G. C. Hadjipanayis

University of Delaware, Delaware, United States

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Publications (666)1136.48 Total impact

  • [Show abstract] [Hide abstract]
    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.
  • 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.
  • 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.
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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.
  • 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.
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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. · 2.21 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 01/2012; 14(6). · 2.18 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). · 2.21 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
  • 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. · 3.84 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; · 2.21 Impact Factor
  • A. M. Gabay, M. Marinescu, W. F. Li, J. F. Liu, G. C. Hadjipanayis
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    ABSTRACT: Dysprosium-added sintered magnets were prepared from blends of Nd15.5(Fe,Co,Ga)78.2B6.3 and Dy2S3 powders; their microstructure and magnetic properties were compared to those of the magnets made with Dy2O3 additions or from single (Nd,Dy)-(Fe,Co,Ga)-B alloys. The addition of Dy2S3 leads to replacement of the neodymium oxide phases in the sintered magnets by the Nd2O2S and NdS phases. The magnets prepared with both the Dy2S3 and Dy2O3 powders exhibited inhomogeneous distribution of Dy within the (Nd,Dy)2Fe14B grains with Dy-rich outer grain regions. However, in a marked difference from the Dy2O3-added and single-alloy magnets, where the grain-boundary oxide phases were Dy-rich, the magnets prepared with Dy2S3 had their sulfur-containing grain-boundary phases depleted of Dy. With the larger fraction of Dy atoms available for alloying the main (Nd,Dy)2Fe14B phase, the magnets prepared with Dy2S3 showed the largest coercivity gain per 1 at.% of the added Dy.
    Journal of Applied Physics 04/2011; 109(8):083916-083916-5. · 2.21 Impact Factor
  • Nilay G. Akdogan, Wanfeng Li, George C. Hadjipanayis
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    ABSTRACT: Anisotropic Nd2Fe14B nanoparticles and nanoflakes have been produced by surfactant-assisted high-energy ball milling (SA-HEBM) of precursor nanocrystalline alloys. A two-stage high-energy ball milling was performed to obtain the nanopowders and nanoparticles; first the coarse powders were subjected to a wet milling followed by a wet surfactant-assisted milling. Different shaped nanoparticles have been obtained by varying the time of the first stage of the milling process and then separated by sonication. For a surfactant-free wet milling of 4 h, followed by the SA-HEBM, the nanopowders consisted of a mixture of Nd2Fe14B flakes with a thickness below 200 nm and an aspect ratio as high as 102–103, and anisotropic square nanoparticles with a size of 10 nm. However, for a shorter wet milling, nearly spherical nanoparticles with a size of 2.7 nm were obtained. Low-temperature coercivities have been obtained with maximum values of 4 kOe for square nanoparticles and 2.5 kOe for the nearly spherical nanoparticles.
    Journal of Applied Physics 04/2011; 109(7):07A759-07A759-3. · 2.21 Impact Factor
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    ABSTRACT: Magnetically isotropic Sm–Co permanent magnets with anisotropic electrical conductivity were produced by hot-pressing SmCo5 ultrathin flakes with or without insulating additions of mica, MoS2, CaF2, and B2O3. The laminated structures of the magnets were assured by the natural packing of the flakes in the die. In the additive-free magnets, the resistivity is increased due to oxidized flake boundaries. Additives with platelet-shaped particles (mica, MoS2) and especially those with low-melting temperature (B2O3) form more continuous insulating layers. On the other hand, the CaF2 additive does not react with the magnetic Sm–Co phase. The characteristic values of the maximum energy product and electrical resistivity, measured in orthogonal directions, were 9.7 MG Oe and 130 μΩ cm for the additive-free magnet, 5.7 MG Oe and 640 μΩ cm for the magnet made with 2 wt. % B2O3 and 5.7 MG Oe and 330 μΩ cm for the magnet made with 10 wt. % CaF2.
    Journal of Applied Physics 03/2011; 109(7):07A719-07A719-3. · 2.21 Impact Factor
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    ABSTRACT: Nanoparticles of YCo2, YCo3, and YCo5 are produced by cluster-deposition and investigated bothstructurally and magnetically. The nanoparticles have sizes of less than 10 nm and aresuperparamagnetic at 300 K, irrespective of stoichiometry. As-produced nanoparticles exhibitdisordered structures with magnetic properties differing from those of the bulk particles.The temperature-dependent magnetization curves of the nanoparticles reveal blocking temperatures from 110 to 250 K, depending on stoichiometry. The magnetic anisotropy constant K1 of disordered YCo5 nanoparticles of 7.8 nm in size is 3.5 × 106 ergs/cm3, higher than those of thedisordered YCo2 (8.9 × 105 ergs/cm3) and YCo3 (1.0 × 106 ergs/cm3) nanoparticles.
    Journal of Applied Physics 03/2011; 109(7):07A707-07A707-3. · 2.21 Impact Factor
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    ABSTRACT: Rare-earth transition-metal (R-TM) alloys show superior permanent magnetic properties in the bulk, but the synthesis and application of R-TM nanoparticles remains a challenge due to the requirement of high-temperature annealing above about 800 °C for alloy formation and subsequent crystalline ordering. Here we report a single-step method to produce highly ordered R-TM nanoparticles such as YCo(5) and Y(2)Co(17), without high-temperature thermal annealing by employing a cluster-deposition system and investigate their structural and magnetic properties. The direct ordering is highly desirable to create and assemble R-TM nanoparticle building blocks for future permanent-magnet and other significant applications.
    Nano Letters 02/2011; 11(4):1747-52. · 13.03 Impact Factor
  • B. Z. Cui, W. F. Li, G. C. Hadjipanayis
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    ABSTRACT: Surfactant-assisted high-energy ball milling (HEBM) of malleable materials has been found to lead to the formation of flakes. However, SmCo5 magnetic materials are inherently brittle and, therefore, are not expected to “flake” during ball milling. In this study, we report the fabrication of high-aspect-ratio single-crystal flakes with a submicron thickness and textured polynanocrystalline flakes with a submicron or nanosize thickness of SmCo5 by one-step surfactant-assisted HEBM in heptane with 15wt.% oleic acid. Single-crystal micron flakes were first formed via a basal cleavage along the easy glide (001) planes of the initial irregular and large single-crystal particles during the first stage of HEBM. Subsequently, single-crystal submicron flakes were formed by continuous cleavage. With further ball milling, polycrystalline submicron flakes with small-angle grain boundaries were formed. Finally, crystallographically anisotropic polycrystalline SmCo5 nanoflakes were formed with [001]-out-of-plane texture, thicknesses of 6–80nm, average grain sizes of 7–8nm, an aspect ratio of 102–103 and coercivities of 16.3–17.7 kOe. The mechanism responsible for unusual formation of single-crystal submicron flakes and anisotropic nanocrystalline nanoflakes from initial inherently brittle polycrystalline ingots with irregular grains of tens of microns in size is discussed.
    Acta Materialia - ACTA MATER. 01/2011; 59(2):563-571.
  • B. Z. Cui, L. Y. Zheng, D. Waryoba, M. Marinescu, G. C. Hadjipanayis
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    ABSTRACT: This work is focused on the evolution of structure, texture and magnetic properties in SmCo5 powders during high energy ball milling (HEBM) in heptane with and without oleic acid (OA). Single-crystal micron SmCo5 flakes were formed in the early stage of HEBM (0-0.5 h). However, HEBM of 1-2 h without OA led to the formation of a mixture of single-crystal microflakes and loosely agglomerated polycrystalline microparticles with the highest coercivity of 15.7 kOe. HEBM of 3-5 h without OA led to the formation of isotropic, more or less equiaxed SmCo5 particles with a size of 2-30 mum and a polycrystalline nanostructure due to the coalescence of flakes and particles. In the case of HEBM in heptane with 15 wt. % OA, prolonged milling (from 1 to 5 h) led first to the formation of SmCo5 single-crystal submicron flakes and later to submicron- and nanosize-nanocrystalline flakes with a [001]-out-of-plane texture. OA plays an essential role in the formation of anisotropic SmCo5 flakes via the impediment of cold welding and agglomeration of flakes. The highest coercivity of 17.7 kOe was obtained in a flake sample milled for 5 h with a thickness of 8-80 nm, an average grain size of 8 nm and an aspect ratio of 102-103.
    Journal of Applied Physics 01/2011; 109. · 2.21 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 01/2011; 48(8):36-41. · 1.32 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 01/2011; 323(20):2470-2473. · 2.00 Impact Factor

Publication Stats

4k Citations
1,136.48 Total Impact Points


  • 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