| 1. |
- Andersson, Gabriella, et al.
(författare)
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Effects of strain on magnetic anisotropy in Fe and Co based heterostructures
- 2008
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Ingår i: Phase Transitions. - : Informa UK Limited. - 0141-1594 .- 1029-0338. ; 81:7 & 8, s. 679-701
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic ordering in transition metals is strongly affected by any strain in the lattice, due to the close connection between strain and anisotropy. This article reviews recent developments, both experimental and theoretical, in the field of strain-induced magnetic anisotropies, with focus on films and multilayers based on Fe and Co. It is demonstrated that the reorientation of preferred magnetization directions can be controlled through both the growth direction and the strain state, which is caused by interleaving with suitable spacers, as well as by tuning the chemical composition if the magnetic material is an alloy. Both in-plane and out-of-plane easy directions can be forced upon the films and superlattices in this manner. Some methods for local structural manipulation are also reviewed.
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| 2. |
- Björck, Matts, et al.
(författare)
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The effect of strain and interfaces on the orbital moment in Fe/V superlattices
- 2007
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 313:1, s. 230-235
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Tidskriftsartikel (refereegranskat)abstract
- The dependence of the Fe orbital moment on strain and interfaces in Fe/V superlattices has been investigated by X-ray magnetic circular dichroism (XMCD). The orbital moment was determined to be lower at the interfaces than in the bulk, which we attribute to Fe–V hybridization. An enhancement of the orbital moment with increasing strain in the Fe layers was observed. This enhancement is attributed to an unquenching of the orbital moment. Consequently, the orbital moment of Fe in Fe/V is concluded to be influenced by two competing parameters. It is lowered by increasing interface density, and enhanced by increasing strain.
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| 3. |
- 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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| 4. |
- Bručas, Rimantas, et al.
(författare)
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Magnetic and transport properties of Ni81Fe19/Al2O3 granular multilayers approaching the superparamagnetic limit
- 2007
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101:7, s. 073907-
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic and transport properties of Ni81Fe 19/Al2O3 granular multilayer films were studied in relation to their structural properties as the nominal thickness t of the permalloy (Ni81Fe19) layer was varied near the percolation limit: in the range of 8≤t≤ 16 Å while keeping the nominal thickness of the Al2O3 layers constant at 16 Å. A good structural quality of the multilayers was demonstrated by low angle x-ray reflectivity measurements, and transmission electron microscopy showed the transition from continuous permalloy layers separated by aluminium oxide layers for t= 16 Å to metal grains dispersed in the insulator at t=8 Å. Magnetization measurements showed the gradual transition from ferromagnetic layers to superparamagnetic clusters and grains that successively become blocked as the temperature decreases. A strong correlation between transport and structural properties was observed in the temperature (T) dependence of the electrical resistance measured with the current in the plane in the range of 2 ≤T≤300 K: a gradual change of behavior from continuous permalloy layers with conducting interlayer connections for t=16 Å. to isolated permalloy grains in a dielectric for the film with t= 10 Å. The percolation occurs between 12 and 10 Å, as deduced both from the magnetic and resistive properties. The discontinuous metal films were analyzed within models for thermally assisted tunneling, yielding estimates of the tunneling barrier for intralayer conduction of about 20 meV for t= 10 Å. A significant magnetic field dependence of the resistance increasing with decreasing temperature was observed in all samples.
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| 5. |
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| 6. |
- Brucas, R., et al.
(författare)
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Magnetic anisotropy and evolution of ground-state domain structures in bcc Fe81Ni19/Co(001) superlattices
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : The American Physical Society. - 1098-0121 .- 1550-235X. ; 78:2, s. 024421-
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic anisotropy and evolution of striped magnetic domain structures in bcc Fe81Ni19/Co(001) superlattices with the total thickness ranging from 85 to 1370 nm has been studied by magneto-optical Kerr effect and magnetic force microscopy. At a thickness of about 85 nm [25 bilayers (BL)] the domains appear as stripe domains, typical for perpendicular anisotropy films, with the weak cubic anisotropy of the in-plane magnetization component stabilizing the stripe direction. The magnetic domain period strongly depends on the thickness of the superlattice. As the thickness increases, the equilibrium magnetization orients at oblique angles with respect to the film plane and continuously varies with the thickness from in-plane to out-of-plane. We first apply a simple phenomenological model which correctly predicts the transition from in-plane to out-of-plane magnetization as well as increasing domain period and saturation field with increasing BL number. The results indicate the presence of partial flux-closure domains at the film surface with the tilt angle continuously varying with the superlattice thickness. By solving a linearized Landau–Lifshitz equation together with Maxwell’s equations in magnetostatic approximation for samples consisting of up to 1000 individual layers, we calculate the spin-wave dispersion and determine the stability conditions for the saturated ferromagnetic state. From these results the dependence of the saturation field on the number of layers is inferred and agrees well with the experiment. The uniaxial bulk anisotropy is attributed to distortions along the c axis and the results further show evidence for the presence of an easy-plane interface anisotropy in these samples.
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| 7. |
- Chacon, C., et al.
(författare)
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Growth and Hydrogen uptake of Mg-Y thin films
- 2005
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 97, s. 104903-
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Tidskriftsartikel (refereegranskat)abstract
- Wedged Mg–Y films with compositions ranging from pure Mg to Mg0.83Y0.17 were grown by dc-magnetron sputtering and hydrogenated. Mg1–xYx forms a substitutional alloy, ranging from 0 to at least 17 at. % in thin films. The c lattice parameter of the film containing 17 at. % of yttrium is determined to be approximatively 1% larger than in pure Mg. Upon exposure to 1 bar of hydrogen at 300 K, the samples switch from shiny metals to colorless semiconductor. Different characteristic hydrogen depth distributions are found for different Y concentrations. At low yttrium contents, a large concentration gradient is observed, with the highest hydrogen concentration close to the Pd/Mg1–xYx interface. For yttrium concentrations larger than 7 at. %, the obtained hydrogen distribution is almost independent of depth. The optical band gap is determined to be 3.6 eV, for all the Y concentrations. The optical transmission is found to decrease for increasing Y content, which is associated with an incomplete hydride formation in the films.
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| 8. |
- Dziekan, Thomas, et al.
(författare)
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Influence of vanadium spin-polarization on the dissolution of hydrogen in vanadium
- 2009
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Ingår i: PHYSICAL REVIEW B. - 1098-0121 .- 1550-235X. ; 79:1, s. 012402-
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Tidskriftsartikel (refereegranskat)abstract
- Transition-metal (TM)/V superlattices (TM=Cr,Fe,Mo) show certain peculiarities under hydrogen uptake. Here we investigate the influence of an induced magnetization of the V layers on the hydrogen dissolution by means of first-principles calculations. We find that below a certain value for the magnetic moment of the V host the hydrogen solubility is slightly reduced, whereas for larger moments the hydrogen dissolution becomes favored. The actual position of this transition depends on the tetragonal distortion of the V layers.
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| 9. |
- Eastwood, D. S., et al.
(författare)
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X-ray scattering from two-dimensionally patterned magnetic thin film nanoscale arrays
- 2007
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Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 41:2-3, s. 163-167
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Tidskriftsartikel (refereegranskat)abstract
- Thin metallic films patterned into nanoscale periodic arrays give rise to coherent satellites on either side of the specular reflection when illuminated at grazing incidence with x-rays. We have used grazing incidence x-ray scattering to probe the crystallography of two-dimensional patterned arrays. Excellent agreement is obtained between experimental scattering profiles and those simulated using a fast Fourier transform method. The technique is appropriate to any shape of patterned element. It is shown that the symmetry of the satellites in an azimuthal rotation map of the scattered intensity provides a sensitive means of measuring symmetry of the two-dimensional order in patterned nanostructures. Furthermore, the in-plane coherence length of the array is found to determine the lineshape of the coherent satellite peaks. We find very good agreement between experimental and simulated rotation maps for the two dimensional array.
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| 10. |
- Eriksson, Anna Karin, et al.
(författare)
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Resistivity changes in Cr/V(0 0 1) superlattices during hydrogen absorption
- 2007
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 446, s. 526-529
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Tidskriftsartikel (refereegranskat)abstract
- The hydrogen induced resistivity changes in Cr/VHx(0 0 1) superlattices where investigated in the concentration range 0<x<0.7. Initially, the resistivity increases with H content, reaching a maximum at H/V≈0.5 atomic ratio. At concentration above 0.5, the resistivity decreases with increasing H concentration. These results are in stark contrast to the H induced resistivity changes in Fe/V(0 0 1) superlattices, in which the resistivity increases monotonically up to H/V≈1. The results unambiguously prove the importance of the interface scattering, which calls for better theoretical description of the H induces changes in the electronic structure in this type of materials.
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