Jürgen Feydt

Interdiffusion in Fe-Pt multilayers

Fe∕Pt multilayers with modulation periods Λ=24.1±0.2 and 37.2±0.1A and [110]‖[111] bcc-fcc texture were fabricated by magnetron sputtering on thermally oxidized Si wafers. The structural evolution of the multilayers with annealing temperature in the range of 300–600K was studied by in situ x-ray diffraction (XRD) and x-ray reflectivity. Two temperature regimes were found from the XRD data. Below 534±4K slow, short-range diffusion is observed without significant broadening of the satellite peaks or changes in the texture. Above 534K fast, long-range diffusion is observed accompanied by significant broadening of the satellites and rapid increase of the misorientations of the grains. The multilayers crystallize at about 583K into the tetragonal FePt phase with a small degree of ordering and strong [111] texture. The transition resembles a first-order phase transition with a critical exponent β=0.48±0.01 which practically does not depend on Λ. The bulk interdiffusion coefficient, determined from the decay of ...

Fabrication and characterization of freestanding NiMnGa films

Freestanding Ni 50 Mn 30 Ga 20 films of 13 μm thickness were fabricated by DC magnetron sputtering and analyzed. Magnetic measurements revealed the influence of the post deposition rapid thermal annealing process on the ferromagnetic hysteresis. Ferromagnetic properties evolved after annealing at 400°C. Thermal annealing of at least 600°C led to polycrystalline films that transformed reversibly and martensitically as shown by structural analysis and differential scanning calorimetry. Transformation temperatures and enthalpies of transformation of the martensitic transformation were strongly influenced by the temperature of the rapid annealing process. It is proposed that the annealing data reflect the evolution of the crystalline state.

Ab initio calculations of the ferromagnetic shape memory alloy Ni–Mn–Al

Results of the first-principles calculations of the magnetic shape-memory alloy NiMnAl are presented. The properties of a series of alloys in the composition range Ni50Mn x Al50− x with 0 ≤ x ≤ 50 were deduced from the ab initio simulations. One essential result is that the alloy is ferromagnetic in the range from 14 to 31 at.% Mn. Furthermore, the martensitic phases 2 M, 10 M, and 14 M with long-periodic structure were calculated. They are metastable in the stoichiometric Ni2MnAl alloy due to additional bonding between specific atomic sites. Their properties are discussed in terms of the density of states and their charge distribution.

Spherical Load Indentation in Submicron NiTiCu Shape Memory Thin Films

Nanoindentation with spherical tipped indenters provides a powerful technique to explore surface and thin film mechanical properties through the application of Hertzian contact mechanics. The full range of mechanical response can be obtained from elastic, through the yield point, to permanent deformation. In this study spherical indentation has been used for probing MBE-grown NiTiCu alloy thin films into superelasticity or stress-induced martensitic transformation. By this way, obstacles typically occurring related to the fabrication of freestanding films (film thickness

Combinatorial Thin Film Synthesis of NiMnAl Magnetic Shape Memory Alloys Using MBE Technique

Composition spreads close to the Heusler alloy Ni 2 MnAl were grown onto 4-inch wafer substrates using molecular beam epitaxy. Compositional variations of up to 10 at.% relative to each constituent enable a direct comparison of the chemical-structural relationship with respect to martensitic transformation and to magnetic ordering as well as an efficient identification of the emerging phase stability regions. In this study, we set the primary focus on the structural aspects of the transformation behavior as observed by X-ray microdiffraction in combination with a specially designed heating stage. Notably, cross-sectional HRTEM imaging of the respective composition areas reveals a laminated two-phase martensitic structure inside the single grains, identified as a sequence of 2M and 14M variants. Stress relief upon transformation as observed by mechanical stress measurements reaches to 400 MPa depending on the composition. Magnetization measurements so far indicate field-induced ordering to occur at low temperatures, here, below 50 K which is assumed to be closely related to a high degree of structural disorder on the Mn-Al sublattice. Single-crystal thin films were realized by means of an epitaxial relationship to MgO (001).