Browsing by Author "Krishnamurthy, VV"

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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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    Evidence for a magnetic moment on Ir in IrMnAl from x-ray magnetic circular dichroism
    (American Physical Society, 2003-12-10) Krishnamurthy, VV; Kawamura, N; Suzuki, M; Ishikawa, T; Mankey, GJ; Raj, P; Sathyamoorthy, A; Joshi, AG; Malik, SK; RIKEN; University of Alabama Tuscaloosa; Japan Synchrotron Radiation Research Institute; Bhabha Atomic Research Center (BARC); Tata Institute of Fundamental Research (TIFR)
    We have observed x-ray magnetic circular dichroism (XMCD) in the helicity modulation mode at Ir L-2,L-3 edges in the ferromagnet IrMnAl (Curie temperature 379 K) at room temperature, 100 K and 30 K. This observation proves that Ir has a 5d magnetic moment in IrMnAl. Using the magneto-optic sum rules, which relate the integrated intensity of the XMCD and x-ray absorption spectra to the expectation values of spin and orbital angular momenta of Ir, the orbital moment to spin moment ratio, the spin moment and the orbital moment of Ir at 30 K have been deduced to be -0.17(2), 0.018(1)mu(B), and -0.0031(8)mu(B), respectively. dc magnetization measurements yielded a total moment of 0.123mu(B)/atom showing that the magnetic moment of Mn is also strongly reduced. We suggest that the extremely small moments of Mn and Ir in IrMnAl are associated with the suppression of local moments and delocalization of 3d and 5d electrons due to their strong hybridization with the sp electrons of Al atoms.
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    Growth and characterization of epitaxial FexPt100-x films on MgO(111)
    (AVS American Institute of Physics, 2005-06-24) Mani, P; Krishnamurthy, VV; Maat, S; Kellock, AJ; Robertson, JL; Mankey, GJ; University of Alabama Tuscaloosa; International Business Machines (IBM); United States Department of Energy (DOE); Oak Ridge National Laboratory
    FexPt100-x films were grown on MgO(111) by co-sputtering Fe and Pt. Composition of the films was determined by Rutherford backscattering spectrometry with an accuracy of 1%. Epitaxy and alloy ordering were quantified by x-ray diffraction and the order parameter was determined to be 0.97 for a film with x=30 and 0.99 for a film with x=25. Neutron diffraction measurements established the presence of an antiferromagnetic phase at T= 100 K in 500 nm FePt3 samples grown on MgO(111). Since FePt3 can be grown as an ordered antiferromagnet and a disordered ferromagnet, these films provide a pathway to grow lattice matched interfaces for exchange bias studies. (c) 2005 American Vacuum Society.
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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.
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    Spin wave excitations in the antiferromagnetic state of Pr0.5Sr0.5MnO3
    (American Institute of Physics, 2004-05-25) Krishnamurthy, VV; Robertson, JL; Mankey, GJ; Cavadini, N; Niedermayer, C; Mitchell, JF; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Oak Ridge National Laboratory; Swiss Federal Institutes of Technology Domain; ETH Zurich; Paul Scherrer Institute; Argonne National Laboratory
    The layered A-type antiferromagnet Pr0.5Sr0.5MnO3 has been investigated by neutron scattering. Antiferromagnetic ordering is observed with a T-N of 152.3 K and an exponent beta of 0.148+/-0.01 that suggests 2d Ising behavior. The spin-wave excitations, measured for the neutron momentum transfer q perpendicular to the ferromagnetic planes at 2 K, show a gap of 2.3+/-0.2 meV at the zone center [0.5 0 0.5]. The dispersion near zone center yielded the spin-wave stiffness coefficient D-SW of similar to100+/-10 meV A(2) which is twice the value found in the isostructural A-type antiferromagnet LaMnO3. The double-exchange interaction, introduced by hole doping, enhances D-SW in Pr0.5Sr0.5MnO3. The spin wave dispersion at low q fits the dispersion of the Heisenberg model with nearest neighbor exchange interaction and single ion anisotropy. (C) 2004 American Institute of Physics.