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Browsing by Author "Maat, S"

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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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    Antiferromagnetic structure of FePt3 films studied by neutron scattering
    (American Physical Society, 2001-03-14) Maat, S; Hellwig, O; Zeltzer, G; Fullerton, EE; Mankey, GJ; Crow, ML; Robertson, JL; International Business Machines (IBM); University of Alabama Tuscaloosa; United States Department of Energy (DOE); Oak Ridge National Laboratory
    The magnetism of antiferromagnetic FexPt1-x thin films (x = 0.27 and 0.30) epitaxially grown onto MgO (110) and a-axis sapphire (alpha -Al2O3) substrates has been studied by elastic neutron and x-ray scattering. Bulk chemically ordered FePt3 exhibits an antiferromagnetic spin structure with a wave vector Q(1) =2 pi /a(1/2 1/2 0) below T(N1)similar to 160 K. For slightly Fe-rich alloys (x greater than or similar to0.26) a spin-reorientation transition ti, a second antiferromagnetic phase with a wave vector Q(2) = 2 pi /a(1/2 00) occurs below T(N2)similar to 100 K at the expense of Q(1). For increased Fe content (x similar to0.30) the Q(1) phase is strongly suppressed with a dominant Q(2) phase. For (111)-oriented films grown on a-axis sapphire the spin structure is the same as that found in the bulk. The a = 0.27 film exhibits transitions at T-N1 and T-N2. The film with x = 0.30 exhibits an almost completely suppressed Q(1) phase and a dominant Q(2) phase with an enhanced ordering temperature of T(N2)similar to 140 K. In contrast FePt3 (110) films grown onto MgO (110) exhibit only the el phase for both compositions x = 0.27 and x = 0.30 with the onset of the Q(2) phase suppressed. The distinct behavior of the films grown onto MgO from those grown onto a-axis sapphire and bulk FePt3 may be explained by higher strain and defect densities incorporated in the films grown onto MgO.
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    Fourier analysis of exchange biased Ni80Fe20/Fe50Mn50/Ni80Fe20 trilayers
    (American Physical Society, 1999-10-01) Maat, S; Shen, L; Mankey, GJ; University of Alabama Tuscaloosa
    Hysteresis loops of polycrystalline and single-crystal exchange biased Ni80Fe20 (Permalloy)/ Fe50Mn50/Ni80Fe20 (Permalloy) trilayers were measured as a function of Fe50Mn50 layer thickness with the longitudinal Kerr effect. The variation of the macroscopic pinning field H-p and the coercivity H-c was observed over a full 360 degrees in plane rotation and Fourier analyzed. The magnetization behavior of both Permalloy layers of the polycrystalline samples was analyzed, and it was found that the pinning field of the bottom layer is always greater than for the top layer, while the situation is reversed for the coercivity due to defects incorporated in the antiferromagnetic layer. The single-crystal samples were prepared on epitaxially grown Cu(111) on Si(110), and the magnetization behavior of the top Permalloy layer was studied. In contrast to the polycrystalline samples, the coercivities peak very sharply at the easy axis, which manifests itself in large higher-order Fourier coefficients. Coercivities and loop shifts show a strong linear dependence on the antiferromagnetic layer thickness. The relation of the biasing direction to the crystal axes had no influence on H-c acid H-p. This behavior is attributed to complete strain relief through the buffer layer and better crystalline growth of the trilayer as compared to the polycrystalline samples. Examination of the results with a Ginzburg-Landau energy functional verified that the Fourier coefficients obey necessary conditions to achieve energetic stability together with spontaneous magnetization. The energy functional was used to model the angular dependence of the loop shift.
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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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    Growth of ultrathin Co/Cu/Si(110) films
    (American Institute of Physics, 2000-07-10) Maat, S; Liu, C; Eads, W; Umlor, MT; Mankey, GJ; University of Alabama Tuscaloosa
    We report the results of a study of the structural properties of Co films deposited on Cu/H-Si(110). A Cu(lll) buffer layer is formed by evaporation or ultrahigh vacuum sputter deposition on the H-terminated Si(110) surface. From consideration of bulk lattice constants, the Cu films undergo a 6% expansion along the [1, -1, 0] direction and a 13% compression along the [1, 1, -2] direction. The structure and annealing behavior of the Cu buffer layer was determined with a combination of low-energy electron diffraction (LEED) and Auger electron spectroscopy. The LEED patterns of Co films evaporated on this buffer layer are compared to Co films grown on a Cu(lll) single crystal. (C) 2000 American Vacuum Society. [S0734-2101(00)03304-2].
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    Magnetic properties of Co films on Cu/Si(110) and Cu(111)
    (American Physical Society, 2000-02-01) Maat, S; Umlor, MT; Orgassa, D; Cho, HS; Koshkina, O; Fujiwara, H; Mankey, GJ; University of Alabama Tuscaloosa
    The magnetic anisotropy of strained epitaxial Co films deposited on strained Cu(lll) on Si(110) and miscut Cu(lll) are reported. First, a Cu(lll) buffer layer with an in-plane strain is formed by epitaxial growth on Si(110). Co films deposited on the buffer layer develop a uniaxial anisotropy axis originating from the strain along the twofold axis of the Si(110) surface. The magnetic anisotropy of this film is compared to that of a Co film deposited on a miscut Cu(lll) single crystal with uniaxial step-induced anisotropy in the Co layer. The measured hysteresis loops are compared to hysteresis loops simulated within a Stoner-Wolfarth model with a varying fraction of uniaxial and triaxial anisotropies. The measurements and the model are in qualitative agreement and reveal uniaxial and triaxial anisotropies of the same order of magnitude for both systems.
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    Noncollinear magnetism in substitutionally disordered face-centered-cubic FeMn
    (American Institute of Physics, 1999-04-21) Schulthess, TC; Butler, WH; Stocks, GM; Maat, S; Mankey, GJ; United States Department of Energy (DOE); Oak Ridge National Laboratory; University of Alabama Tuscaloosa
    We use first principles electronic structure techniques to study the magnetic structure of gamma-FeMn using the Korringa-Kohn-Rostocker multiple-scattering approach in conjunction with an extension of the single site coherent potential approximation to noncollinear magnetic structures. Our results show that the noncollinear 3Q and 2Q structures are both stable solutions with the former being slightly lower in energy. The collinear solutions could only be converged in a traditional spin-polarized calculation and are unstable in a noncollinear treatment. (C) 1999 American Institute of Physics. [S0021-8979(99)59008-7].
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    Optical interference in magneto-optic Kerr-effect measurements of magnetic multilayers
    (American Institute of Physics, 1999-01-15) Maat, S; Shen, L; Hou, C; Fujiwara, H; Mankey, GJ; University of Alabama Tuscaloosa
    Hysteresis loops of exchange-biased permalloy/Fe50Mn50/permalloy trilayers on glass were measured as a function of Fe50Mn50 and permalloy thicknesses with the longitudinal Kerr effect employing a coherent light source. Kerr signals originate from both permalloy layers and give a superposition of hysteresis loops. In vibrating sample magnetometer or looptracer measurements the contribution of a particular layer to a major hysteresis loop cannot be identified. With the Kerr setup presented it is possible to identify the contribution of each layer individually, since the finite optical path through the trilayer gives rise to optical attenuation and interference. For an increasing total thickness of the trilayer, the signal of the buried permalloy layer will become weaker due to attenuation. Kerr measurements of trilayers up to 40 nm show a superposition of two equally oriented loops. Major loops for trilayers of thickness greater than 40 nm show a superposition of two oppositely oriented hysteresis loops. The transition is dependent only on the total thickness of the trilayer, rather than the thickness of each individual layer. This unusual effect can be explained by the phase difference of the two Kerr signals. Additional measurements performed from the glass side of the samples and measurements of a Fe50Mn50/permalloy bilayer confirm that the sense of a hysteresis loop can change for a buried layer due to optical effects. (C) 1999 American Institute of Physics. [S0021-8979(99)06903-0].
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    Spin-dependent band structure, Fermi surface, and carrier lifetime of permalloy
    (American Institute of Physics, 1998-12-02) Petrovykh, DY; Altmann, KN; Hochst, H; Laubscher, M; Maat, S; Mankey, GJ; Himpsel, FJ; University of Wisconsin System; University of Wisconsin Madison; University of Alabama Tuscaloosa
    Angle-resolved photoemission is used to determine the energy bands of permalloy (Ni0.8Fe0.2) and compare them to Ni, Co, and Cu. The energy and momentum resolution (approximate to 0.01 eV and approximate to 0.01 Angstrom(-1)) is high enough to resolve the magnetically split bands at the Fermi level that are responsible for spin-dependent conductivity and tunneling. For the Sigma(1) band we find the magnetic exchange splittings delta E-ex = 0.27 eV (0.23 eV for Ni), delta k(ex) = 0.16 +/- 0.02 Angstrom(-1) (0.12 +/- 0.01 Angstrom(-1) for Ni), the Fermi velocity upsilon(F up arrow) = (0.22 +/- 0.02) 10(6) m/s (0.28 x 10(6) m/s for Ni, 0.33 x 10(6) m/s for fcc Co), and the widths delta k(up arrow) less than or equal to 0.11 Angstrom(-1) and delta k(down arrow) = 0.24 Angstrom(-1). Compared to Ni, permalloy features a 27% larger magnetic splitting of the Fermi surface and an extremely short mean free path of 4-8 Angstrom for minority spins. (C) 1998 American Institute of Physics. [S0003-6951(98)01249-2].