of 1000–5000 A. The origin of the high coercivity is believed to be due to the fine grain microstructure leading to a single‐domain particle‐type behavior.

The effects of vanadium substitution on the magnetic properties of Sm‐Fe‐Ti‐V melt‐spun ribbons and the magnetic properties and microstructure of sintered Sm‐Fe‐Ti based magnets are reported. The highest coercivity, Hc =10.65 kOe, was obtained in a heat‐treated melt‐spun Sm8 Fe75.5 Ga0.5 Ti8 V8 alloy. This is the highest value of the coercivities reported in alloys with the ThMn12 ‐type structure. Bulk magnets were prepared by the usual powder metallurgy technique. However, their highest coercivity (∼2 kOe) was much less than that of the corresponding ribbons. The low coercivity of sintered magnets has been attributed to their nonuniform microstructure.

The results of structural and magnetic measurements on a series of films produced by cosputtering Pr21Fe72B7 and Pr are reported. The materials were deposited serially, 10–200 A at a time, onto 700 °C Ta substrates attached to a computer‐controlled stepping motor. The structure of the resulting films was examined by large‐ and small‐angle x‐ray diffraction, scanning electron microscopy, and by microprobe analysis. Magnetic measurements were made at temperatures from 4.2 to 300 K, in fields up to 80 kOe in a vibrating sample magnetometer. The films appear to consist of 1‐μm grains of Pr2Fe14B with their c axes oriented perpendicular to the film plane, surrounded by a Pr‐rich phase. The maximum room‐temperature coercivity discovered so far is almost 20 kOe, which is close to the values reported for bulk Pr2Fe14B magnets.

The magnetic hysteresis behavior of ultrafine Fe particles has been examined by SQUID magnetometry and Mossbauer spectroscopy. Ultrafine Fe particles have been prepared by vapor deposition under argon atmosphere in pressures from 1 to 6 Torr and passivated by exposure to oxygen at low pressure. Particles with size below 10 nm showed a small coercivity (∼100 Oe) at room temperature, which increased drastically upon cooling to cryogenic temperatures (1000–1500 Oe). The larger particles had the highest coercivity at room temperature (∼1000 Oe) which increased by 100% at 10 K. Mossbauer studies revealed that the Fe particles are surrounded by an Fe oxide layer composed of ultrafine Fe3O4 or γ‐Fe2O3 grains. At room temperature the oxide component gives a very broad absorption superimposed on an α‐Fe spectrum. The enhanced coercivity at room temperature may be attributed to anisotropy induced by exchange interaction between the ferromagnetic core and the ferri‐ or antiferromagnetic oxide layer.

Ultrafine particles of cobalt were prepared under argon by reduction of cobalt halide solutions in diglyme ether with metallic lithium and a naphthalene catalyst. The fine particles had a particle diameter in the range 50–1200 A. The size was dependent on the reaction temperature. X‐ray diffraction showed crystalline particles of α‐Co and CoO. Because the ratio of the metal core to oxidized shell decreases with particle size, the smaller particles displayed much lower magnetization and higher coercivity at low temperature. The anisotropy constant K changes slightly with particle size and has a value close to the bulk value.

Ultrafine Fe‐Ni‐B particles were synthesized by reducing iron and nickel salts with NaBH4 in aqueous solutions. The particles had diameters in the range of 100–300 A. X‐ray diffraction patterns showed that particles with boron concentration 18 at. % or more were amorphous; otherwise they were crystalline. It was also found that the amorphous particles contain less than 50 at. % of Fe. The coercivity of Fe‐B particles was increased by an addition of a small amount of Ni to the system. The room‐temperature coercivity and saturation magnetization for particles of composition Fe63Ni24B13 and of diameter 100 A were 1275 Oe and 70 emu/g, respectively.

3 pages, 5 figures., Hard nanocrystalline Sm–Co thin films having a thickness of 500 nm have been prepared by heat-treating magnetron sputtered amorphous samples deposited over a 300 nm Cr buffer layer onto room temperature water cooled Si substrates. The thermal treatments resulted in the nanocrystallization of the precursor films and in the development of room temperature coercivities covering a range going from a few kOe, measured in the samples treated at low temperature, up to some tens of kOe, observed in the case of the films annealed at high temperatures (the room temperature coercivity of the sample treated at 825 K was 26 kOe). Our analysis of the influence of the thermal treatments on the particularities of the reversal process was based on the measurement of the temperature dependence of the coercive force. It was carried out in the framework of the micromagnetic model, which allowed us to evaluate two parameters accounting for the local anisotropy reduction and the magnitude of the local dipolar fields, respectively. Our results show that the high temperature treatments result in a better degree of crystallization (lower local anisotropy deterioration). That crystallinity improvement is associated to an increase of the local dipolar interactions.

The properties of magnetic nanoparticles is an area of current active interest with many problems of fundamental importance [1–4]. The nanoscale places particles at the single domain size where the coercivity can have a maximum or below this size where the particles eventually become superparamagnetic with no coercivity [5]. The density of states of various excitations in nanoparticles is affected by size through coarsening and/or introduction of a finite wavelength cut off. And, perhaps most importantly, nanoparticles have a large fraction of surface atoms which may have properties distinctly different than those in the bulk. Moreover, the exposed nature of the surface makes it susceptible to interfacial materials which might modify the properties of the surface atoms.

CoPt/Ag nanocomposite films have been prepared with magnetic properties promising for potential application in high density magnetic recording media. The films were initially deposited by sputtering, in a multilayer form and found to crystallize in the magnetically soft face centered cubic phase. Annealing leads to a microstructure consisting of the magnetically hard face centered tetragonal CoPt nanoparticles embedded in a Ag matrix. Microstructure and coercivity can be tailored by adjusting the annealing conditions and the relative CoPt/Ag composition. Values of coercivity in the range of 1-17 kOe with grain sizes in the range of 7-100 nm, respectively, have been obtained. Henkel plots of optimally annealed samples show only weak interactions of dipolar nature, characteristic of magnetically isolated single domain particles. (C) 1999 American Institute of Physics. [S0021-8979(99)50308-3]. Journal of Applied Physics

Magnetic nanoparticles represent a nanotechnology for the treatment of cancer as a result of the localized thermal gradients produced in response to an oscillating magnetic field. Using a sodium borohydride reduction of ferrous chloride in the presence of carboxyl terminated polyethylene glycol this study looks at the contribution of particle size to the achievable temperature using a 500 kHz excitation field. By magnetically separating the as-synthesized sample it was found that the smaller particles within the sample are more effective heating elements.

Preface. Theory. Coherent and Incoherent Magnetization Processes in Non-Interacting Particles A. Aharoni. Interaction Effects in Fine Particle Systems R.W. Chantrell. Monte Carlo Simulations of Small Interacting Magnetic Particles K.N. Trohidou, et al. A Monte Carlo Simulation of the Hysteresis Behavior of Small Magnetic Particles X.D. Zianni, K.N. Trohidou. Micromagnetics of Thin Films and Multilayers T. Schrefl. Micromagnetism of Small Structures S.T. Chui. Magnetic Hysteresis in Bilayer H.S. Darendelioglu. Micromagnetism and the Microstructure of Modern Magnetic Materials H. Kronmuller. Stochastic Model for Domain Wall Dynamics and Ferromagnetic Hysteresis C. Beatrice, et al. Influence of Defects Shape and Period on Domain Wall Coercive Field K.L. Metlov, I. Tomas. Avalanches Size Distribution in Model Hard Magnetic Materials Having Different Textures O.A. Chubykalo, et al. Domain Wall Dynamics and Preisach Modeling G. Bertotti. The Effect of Thermally Induced Relaxation on the Remanent Magnetization in a Moving Preisach Model R.M. Roshko, et al. A Vectorial Preisach-Type Model for Magnetic Hysteresis for Interacting Stoner-Wohlfarth Particles C. Michelakis, et al. An Experimental System of the Classical Preisach Model of Magnetic Hysteresis M. Pardavi-Horvath. Analysis of the Identification Methodologies in Preisach Modeling A. Stancu. First Principles Determination of Hyperfine Fields in Pt/Fe(111) Multilayers G. Lu, et al. Critical Behavior of the Thermal Conductivity Near a Magnetic Phase Transition A. Rassili, M. Ausloos. Characterization Techniques. Imaging Magnetic Structures in the Transmission Electron Microscope J.N. Chapman, K.J. Kirk. Domains, Domain Walls and theMagnetisation Reversal Process J.N. Chapman, K.J. Kirk. Magnetic Force Microscopy of Sintered NdFeB L. Folks, et al. Magnetic Viscosity and Hysteresis Phenomena J. Tejada, et al. Quantum Hysteretic Phenomena in Mn12-Acetate Magnetic Clusters J. Tejada, et al. Anomalous Magnetic Aftereffect K.- H. Muller, et al. NMR as a Tool in Structural Studies of Modern Magnetic Materials E. Jedryka, et al. Novel Materials and Properties. Small Particles. Particulate Magnetic Recording Media D.E. Speliotis. Angular Dependence of Magnetization Reversal and Neel Brown Theory W. Wernsdorfer, et al. Anomalous Hysteresis Properties in Ferrimagnetic and Antiferromagnetic Nanoparticles A.E. Berkowitz, et al. Magnetisation Reversal in Cobalt and Permalloy Nano-Elements K.J. Kirk, et al. Magnetization Dependence on Temperature and Grain Size in Nanostructured Samples J.M. Gonzalez, et al. Effect of Preparation Conditions on the Structural and Magnetic Properties of Fe-SiO2 J.A. Christodoulides, et al. Magnetic Characterization of Ni Nanoparticles Dispersed in Silica J.M. Gonzalez, et al. Fine Particle Magnetism in SmCo5 Produced by Ball Milling E. Kirkpatrick, et al. The Application of Spark Erosion to the Synthesis of Thin Films Containing Ultrafine Particles N.B. Shevchenko, et al. Compaction of Chemically Prepared Amorphous Fe-B Nanoparticles P.V. Hendriksen, et al. Magnetic Behaviour of g-Fe2O3 Nanoparticles B. Martinez, et al. Hysteresis Loops and Remanence Curves of Small g-Fe2O3 Particles with Different Packing Densities M. Grigorova, et al. Irreversible and Time

We have investigated a new method of preparing granular thin films, which are composed of fine metallic particles embedded in a uniform matrix material. By our method, fine metallic particles are created by spark erosion in an inert gas environment and directly deposited onto a cooled substrate. Magnetron sputtering is employed concurrently thereby building up the matrix. Using such a deposition method, granular thin films are created without the need of any heat treatments and can be made with virtually any material and composition. Furthermore it is even possible to create granular thin films from miscible phases. The size of the particles can be controlled by varying the pressure under which the spark erosion is conducted up to a few Torr of Ar. We have applied our method to the case of magnetic particles in a nonmagnetic matrix. Preliminary TEM measurements on Fe particles in a Cu matrix showed 10 nm diameter Fe particles. SAD patterns show that the film is composed of only the Fe and Cu phases. Similarly for the Gd-Ag system, 10 nm Gd particles were observed in an Ag matrix. Gd-Ag compounds that would be expected to form were not observed. Magnetic measurements for these systems agreed with the nanometer scale microstructure that was observed.

The stoichiometry, microstructure and magnetic properties of reactively sputtered Fe-O films were studied as a function of the amount of O2 during the deposition. Films which were comprised of mixtures of oxides resulted for most of the O2/Ar ratios. However, for some conditions pure FeO, Fe3O4 and α - Fe2O3 films were produced. The magnetic properties of Fe3O4 films were consistent with the results of other studies. In particular the Verwey transition was clearly observed in the coercivity versus temperature curve and in the thermomagnetic data. FeO and α - Fe2O3 films showed ferromagnetic-like behavior, in contrast to their well established antiferromagnetic properties in bulk. The unusual magnetic properties are attributed to the formation of clusters of tetrahedraly coordinated Fe3+ ions in FeO and to uncompensated surface spins, in α - Fe2O3 films as suggested by Mossbauer studies.

Rutherford backscattering spectroscopy (RBS) and low angle X-ray diffraction (LAXRD) were used to determine the density of thin Ta, Ag, Mo and Fe films as compared to the bulk value. An exact formula for the positions of the diffraction peaks as a function of the thickness of the film and its index of refraction was derived. A computer program utilizing this formula was written and used to do a least square fit to the low angle diffraction spectra and to obtain the thickness of the films. The densities of the films were found to be in the range between 84 and 98% of the bulk value. A detailed analysis of conditions that need to be fulfilled in order for the method to give reliable results is also presented.

Nanoparticles of the magnetically hard Nd2Fe14B phase embedded in a W matrix were successfully prepared for the first time by sputter depositing the two materials in a multilayer fashion and then annealing the samples to break the multilayer structure. TEM studies showed that the grains of the hard magnetic phase are isolated in the samples with higher W content, whereas in the rest of the samples they form clusters of a size of 50 nm, with an individual grain size of 10 nm. Besides the tetragonal Nd2Fe14B phase, the orthorhombic intergranular WFeB phase was identified in the selected-area diffraction patterns. The coercivity of the Nd2Fe14B/W broken multilayers is increased compared with Nd2Fe14B single layers prepared under the same conditions. Journal of Magnetism and Magnetic Materials

The stoichiometry, structural and magnetic properties, and Mössbauer spectra of reactively sputtered Fe-O films were studied as a function of the O2 partial pressure during the deposition. By increasing the amount of O2 films with the following crystallographic structures and stoichiometry were fabricated; amorphous Fe-O, mixture of Fe and FeO, offstoichiometric single-phase FexO, mixture of FeO and Fe3O4, single-phase Fe3O4, mixture of Fe3O4 and γ-Fe2O3, mixture of Fe3O4 and α-Fe2O3, and single-phase α-Fe2O3. The Verwey transition in Fe3O4 films was observed in the coercivity versus temperature curve and in the thermomagnetic data. FeO and α-Fe2O3 films showed anomalous ferromagnetic-like behavior, in contrast to their anti ferromagnetic nature in bulk. The unusual magnetic properties were attributed to the formation of clusters of tetrahedraly coordinated Fe3+ ions in FeO and to uncompensated surface spins, in α - Fe2O3 films.

The large coercivities (exceeding 2 kOe) obtained in Fe based ceramic granular films are found to be sensitive to the type of matrix material used, being higher in the SiO2 matrix. In this study the hysteresis behavior of Fex(SiO2)1-x granular films was studied as a fiinction of the preparation technique and conditions. The films were prepared by dc and rf magnetron sputtering over the composition range of x v = 0.1–0.9 volume fraction of Fe. Tandem deposition method and deposition from a composite target both with and without titanium sublimation, were used to prepare the films. Magnetic properties showed that by varying the deposition method and some of the sputtering parameters (sputtering rate, argon flow, film thickness and substrate temperature), it is possible to switch from a relatively magnetically hard sample (H c ~ 700 Oe) made by the tandem technique, to a soft sample (H c ~ 20 Oe) made from a composite target. A dramatic increase in H c has been observed at cryogenic temperatures in the former samples. This result along with the Mossbauer data, which show the presence of an Fe-Si-O phase (Fe2SiO4), suggest a shell/core granule morphology with an Fe and/or (Fe-Si) core surrounded by Fe-Si-O. At high temperatures Fe-Si-O is paramagnetic and does not affect much the coercivity of core. However, below a critical temperature Fe2SiO4 orders antiferromagnetically and pins the interface spins through exchange anisotropy with the interface core spins leading to large increases of H c at cryogenic temperatures.

Sm 2 Fe 17 melt-spun powders were subjected to the nitriding process and followed by surfactant-assisted high-energy ball milling (HEBM). The microstructures, morphology and magnetic properties were also investigated by X-ray diffractometer, scanning electron microscope and vibrating sample magnetometer. The results showed that the coercivities of the nitrided Sm 2 Fe 17 powders were 1.64 kOe and 3.65 kOe when the nitriding temperatures were 350°C and 450°C, respectively. When the nitriding temperature was 350°C, there was a wasp-shaped hysteresis loop, due to the soft phase of iron, formed during the nitrogenation process. The subsequent surfactant-assisted HEBM can further improve the magnetic properties of the nitrided Sm 2 Fe 17 powders and a 3 h milling process increased the coercivity of the sample nitrided at 450°C reached a high value of 6.97 kOe.

Cryomilling was employed for Mn50+xBi50−x (x = 0,5) alloys to synthesize nanocrystalline powders with particle size of 400–500 nm without much increase of the unwanted secondary Bi phase. At room temperature, the cryomilled powders showed large coercivity (Hc) values of 18.5 kOe and 20.7 kOe for Mn50Bi50 and Mn55Bi45, respectively. Cryomilled powders were hot compacted to form a bulk nanostructured MnBi magnet. The Mn50Bi50 hot compacted magnet exhibits Hc of 12.9 kOe with remanence value of 26 emu g−1 at room temperature. The microstructure of hot compacted magnet illustrates uniform grains with an average size of 40 nm. Furthermore, the magnet shows a large positive temperature coefficient of Hc which reaches a value of more than 30 kOe above 450 K.

Co(Zr) nanoclusters having a small fraction of Zr (≤7.8 at. %) were produced using a cluster-deposition method and aligned using a magnetic field of about 5 kOe prior to deposition. This study shows that Zr addition to Co nanoclusters improves the fraction of hexagonal close-packed structure, magnetic anisotropy, and easy-axis alignment process. Co(Zr) nanoclusters having 7.8 at. % of Zr exhibit a considerably enhanced magnetic anisotropy constant K1 ≈ 6.7 Mergs/cm3 and coercivity Hc ≈ 700 Oe at 300 K as compared to those of Co nanoclusters (K1 ≈ 2.9 Mergs/cm3 and Hc ≈180 Oe).

We report the synthesis and magnetic properties of core/shell FeO/Fe3O4 nanoparticles with an average size of 30 nm in a complex quasi-octopod shape. FeO nanoparticles were synthesized by a wet chemical synthesis route followed by partial oxidation to form core/shell structured FeO/Fe3O4 octopods. X-ray diffraction and transmission electron microscopy confirmed the presence of iron oxide phases and the formed core/shell FeO/Fe3O4 morphology. Magnetic measurements revealed two distinct temperatures corresponding to the thermally activated Verwey transition (TV ∼ 120 K) of the ferrimagnetic Fe3O4 shell and the Neel temperature (TN ∼ 230 K) of the antiferromagnetic FeO core. The nanoparticles exhibited a strong horizontal shift in the field-cooled hysteresis loop (the so-called exchange bias (EB) effect) accompanied by enhanced coercivity. The Meiklejohn-Bean model has been implemented to quantify the amount of frozen spins that locate at the interface between FeO and Fe3O4 and are responsible for the observed EB effect.

Giant negative magnetoresistance (GMR) has been observed in melt-spun ${\mathrm{Cu}}_{\mathit{x}}$${\mathrm{Mn}}_{\mathit{y}}$${\mathrm{Al}}_{\mathit{z}}$ ribbons (x=50--65, y=5--25, and z=25--30), with values up to 15% at 30 K. GMR was observed in all samples with off-stoichiometric composition which had a tweed structure consisting of a mixture of Mn-rich and Mn-poor ${\mathrm{Cu}}_{2}$MnAl-type (2:1:1) regions and/or of a fine mixture of magnetic 2:1:1 and nonmagnetic ${\mathrm{Cu}}_{9}$${\mathrm{Al}}_{4}$ (9:4) phases. The highest value was obtained in samples with the 2:1:1 phase as a majority phase. The observed GMR is attributed to interfacial scattering at these fine structural mixtures. \textcopyright{} 1996 The American Physical Society.

The magnetic properties of nanocrystalline melt-spun single-phase (Nd2Fe14B-type) and composite (Nd2Fe14B + alpha-Fe or Nd2Fe14B + Fe3B + alpha-Fe) magnets have been studied systematically in an attempt to understand better their 'exchange spring' behavior. The reversibility of the recoil demagnetization curves has been found to increase with increasing content of the soft phase giving rise to the characteristic exchange spring behavior only in nanocomposite samples. delta M plots indicate positive interactions of the exchange type for small fields; for fields higher than the remanence coercivity, magnetostatic interactions become dominant. The relative strength of the magnetostatic interactions is increased in samples with higher soft phase contents. Journal of Magnetism and Magnetic Materials

We have successfully prepared a new nanocomposite magnet consisting of a mixture of a magnetically hard Pr2Co14B phase and a soft Co phase using the technique of melt spinning. The final microstructure with the above phases is formed after annealing the melt-spun ribbons through an intermediate transformation to the metastable TbCu7-type structure. Optimum coercivity of 4.3 kOe was observed in a sample with the composition Pr8Co83Nb1B8 after heating above 775 degrees C. (C) 1996 American Institute of Physics. Journal of Applied Physics

Coercivity optimization studies were done on melt-spun nanocomposite Nd(4)R(2)Fe(87-x)NbT(x)B(6) (R=Nd,Y,Dy; T=Ag,Cu) isotropic ribbon samples. The maximum attainable coercivities, after adjusting the annealing time, were found to be very sensitive to the annealing temperatures. The optimum magnetic properties [H-c=3.9 kOe, (BH)(max)=10 MGOe] were obtained by annealing at 750-775 degrees C for a few minutes. Optimization by flash annealing gave similar results. Microstructural studies show that the grain size is greater than the theoretically predicted grain size for optimum coupling between the hard and the soft phase. With the annealing conditions used, Nd4Dy2Fe87NbB6 samples gave moderate coercivities and in Nd4Y2Fe87NbB6 samples the coercivity was reduced more than the expected reduction in the anisotropy field due to the presence of Y. (C) 1996 American institute of Physics. Journal of Applied Physics

We report a low-temperature (150 degrees C) and simple synthesis of quasi-monodispersed and uniform hexagonal (Wurtzite) ZnS nanocrystals in ethylene glycol medium. The samples structures were characterized with X-ray diffraction technique and transmission electron microscopy. It is believed that ethylene glycol medium plays a key role in forming hexagonal ZnS which is a stable phase at high temperatures.

The microstructure and microchemistry of Cu-substituted SmCo 5 - and PrCo 5 -based cast alloys have been investigated using high-resolution transmission electron microscopy and nanoprobe energy dispersive spectroscopy. Nanoscale precipitates with the hexagonal crystal structure have been observed in the Cu-substituted 1:5 alloys. Results suggest that the coercivity of Cu-substituted 1:5 alloys depends on both the Cu content of the 1:5 matrix phase and the crystal structure of the precipitates.

The magnetic viscosity of FePt/C granular thin films was studied in the temperature range from 2 to 300 K in order to examine the thermal stability of the nanoparticles. The magnetic viscosity coefficient ( S max ) was found to decrease with temperature because of decreased thermal activation. At low temperatures, S max showed an almost linear dependence on temperature. However, S max does not extrapolate to zero but seems to have a finite value at cryogenic temperatures.

CoPt nanoparticles with a size of 1– 2 nm were synthesized by chemical reduction using the solvents of water and alcohol. A phase transformation from the originally disordered face centered cubic (FCC) structure to an ordered face centered tetragonal L1 0 structure occurred after annealing, which results in the coercivity up to 9 kOe because of the high anisotropy of the tetragonal phase ( K ≈2– 4×10 7 erg / cm 3 ) (Philos. Mag. 13 (1966) 567; IEEE Trans. Magn. 20 (1984) 1625). HREM images of as-grow Co 48 Pt 52 shows the single-crystalline FCC structure with the shape of columnar and spherical.

The structural and magnetic properties of a fully dense anisotropic MnBi magnet prepared by hot compaction are investigated. Arc-melting and low energy ball milling are employed to synthesize highly anisotropic MnBi powders with very high remanence ratio (Mr/Ms) of 0.97, coercivity (Hc) of 11.7 kOe and maximum energy product [(BH)max] of 9 MG Oe. The bulk magnet fabricated from these powders displays anisotropic characteristics with high Mr/Ms ratio of 0.91 and a (BH)max of 5.8 MG Oe at room temperature. The Hc of bulk magnet increases linearly from 6.5 kOe at 300 K to 28.3 kOe at 530 K with a (BH)max of 3.6 MG Oe at 530 K. Analysis of the temperature dependence of Hc suggests the nucleation of reversed domains as the dominant mechanism for the magnetization reversal.

Ultrafine magnetically hard particles are needed for the bottom­up fabrication of anisotropic exchanged­coupled permanent magnets. In this study we have chosen Sm2 Fe17 Nx because of its high anisotropy field and large saturation magnetization. A multi­stage planetary ball milling process was used. The key is to find the right balance of energy used to mill the precursor particles; it must be high enough to break the particles and reduce their size but, not so high as to destroy the crystal structure of the nanoparticles that would lead to deterioration of the magnetic properties. First the coarse powders were subjected to a wet milling with 2.0 mm diameter media. In each subsequent stage the media size was reduced to maintain the milling balance. Using such a process produced particles in a size range from 100 - 800 nm with a coercivity as high as 10 kOe at room temperature.

A comprehensive and systematic study has been made on Sm(CO(sub bal)Fe(sub v)Cu(sub y)Zr(sub x))(sub z) magnets to completely understand the effects of composition and processing on their magnetic properties with the aim to increase their operating temperature. The homogenized Sm(Co, Fe, Cu, Zr)(sub z) magnets have a featureless microstructure. A cellular/lamellar (2:17R cells surrounded by 1:5 cell boundaries superimposed on 2:17H lamellar) microstructure develops after 2-3 hours of aging at 800-850 deg C, but the coercivity increases only after a subsequent slow cooling to 400 deg C. During the cooling, diffusion takes place and Cu is concentrated in the 1:5 cell boundaries and Fe in the 2:17R cells. Our studies have led to compositions with operating temperatures above 400 deg C and with coercivities as high as 10 kOe at 500 deg C(sup i). The high temperature performance of these magnets is very sensitive to the Cu content, which is believed to control both the coercivity and its temperature dependence. Furthermore, we developed a more economical heat treatment processing for higher Cu content magnets.

We have used a microemulsion technique to synthesize metallic cobalt particles in the size range 18 to 44 \AA{} diameter. The particles are spherical, not aggregated due to their surfactant coating, and free of oxide. Magnetic properties such as total moment per particle, blocking temperature, and hysteresis all show reasonable size dependencies. The effects of small size are seen in: (1) the anisotropy constant increased markedly as particle size decreased, and (2) the total magnetic moment per atom in the Co particles was enhanced with decreasing particle size by as much as 30% over the bulk value. Magnetization versus applied field curves indicate the particles are heterogeneous with two magnetic phases, possibly a core-shell structure. The core phase has a large total moment and the shell phase is superparamagnetic with an effective moment of 7.5\ifmmode\pm\else\textpm\fi{}1${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for all sizes. We propose that the shell phase is responsible for the enhanced anisotropy and magnetization.

Fe-O and Co-O films were prepared by DC magnetron sputtering in a mixture of Ar and O2 gases. By varying the oxygen to argon ratio, oxide films with stoichiometry FeO, Fe3O4, α-Fe2O3, CoO and Co3O4 were produced. TEM studies showed that the Fe – oxide films were polycrystalline consisting of small almost spherical grains, about 10 nm in size. Co-O films had different microstructure with grain size and shape dependent on the amount of oxygen. X-ray diffraction studies showed that the grains in Fe-O films were randomly oriented in contrast to Co-O films in which a texture was observed. Pure FeO and α-Fe2O3 films were found to be superparamagnetic at room temperature but strongly ferromagnetic at low temperatures in contrast to the antiferromagnetic nature of bulk samples. A very large shift in the hysteresis loop, about 3800 Oe, was observed in field cooled Co-CoO films indicating the presence of a large unidirectional exchange anisotropy.