G. C. Hadjipanayis

University of Delaware, Delaware, United States

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Publications (650)1178.22 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The effect of Mo and MoS2 additions on the magnetic and microstructure properties has been investigated in Nd–Fe–Ga–B sintered magnets. Coercivity can be increased by both the additions, but the MoS2 addition provides the larger increase per Mo atom for up to 0.6 at.% Mo. Microstructure investigation reveals a new amorphous intergranular Ga rich phase. This phase forms a thin layer in the grain boundaries and leads to a wetting behavior of the grain boundary phase, therefore increasing the coercivity. Molybdenum addition in the form of MoS2 is found to modify the Nd2Fe14B phase, rather than form new minority phases, and the coercivity enhancement of the magnet is due to the increased anisotropy field of the hard magnetic phase.
    Journal of Magnetism and Magnetic Materials 04/2012; 324(7):1391–1396. · 2.00 Impact Factor
  • O. Akdogan, W. Li, G. C. Hadjipanayis, R. Skomski, D. J. Sellmyer
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    ABSTRACT: In this work, single crystal L10 FePt nanocubes have been successfully produced by a cluster beam deposition technique without the need of post annealing. Particles have been deposited by dc magnetron sputtering using high Ar pressures on both single crystal Si substrates and Au grids for the measurement of magnetic and structural properties, respectively. The nanocubes have a uniform size distribution with an average size of 5 nm. At 1 Torr, the particles have the L10 structure with an order parameter of 0.5 and a RT coercivity of 2 kOe with high switching fields observed in the hysteresis loop. Further annealing increased the particle size to 20 nm and the RT coercivity to 10.2 kOe with perfect chemical ordering. In addition to these nanocubes, micron size rods with the L10 structure have been observed near the cluster gun. SEM analysis showed that these rods consist of nanoparticles with 20 nm average size. Surfactant assisted high-energy ball milling has been used to separate the nanoparticles from the rods. After one hour of milling, these 20 nm particles showed a room temperature coercivity of 9 kOe with an order parameter of 0.85. These FePt nanocubes have a potential for use in the development of future high-density magnetic recording media because of their high coercivity, good shape and very narrow size distribution.
    Journal of Applied Physics 03/2012; 111(7). · 2.19 Impact Factor
  • Liyun Zheng, Baozhi Cui, Wanfeng Li, George C. Hadjipanayis
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    ABSTRACT: Sm-Co hard nanoparticles have been synthesized by a modified mechanochemical processing. The phases, microstructure, particle sizes and magnetic properties of the synthesized Sm-Co nanoparticles have been investigated. The results showed that the precursors and milling times had great effects on the structure and magnetic properties of the synthesized Sm-Co nanoparticles. It is interesting to find that either Sm2Co7, SmCo5 or Sm2Co17 hard single phases can be obtained by manipulating the amount and ratio of starting materials of Sm2O3 and Co. After the mixture of Sm2O3, Co, Ca, and CaO powders were milled for 2–12 h, no Sm-Co based hard phases was formed, whereas the Sm-Co hard nanocrystallites were formed by a subsequent annealing at 650 °C for 1 h. The maximum coercivities of the synthesized Sm2Co7, SmCo5, and Sm2Co17 were 37.1, 35.5, and 10.8 kOe, respectively. Separated Sm2Co17 nanoparticles were obtained after washing the annealed powder with a solution of acetic acid aqueous solution. The washed Sm2Co17 particles were single crystals and had an average particle size of 81 nm.
    Journal of Applied Physics 03/2012; 111(7). · 2.19 Impact Factor
  • B. Z. Cui, L. Y. Zheng, M. Marinescu, J. F. Liu, G. C. Hadjipanayis
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    ABSTRACT: Morphology, structure, and magnetic properties of the [001] textured Nd2Fe14B nanocrystalline flakes prepared by surfactant-assisted high energy ball milling (HEBM) and subsequent annealing were studied. These flakes have a thickness of 80–200 nm, a length of 0.5–10 μm, and an average grain size of 10–14 nm. The addition of some amount of Dy, Nd70Cu30 alloy, and an appropriate post annealing increased the coercivity iHc of the Nd2Fe14B flakes. iHc was 3.7, 4.3, and 5.7 kOe for the Nd15.5Fe78.5B6, Nd14Dy1.5Fe78.5B6 and 83.3 wt.% Nd14Dy1.5Fe78.5B6 + 16.7 wt.% Nd70Cu30 flakes prepared by HEBM for 5 h in heptane with 20 wt.% oleylamine, respectively. After annealing at 450 °C for 0.5 h, their iHc increased to 5.1, 6.2, and 7.0 kOe, respectively. Anisotropic magnetic behavior was found in all of the as-milled and annealed flakes. Both, the thickening of Nd-rich phase at grain boundaries via diffusion of Nd70Cu30 and the surface modification of the Nd2Fe14B flake could be the main reasons for the coercivity enhancement in the as-milled and annealed Nd70Cu30-added Nd2Fe14B flakes.
    Journal of Applied Physics 03/2012; 111(7). · 2.19 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Sm-Co bulk alloys have shown superior permanent-magnet properties, but research on Sm-Co nanoparticles is challenging because of the need to control particle size, size-distribution, crystalline ordering, and phase purity. In the present study, a cluster-deposition method was used to produce Sm-Co nanoparticles having desired crystal structures without the requirement of subsequent high-temperature thermal annealing. Poorly crystallized SmCo5 nanoparticles exhibit a low room-temperature coercivity of only 100 Oe, whereas crystalline SmCo5 and Sm2Co17 nanoparticles show room-temperature coercivities of 2000 and 750 Oe, respectively. The direct synthesis of Sm-Co nanoparticles having sizes of less than 10 nm and a high degree of atomic ordering is an important step toward creating nanoparticle building blocks for permanent-magnets and other significant applications.
    Journal of Applied Physics 03/2012; 111(7). · 2.19 Impact Factor
  • Nilay G. Akdogan, Wanfeng Li, George C. Hadjipanayis
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    ABSTRACT: High-energy ball milling has been shown to be a promising method for the fabrication of rare earth—transition metal nanopowders. In this work, NdCo5 nanoflakes and nanoparticles have been produced by a two-stage high-energy ball milling (HEBM), by first using wet HEBM to prepare precursor nanocrystalline powders followed by surfactant-assisted HEBM. NdCo5 flakes have a thickness below 150 nm and an aspect ratio as high as 102–103; the nanoparticles have an average size of 7 nm. Both the nanoparticles and nano-flakes exhibited high coercivities at low temperatures, with values at 50 K of 3 and 3.7 kOe, respectively. The high values of coercivity can be attributed to the large surface anisotropy of nanoparticles that leads to an effective uniaxial-type of behavior in contrast to the planar anisotropy of the bulk samples. Angle-dependent magnetization measurements at different temperatures were used to determine the spin reorientation transitions in the nanopowders and nanoparticles. The nanoparticles showed spin reorientation temperatures, T SR1 = 276 and T SR2 = 237 K which are lower when compared with the values of 290 and 245 K, respectively for bulk.
    Journal of Nanoparticle Research 02/2012; 14(2). · 2.28 Impact Factor
  • Xiaocao Hu, Ozan Akdogan, Wanfeng Li, George Hadjipanayis
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    ABSTRACT: FePt particles with the tetragonal L10 structure are attractive for high density recording media. In this study, we have fabricated well-ordered and separated L10 FePt islands on MgO (100) substrates by DC sputtering at temperatures varying from room temperature to 700 ^oC. The dependence of particle size, degree of ordering and magnetic properties on sputtering time (5-20s), power (5-20W) and substrate temperature (300-1000K) were investigated. Electron diffraction patterns from TEM showed that the islands had the L10 tetragonal structure. TEM data revealed that a higher substrate temperature significantly increased the size of the islands from 2 to 20 nm whereas a higher sputtering power and longer sputtering times did not change the island size much but made the islands more connected with each other. Larger size islands and inter-connected islands showed a higher degree of ordering with an ordering parameter of 0.8 achieved at 600 ^oC. The magnetic properties are currently being measured and the results will be reported.
    02/2012;
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    ABSTRACT: This study is focused on a comparison of magnetic properties of chemically synthesized core/shell structured iron/iron-oxide nanoparticles with different core sizes and comparable shell thickness. Particles were synthesized by high temperature decomposition of iron organometallic compounds. Thermomagnetic data revealed that particles are superparamagnetic at room temperature. The field cooled hysteresis loops showed interesting features of enhanced coercivity and horizontal and vertical shifts along directions of the cooling field, all strongly depend on temperature, indicative of an exchange-bias-like phenomenon. These effects were more pronounced in smaller core size nanoparticles with an exchange bias field of 4098 Oe. The spin-glass-like phase with high-field irreversibility in the iron oxide shells played the role of the fixed phase in the core/shell system and provided the pinning force to the reversible spins. The magnetic domains and higher contributions from the surface anisotropy in the hollow nanoparticles caused enormous magnetic frustration that is the origin of high field irreversibility and vertical shift of hysteresis loop in these particles.
    02/2012;
  • Andrew Gallagher, Levent Colak, Ozan Akdogan, George Hadjipanayis
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    ABSTRACT: The major challenge for the application of chemically synthesized FePt nanoparticles (NPs) in magnetic storage media is the sintering problem encountered during the required high temperature annealing to obtain the high anisotropy L10 phase. In this work, we have used two methods to avoid sintering: coating the NPs with a protective layer of silica (SiO2) and using porous aluminum oxide (Al2O3) as a template to hold the NPs. The NPs were synthesized via the synthesis method of Sun et al.^[1] The NPs were added to the Al2O3 by in-situ suctioning of the reaction solution into the porous Al2O3 template. Monodispersed FePt NPs with a size of 5.8 and 15 nm were coated with SiO2 shells using a water-in-oil microemulsion method. High room temperature coercivities were only obtained after annealing the samples at 900 C for long times (24-48 h) under forming gas flow as compared to the usual 600-700 C. Values of 4.7 and 7.8 kOe were observed in SiO2 and Al2O3 samples, respectively after annealing for 24 h at 900 C. This behavior suggests that the restricted geometry of the samples suppresses the phase transformation drastically. [4pt] [1] S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser Science 2000, 287, 1989.
    02/2012;
  • B.Z. Cui, L.Y. Zheng, W.F. Li, J.F. Liu, G.C. Hadjipanayis
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    ABSTRACT: This paper reports on the fabrication, structure and magnetic property optimization of Nd2Fe14B single-crystal and [0 0 1] textured poly-nanocrystalline flakes prepared by surfactant-assisted high-energy ball milling (HEBM). Single-crystal Nd2Fe14B flakes first with micron and then with submicron thicknesses were formed via continuous basal cleavage along the (1 1 0) planes of the irregularly shaped single-crystal microparticles during the early stage of HEBM. With further milling, [0 0 1] textured polycrystalline submicron Nd2Fe14B flakes were formed. Finally, crystallographically anisotropic polycrystalline Nd2Fe14B nanoflakes were formed after milling for 5–6 h. Anisotropic magnetic behavior was found in all of the flake samples. Nd2Fe14B flakes prepared with either oleic acid (OA) or oleylamine (OY) as the surfactant exhibited similar morphology, structure and magnetic properties. Both the addition of some low-melting-point eutectic Nd70Cu30 alloy and an appropriate post-annealing can increase the coercivity of the Nd2Fe14B flakes. The coercivity of Nd2Fe14B nanoflakes with an addition of 16.7 wt.% Nd70Cu30 by milling for 5 h in heptane with 20 wt.% OY increased from 3.7 to 6.8 kOe after annealing at 450 °C for 0.5 h. The mechanism for formation and coercivity enhancement of Nd2Fe14B single-crystal and textured poly-nanocrystalline flakes with a submicron or nanosize thickness was discussed.
    Acta Materialia 02/2012; 60(4):1721–1730. · 3.94 Impact Factor
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    ABSTRACT: In this work, single crystal fct FePt nanocubes have been successfully produced by a cluster beam deposition technique without the need of post annealing. Particles have been deposited by DC magnetron sputtering using high Ar pressures (0.5 to 2 Torr) on both single crystal Si substrates and Au grids for the measurement of magnetic and structural properties, respectively. The nanocubes have a uniform size distribution with an average size of 6.5 nm. At 1 Torr, the particles have the fct structure with an order parameter of 0.5 and a RT coercivity of 2 kOe with high switching fields seen in the hysteresis loop. Particle size was controlled by changing the pressure and power and also by ex-situ annealing. In addition to these nanocubes, micron size rods (which consist of 20 nm nanoparticles) with the fct structure have been observed near the cluster gun. These particles show a room temperature coercivity of 8 kOe with an order parameter of 0.85. Intrinsic magnetic properties (Curie temperature, HA, MS and magnetic viscosity) of the nanocubes and the nanoparticles (separated from the rods) have been extensively studied and the results will be reported.
    02/2012;
  • George Hadjipanayis, Nilay Gunduz Akdogan, Wanfeng Li
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    ABSTRACT: High temperature magnetic ordering studies on rare earth transition-metal nanoparticles and nanoflakes present a great challenge due to the very high reactivity of these materials. It is well known that Mn substitution for Fe in Nd2Fe14B compound decreases the Curie temperature to a temperature range that allows for reliable measurements to be made. In this work, we have studied the magnetic properties of Mn substituted Nd2Fe14B particles in the temperature range of 50-400 K. Nd2Fe14-xMnxB nanoparticles and nanoflakes have been produced by surfactant-assisted high-energy ball milling (SA-HEBM). Different size nanoparticles have been obtained by varying the milling conditions. Anisotropic Nd2Fe14-xMnxB nanoparticles have been found with a size from 13 to 25 nm. Both the nanoparticles and nano-flakes showed high coercivities at low temperatures, with values at 50 K of 2.4 kOe and 5.5 kOe, respectively. The Curie temperature was determined from the temperature dependence of magnetization. We have observed a different magnetic ordering behavior in the nanoparticles with Curie temperatures that are higher when compared to the bulk values.
    02/2012;
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    ABSTRACT: Determination of spin-dependent Seebeck coefficients of CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars J. Appl. Phys. 111, 07C520 (2012) Effect of packing fraction on ferromagnetic resonance in NiFe2O4 nanocomposites J. Appl. Phys. 111, 07E348 (2012) General approach to the magnetostatic force and interaction between cylindrically shaped nanoparticles J. Appl. Phys. 111, 07D131 (2012) Effect of rounded corners on the magnetic properties of pyramidal-shaped shell structures The performance of hard-magnetic nanostructures is investigated by analyzing the size and geometry dependence of thin-film hysteresis loops. Compared to bulk magnets, weight and volume are much less important, but we find that the energy product remains the main figure of merit down to very small features sizes. However, hysteresis loops are much easier to control on small length scales, as epitomized by Fe-Co-Pt thin films with magnetizations of up to 1.78 T and coercivities of up to 2.52 T. Our numerical and analytical calculations show that the feature size and geometry have a big effect on the hysteresis loop. Layered soft regions, especially if they have a free surface, are more harmful to coercivity and energy product than spherical inclusions. In hard-soft nanocomposites, an additional complication is provided by the physical properties of the hard phases. For a given soft phase, the performance of a hard-soft composite is determined by the parameter (M s -M h)/K h . V C 2012 American Institute of Physics. [doi:
    Journal of Applied Physics 01/2012; 111(111):7-345. · 2.19 Impact Factor
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    ABSTRACT: The effect of magnetic annealing on the texture is investigated in a series of CoPt films of different thickness obtained by annealing of Co/Pt multilayers. Magnetic field is found to promote the (001) texture by selective (001) grain growth driven by the magnetic field and biaxial strain. A systematic correlation between in-plane biaxial strain and the degree of (001) texture is found only for the magnetically annealed samples.
    Journal of Applied Physics 10/2011; 110(8). · 2.19 Impact Factor
  • Liyun Zheng, Baozhi Cui, George C. Hadjipanayis
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    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 [001] 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.
    Acta Materialia 10/2011; 59(17):6772-6782. · 3.94 Impact Factor
  • A. M. Gabay, W. F. Li, G. C. Hadjipanayis
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    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 [001] 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 10/2011; 323(20):2470-2473. · 2.00 Impact Factor
  • George Hadjipanayis, Alexander Gabay
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    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.
    IEEE Spectrum 08/2011; 48(8):36-41. · 0.94 Impact Factor
  • Hafsa Khurshid, Wanfeng Li, Vasillis Tzitzios, George C Hadjipanayis
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    ABSTRACT: In this work, we report a detailed study of the formation of hollow nanostructures in iron oxides. Core/shell Fe/Fe-oxide nanoparticles were synthesized by thermal decomposition of Fe(CO)(5) at high temperature. It was found that 8 nm is the critical size above which the particles have a core/shell morphology, whereas below this size the particles exhibit a hollow morphology. Annealing the core/shell particles under air also leads to the formation of hollow spheres with a significant increase in the average particle size. In the case of the thermally activated Kirkendall process, the particles do not fully transform into hollow structures but many irregular shaped voids exist inside each particle. The 8 nm hollow particles are superparamagnetic at room temperature with a blocking temperature of 70 K whereas the core/shell particles are ferromagnetic.
    Nanotechnology 07/2011; 22(26):265605. · 3.67 Impact Factor
  • Liyun Zheng, Wanfeng Li, Baozhi Cui, George C. Hadjipanayis
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    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 05/2011; 509(19):5773-5776. · 2.73 Impact Factor
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    ABSTRACT: In this study, we discuss the effects of a type of surfactant (oleylamine, oleic acid, and trioctylamine) and hot pressing on the hard magnetic properties of crystallographically anisotropic SmCo 5 nanoflakes prepared by surfactant-assisted high energy ball milling. The phase, microstructure, and magnetic properties of the hot-pressed SmCo 5 were investigated by using x-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry. The coercivities of the precursor flakes prepared using oleylamine, oleic acid, and trioctylamine were 14.9, 15.8, and 15 kOe, respectively. Hot-compacted SmCo 5 magnets prepared from the nanoflakes milled with oleic acid had the lowest coercivity of 8.1 kOe. It is believed that even after repeated washing in an ultrasonic bath with different solvents, the remaining oleic acid in the SmCo 5 nanoflakes led to oxidation of SmCo 5 at the surface/interface of nanoflakes during the hot-pressing process. The compacted SmCo 5 magnets prepared from the nanoflakes milled with trioctylamine and oleylamine had higher coercivity values of 14.8 and 12.8 kOe, respectively. Unlike oleic acid, oleylamine and trioctylamine contain no oxygen atoms, and therefore there was less oxidation during the process. The compaction temperature also influences the coercivity. The SmCo 5 magnets hot pressed at 550 °C had the highest coercivity, whereas those hot pressed at 650 °C had the lowest coercivity; this is attributed to the change of phases and composition of SmCo 5 alloys during the hot-pressing process.
    Journal of Applied Physics 05/2011; · 2.19 Impact Factor

Publication Stats

6k Citations
1,178.22 Total Impact Points

Institutions

  • 1989–2014
    • University of Delaware
      • Department of Physics and Astronomy
      Delaware, United States
  • 2013
    • Himachal Pradesh University
      Simla, Himachal Pradesh, India
  • 1980–2011
    • University of Nebraska at Lincoln
      • Department of Physics and Astronomy
      Lincoln, NE, United States
  • 2010
    • CSU Mentor
      • Department of Chemistry
      Long Beach, California, United States
  • 2008–2010
    • Emory University
      Atlanta, Georgia, United States
  • 2004
    • University of Dayton
      Dayton, Ohio, United States
  • 2002–2003
    • Aristotle University of Thessaloniki
      • School of Electrical and Computer Engineering
      Thessaloníki, Kentriki Makedonia, Greece
  • 2001–2002
    • University of Ioannina
      • Τμήμα Φυσικής
      Yannina, Epirus, Greece
  • 1984–2000
    • Kansas State University
      • Department of Physics
      Manhattan, KS, United States
  • 1994
    • Northeast Institute of Geography and Agroecology
      • Institute of Physics
      Beijing, Beijing Shi, China
  • 1992
    • University of Missouri
      • Department of Chemistry
      Columbia, Missouri, United States