G. C. Hadjipanayis

University of Delaware, Ньюарк, Delaware, United States

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Publications (703)1294.9 Total impact

  • O. Akdogan · W. Li · B. Balasubramanian · David J Sellmyer · G. C. Hadjipanayis ·
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    ABSTRACT: Single crystal SmCo5 nanoparticles with an average size of 3.5 nm are produced by cluster-beam deposition. When deposited without matrix, the nanoparticles showed a super-paramagnetic behavior with a blocking temperature of 145 K. Dispersion of the SmCo5 nanoparticles in a carbon matrix results in an increase in both the coercivity and the blocking temperature. Room temperature coercivities as high as 12 kOe are obtained for the first time in mono-layers of SmCo5 nanoparticles dispersed in C matrix. δM plots show that the interactions in the samples are of exchange type, which can decrease the overall effective anisotropy and coercivity according to the random-anisotropy model. Coercivity is found to be inversely proportional to the packing density of the particles. SmCo5 nanoparticles with high coercivity are potential candidates for the next generation ultra high-density magnetic recording media.
    Advanced Functional Materials 07/2013; 23(26). DOI:10.1002/adfm.201201353 · 11.81 Impact Factor
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    Ralph Skomski · Pankaj Kumar · George C. Hadjipanayis · David J Sellmyer ·
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    ABSTRACT: It is investigated how magnetic hysteresis is affected by finite-temperature excitations, using soft regions in hard-magnetic matrices as model systems. In lowest order, magnetization processes are described by the traditional approach of using finite-temperature materials. Nanoscale excitations are usually small perturbations. For example, a Bloch summation over all magnon wave constants such as vectors shows that remanence is slightly enhanced, because long-wavelength excitations are suppressed. However, a reverse magnetic field enhances the effect of thermal excitations and causes a small reduction of the coercivity. To describe such effects, we advocate micro-magnetic calculations where finite-temperature fluctuations are treated as small corrections to the traditional approach, as contrasted to full-scale Monte Carlo simulations.
    IEEE Transactions on Magnetics 07/2013; 49(7):3229. DOI:10.1109/TMAG.2013.2247386 · 1.39 Impact Factor
  • N. V. Rama Rao · Alexander M. Gabay · G. C. Hadjipanayis ·
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    ABSTRACT: MnBi/Sm2Fe17Nx bulk anisotropic hybrid magnets have been fabricated employing hot compaction of mechanically milled powders. The effects of Sm2Fe17Nx content on the magnetic properties, density and texture of the hybrid magnets were investigated. With increasing Sm2Fe17Nx content, the density of hybrid magnets decreases, their coercivity (H-c) remains nearly constant, while the remanence (M-r) increases drastically. TheMnBi/40% Sm2Fe17Nx magnet displays an H-c value of 5.7 kOe with M-r of 69 emu/g; these values for pure MnBi magnet are 6.5 kOe and 45 emu/g. An energy product [(BH)(max)] of 7 MGOe was obtained in the hybrid magnet with 40% Sm2Fe17Nx, which is 20% higher than that of single phase MnBi magnet [5.8 MGOe].
    IEEE Transactions on Magnetics 07/2013; 49(7):3255-3257. DOI:10.1109/TMAG.2013.2240274 · 1.39 Impact Factor
  • Alexander M. Gabay · Wanfeng Li · George C. Hadjipanayis ·
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    ABSTRACT: Anisotropic Sm0.85Pr0.15(Co1-infinity Fe infinity)(z) alloy powders with x = 0.2-0.5, consisting mostly of the rhombohedral (Sm,Pr)(2)(Co,Fe)(17) compound, were prepared from a mixture of cobalt, ferric oxide and rare earth oxides subjected to high-energy ball-milling in the presence of calcium reducing agent and calcium oxide dispersant. The milling was followed by a short-time annealing and multi-step washing. The resulting powders were polydispersed, but their average particle size could be tuned from 80 to 300 nm by controlling the excess Ca metal and the annealing temperature. A nanometer-thin oxide shell was observed on a surface of the particles, and it may be, at least in part, responsible for their excellent stability to air. The significant effect of the excess Ca on the particle size, anisotropy and chemical/structural homogeneity is discussed based on the supposed solution-precipitation mechanism of particle growth. Intrinsic coercivity H-c up to 14 kOe was obtained; powders with x = 0.4 and an average particle size of 121 nm exhibited a remanence of 106 emu/g and H-c of 8.3 kOe.
    IEEE Transactions on Magnetics 07/2013; 49(7):3225-3228. DOI:10.1109/TMAG.2013.2245636 · 1.39 Impact Factor
  • N. V. Rama Rao · A M Gabay · W F Li · G C Hadjipanayis ·
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    ABSTRACT: 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.
    Journal of Physics D Applied Physics 06/2013; 46(26):265001. DOI:10.1088/0022-3727/46/26/265001 · 2.72 Impact Factor
  • W. F. Li · A. M. Gabay · X. C. Hu · C. Ni · G. C. Hadjipanayis ·
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    ABSTRACT: In this work we report fabrication and microstructural characterization of single crystalline Sm2Co17 nanoparticles with an average size of 103.7 nm. These particles are fabricated using mechanochemical method and can be used for fabrication of high performance permanent magnets due to their high coercivity (20 kOe). Microstructure analysis reveals the inhomogeneity and defects in the nanoparticles. The origin of these defects was analyzed and discussed by systematic microstructural investigation of the as-milled, annealed, and washed samples. On the basis of these results, by further optimizing the processing parameters, properties of the nanoparticles can be improved.
    The Journal of Physical Chemistry C 05/2013; 117(20):10291–10295. DOI:10.1021/jp401836w · 4.77 Impact Factor
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    ABSTRACT: We report the use of metallic iron-based nanoparticles for magnetic resonance imaging (MRI) applications. Core/shell structured iron-based nanoparticles prepared by thermally decomposing organo-metallic compounds of iron at high temperature in the presence of hydrophobic surfactants were coated and stabilized in the aqueous solvent using the newly developed polysiloxane PEO–b–PγMPS (poly(ethylene oxide)–block–poly (γ methacryloxypropyl trimethyl oxysilane)) diblock copolymers. Particles are well suspended in water and retain their core–shell morphology after coating with the copolymer. In comparison to the conventionally used iron-oxide nanoparticles, core/shell structured iron/iron-oxide nanoparticles offer a much stronger T2 shortening effect than that of iron-oxide with the same core size due to their better magnetic properties.
    Journal of Magnetism and Magnetic Materials 04/2013; 331:17–20. DOI:10.1016/j.jmmm.2012.10.049 · 1.97 Impact Factor
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    ABSTRACT: 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).
    Journal of Applied Physics 03/2013; 113(17):17B509. DOI:10.1063/1.4795318 · 2.18 Impact Factor
  • Jason Huynh · Angshuman Pal · Hafsa Khurshid · George Hadjipanayis ·
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    ABSTRACT: In this work, we have used a low temperature method that reduces Pt(acac)2 and thermally decomposes Fe(CO)5 in the presence of oleic acid (OA) and oleyl amine (OY) to fabricate monodispersed fcc FePt nanoparticles ^[1]. Adjusting the parameters, such as injection temperature and heating rate, could control the size and shape, respectively. The nanoparticles made for this study had a cubic shape and an average diameter of 5.3 nm. The particles were found to be superparamagnetic at room temperature with low coercivity. The aim of this project was to disperse nanoparticles in aqueous solutions. The surfactants (OA/OY) were then exchanged with tetramethylammonium hydroxide (TMAOH) to allow water phase transfer^[2]. After the transformation, the FePt nanoparticles were dispersed in 20 mL of a 0.01-wt % TMAOH solution. The structural and magnetic properties were maintained as shown by X-Ray Diffraction and Vibrating Sample Magnetometer data. From these findings, the FePt nanoparticles are being modified for higher coercivity for potential use in hyperthermia studies. [1] L. Colak and G. C. Hadjipanayis, Nanotechnology 20 (2009) 485602. [2] V. Salgueiriño-Maceira, L. M. Liz-Marzán and M. Farle, Langmuir 20 (2004) 6947.
  • Lixin Zhao · Nilay G. Akdogan · George C. Hadjipanayis ·
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    ABSTRACT: Sm2Fe17Nx nitride is usually synthesized at a range of temperature 350-500 degrees C. In this paper, hard magnetic Sm2Fe17N3 flakes have been synthesized by surfactant-assisted high energy balling milling (HEBM) followed by nitirding in 3 psi of N-2 gas at 300-350 degrees C for 3 h. The synthesized Sm2Fe17N3 flakes still have a texture and are obtained a coercivity of 3.56 kOe. The microstructure, phase structure, properties and the nitrogenation mechanism at lower temperature has been discussed.
    Journal of Alloys and Compounds 03/2013; 554:147–149. DOI:10.1016/j.jallcom.2012.11.153 · 3.00 Impact Factor
  • George Hadjipanayis · Alexander Gabay · Wanfeng Li ·
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    ABSTRACT: Bottom-up fabrication of nanocomposite permanent magnets with enhanced maximum energy product requires large quantities of high-coercivity powder with crystallographically anisotropic particles tens of nanometers in size. In this work, we report a systematic study aimed to employ combination of intensive mechanical milling and calciothermic reduction for preparation of polydispersed (Sm,Pr)2(Co,Fe)17 powders with a predominant-to-significant part of the particles smaller than 100 nm. In addition to the effects of Pr and Fe on the hard magnetic properties of the particles, the study analyzes the influence of excess reducing agent Ca and that of the heat treatment on the particle size distribution, their chemical/structural homogeneity and crystallographic anisotropy. Emphasized is the likely role of the excess Ca facilitating the diffusion-enabled particle growth. Remanent magnetization up to 106 emu/g and intrinsic coercivity up to 14 kOe were obtained.
  • Ozlem Koylu-Alkan · George C. Hadjipanayis · Dimitris Niarchos ·
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    ABSTRACT: The bottom-up fabrication of anisotropic exchange-coupled nanocomposites brings out the necessity of fabrication of magnetically hard nanoparticles with high coercivity. In this study, we have fabricated Nd2Fe14B nanoparticles from die-upset Nd-Fe-B (MQ3) precursor materials using planetary milling. The MQ3 alloy consists of platelets which are approximately 80 nm in thickness and 500 nm in diameter. Using planetary ball milling we were able to produce Nd2Fe14B nanoparticles with a size down to 20 nm. However, the size distribution of the milled particles is very broad ranging between 20 nm and 20 μm. A sedimentation experiment was used to separate the different size particles. By allowing bigger particles to sediment in a viscous liquid, we were able to separate different size nanoparticles with a size smaller than 200 nm. The coercivity of particles is found to decrease with particle size. After 60 min sedimentation the collected particles had an average size 100 nm with a coercivity value of 5.4 kOe. The objective of this study is to obtain nanoparticles with a size below 100 nm and a coercivity greater than 10 kOe for the fabrication of anisotropic exchange-coupled nanocomposites.
  • Xiaocao Hu · Ryan Gallagher · George Hadjipanayis ·
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    ABSTRACT: FePt particles with tetragonal L10 structure have been of great interest as one of the most promising candidate for ultra-high density recording media. Chemical synthesis is one of the two major methods to fabricate FePt nanoparticles because it can lead to high uniformity and patterned assembly. However, traditional approaches require post annealing above 500^o to transform the FePt nanoparticles from the disordered face-centered cubic (fcc) to the ordered L10 phase which introduces undesirable agglomeration and sintering. In this study, we have fabricated ordered L10 FePt nanoparticles using one-step chemical synthesis without post annealing. The traditional synthesis method of reduction of Pt(acac)2 and Fe(CO)5 was used at higher temperatures in the range of 300 to 400^o . Monodispersed Au nanoparticles with average size of 10 nm were used as catalysts. X-ray diffraction (XRD) spectra and selected area electron diffraction (SAED) patterns revealed that the FePt nanoparticles are in L10 phase. The highest coercivity obtained was 8 kOe at room temperature and 11 kOe at 50 K and is achieved at the reaction temperature of 400^o. Transmission electron microscopy (TEM) images showed that FePt nanoparticles are partially agglomerated which needs further improvement.
  • Liyun Zheng · Baozhi Cui · Lixin Zhao · Wanfeng Li · George C. Hadjipanayis ·
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    ABSTRACT: CaF2-coated single-crystal and textured polycrystalline flakes have been synthesized by a solid surfactant-assisted high energy ball milling (HEBM), which is totally different from the traditional liquid surfactant-assisted HEBM. The results show that as-milled SmCo5 specimens were crystallographically anisotropic and coated very well with CaF2. Single-crystal SmCo5 flakes with a thickness of 150–700 nm were formed after HEBM for 2 h with 40 wt.% CaF2. After HEBM for 5 h, [0 0 1] textured poly-nanocrystalline SmCo5 flakes were mainly formed. The c-axes of most of the grains is perpendicular to the flake surface. The texture intensities reduced with either increasing the milling time or reducing the amount of CaF2.
    Journal of Alloys and Compounds 02/2013; 549:22–25. DOI:10.1016/j.jallcom.2012.08.057 · 3.00 Impact Factor
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    A.Tomou · I. Panagiotopoulos · V. Tzitzios · Wanfeng Li · G.C. Hadjipanayis ·
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    ABSTRACT: Cr–Pt nanoparticles of different compositions around the ferrimagnetic Pt3Cr have been synthesized with the thermolytic method and proper adjustment of the Pt(acac)2 and Cr(acac)2 mole ratios. The as prepared particles have a size of (5.5±1.0) nm but after heat-treatment at 850 °C for 2 h an inhomogeneous microstructure consisting of clustered nanoparticles with variable sizes 5–50 nm is obtained. Chemical ordering to the L12 phase up to S=0.83 has been achieved for the nanoparticles with optimized stoichiometry while moderate coercivity values up to 380 Oe are achieved.
    Journal of Magnetism and Magnetic Materials 01/2013; 334:107-110. DOI:10.1016/j.jmmm.2013.01.025 · 1.97 Impact Factor
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    N. V. Rama Rao · A M Gabay · G C Hadjipanayis ·
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    ABSTRACT: 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.
    Journal of Physics D Applied Physics 01/2013; 46(6):062001. DOI:10.1088/0022-3727/46/6/062001 · 2.72 Impact Factor
  • G. Giannopoulos · Th. Speliotis · W. F. Li · G. Hadjipanayis · D. Niarchos ·
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    ABSTRACT: In this work structural and magnetic properties of (L10-FePt/A1-FePt) exchange coupled nanocomposites are presented. Semi spherical “dome-like” nanocomposites with L10 FePt isolated nanoparticles and A1 FePt (fcc) cap layers were obtained by depositing A1-FePt on type L10 FePt nanoparticles in order to understand the influence of the soft magnetic layer thickness on the magnetic properties of the system. Epitaxial growth is confirmed by X-ray diffraction and TEM, while the coercivity decreases dramatically for the L10/A1-FePt system when the thickness of the A1-FePt cap layers is increased. This result can be used to realize ultrahigh magnetic recording media with tunable coercivity, suitable for conventional write heads.
    Journal of Magnetism and Magnetic Materials 01/2013; 325:75–81. DOI:10.1016/j.jmmm.2012.08.003 · 1.97 Impact Factor
  • Alexander M. Gabay · Melania Marinescu-Jasinski · Jinfang Liu · George C. Hadjipanayis ·
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    ABSTRACT: Dielectric CaF2 layers were introduced into Nd-Fe-Co-Ga-B green compacts which were subsequently sintered into internally segmented anisotropic magnets. Layers with the nominal area density of 0.2-0.3 mg CaF 2 per mm 2 were found to increase the electrical resistivity by two orders of magnitude while still allowing the magnets to be cut and polished. The internal segmentation with CaF 2 fully preserves the coercivity of the magnets and is accompanied by only a small decline of the magnet remanence. The fcc Nd1-x Cax (F,O)δ phase produced by the reaction between the CaF 2 phase and the Nd-rich phase is believed to improve the metallurgical bond between the dielectric layers and magnet matrix. The internal segmentation may be a cost-effective way of manufacturing Nd-Fe-B sintered magnets with low eddy current losses, suitable for motor/generator applications.
    IEEE Transactions on Magnetics 01/2013; 49(1):558-561. DOI:10.1109/TMAG.2012.2207734 · 1.39 Impact Factor

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    ABSTRACT: Mn55Bi45 + x at.% Mn (x = 0, 7 and 11) magnetic powders were prepared by low energy ball milling technique. The x-ray diffraction pattern showed that the samples mainly contain over 90% of the low temperature Mn-Bi phase for x = 0 and 7 with a very small amount of Bi phase. Microstructural analysis showed that the amount of sub-micron particles increases with milling time. The milling time dependent magnetic properties were studied for all the compositions. An intrinsic coercivity (H-ci) of 12.2 kOe, remanent magnetization (4 pi M-r) of 7.1 kG, and energy product (BH)(max) of 11 MGOe have been obtained for the 8 hours milled Mn55Bi45 + 7 at.% Mn sample. These Mn-Bi powders have potential use as precursor for rare earth free permanent magnets.
    IEEE Transactions on Magnetics 11/2012; 48(11):3641-3643. DOI:10.1109/TMAG.2012.2201146 · 1.39 Impact Factor

Publication Stats

11k Citations
1,294.90 Total Impact Points


  • 1989-2015
    • University of Delaware
      • Department of Physics and Astronomy
      Ньюарк, Delaware, United States
  • 1980-2015
    • University of Nebraska at Lincoln
      • Department of Physics and Astronomy
      Lincoln, Nebraska, United States
  • 2010
    • Emory University
      Atlanta, Georgia, United States
  • 1984-2000
    • Kansas State University
      • • Department of Physics
      • • Department of Chemistry
      Manhattan, KS, United States
  • 1993-1998
    • University of Ioannina
      • Department of Physics
      Yannina, Epirus, Greece
  • 1992
    • University of Missouri
      • Department of Chemistry
      Columbia, Missouri, United States
  • 1988
    • Carnegie Mellon University
      • Department of Materials Science and Engineering
      Pittsburgh, Pennsylvania, United States