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ABSTRACT: Single-phase rhombohedral Sm2(CobalFe0.2My)17Bx with M = Cr or Mn substituting for Co have been prepared and the influence of transition metals on crystal structure, anisotropy field, and Curie temperature is examined. Sm2Co17 as-cast bulk sample has the 2 : 17 hexagonal structure (type Th2Ni17), while boron addition stabilizes the 2 : 17 rhombohedral structure (type Th2Zn17). Anisotropy field for Cr-doped as-cast sample is 74 kOe. Secondary phases of fcc-Co or Fe-Co are observed in some cases. A Cr-doped sample with y = 0.1 after annealing at 1190°C for 26 h is single phase. Curie temperatures of as-cast samples vary from ~849°C to ~687°C depending on the composition. Mechanical alloying and melt spinning were used to develop coercivity. As-spun ribbons at 53 m/s with Cr present a nanocomposite structure with Hc = 5.2 kOe. Samples with Mn have a coercive field close to 0.4 kOe. Scanning electron microscopy studies show different microstructure for Mn and Cr substituted ribbon samples.
IEEE Transactions on Magnetics 09/2003; 39:2872-2874. · 1.36 Impact Factor
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ABSTRACT: High coercivity, the highest for Cu-free 2 : 17 Sm-Co ribbons, has been obtained in as-spun (Hc=21.1 kOe) and short time annealed (Hc=23.2 kOe) samples of Sm(CobalFezZryBx)7.5 alloys, with varying B, Zr, and Fe content (x = 0-0.06, y=0-0.16, z = 0.08-0.3) and wheel speed. In as-spun samples, the TbCu7 type structure and in annealed samples the Th2Zn17 and CaCu5 type structures is observed, plus fcc Co as minority phase is observed. Reduced remanence (Mr/Ms) is higher than 0.7. High-temperature magnetic measurements show very good stability above 300°C with coercive field as high as 5.2 kOe at 330°C. For annealed Sm(CobalFe0.3Zr0.02B0.04)7.5, very good loop squareness and high maximum energy product of 10.7 MGOe have been obtained. Increasing Zr content results in less uniform microstructure of annealed ribbons.
IEEE Transactions on Magnetics 09/2003; 39:2869-2871. · 1.36 Impact Factor
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ABSTRACT: Phosphorus substituted ( Pr,Tb )<sub>8</sub>( Fe,Nb,Zr,P )<sub>88</sub> B <sub>4</sub> nanocomposites have been produced by melt-spinning. The effects of phosphorus substitution as well as wheel speed on the crystallization behavior and magnetic properties of the melt-spun samples have been investigated. With the substitution of phosphorus, the crystallization temperature of amorphous phase increases. The optimum wheel speed was found to be around 25 m/s for as-spun ribbons and 40 m/s for the annealed samples, both of which present excellent second quadrant hysteresis loop shapes due to the fine grain size of α-Fe which is around 20 nm. The addition of phosphorus also greatly improves the coercivity of Pr–Fe–B nanocomposites without a significant loss of saturation magnetization. A higher coercivity of 9.2 kOe in P-substituted samples was obtained as compared to 8.1 kOe in P-free samples. This is attributed to a narrower temperature span between the crystallization into TbCu <sub>7</sub> structure and the transformation into the 2:14:1 phase caused by the phosphorus substitution. © 2003 American Institute of Physics.
Journal of Applied Physics 06/2003; · 2.17 Impact Factor
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ABSTRACT: The formation of FePt nanoparticles by annealing FePt/C multilayer in the range of 600–800 °C was studied. During annealing at 700 °C, the coercivity of FePt (5 Å)/C (5 Å) sample increases from 0.2 to 6 kOe after 2 min of annealing, reaching 14.5 kOe after 1 h, while the coercivity of FePt (5 Å)/C (20 Å) sample shows a much slower change with annealing, exhibiting a value of 3.5 kOe after 1 h of annealing. Transmission electron microscopy results did not show a layer structure in as-deposited 5 Å/5 Å samples, and a clear superlattice reflection was observed in 5 Å/5 Å samples subjected to 2 min of annealing, indicative of the L10 fct phase. The degree of atomic ordering of the fct structure increases with further annealing. After 1 h of annealing, the 5 Å/5 Å samples show aggregates of particles with a well-ordered structure and a wide particle size distribution. In the as-deposited 5 Å/20 Å sample, a layer structure was observed with a uniform particle size distribution. After 5 min annealing at 700 °C, the layers are completely broken with the appearance of weak superlattice reflections. Both particle size and degree of atomic ordering increase slowly with further annealing in the 5 Å/20 Å sample. After 1 h annealing, isolated particles were observed with an average particle size of 6 nm. Depositing thicker carbon layers appear to restrict the growth of particles and delay the onset of atomic ordering with annealing. © 2003 American Institute of Physics.
Journal of Applied Physics 05/2003; 93(10):7175-7177. · 2.17 Impact Factor
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ABSTRACT: Ordered FePt nanoparticles with unique magnetic properties were obtained by direct deposition of FePt and C onto heated substrates at temperature above 450 °C. The FePt particle sizes were controlled in the range from a few nanometers to 20 nm by adjusting the sputtering time from both the FePt and C targets. The tiny FePt nanoparticles (less than 3 nm) showed superparamagnetic behavior at room temperature. However, larger particles showed huge coercivities at room temperature (23 and 34 kOe for particles with average sizes of around 8 and 15 nm, respectively). For a certain FePt to C ratio, the films can show strong perpendicular anisotropy which is favorable for high density recording media. © 2003 American Institute of Physics.
Journal of Applied Physics 05/2003; 93(10):7172-7174. · 2.17 Impact Factor
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ABSTRACT: In this study we present a technique to obtain ordered fct FePt particles embedded in a C matrix. FePt nanoparticles are formed inside a high-pressure sputtering cell, called a particle gun (PG), and subsequently deposited on the substrate through a small orifice. These particles have a uniform size distribution with an average particle size that can be controlled in the range of 3–10 nm by adjusting the sputtering cell pressure, power, distance between the magnetron and the orifice, and by using a liquid nitrogen cooling jacket. The particles are converted to the L10 phase as they pass through a specially designed heating stage, attached to the top of the PG, heated by halogen lamps, thus avoiding alloying and oxidation effects. A strong dependence of coercivity on both the particle size and temperature was observed. © 2003 American Institute of Physics.
Journal of Applied Physics 05/2003; 93(10):7190-7192. · 2.17 Impact Factor
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ABSTRACT: The influence of Co substitution on the crystallization behavior and magnetic properties of (Pr,Tb)2(Fe,Nb,Zr)14B/α-Fe nanocomposites was investigated in melt-spun Pr7Tb1Fe87Nb0.5Zr0.5B4 and Pr7Tb1Fe57Co30Nb0.5Zr0.5B4 samples. Crystallographic textures were observed for all constituent phases in both sample S spun at wheel speeds less than 15 m/s. Both alloys show a fully amorphous structure when spun at a speed faster than 40 m/s which crystallizes into the TbCu7 structure before it finally transforms into the 2:14:1 phase. The Curie temperature of α-Fe and Pr2Fe14B phase increases greatly with the substitution of 30% Co for Fe. However, both the coercivity and energy product decrease. Optimum magnetic properties are found in the Pr7Tb1Fe87Nb0.5Zr0.5B4 sample with coercivity Hc=6.84 kOe and (BH)max=11.8 MG Oe. © 2003 American Institute of Physics.
Journal of Applied Physics. 05/2003; 93(10):7978-7980.
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01/2003;
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ABSTRACT: A multi-step crystallization evolution has been observed in melt-spun nanocomposite Pr8Fe86B6 alloys, resulting in drastically different magnetic properties. The ribbons spun at different wheel speeds consist of different volume fraction of an amorphous structure. Upon annealing the partially amorphous ribbons, a single transformation directly into Pr2Fe14B phase has been detected since the pre-existed 2 : 14 : 1 nanocrystallites serve as nucleation centres. However, a multiple-stage structural evolution was observed in the fully amorphous ribbons that crystallize first into metastable TbCu7-type phase and then Pr2Fe23B3 phase before they finally transform into a mixture of Pr2Fe14B and α-Fe. The microstructure in the samples with partially amorphous precursors is much homogeneous and finer with the grain size around 10–30 nm.
Journal of Physics D Applied Physics 11/2002; 35(22):2893. · 2.54 Impact Factor
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ABSTRACT: The structural and magnetic properties of nanocomposite melt-spun Sm(Co<sub>0.74-x</sub>Fe<sub>0.1</sub>Cu<sub>0.12</sub>Zr<sub>0.04</sub>B<sub>x</sub>)<sub>7.5</sub> magnets have been investigated as a function of boron content (x = 0.005 0.05), wheel speed and annealing conditions. The as-spun ribbons are nanocrystalline with fine microstructure and average grain size of 60-100 nm. X-ray diffraction indicates that the as-spun samples have the metastable hexagonal TbCu<sub>7</sub>-type structure phase and fcc-Co as a secondary soft phase. Magnetization at nonsaturating 5 T field is 45-72 emu/g and the reduced remanence (Mr/Ms) is above 0.8. The loop shape exhibits a characteristic step due to the soft magnetic phase. At room temperature, Hc values of 20-28 kOe are obtained for as spun samples, with a record value of 38.5 kOe for x = 0.04. At 380°C Hc values higher than 5 kOe are observed. Coercivity and loop shape are strongly dependent on annealing conditions.
IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
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ABSTRACT: An effort has been made to improve the coercivity of nanocomposite Nd<sub>10</sub>Fe<sub>82</sub>C<sub>6</sub>B<sub>2</sub> alloys, using small amounts (up to 1 at%) of Cr, Ti, Nb, Zr, and Ga. X-ray studies revealed that the amount of α-Fe phase in both as-spun and annealed ribbons was significantly reduced for Nb and Zr substitutions, but increased for all the other substitutions. From the magnetic properties point of view, Zr (1 at%) was proven to be the most advantageous of all, since it increased the coercivity of annealed ribbons from 3.2 to 4.8 kOe and the maximum energy product from 5.8 to 13 MGOe. This improvement is associated with the much finer and more uniform microstructure as was revealed by transmission electron microscopy.
IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
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ABSTRACT: CoPt nanoparticles embedded in nonmagnetic matrices M (M = C, BN) have been obtained by annealing multilayer precursors prepared by sputtering from Co<sub>50</sub>Pt<sub>50</sub> and M targets in a tandem deposition mode onto Si substrate. The structure and microstructure development of the CoPt/C films has been studied at different annealing stages. Magnetic properties of CoPt with different particle size are investigated with respect to bilayer thickness, annealing conditions, and temperature. The possibility for high density recording media is discussed.
IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
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ABSTRACT: The abnormal temperature dependence of coercivity with a positive temperature coefficient is characteristic in Sm-Co magnets with a cellular (or cellular-like) microstructure. The H<sub>c</sub>(T) exhibits a transformation from abnormal to normal behavior with an increase in the ratio of z from 7.0 to 8.5. For a given z, there exists a critical Cu amount below which the abnormal H<sub>c</sub>(T) is observed. This critical amount of Cu increases with decreasing z ratio. Additionally, the temperature of the H<sub>c</sub> peak at high temperature decreases with increasing Cu content. The magnets below the critical Cu value with different Zr also exhibit an abnormal H<sub>c</sub>(T) behavior. However, Zr does not cause an obvious change of H<sub>c</sub>(T). In the magnets with different Fe, the room temperature coercivity increases, but the peak of H<sub>c</sub> at high temperature decreases with increasing Fe content. An increase in the amount of Fe lowers the anisotropy field and the Curie temperature of the 2:17 matrix phase, resulting in a change of H<sub>c</sub>(T).
IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
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ABSTRACT: FePt single-layer and FePt-Fe multilayer thin films are prepared by magnetron sputtering. By varying the Pt content, FePt, Fe<sub>3</sub>Pt, or a mixture of FePt and Fe<sub>3</sub>Pt can be obtained in Fe-Pt single layers. In annealed FePt-Fe multilayers, the coercivities decrease with the introduction of Fe layers compared to FePt single layers, while the magnetization increases. The single-phase behavior of the hysteresis loops of FePt-Fe multilayers indicates the existence of exchange coupling in these materials. For one FePt-Fe sample with optimized exchange coupling, the intrinsic properties correspond to an energy product of 19 MGOe. The texture of the magnets is determined by the [111] orientation of the crystallites. This means that the easy magnetization directions of the FePt grains form an angle of 54.7° with the film normal, but are randomly oriented in the film plane. It is analyzed how this easy-axis distribution affects the magnetic hysteresis.
IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
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ABSTRACT: In this work we examine the effect of small boron substitution (x=0, 0.005, 0.010, 0.015) on the structural and magnetic properties of Sm(Co <sub>0.86-x</sub> Fe <sub>0.1</sub> Zr <sub>0.04</sub> B <sub>x</sub>)<sub>7.5</sub> and Sm(Co <sub>0.74-x</sub> Fe <sub>0.1</sub> Cu <sub>0.12</sub> Zr <sub>0.04</sub> B <sub>x</sub>)<sub>7.5</sub> melt-spun samples, as a function of wheel speed and annealing conditions. Boron substituted as-spun ribbons are found to have increased coercivity, Hc ≫5 kOe , and small grain size of 60–100 nm. For copper containing samples, the highest coercivity ( Hc =16.3 kOe ) was obtained in as-spun ribbons with x=0.015. In samples without copper the coercivity increased after short annealing ( Hc =12 kOe for x=0.015). The large coercivities are attributed to a fine microstructure consisting mainly of hexagonal TbCu <sub>7</sub> -type phase and a small amount of soft-phase grains. © 2002 American Institute of Physics.
Journal of Applied Physics 06/2002; · 2.17 Impact Factor
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ABSTRACT: A systematic study has been undertaken to understand the microstructure and coercivity in Ni substituted Sm ( Co <sub> bal </sub> Fe <sub>0.1</sub> Ni <sub>y</sub> Zr <sub>0.04</sub>)<sub>8.5</sub> magnets using magnetometry and transmission electron microscopy. These samples exhibit a cellular and lamellar microstructure which is typical of Cu substituted Sm(Co,Fe,Cu,Zr) <sub>z</sub> magnets. However, the coercivity obtained by Ni substitution is very low. With increasing Ni content y from 0.04 to 0.24, the coercivity increases slightly from 0.4 to 1.5 kOe. Thermomagnetic analysis shows that Ni goes into both the Sm <sub>2</sub> Co <sub>17</sub> and SmCo <sub>5</sub> phases. This makes it impossible to obtain a large domain wall energy gradient at the interface of the 2:17 and 1:5 phases, leading to low room temperature coercivity. The abnormal temperature dependence of coercivity H<sub>c</sub>(T), which is found in R <sub>2</sub> Co <sub>17</sub> magnets consisting of cellular/lamellar microstructure, is also observed in these magnets. A maximum coercivity of 3.6 kOe was obtained at 480 °C in Sm ( Co <sub> bal </sub> Fe <sub>0.1</sub> Ni <sub>0.18</sub> Zr <sub>0.04</sub>)<sub>8.5</sub>. The abnormal behavior in H<sub>c</sub>(T) can be explained by both the domain wall pinning and nucleation models. © 2002 American Institute of Physics.
Journal of Applied Physics 06/2002; · 2.17 Impact Factor
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ABSTRACT: In this study we have fabricated by electrodeposition CoPt(FePt) nanowires embedded inside an array of empty holes in anodized aluminum disks. By adjusting the current density and solution composition, one can control the composition of the film to 50:50 in order to obtain the high anisotropy CoPt and FePt face centered tetragonal (L10) phases. The as-made films are magnetically soft. Magnetic hardening is developed after annealing at 700 °C with coercivity typically in the range of 3–6 kOe. The scanning electron microscopy and transmission electron microscopy studies showed that nanowire structured CoPt and FePt with near stoichiometric composition inside an array of holes which are about 25–100 nm in diameter. A preferred perpendicular anisotropy is observed in the CoPt nanowires. © 2002 American Institute of Physics.
Journal of Applied Physics 05/2002; 91(10):6869-6871. · 2.17 Impact Factor
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ABSTRACT: The microstructure and magnetic properties of Pr2Fe14B/α-Fe nanocomposites with small amount of Tb, Nb, and Zr additions have been investigated. A c-axis texture in the 2:14:1 phase has been observed in ribbons spun at speeds below 14 m/s. Optimal magnetic properties were found in annealed samples spun at 14–17 m/s. With small additions of Nb and Zr, the coercivity significantly increases. Nb substitution leads to a poor loop squareness, which can be significantly improved with Zr addition due to a refinement in microstructure, resulting in a drastic increase in the energy product. Optimum magnetic properties with a room temperature coercivity of 8.2 kOe, and (BH)m=20.3 MGOe have been obtained in Pr7Tb1Fe87Nb0.5Zr0.5B4 ribbons. Transmission electron microscopy on this sample revealed a much finer and homogeneous microstructure with an average grain size of 20 nm. © 2002 American Institute of Physics.
Journal of Applied Physics 05/2002; 91(10):8165-8167. · 2.17 Impact Factor
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