Browsing by Author "Zoto, I"

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    Antiferromagnetic phase transitions in an ordered Pt3Fe(111) film studied by neutron diffraction
    (American Physical Society, 2004-07-30) Krishnamurthy, VV; Zoto, I; Mankey, GJ; Robertson, JL; Maat, S; Fullerton, EE; Nwagwu, I; Akujieze, JK; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Oak Ridge National Laboratory; Chicago State University
    Neutron diffraction has been used to investigate the critical behavior at the onset of antiferromagnetic phase transitions in a (111) oriented Pt73Fe27 film grown on an a-axis oriented sapphire (alpha-Al2O3) substrate. As in the bulk, there is an antiferromagnetic reorientation transition from the Q(1)=2pi/a(1/2,1/2,0) phase to the Q(2)=2pi/a(1/2,0,0) phase upon cooling. The temperature dependence of the integrated intensity of the (1/2,1/2,0) and the (1/2,0,0) antiferromagnetic Bragg peaks yielded the Neeel temperature of 160.25+/-0.2 K and a reorientation transition temperature of 95+/-0.2 K. The magnetization critical exponent beta is found to be 0.368+/-0.013 for the Q(1) phase and 0.37+/-0.02 for the Q(2) phase. These critical exponents are in excellent agreement with the predictions of the 3d Heisenberg universality class. A comparison of the transition temperatures and the exponents in the film and in single crystal at the same alloy composition is presented.
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    Shear- and magnetic-field-induced ordering in magnetic nanoparticle dispersion from small-angle neutron scattering
    (American Physical Society, 2003-05-23) Krishnamurthy, VV; Bhandar, AS; Piao, M; Zoto, I; Lane, AM; Nikles, DE; Wiest, JM; Mankey, GJ; Porcar, L; Glinka, CJ; University of Alabama Tuscaloosa; National Institute of Standards & Technology (NIST) - USA
    Small-angle neutron scattering experiments have been performed to investigate orientational ordering of a dispersion of rod-shaped ferromagnetic nanoparticles under the influence of shear flow and static magnetic field. In this experiment, the flow and flow gradient directions are perpendicular to the direction of the applied magnetic field. The scattering intensity is isotropic in zero-shear-rate or zero-applied-field conditions, indicating that the particles are randomly oriented. Anisotropic scattering is observed both in a shear flow and in a static magnetic field, showing that both flow and field induce orientational order in the dispersion. The anisotropy increases with the increase of field and with the increase of shear rate. Three states of order have been observed with the application of both shear flow and magnetic field. At low shear rates, the particles are aligned in the field direction. When increasing shear rate is applied, the particles revert to random orientations at a characteristic shear rate that depends on the strength of the applied magnetic field. Above the characteristic shear rate, the particles align along the flow direction. The experimental results agree qualitatively with the predictions of a mean field model.
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    Sintering behavior of spin-coated FePt and FePtAu nanoparticles
    (American Institute of Physics, 2006-04-19) Kang, SS; Jia, Z; Zoto, I; Reed, D; Nikles, DE; Harrell, JW; Thompson, G; Mankey, G; Krishnamurthy, VV; Porcar, L; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Oak Ridge National Laboratory; National Institute of Standards & Technology (NIST) - USA
    FePt and [FePt](95)Au-5 nanoparticles with an average size of about 4 nm were chemically synthesized and spin coated onto silicon substrates. Samples were subsequently thermally annealed at temperatures ranging from 250 to 500 degrees C for 30 min. Three-dimensional structural characterization was carried out with small-angle neutron scattering (SANS) and small-angle x-ray diffraction (SAXRD) measurements. For both FePt and [FePt](95)Au-5 particles before annealing, SANS measurements gave an in-plane coherence length parameter a=7.3 nm, while SAXRD measurements gave a perpendicular coherence length parameter c=12.0 nm. The ratio of c/a is about 1.64, indicating the as-made particle array has a hexagonal close-packed superstructure. For both FePt and FePtAu nanoparticles, the diffraction peaks shifted to higher angles and broadened with increasing annealing temperature. This effect corresponds to a shrinking of the nanoparticle array, followed by agglomeration and sintering of the nanoparticles, resulting in the eventual loss of positional order with increasing annealing temperature. The effect is more pronounced for FePtAu than for FePt. Dynamic coercivity measurements show that the FePtAu nanoparticles have both higher intrinsic coercivity and higher switching volume at the same annealing temperature. These results are consistent with previous studies that show that additive Au both lowers the chemical ordering temperature and promotes sintering. (C) 2006 American Institute of Physics.
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    Sintering behavior of spin-coated FePt and FePtAu nanoparticles
    (American Institute of Physics, 2006-04-19) Kang, SS; Jia, Z; Zoto, I; Reed, D; Nikles, DE; Harrell, JW; Thompson, G; Mankey, G; Krishnamurthy, VV; Porcar, L; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Oak Ridge National Laboratory; National Institute of Standards & Technology (NIST) - USA
    FePt and [FePt](95)Au-5 nanoparticles with an average size of about 4 nm were chemically synthesized and spin coated onto silicon substrates. Samples were subsequently thermally annealed at temperatures ranging from 250 to 500 degrees C for 30 min. Three-dimensional structural characterization was carried out with small-angle neutron scattering (SANS) and small-angle x-ray diffraction (SAXRD) measurements. For both FePt and [FePt](95)Au-5 particles before annealing, SANS measurements gave an in-plane coherence length parameter a=7.3 nm, while SAXRD measurements gave a perpendicular coherence length parameter c=12.0 nm. The ratio of c/a is about 1.64, indicating the as-made particle array has a hexagonal close-packed superstructure. For both FePt and FePtAu nanoparticles, the diffraction peaks shifted to higher angles and broadened with increasing annealing temperature. This effect corresponds to a shrinking of the nanoparticle array, followed by agglomeration and sintering of the nanoparticles, resulting in the eventual loss of positional order with increasing annealing temperature. The effect is more pronounced for FePtAu than for FePt. Dynamic coercivity measurements show that the FePtAu nanoparticles have both higher intrinsic coercivity and higher switching volume at the same annealing temperature. These results are consistent with previous studies that show that additive Au both lowers the chemical ordering temperature and promotes sintering. (C) 2006 American Institute of Physics.