Gabriella Andersson

GenX : an extensible X-ray reflectivity refinement program utilizing differential evolution

GenX is a versatile program using the differential evolution algorithm for fitting X-ray and neutron reflectivity data. It utilizes the Parratt recursion formula for simulating specular reflectivity. The program is easily extensible, allowing users to incorporate their own models into the program. This can be useful for fitting data from other scattering experiments, or for any other minimization problem which has a large number of input parameters and/or contains many local minima, where the differential evolution algorithm is suitable. In addition, GenX manages to fit an arbitrary number of data sets simultaneously. The program is released under the GNU General Public License.

Metallic superlattices: Quasi two-dimensional playground for hydrogen

Abstract Hydrogen in superlattices, whose constituents have different hydrogen affinities, comprises a new class of hydrogen absorbing materials. The period of the absorption potential can be made as small as two monolayers, which implies highly anisotropic host mediated hydrogen–hydrogen (H–H) interactions. As the correlation between the hydrogen atoms can be influenced in an almost arbitrary way, it is possible to create an ensemble whose nature is neither three- nor two-dimensional, but something in between. By changing the strain state of the constituents, the interaction between the hydrogen atoms in vanadium can be changed from being attractive (for biaxial tensile strain) to repulsive (for biaxial compressive strain). This has been observed experimentally for, e.g., 2 nm vanadium layers in Mo–V and Fe–V superlattices. The biaxial strain state is shown to have a gigantic effect on the hydrogen induced lattice expansion. By changing the strain state from tensile to compressive, in ≈2 nm V layers, the hydrogen induced lattice expansion is altered by a factor of ten. This feature is expected to affect the diffusion properties of hydrogen in these structures dramatically. The principal orientation of the crystal axis of the host material, with respect to the boundary planes, is found to have large impact on the host mediated H–H interactions. The theoretical understanding of hydrogen in quasi-two-dimensional potentials is still rudimentary, and more theoretical and experimental work is required for a detailed understanding of these problems. In the present work, we will treat the implications of the extension and amplitude of the hydrogen absorption potential, and we will demonstrate the existence and impact of weak and strong modulation of the host potential on the hydrogen uptake. The influence of the strain state on the hydrogen absorption and the host mediated H–H interactions will also be treated, and we will show that the phase diagram can, in some cases, be altered in a controllable way.

Imprinting layer specific magnetic anisotropies in amorphous multilayers

We demonstrate how layer specific in-plane magnetic anisotropy can be imprinted in amorphous multilayers. The anisotropy is obtained by growing the magnetic layers in the presence of an external field and the anisotropy direction can thereby be arbitrarily chosen for each of the magnetic layers. We used Co68Fe24Zr8 and Al70Zr30 layers as building blocks for demonstrating this effect. The imprinting is expected to be obtainable for a wide range of amorphous materials when grown at temperatures below the magnetic ordering temperature.

Magnetic anisotropy of tetragonal FeCo/Pt (001) superlattices

The magnetic properties of tetragonally strained Fe0.36Co0.64 alloys in the form of FeCo/Pt(001) superlattices have been investigated experimentally. The strain was controlled by varying the indivi ...

Hydrogen uptake of thin epitaxial vanadium (001) films

Abstract Hydrogen solubility isotherms between 170°C and 240°C in 50–100 nm thick epitaxial V (001) films were measured resistometrically. Hydrogen concentrations up to H/V≈0.9 (atomic ratio) were obtained without the crystalline structure deteriorating. At concentrations around 0.65–0.7, the hydrogen–hydrogen interaction switched from attractive to repulsive, indicating a phase transition with a critical temperature lower than for the bulk V–H system. The number of available hydrogen sites per V atom in the disordered phase, as deduced from the entropy change at low concentrations, is close to the bulk value.

Tunable giant magnetic anisotropy in amorphous SmCo thin films

SmCo thin films have been grown by magnetron sputtering at room temperature with a composition of 2–35 at. % Sm. Films with 5 at. % or higher Sm are amorphous and smooth. A giant tunable uniaxial in-plane magnetic anisotropy is induced in the films which peaks in the composition range 11–22 at. % Sm. This cross-over behavior is not due to changes in the atomic moments but rather the local configuration changes. The excellent layer perfection combined with highly tunable magnetic properties make these films important for spintronics applications.

Probing the local electronic structure in the H induced metal - insulator transition of Y

The occupied acid unoccupied valence band states of yttrium (Y) metal, Y dihydride and Y trihydride were measured by soft x-ray emission and absorption spectroscopy. The experiments were complement ...

Structure of Fe-Co/Pt(001) superlattices : a realization of tetragonal Fe-Co alloys

The structural properties of a tetragonally distorted Fe1−xCox alloy, in the form of Fe1−xCox/Pt(001) superlattices with x = 0.64, have been investigated experimentally. The study follows recent theoretical predictions on the enhanced uniaxial magnetocrystalline anisotropy of such alloys with specific combinations of chemical composition and tetragonal distortion. The ratio between out-of-plane and in-plane lattice parameters in the Fe0.36Co0.64 layers, c/a, was found to vary between 1.18 and 1.31, depending on the thickness ratio between the alloy and the spacer. This covered the range of interest c/a = 1.20–1.25 in the previous calculations and should be compared to c/a = 1 in the original bcc alloy lattice. Simulations of x-ray diffraction patterns as well as density functional calculations support the derivation of the Fe0.36Co0.64 lattice parameters.

Influence of interface mixing on the magnetic properties of BCC Fe0.82Ni0.18/V (0 0 1) superlattices

Influence of interface mixing on the magnetic properties of BCC Fe0.82Ni0.18/V (0 0 1) superlattices

Long-range magnetic interactions and proximity effects in an amorphous exchange-spring magnet

Low-dimensional magnetic heterostructures are a key element of spintronics, where magnetic interactions between different materials often define the functionality of devices. Although some interlayer exchange coupling mechanisms are by now well established, the possibility of direct exchange coupling via proximity-induced magnetization through non-magnetic layers is typically ignored due to the presumed short range of such proximity effects. Here we show that magnetic order can be induced throughout a 40-nm-thick amorphous paramagnetic layer through proximity to ferromagnets, mediating both exchange-spring magnet behaviour and exchange bias. Furthermore, Monte Carlo simulations show that nearest-neighbour magnetic interactions fall short in describing the observed effects and long-range magnetic interactions are needed to capture the extent of the induced magnetization. The results highlight the importance of considering the range of interactions in low-dimensional heterostructures and how magnetic proximity effects can be used to obtain new functionality.