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Fabrication of magnetic nanostructures by e-beam lithography for X-ray tomography

AuthorsMartín, José Ignacio ; Quirós, Carlos ; Hierro-Rodríguez, A. ; Sorrentino, Andrea; Álvarez-Prado, L. M. ; Alameda, J. M. ; Pereiro, Eva; Vélez, María ; Ferrer, S.
Keywordse-beam lithography
Magnetic nanostructures
X-ray microscopy
Issue Date2017
Citation43rd International Conference on Micro and Nano Engineering (2017)
AbstractThe magnetic behavior of planar nanostructures is determined by the competition between shape, magnetic anisotropy and exchange resulting in many different possible 2D magnetic textures. These can be characterized with a variety of microscopy techniques such as magnetic force microscopy, magnetooptical microscopy or magnetic X-ray microscopy. As film thickness increases, magnetic configuration becomes 3D and resolving the magnetic state of buried layers becomes increasingly challenging. X-ray microscopy with its long penetration depth (over 100 nm) has been used to detect buried magnetic singularities in magnetic multilayers, taking advantage of its element selectivity to separate the contributions at different sample depths. However, the possibility to reconstruct a 3D map of the magnetic configuration, i.e. the magnetic analogy of scalar tomography, is still under development. In this work we have fabricated a series of permalloy magnetic discs and prisms with decreasing aspect ratio (from 10:1 to 3:1) to study the evolution of their 3D magnetic configuration by magnetic X-ray microscopy at the full field transmission microscope of the Mistral Beamline at Alba synchrotron. 300 nm thick permalloy (Ni80Fe20) elements were fabricated by e-beam lithography and lift-off on 50 nm thick N4Si3 membranes with 0.75 mm × 0.75 mm windows to allow for enough aperture at oblique incidence during X-ray microscopy measurements. Resist thickness was optimized to allow for the lift-off process of 300 nm thick metallic layers minimizing mechanical stress on the 50 nm thick membranes. Figure 1 shows the SEM image of a series of prisms, disks and rings fabricated with this method. 3D magnetic configuration of the permalloy prisms was simulated with MuMax code with saturation magnetization MS=8.5×105 A/m and exchange constant A = 1.3×10-11 J/m. Magnetic anisotropy has both in plane Ku=850 J/m3 and out-of-plane KN-104 J/m3 components due to strains during sample growth by sputtering. Figure 2 shows a typical remanent state for a 2 μm×1.4 μm×0.3 μm prism: it consists of a few linear up/down magnetic domains at the central plane and a relative large closure domain structure near the top/bottom sample surfaces. Magnetic dichroic images were acquired at remanence by tuning the circularly polarized photon beam energy to the Fe L3 atomic absorption edge, since the lower proportion of Fe in the Ni80Fe20 alloy allows to increase the effective penetration depth. The X-ray angle of incidence was varied from θ = -50º to +50º from the film normal, in order to characterize in-plane and out-of-plane magnetization components. Figure 3 shows a typical transmission X-ray microscopy image of a small 2 μm × 1.4 μm × 0.3 μm permalloy prism acquired at normal incidence (only sensitive to out-of-plane magnetization). The structure of up/down domains (bright/dark contrast) clearly shows up in the image. These domains look curved near the sample edge, indicative of their 3D character.
DescriptionResumen del trabajo presentado a la 43rd International Conference on Micro and Nano Engineering (MNE), celebrada en Braga (Portugal) del 18 al 22 de septiembre de 2017.
Appears in Collections:(CINN) Comunicaciones congresos
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