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| 32. |
- Blixt, Anna Maria, 1969-
(författare)
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Growth and Magnetic Properties of Fe- and FeNi-based Thin Films and Multilayers
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns the growth and magnetic properties of thin films and multilayers. The samples were grown by magnetron sputtering, and characterized structurally mainly by x-ray diffraction and reflectivity. The magnetic characterization of the multilayers was done by magneto-optical Kerr technique, SQUID magnetometry and, in two samples, by neutron reflectometry.Arrays of small elements of polycrystalline permalloy (FeNi alloy with 19 wt% Fe) are of interest as a component in non-volatile magnetic random access memories (MRAM). Here the shape dependence of the domain structure in such elements was studied by magnetic force microscopy (MFM) and in thin ring magnets the 'onion' state could be seen for the first time. Also, by post-annealing in hydrogen atmosphere the number of domains decreased in each element due to enhanced relaxation and defect reduction.Furthermore, permalloy-based anisotropic magnetoresistance (AMR) in read heads are nowadays replaced by material combinations that have a giant magnetoresistance (GMR) effect. In this work Fe/V(001) and Fe0.82Ni0.18/V(001) superlattices, i.e. single-crystal-like multilayers, were investigated. These systems showed much smaller GMR effect compared to the Fe/Cr system. However, by introducing Ni into the Fe layers the magnetic anisotropy and the interlayer exchange coupling (IEC) decreased, thereby increasing the sensitivity, which is a key property for a magnetic sensor. The interface region showed a reduced magnetic moment, and the influence of the structural quality was modelled and investigated theoretically in the Fe0.82Ni0.18/V case. Also, in the Fe(2-3 ML)/V(x ML) superlattices (ML=monolayers) the transition temperature from long-range magnetic order to paramagnetic order oscillated with the V layer thickness (x) as a result of the oscillatory behaviour of the IEC.The introduction of hydrogen in the non-magnetic layers of, for example, Fe/V(001) superlattices is a way to tune the IEC strength. Here the tuning was used as a tool to study the magnetic order in a low-dimensional magnet. At the critical hydrogen concentration =0.022 the Fe layers in an Fe(2 ML)/V(13 ML) superlattice became decoupled. Then the system behaved as a two-dimensional Ising magnet with a finite ordering temperature of about 60 K.
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| 33. |
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| 34. |
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| 35. |
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| 36. |
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| 37. |
- Bloch, J., et al.
(författare)
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Hydrogen-vanadium system in thin films : Effect of film thickness
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:24, s. 245428-
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Tidskriftsartikel (refereegranskat)abstract
- The absorption of hydrogen in thin V(001) films under pressures of 1-10(5) Pa H-2 and at temperatures between 350 and 530 K was studied as a function of film thickness between 50 and 5 nm using in situ electrical resistivity measurements. The critical temperatures for the order-disorder transitions taking place in the V-H system are decreased with decreasing film thickness. At 370 K, the high-concentration (epsilon-VH) phase disappears as the thickness of the film is reduced from 50 to 10 nm and the low-concentration (beta-V2H) phase follows when the film thickness is further decreased to 5 nm. The difference in solubility at 530 K of H in the alpha phase of V films down to 10 nm is small but it increases for lower temperatures. At low concentrations the heat of solution in the 10 nm film is somewhat lower than in the bulk, but around H/V = 0.07, the values of Delta H-H in the film approach those of the bulk. The values of Delta S-H(nc) of the film are rather close to those of the bulk. Significant difference is found in the pressure-resistivity isotherms above the maximum of the residual resistivity, Delta R-max. For the 10 nm film, in contrast to the 50 nm film, a minimum is located just beyond Delta R-max for the whole temperature range but the difference between the maximum and minimum starts to decrease above 420 K. This results in, by extrapolation, the vanishing of the maximum around 520 K. We suggest that this phenomenon is the result of an Oz -> T transition due to a final size effect. This transition means additional randomly distributed interstitial sites available for the H atoms in the lattice of the V film leading to a continuous increase in the residual resistivity with H concentration.
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| 38. |
- Bloch, J., et al.
(författare)
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Prediction and Hydrogen Acceleration of Ordering in Iron-Vanadium Alloys
- 2012
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 108:21, s. 215503-
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio calculations of binary metallic systems often predict ordered compounds in contrast to empirical reports of solid solutions or disordered phases. These discrepancies are usually attributed to slow kinetics that retains metastable structures at low temperatures. The Fe-V system is an example of this phenomenon, in which we predict two ordered stable ground states, Fe3V and FeV3, whereas a disordered sigma phase is reported. We propose to overcome this difficulty by hydrogen absorption, which facilitates metal atom mobility through vacancy formation and separation between the two elements due to their opposite affinities towards it, thus accelerating transformation kinetics. Hydrogen also increases the relative stability of the ordered structures compared with that of the sigma phase without affecting the shape of the phase diagram. The hydrogen-induced formation of the ordered structures is expressed by a reversible decrease of the electrical resistivity with increasing hydrogen pressure. Such behavior has not been reported before in thin H absorbing films. Formation of the ordered structures is further substantiated by the kinetics of the resistivity changes upon variation of the hydrogen pressure, where two stages are distinguished: a fast initial stage and a much slower subsequent process in which the resistivity changes direction, associated with hydrogen dissolution and phase transformation, respectively.
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| 39. |
- Bloch, J., et al.
(författare)
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Reversible structural change and thermodynamic properties of hydrogen in thin vanadium films
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:16, s. 165418-
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Tidskriftsartikel (refereegranskat)abstract
- The absorption of H in 50nm V(001) films under pressures of 1–104Pa H2 and at temperatures between 310 and 530K was studied using in situ combined x-ray diffraction (XRD) and electrical resistivity measurements. The H-induced XRD peak shifts clearly indicate a fully reversible and nondeteriorating first-order phase transformation for the film. The H-induced residual resistivity changes in the V film demonstrate several extreme points (maxima and minima) apparently associated with some special points on the pressure isotherms of the film both below and above the critical temperature (Tc=460±20K) . Correlating these extreme points with the known V(bulk)-H phase diagram indicates that, in addition to the first-order transformation, two additional reversible second-order transitions exist in the film. The enthalpy and entropy of this transformation, obtained from the temperature dependence of the plateau pressures, are ‑39±1kJ/mol°K , and ‑60±2J/mol , respectively. The thermodynamic constants and the lattice parameters of the phases for the first-order transformation in the film closely correspond to the α→β phase transformation in bulk V.
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| 40. |
- Blomqvist, Andreas, et al.
(författare)
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Significance of self-trapping on hydrogen diffusion
- 2010
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 105:18, s. 185901-
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Tidskriftsartikel (refereegranskat)abstract
- The diffusion rate of hydrogen in Nb was calculated using ab initio molecular dynamics simulations. At low temperatures the hydrogen is strongly trapped in a local strain field which is caused by the elastic response of the lattice. At elevated temperatures, the residence time (τ) of hydrogen in an interstitial site is not sufficient for fully developing the local strain field. This unbinding of the interstitial hydrogen and the strain field increases the hopping rate (1/τ) at elevated temperatures (>400 K). These results call for a revision of the conceptual framework of diffusion of hydrogen in transition metals at elevated temperatures.
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