G. C. Hadjipanayis

University of Delaware, Ньюарк, Delaware, United States

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Publications (678)1247.54 Total impact

  • 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 · 2.73 Impact Factor
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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 · 2.00 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 · 2.52 Impact Factor
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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 · 1.21 Impact Factor
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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 · 2.00 Impact Factor
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    ABSTRACT: Anisotropic SmCo5 nanoflakes prepared by high-energy ball-milling with surfactants have great potential in applications for high-performance nanocomposite magnets. For such “nanocomposite” applications, the surface structure and chemistry of nanoflakes are crucial for achieving high coercivity. In this study, hot-pressed samples from anisotropic SmCo5 nanoflakes, ball-milled with different surfactants, oleic acid (OA) and oleylamine (OY), were investigated. Interface layers between the SmCo5 nanoflakes were found to consist of samarium oxides and a soft magnetic Co phase. These surface layers contribute to the degradation of hard magnetic performance, which is confirmed by scanning transmission electron microscopy-energy dispersive X-ray spectroscopy analysis of the cross-section of a single flake ball-milled with OA. Samples milled with OY show a much thinner interface layer in compacted samples, which means that the surface degradation during ball-milling with OY is much less than that with OA. The results show clearly that the choice of proper surfactant and the control of processing parameters are the key factors for improving the surface condition of the nanoflakes and the resulting hard magnetic properties.
    Acta Materialia 11/2012; 60(19):6685–6691. DOI:10.1016/j.actamat.2012.08.038 · 3.94 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 · 1.21 Impact Factor
  • Liyun Zheng, Baozhi Cui, Lixin Zhao, Wanfeng Li, George C. Hadjipanayis
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    ABSTRACT: Synthesis of rare-earth-based magnetically hard single-crystal nanoparticles with significant magnetic properties and large quantities is highly challenging. Surfactant-assisted high energy ball milling (HEBM) is usually used for the preparation of separated Sm–Co-based hard nanoparticles. In this study, Sm2Co17 hard nanoparticles have been synthesized by the surfactant-assisted HEBM SmCo5 precursors. Phases, microstructure, particle sizes and magnetic properties of the synthesized Sm2Co17 nanoparticles have been investigated in detail. The results showed that the precursors had great effects on the microstructure of the synthesized Sm2Co17 nanoparticles. The Sm2Co17 nanoparticles prepared by one-step HEBM of ingot powders in heptane for 5 h using oleic acid as surfactant had a single-crystal and a coercivity of 8.3 kOe. The majority of the Sm2Co17 nanoparticles synthesized using oleylamine as a surfactant showed single-crystal, accompanied by the presence of a few of polycrystalline. In the case of two-step milling (initial low energy ball milling of ingot powders for 18 h and then HEBM for 5 h), the obtained nanoparticles were polycrystalline and showed a low coercivity. When the precursor was jet-milled SmCo5 powders, the synthesized Sm2Co17 nanoparticles had both single-crystal and polycrystalline nanostructures.
    Journal of Alloys and Compounds 10/2012; 539:69–73. DOI:10.1016/j.jallcom.2012.06.011 · 2.73 Impact Factor
  • Liyun Zheng, Baozhi Cui, Lixin Zhao, Wei Li, Minggang Zhu, George C. Hadjipanayis
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    ABSTRACT: Separated core/shell SmCo5/Sm2O3 composite magnetic nanoparticles have been fabricated by a mechanochemical process, heat treatment and washing in this study. Phases, microstructure, particle sizes, and magnetic properties of the as-synthesized SmCo5 and SmCo5/Sm2O3 nanoparticles and the influence of annealing temperature and time have been investigated. The results showed that the annealing temperatures had great effects on particle sizes and magnetic properties while annealing time had a little effect on the particle size of the annealed SmCo5 nanoparticles. The average particle sizes of the annealed SmCo5 nanoparticles changed from 176 to 362 nm when the annealing temperatures increased from 450 to 650 °C and annealing time was 1 h. The coercivity of the synthesized SmCo5 nanoparticles annealed at 450 °C for 1 h was 21 kOe. It increased with the increase of annealing temperatures and reached a maximum value of 35.5 kOe at 600 °C and then decreased. It is interesting to find that the core/shell SmCo5/Sm2O3 hard magnetic nanoparticles were formed after washing with ethanol and acetic acid aqueous solution. Magnetic measurements of these particles exhibited a higher coercivity of 23.7 kOe. The SmCo5 core reserves a single-crystal structure and a 68–75 nm size while the Sm2O3 has a thickness of 5 nm.
    Journal of Nanoparticle Research 09/2012; 14(9). DOI:10.1007/s11051-012-1129-5 · 2.28 Impact Factor
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    ABSTRACT: Processes of high-energy ball milling of SmCo5 alloys were compared for three single-liquid environments without using additional surfactants. Both coarsely grained as-cast and nanocrystalline pre-milled SmCo5 precursors showed tendency toward formation of thin flakes if milled in polar liquids (acetone and ethanol) in a marked contrast to milling in non-polar heptane. CaF2 dielectric powder added prior to milling in the polar liquids tends to become attached on the flake surfaces. Milling in heptane in the presence of CaF2 produces flake-like SmCo5 particles which with increasing the milling time are found to incorporate an increasing amount of CaF2. The SmCo5—5 wt% CaF2 mixtures milled for the optimum time in both the polar and non-polar liquids were successfully hot-pressed into laminated composite magnets having intrinsic coercivity of 25–30 kOe, maximum energy product of approximately 6.5 MG Oe and electrical resistivity of 500–600 μΩ cm, which is more than 7 times the resistivity of conventional Sm–Co magnets.
    Journal of Magnetism and Magnetic Materials 09/2012; 324(18):2879–2884. DOI:10.1016/j.jmmm.2012.04.033 · 2.00 Impact Factor
  • B. Balamurugan, D. J. Sellmyer, G. C. Hadjipanayis, R. Skomski
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    ABSTRACT: Magnetic nanoparticles smaller than ∼15 nm in diameter and with high magnetocrystalline anisotropies K1 ⩾ 1 MJ m−3 can be used as building blocks for next-generation permanent magnets. Advances in processing steps are discussed, such as self-assembly, alignment of the easy axes and appropriate nanostructuring that will enable the fabrication of densely packed nanoparticle assemblies with improved permanent-magnet properties. This study also proposes an idealized nanocomposite structure for nanoparticle-based future permanent magnets with enhanced energy products.
    Scripta Materialia 09/2012; 67(6):542–547. DOI:10.1016/j.scriptamat.2012.03.034 · 2.97 Impact Factor
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    Angshuman Pal ⇑, Alexander Gabay, George C. Hadjipanayis
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    ABSTRACT: High coercivity nanocrystalline Nd2Fe14B particles were synthesized by mechanochemical processing involving the reduction of Nd2O3 with Ca in presence of Fe–B and Fe. The as-milled powder shows amorphous structure and subsequent heat treatment results in formation of nanocrystalline Nd2Fe14B. The resulting powder does not only consist of Nd2Fe14B but also some Nd-rich phase. The optimally heat treated sample exhibits coercivity of more than 12 kOe.
    Journal of Alloys and Compounds 07/2012; 543. DOI:10.1016/j.jallcom.2012.07.114 · 2.73 Impact Factor
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    ABSTRACT: A detailed study is presented on Fe/γ-Fe2O3 core-shell structured nanoparticles (mean size ∼10 nm) to understand the spin dynamics of the core and shell independently and their role in triggering exchange bias (EB) phenomena. The particle dynamics critically slow down at Tg ∼ 68 K, below which they exhibit memory effect in field-cooled and zero-field-cooled protocols associated with a superspin glass state. The field dependence of mean blocking temperature fits the de Almeida-Thouless line and shows two different linear responses in the low and high field regimes corresponding to the core and shell, respectively. We show that the energy barrier distribution estimated from the temperature decay of isothermal remanent magnetization shows two maxima that mark the freezing temperatures of the core (Tf-cr ∼ 48 K) and shell (Tf-sh ∼ 21 K). Last, hysteresis measurements after field cooling reveal strong EB indicated by a loop shift associated with unidirectional anisotropy. The onset of EB is at 35 K when the ferromagnetic core is frozen and the moments in the ferrimagnetic shell begin to block, resulting in enhanced exchange coupling.
    Physical review. B, Condensed matter 07/2012; 86(1). DOI:10.1103/PhysRevB.86.014426 · 3.66 Impact Factor
  • Ozan Akdogan, Wanfeng Li, George Hadjipanayis
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    ABSTRACT: In our research work to study the spinodal decomposition of Alnico thin films prepared by sputtering on Si substrates, it has been discovered that Si diffuses into the films and gives rise to a new magnetically hard phase with T C = 305 °C. As-made thin films with the “α phase” of the Alnico were heat treated by annealing at different temperatures in the range of 600–900 °C. The heat treatment gave a room temperature coercivity of 6.5 kOe after annealing at 900 °C. The maximum coercivity observed is approximately ten times larger than the bulk Alnico V value. Planar electron diffraction patterns can be mostly indexed to an FCC spinel phase with a = 7.79 Å. On the other hand, 80 nm deep inside the film a separate layer is observed with smaller grains having BCT structure with lattice constants a = 2.65 and c = 3.19 Å. We strongly believe that this phase is related to the observed high coercivities in the samples. This discovery is very important in view of the recent “rare earth problem” and may lead to alternative to rare earth materials for the development of high performance magnets.
    Journal of Nanoparticle Research 06/2012; 14(6). DOI:10.1007/s11051-012-0891-8 · 2.28 Impact Factor
  • Energy Technology 2012: Carbon Dioxide Management and Other Technologies, 05/2012;
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    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. DOI:10.1016/j.jmmm.2011.11.049 · 2.00 Impact Factor

Publication Stats

9k Citations
1,247.54 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
  • 1992–2001
    • University of Ioannina
      • Department of Physics
      Yannina, Epirus, Greece
    • University of Missouri
      • Department of Chemistry
      Columbia, Missouri, United States
  • 1984–2000
    • Kansas State University
      • • Department of Physics
      • • Department of Chemistry
      Manhattan, KS, United States
  • 1999
    • Complutense University of Madrid
      Madrid, Madrid, Spain
  • 1988
    • Carnegie Mellon University
      • Department of Materials Science and Engineering
      Pittsburgh, Pennsylvania, United States