[show abstract][hide abstract] ABSTRACT: Ordered faced-centered tetragonal (fct) FePt nanoparticles were successfully synthesized by a chemical method using presynthesized Au nanoparticles as the "catalyst." The reaction temperature was also studied and it seems that there is an optimum value of 360 C where the fct structure is formed. The particles have a mean diameter of 3.5–15 nm and the x-ray diffraction patterns exhibited (001) and (110), which signified the tetragonal phase formation. Room temperature magnetic hysteresis loops show that the FePt particles have coercive fields between 0.68 and 2.8 kOe. V C 2011 American Institute of Physics. [doi:
Journal of Applied Physics 01/2011; 109. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nanocrystalline PrCo5, SmCo5 and Sm2(Co,Fe,Mn)17 alloys were subjected to a high-degree plastic deformation at 950°C with the height reduction ranging from 70% to 95%. With increasing degree of deformation, the PrCo5 and SmCo5 magnets showed improvement of the deformation-induced  texture. The PrCo5 alloys, known to develop a superior texture at the lower degrees of deformation, showed only modest improvement and their magnetic performance was undermined by a low coercivity. The SmCo5 alloys had their texture markedly enhanced and, after height reduction by 94.5%, they exhibited a remanence of 8.6kG, maximum energy product of 18MGOe and an intrinsic coercivity of 22.8kOe. No induced texture was found in the alloys based on the Sm2Co17 structure. The microstructures of the hot-deformed alloys were studied with a transmission electron microscopy, and possible mechanisms of the texture development in the RCo5 alloys (R=Pr, Sm) are briefly discussed.
Journal of Magnetism and Magnetic Materials 01/2011; 323(20):2470-2473. · 1.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nanoparticulate FeCo alloys have been synthesized by Fe(CO)5 and Co2(CO)8 thermal decomposition in paraffin oil in the presence of oleic acid and oleyl amine. The crystal structure and morphology of the nanoparticles were confirmed by powder x-ray diffraction and transmission electron microscopy. The size and crystallinity of the particles was found to depend on the reaction conditions such as the precursors concentration, reaction time, and carbonyls injection temperature. Also the Fe/Co ratio can be easily controlled by controlling the Fe and Co carbonyls ratio. The as-made nanoparticles were annealed at 500 °C under CH4 stream in order to be protected from future oxidation. This treatment leads to the formation of a graphitic shell around the particles which also protects them from sintering. Additionally, these particles can be functionalized with 1-pyrenebutiric acid in order to be soluble in various solvents.
Journal of Applied Physics 01/2011; 109(7):07A313-07A313-3. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The TbCu7-type structure in bulk Pr–Hf–Co alloys was obtained by high-temperature homogenization of alloys adjacent to the PrCo5 composition. At 1150°C, the homogeneity range of binary PrCo5+δ structure stretches to 84.5at.% Co; addition of up to 1.5at.% Hf extends this range to 85.3at.% Co. The lattice constants, Curie temperature and room-temperature saturation magnetization of the PrCo5+δ phase approximately follow the Vegard's law as if this phase were transitional between the stoichiometric PrCo5 phase and the hypothetical disordered Pr2Co17 phase. Hafnium decreases the saturation magnetization of the Pr1−xHfxCo5+δ phase, but has no significant effect on its Curie temperature. The highest saturation magnetization of 12.9kG was observed in the off-stoichiometric binary alloy Pr84.5Co15.5. The solubility of Co in the PrCo5 structure was found to decrease with decreasing the homogenization temperature; only Pr1−xHfxCo5 alloys exhibited the single-phase structure after annealing at 900°C.
Journal of Alloys and Compounds 01/2011; 509(5):2562-2566. · 2.39 Impact Factor
[show abstract][hide abstract] ABSTRACT: Tb0.3Dy0.7Fe1.92 nanoflakes with larger specific surface and higher resistance to oxidation have been prepared by high energy ball milling (HEBM) with oleic acid. The morphology, structure and magnetic properties of Tb0.3Dy0.7Fe1.92 have been investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and vibrating sample magnetometry, respectively. The experimental results showed that the as-made samples had sustained little oxidation after surfactant-assisted HEBM suggesting that oleic acid was efficient to protect the powders from oxidation. These Tb0.3Dy0.7Fe1.92 nanoflakes have great potential to be used for the preparation of high performance Terfenol composites with reduced eddy current heating.
Journal of Alloys and Compounds 01/2011; 509(19):5773-5776. · 2.39 Impact Factor
[show abstract][hide abstract] ABSTRACT: In ancient times, they were considered magical objects with supernatural powers. These days, we just stick them on our refrigerators. Yet those little magnets deserve our admiration more than ever. Take laptop computers, with their slim hard drives. It became possible to manufacture the motors for those drives only after the development of especially powerful permanent magnets in the early 1980s. Such muscular magnets are now found in many other places as well-various household appliances, cellphones, and the small electric motors that operate accessories in our cars, to name a few. They are also critical in the brawny electric motors that propel hybrid vehicles and in the generators attached to many wind turbines. So they can help both to reduce energy consumption and produce green electric power. Because the magnets themselves are hidden away, many of us tend to take them for granted. We shouldn't, especially not now. The manufacture of most high-performance magnets requires neodymium, a rare earth element that's in short supply. Almost all of the world's production comes from China, which has increasingly restricted exports to ensure that it has enough to satisfy its own needs. So the price of neodymium has been skyrocketing. If the trend continues, pretty soon we'll have a real crisis on our hands.
[show abstract][hide abstract] ABSTRACT: This work is focused on the evolution of structure, texture and magnetic properties in SmCo5 powders during high energy ball milling (HEBM) in heptane with and without oleic acid (OA). Single-crystal micron SmCo5 flakes were formed in the early stage of HEBM (0-0.5 h). However, HEBM of 1-2 h without OA led to the formation of a mixture of single-crystal microflakes and loosely agglomerated polycrystalline microparticles with the highest coercivity of 15.7 kOe. HEBM of 3-5 h without OA led to the formation of isotropic, more or less equiaxed SmCo5 particles with a size of 2-30 mum and a polycrystalline nanostructure due to the coalescence of flakes and particles. In the case of HEBM in heptane with 15 wt. % OA, prolonged milling (from 1 to 5 h) led first to the formation of SmCo5 single-crystal submicron flakes and later to submicron- and nanosize-nanocrystalline flakes with a -out-of-plane texture. OA plays an essential role in the formation of anisotropic SmCo5 flakes via the impediment of cold welding and agglomeration of flakes. The highest coercivity of 17.7 kOe was obtained in a flake sample milled for 5 h with a thickness of 8-80 nm, an average grain size of 8 nm and an aspect ratio of 102-103.
Journal of Applied Physics 01/2011; 109. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The effect of the amount and types of surfactants and milling time on the formation of SmCo5 nanoflakes by surfactant-assisted high-energy ball milling in heptane has been investigated. The majority of the as-milled SmCo5 samples had the shape of flakes with a high degree of texture. Among the surfactants used, oleylamine (OY), trioctylamine (TOA) and oleic acid (OA), OA and OY have similar effects on the formation of SmCo5 nanoflakes, with the highest coercivity of 20.1 kOe obtained after 5h of milling with 2 wt.% OA. Textured poly-nanocrystalline SmCo5 nanoflakes were formed even with as little as 2 wt.% OA. The coercivity of the specimens prepared with 2–10 wt.% of OA increased from 10.8 to 20.1 kOe when the milling time increased from 0.25 to 5h. However, the same amount of OA had little effect on the coercivity and grain size when the milling time was kept for 5h. In the case of TOA, a higher amount of surfactant is required to form the nanoflakes. The evolution of thickness, structure and magnetic properties of SmCo5 flakes with increasing milling time has a similar trend with all surfactants used. The  texture was found to be reduced either with decreasing the amounts of OA from 10 to 2 wt.% when the milling time was 5h or with increasing the milling time from 0.25 to 5h when the amount of OA was kept at 5 wt.%. Another interesting result is that the majority of jet-milled SmCo5 powders became more uniform and had a smaller size in the range of 0.3–3μm when using milling balls with a single diameter of 4mm. The small size and uniform nanoflakes are very promising for the development of anisotropic nanocomposite magnets.
[show abstract][hide abstract] ABSTRACT: High energy ball milling has been shown to be a promising method for large-scale fabrication of rare earth-transition metal nanoparticles. In this work, magnetically hard Nd-Fe-B nanopowders with a coercivity in the range of 1.2-4 kOe have been produced by surfactant-assisted ball milling of nanocrystalline precursor alloys. The nanopowders consisted of Nd(2)Fe(14)B flakes with a thickness below 100 nm and an aspect ratio as high as 10(2)-10(3) and anisotropic square nanoparticles with a size of 11 nm. Both the nanoparticles and nanoflakes showed a strong  out-of-plane texture. The nanoparticles showed a spin reorientation temperature which is lower (117 K) than the bulk value (135 K). The successful fabrication of Nd-Fe-B nano-thin flakes and anisotropic nanoparticles provides hope for the development of nanocomposite permanent magnets with an enhanced energy product.
[show abstract][hide abstract] ABSTRACT: The effect of nanostructuring on magnetostatic interactions in permanent magnets is investigated by model calculations. Emphasis is on the energy product as a function of packing fraction of the magnetic phase, of the magnet’s macroscopic shape, and of the nanoscale feature size. The main difference between nanostructured and macroscopic magnetic bodies, namely, the transition between coherent and incoherent reversal, has a far-reaching impact on demagnetizing field and energy product. For small magnet sizes, the energy product is substantially enlarged, up to μ<sub>0</sub>M<sub> s </sub><sup arrange="stagger">2</sup>/4 for soft magnetic materials, but this effect is difficult to exploit in real devices. In bulk magnets, the energy product depends on the packing fraction f of the soft phase and exhibits a maximum μ<sub>0</sub>M<sub> s </sub><sup arrange="stagger">2</sup>/12 for f=2/3 . Nanoscale magnetization processes involve demagnetizing factors different from the macroscopic ones used to determine the optimum shape of permanent magnets. Confusion of these two types of demagnetizing fields yields unphysical mechanisms, such as hysteresis-loop overskewing and the addition of self-interaction fields to the external field.
Journal of Applied Physics 06/2010; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Crystallographically anisotropic SmCo <sub>5</sub> nanoflakes were fabricated directly by one-step surfactant-assisted high energy ball milling (HEBM) of Sm <sub>17</sub> Co <sub>83</sub> ingot powders for 5 h in heptane and oleic acid (OA) without preprocessing or further annealing. The SmCo <sub>5</sub> nanoflakes have a strong  out-of-plane texture. The thickness of nanoflakes is in the range of 8–80 nm while their length is 0.5–8 μ m . The surfactant OA plays an important role in the formation of SmCo <sub>5</sub> nanoflakes. HEBM of SmCo <sub>5</sub> ingots in heptane without OA resulted in the formation of magnetically isotropic more or less equiaxed SmCo <sub>5</sub> particles with a size of 2–30 μ m . Closely packed “kebablike” SmCo <sub>5</sub> nanoflakes were formed by HEBM in heptane with 15 wt % OA. HEBM in 150 wt % OA led to well-separated nanoflakes instead of the closely packed kebablike nanostructure. This resulted in the enhanced  out-of-plane texture. In-plane transmission electron microscope examination showed that the SmCo <sub>5</sub> nanoflakes were composed of grains with sizes in the range of 4–8 nm. Coercivities of about 18.0 kOe were obtained for the anisotropic SmCo <sub>5</sub> nanoflakes.
Journal of Applied Physics 06/2010; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Indium addition in PrCo <sub>5</sub> magnets can facilitate their sintering and improve their coercivity significantly. Transmission electron microscope studies showed the presence of indium-rich phase PrCo <sub>2</sub> In , at the intergranular region in the sintered Pr <sub>18</sub> Co <sub>81.5</sub> In <sub>0.5</sub> magnets annealed at 800 and 950 ° C . Another new ternary phase, Pr <sub>3</sub> Co <sub>9</sub> In <sub>2</sub> , appeared only in magnets annealed at 950 ° C . Both of these phases are believed to be helpful to improve the sintering behavior of the magnets. Especially, the Pr <sub>3</sub> Co <sub>9</sub> In <sub>2</sub> phase is observed in the form of a thin layer along the grain boundaries, which is believed to improve the magnetic performance of the magnet.
Journal of Applied Physics 04/2010; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: High-energy ball milling has been shown to be a promising method for large-scale fabrication of rare earth-transition metal nanoparticles. In this work, we report crystallographically anisotropic SmCo(5), PrCo(5) and Sm(2)(Co, Fe)(17) nanoparticles (particle size smaller than 10 nm) obtained by surfactant-assisted ball milling and study their size and properties as a function of the milling conditions. By milling nanocrystalline precursor alloys, we obtained SmCo(5) platelets (flakes) approximately 100 nm thick with an aspect ratio as high as 10(2)-10(3). The unusual shape evolution of this brittle material is attributed to its increased plasticity in the nanocrystalline state. The nanoflakes are susceptible to re-crystallization annealing and exhibit a room-temperature coercivity of up to 19 kOe. The successful fabrication of rare earth-cobalt nanoparticles and ultra-thin flakes provides hope for the development of nanocomposite permanent magnets with an enhanced energy product.
[show abstract][hide abstract] ABSTRACT: Element-strategic considerations have sparked renewed interest in rare-earth-free permanent magnets, but the prediction of the magnetocrystalline anisotropy from the atomic structure is still in its infancy, and there are no rules predicting the d anisotropy as function of the atomic structure. We have obtained tight-binding estimates for a variety of clusters and etxtended structures of different symmetry and d-band filling. A expected, the anisotropy strongly oscillates as a function of the d-band filling. Our calculations indicate that nearly filled d bands tend to yield anisotropy parallel to the pair axis. Sites with trigonal symmetry support bigger anisotropies than cubic and tetragonal environments, but this is a crystal- field effect similar to that in BaFe12O19 rather than a band-structure effect. Shape anisotropy is important in alnico- type nanostructured permanent magnets. We find a maximum of the energy product as a function of packing fraction, namely a maximum value of muoMs^2/12 realized at a volume fraction of 2/3. For Fe65Co35, this yields an upper limit of 390 kJ/m^3 [49 MGOe].
[show abstract][hide abstract] ABSTRACT: The spinodal decomposition has been investigated in Alnico thin films prepared by sputtering. The as-made bcc thin films were heat treated both by the complex heat treatment of bulk Alnico magnets and by a simple annealing at different temperatures in the range of 600-900 °C. The simple heat treatment gave a coercivity of 2.6 kOe at 600°C and 6.9 kOe at 800°C. The maximum coercivity observed is approximately ten times larger than the bulk Alnico V value. Electron diffraction patterns can be mostly indexed to a new fcc phase with a=7.79 Å. The traditional Alnico heat treatment gave a similar coercivity (6.7 kOe after full heat treatment) and similar diffraction patterns. The coercivity was found to depend strongly on the film thickness. In samples with thickness above 100 nm, the coercivity declined dramatically and at 150 nm it was only 500 Oe. These findings suggest structural transformations in films which are drastically different from the spinodal decomposition observed in bulk Alnico.
Journal of Physics Conference Series 02/2010; 200(7):072001.
[show abstract][hide abstract] ABSTRACT: This study is focused on the fabrication and characterization of core-shell structured iron/iron-oxide nanoparticles for potential use in biomedical applications. In particular, we have investigated the effect of Pt seeding and the injection temperature of Fe(CO)5 precursor on the particle's morphology and magnetic properties. Injection of the iron precursor at low temperature led to a mixture of core-shell structured particles and separate Fe-oxide particles. Whereas, injection at high temperatures led to only core-shell nanoparticles without any separate iron oxides. The importance of the use of Pt(acac)2 to achieve more uniform and oxide free particles is investigated in detail.
Journal of Physics Conference Series 02/2010; 200(7):072049.
[show abstract][hide abstract] ABSTRACT: Composite magnets containing Sm-Co and Co phases were prepared from high-energy-milled Sm-Co and Sm-(Co,Fe,Mn) powders with the 1:5 and 2:7 stoichiometries blended with a micron-size Co powder. The blending was done via high-energy milling in argon or heptane; the blends were subsequently consolidated at 650 °C and subjected to a 65% plastic deformation at 800 °C. Both the premilling of Sm-Co component and the processing environment markedly influenced the microstructure and magnetic properties of the resulting composites. After blending in heptane, the composites made from the 1:5 and Co phases still contained these two phases in addition to the emerging 2:17 phase. Addition of oleic acid as a surfactant led not only to a more uniform and refined microstructure but also to a significant oxidation. After blending in argon, the composites made from the 1:5(2:7) and Co phases exhibited a two-phase structure with 80 nm Co inclusions embedded in a 2:17 matrix. Unfortunately, the newly formed 2:17 phase did not inherit the texture induced in the 1:5(2:7) precursors by the hot-deformation. Although the argon-blended composites were magnetically superior to the heptane-blended ones, they exceeded the maximum energy product of the single-alloy magnets with the same overall composition only when the Sm(Co,Fe,Mn)5 powder had been used as a precursor. The highest properties obtained were 8.8 kG, 6.3 kOe, and 13.1 MGOe for the remanence, coercivity, and energy product, respectively.
Journal of Applied Physics 01/2010; 107. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Exchange coupling between hard and soft magnetic materials has implications for both permanent magnet and magnetic recording technologies. This paper looks at exchange coupling of FCC FePt epitaxially grown onto (0 0 1) oriented L10 FePt deposited on (0 0 1) MgO substrates at elevated temperature. By varying the thickness of the FCC layer there is a relaxation of the single crystal FCC layer that produces a polycrystalline microstructure as evidenced by the development of rings in the electron diffraction pattern. A concurrent decoupling of the layers is apparent from magnetization curves with two distinct switching fields as the FCC layer thickness is increased above 20 nm. The results shown here confirm the importance of the epitaxial relationship between materials of disparate anisotropies in maintaining strong exchange coupling.
Journal of Magnetism and Magnetic Materials 01/2010; 322(13):1811-1815. · 1.83 Impact Factor