14 Externe wissenschaftliche Einrichtungen
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Item Open Access Magnetism of amorphous and highly anisotropic multilayer systems on flat substrates and nanospheres(2008) Amaladass, Edward Prabu; Schütz, Gisela (Prof. Dr.)Fe/Gd multilayers with monolayer and submonolayer thicknesses have been systematically studied on flat substrates and on self-assembled nanospheres with high spatial resolution. A series of Fe/Gd multilayers consisting of 75 bilayers have been prepared on flat silicon substrate with sublayer thicknesses from 0.36 to 0.60 nm. A thickness dependent spin reorientation transition (SRT) from an in-plane to a perpendicularly magnetized state is observed by decreasing the sublayer thickness. These results are quantitatively explained by the contribution of the stray field and by the dipolar pair ordering model. From transmission electron microscopy (TEM) analysis it has been seen that inhomogeneous interfaces cause an in-plane magnetization, whereas almost homogeneous interfaces lead to a perpendicular magnetic anisotropy (PMA). The layer thickness (total thickness), preparation techniques, and the choice of the substrate also have a strong influence on magnetic properties of the system. The domain configuration during a temperature driven SRT is studied by photoemission electron microscopy (PEEM). A sample with the composition 75×[Fe (0.36 nm)/Gd (0.36 nm)] initially shows an in-plane magnetization at room temperature and upon heating the sample, the in-plane domains change to a maze like, perpendicular domains. Between these transformations, a coexistence of in-plane and out-of-plane domain configurations is seen in a broad temperature range of 57° - 81° C. Micromagnetic simulations reproduced a similar coexistence phase. The presence of two local minima in the total energy, due to the different temperature behaviors of the surface anisotropy (KS) and the volume anisotropy (KV ) is believed to cause this coexistence phase. After several SRT cycles the perpendicular state of the sample became more prominent at room temperature. TEM images recorded before and after annealing show significant changes in the layered structure. This infers that the chemical and structural changes in the sample upon annealing are responsible for the appearance of PMA. Upon depositing Fe/Gd multilayers on self-assembled silica or polystyrene nanospheres a new class of magnetic nanostructures with a crescent shaped cross section is produced. The influence of the thickness gradient and the curved nature of the substrate on the properties of the magnetic multilayers are studied with respect to the diameter of the spheres and the thickness of the films. A 36 nm Fe/Gd film with a composition of 50×[Fe (0.36 nm)/Gd (0.36 nm)] has been deposited on nanospheres with diameters of 800, 330, and 160 nm. Upon increasing the diameter of the spheres to 800 nm a dramatic change in the spin structure and the reversal mechanism is observed. Local hysteresis loops of the spherules, deduced from scanning transmission x-ray microscopy (STXM) images reveal an S-shaped loop for 800 nm spherules, while for the 330 and 160 nm spherules a squared loop with a sharp switching field accompanied by a change in the magnetic anisotropy from radial to uniaxial is observed. The exchange energy that favors the parallel alignment of the spins overcomes the radial anisotropy and leads to such parallel orientation of the moments upon decreasing the diameter of the sphere. The hysteresis loops and the reversal mechanism obtained by simulation are in good agreement with the experimental results. In addition, element selective hysteresis loops on 330 and 160 nm spherules show an asymmetric behavior at Fe and Gd edges. At the maximum field of about 400 Oe Gd moments align with the field direction, whereas Fe moments tilt away from the field direction. These changes suggest a twisted magnetic state due to the increase in the curvature and the magnetic field. Furthermore, the thickness dependence of the magnetic properties is addressed. Spherules with a 36 nm thick Fe/Gd film, i.e. 50 bilayers, show an S-shaped loop. Spherules of same diameter of 800 nm but with 75 bilayers system show a squared loop with prominent steps and a strong increase in the coercive field by a factor of four. Magnetization reversal takes place through an intermediate flower state that shows up as a step in the hysteresis loop. This thickness dependent reversal mechanism is explained by the interplay between the different anisotropy terms and the thickness dependent switching field. Similar changes are not observed for the films deposited on a flat substrate. A shape induced SRT is observed for an in-plane magnetized Fe/Gd film deposited on nanospheres. A hard axis loop is observed on a flat substrate, whereas the film on spherules with a diameter of 800 nm showed a rectangular easy axis loop. This is explained by the change in the shape anisotropy. In addition to Fe/Gd system, Co/Pt multilayer system is also studied with high spatial resolution. The thickness gradient on the spheres is used to induce a thickness dependent SRT locally on every nanospheres. STXM images measured in an in-plane and an out-of-plane geometry reveal two different magnetic states at the rim and at the center of the spherules. Local hysteresis loops deduced on a ≈ 50 nm scale show a transformation of the magnetization curve from an easy axis loop to a hard axis loop with increasing the elevation angle on the spheres. The SRT from perpendicular to in-plane magnetization has been seen in the interval 56° < θ < 70° within a distance of less than 90 nm. These results are in good agreement with micromagnetic simulations.