G. C. Hadjipanayis

University of Delaware, Ньюарк, Delaware, United States

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Publications (715)1326.44 Total impact

  • N. V. Rama Rao · A. M. Gabay · X. Hu · G. C. Hadjipanayis
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    ABSTRACT: MnBi nanoparticles with size in the range 100-300 nm were fabricated from a mixture of Bi2O3 and Mn in the presence of Ca as reducing element by a mechanochemical process. Heat treatment of as-milled powder and subsequent washing yielded nanoparticles with composite morphology of MnBi/Bi. Nanoparticles were crystallographically anisotropic and displayed high remanence ratio (M-r/M-s) of 0.94 and high coercivity (H-c) of 16.3 kOe at room temperature. The spin reorientation temperature of the MnBi nanoparticles was found to be at 119 K showing an increase by 10 K as compared to the bulk sample.
    No preview · Article · Feb 2014 · Journal of Alloys and Compounds
  • M. Gjoka · H. Sarafidis · Ch. Wang · G. Hadjipanayis · O. Kalogirou · D. Niarchos

    No preview · Article · Jan 2014 · IEEE Transactions on Magnetics
  • A.M. Gabay · X.C. Hu · G.C. Hadjipanayis
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    ABSTRACT: Polydispersed single crystal particles of the Nd2Fe14B and (Nd0.75Dy0.25)2Fe14B compounds with a short axes in the 60–140 nm range were prepared by high-energy ball milling and subsequent annealing of rare-earth oxides, iron oxide and boron oxide in the presence of a calcium reducing agent and a calcium oxide dispersant. The particles embedded in the CaO/Ca matrix in the presence of excess rare earth exhibit coercivity Hc of 14.7 and 18.9 kOe. Removal of the matrix and excess rare earth is accompanied by interstitial modification of the 2:14:1 compounds with hydrogen; the Hc of the Dy-free and Dy-containing particles declines with the washing to 1.4 and 3.2 kOe, respectively. After desorption of the hydrogen through vacuum annealing, the Hc increases, but only to 4.6 and 6.2 kOe. This incomplete recovery is tentatively attributed to local anisotropy defects caused by loss of the rare earth atoms from the particle surfaces. If correct, this model implies that a much higher Hc can be expected if the 2:14:1 particles are coated with or sintered in the presence of a rare-earth-rich phase.
    No preview · Article · Oct 2013 · Journal of Alloys and Compounds
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    ABSTRACT: The development of positive magnetic entropy change in the case of ferromagnetic (FM) nanostructures is a rare occurrence. We observe positive magnetic entropy change in core/shell (Fe/γ-Fe2O3) and hollow (γ-Fe2O3) nanoparticles and its origin is attributed to a disordered state in the nanoparticles due to the random distribution of anisotropy axes which inhibits any long range FM ordering. The effect of the energy barrier distribution on the magnetic entropy change and its impact on the universal behavior based on rescaled entropy change curves for core/shell and hollow nanostructures is discussed. Our study emphasizes that the magnetic entropy change is an excellent parameter to study temperature and field dependent magnetic freezing in such complex nanostructures.
    Full-text · Article · Sep 2013 · Journal of Physics Condensed Matter
  • Lixin Zhao · Liyun Zheng · George C. Hadjipanayis
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    ABSTRACT: Sm2Fe17 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 Sm2Fe17 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 Sm2Fe17 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.
    No preview · Article · Aug 2013 · Functional Materials Letters
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    W. F. Li · X. C. Hu · B. Z. Cui · J. B. Yang · J. Z. Han · G. C. Hadjipanayis
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    ABSTRACT: In this work we report the microstructure and magnetic property of single crystalline Nd2Fe14B ultrafine particles ball milled from HDDR Nd–Fe–B alloys. The average size of the particles is 283 nm, and TEM observation reveals that these particles are single crystalline. The coercivity of these particles is 6.0 kOe, which is much higher than that of the particles ball milled from sintered and hot pressed Nd–Fe–B magnets. Micromagnetic analysis shows that the coercivity degradation is caused by surface damage during ball milling.
    Full-text · Article · Aug 2013 · Journal of Magnetism and Magnetic Materials
  • 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.
    No preview · Article · Jul 2013 · Advanced Functional Materials
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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.
    Full-text · Article · Jul 2013 · IEEE Transactions on Magnetics
  • 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].
    No preview · Article · Jul 2013 · IEEE Transactions on Magnetics
  • 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.
    No preview · Article · Jul 2013 · IEEE Transactions on Magnetics
  • 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.
    No preview · Article · Jun 2013 · Journal of Physics D Applied Physics
  • 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.
    No preview · Article · May 2013 · The Journal of Physical Chemistry C
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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.
    Full-text · Article · Apr 2013 · Journal of Magnetism and Magnetic Materials
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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).
    Full-text · Article · Mar 2013 · Journal of Applied Physics
  • 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.
    No preview · Article · Mar 2013
  • 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.
    No preview · Article · Mar 2013 · Journal of Alloys and Compounds
  • 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.
    No preview · Article · Mar 2013
  • 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.
    No preview · Article · Mar 2013
  • 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.
    No preview · Article · Mar 2013
  • 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.
    No preview · Article · Feb 2013 · Journal of Alloys and Compounds

Publication Stats

12k Citations
1,326.44 Total Impact Points

Institutions

  • 1989-2016
    • University of Delaware
      • Department of Physics and Astronomy
      Ньюарк, Delaware, United States
  • 2010
    • Emory University
      Atlanta, Georgia, United States
  • 2002
    • Autonomous University of Barcelona
      • Department of Physics
      Cerdanyola del Vallès, Catalonia, Spain
  • 1980-2002
    • University of Nebraska at Lincoln
      • Department of Physics and Astronomy
      Lincoln, NE, 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