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The spontaneous crystal surface reconstruction of M-plane Al₂O₃ is employed for nanopatterning and nanofabrication in various fields of research including, among others, magnetism, superconductivity, and optoelectronics. However, investigating this reconstruction process from a planar surface to one with a nanoscale ripple topography is challenging, since it occurs at high temperatures above approximately 1000 °C. This contribution presents an approach combining ex-situ Atomic Force Microscopy with in-situ and simulated Grazing Incidence Small Angle X-ray Scattering to further elucidate this morphological transition. It provides time-resolved information on all relevant morphological characteristics required to trace the surface topography during recon- struction and thereby offers a comprehensive picture of this process on a nanometer scale.

Nuclear resonant reflectivity (NRR) from an Fe60Al40 film was measured using synchrotron radiation at several grazing angles near the critical angle of the total external reflection. By laterally resolved measurements after 20 keV Ne+ ion irradiation with gradually varied fluences of 0 - 3.0 x 10^14 ions/cm2, the progressive creation of the ferromagnetic A2 phase with increasing ion fluence was confirmed. The observed depth-selectivity of the method has been explained by application of the standing wave approach. From the time spectra of the nuclear resonant scattering in forward or diffraction directions the depth-profiles for different hyperfine fields were extracted. The results evidence that the highest magnetic hyperfine fields (~ 18 % 23 T) are initially created at the center part of the film and partially at the bottom interface with SiO2 substrate. The evolution of the ferromagnetic onset, commencing at a fixed depth within the film and propagating towards the interfaces has been directly observed. At higher fluence (3.01014 ions/cm2) the depth distribution of the ferromagnetic fractions became more homogeneous across the film depth in accordance with previous results.

Potential technological applications of periodic nanostructure arrays range from photovoltaics augmented by light trapping [1] to high-sensitivity biomolecule detection using plasmonic signal enhancement [2] and high-speed low-energy information technology based on magnonic crystals [3]. Industrial-scale fabrication of such devices requires nanopatterning processes which are cost-effective, scalable, and highly reproducible. These demands can be met by a versatile bottom-up approach based on ion irradiation of semiconductor surfaces and well-established thin film deposition techniques. Reverse epitaxy, i.e. the non-equilibrium self-assembly of vacancies and ad-atoms under ion irradiation, leads to nanoscale surface patterning with well-defined lateral periodicity on semiconductor substrates [4]. The GaAs(001) and InAs(001) surfaces exhibit highly regular faceting and therefore lend themselves to transferring this pattern regularity to other materials. The nanofaceted surfaces with a sawtooth profile can for instance be employed as substrates for molecular beam epitaxy under grazing incidence, producing periodic arrays of nanowires, periodically corrugated thin films, or combinations thereof by geometrical shading. They can also induce long-range chemical ordering in diblock copolymer thin films, which may then serve as highly ordered chemical templates for metal nanostructure growth in a variety of pattern morphologies [5]. Furthermore, separated semiconductor nanostructures can be fabricated by introducing an interlayer before ion irradiation. In this contribution, we outline the reverse epitaxy mechanism and present examples of how it can be employed in the fabrication of large-area nanostructure arrays. We hope to stimulate discussion of further applications by emphasizing the simplicity and versatility of this bottom-up approach. [1] H.A. Atwater and A. Polman, Nature Materials 9 (2010); [2] J. Vogt et al., Phys. Chem. Chem. Phys. 17 (2015); [3] D. Grundler, Nature Physics 11 (2015); [4] X. Ou et al., Nanoscale 7 (2015); [5] D. Erb et al., Science Advances 1 (2015)

Magnetoelectronic multilayer devices are widely used in today's information and sensor technology. Their functionality, however, is limited by the inherent properties of magnetic exchange or dipolar coupling which constrain possible spin configurations to collinear or perpendicular alignments of adjacent layers. Here, a deposition procedure is introduced that allows for a new class of layered materials in which complex spin structures can be accurately designed to result in a multitude of new and precisely adjustable spintronic and magnetoresistive properties. The magnetization direction and coercivity of each individual layer are determined by the deposition process in oblique incidence geometry and can be completely decoupled from neighboring layers. This applies for layers of any ferromagnetic material down to layer thicknesses of a few nm and lateral dimensions of a few 100 nm, enabling the design of efficient and compact magnetoelectronic devices, encompassing precision magnetoresistive sensors as well as layer systems with multiple addressable remanent states for magnetic memory applications.

Nanopatterning of different materials is a key requirement in research fields as diverse as magnetism, plasmonics, optics or catalysis. Potential technological applications range from photovoltaics augmented by light trapping [1] to high-sensitivity biomolecule detection using plasmonic signal enhancement [2] and high-speed low-energy information encoding, transmission, and processing based on magnonic crystals [3]. Industrial-scale fabrication of such devices for energy harvesting, medical diagnostics, or information technology requires nanopatterning processes which are fast, facile, cost-effective, scalable, and highly reproducible. A versatile bottom-up nanopatterning approach which can meet these demands is based on ion irradiation of semiconductor surfaces and well-established thin film deposition techniques. On crystalline semiconductor substrates, nanoscale surface patterns with well-defined lateral periodicity form via the mechanism of reverse epitaxy, i.e. the non-equilibrium self-assembly of vacancies and ad-atoms under ion irradiation [4]. The GaAs(001) surface exhibits highly uniform faceting and therefore lends itself to transferring this pattern regularity to other materials. The nanopatterned GaAs surface can for instance be employed as a substrate for molecular beam epitaxy under grazing incidence, producing arrays of nanodots, nanowires, periodically corrugated thin films, or combinations thereof by geometrical shading. It can also be the basis for hierarchical self-assembly: here, the topography of the GaAs surface provides a preferential direction for the chemical microphase separation in a diblock copolymer thin film. This flat film then serves as a highly ordered chemical template for metal nanostructure growth in a variety of pattern morphologies [5]. The large-area periodically nanopatterned sample systems are especially very well suited for x-ray and neutron scattering experiments. In this contribution, we outline the reverse epitaxy mechanism and present examples of how the resulting surface nanopatterns can be employed in the fabrication of nanostructure arrays. We hope to stimulate discussion of further applications by emphasizing the simplicity and versatility of this bottom-up approach. [1] H.A. Atwater and A. Polman, Nature Materials 9 (2010) [2] J. Vogt et al., Phys. Chem. Chem. Phys. 17 (2015) [3] D. Grundler, Nature Physics 11 (2015) [4] X. Ou et al., Nanoscale 7 (2015) [5] D. Erb et al., Science Advances 1 (2015)

Nanopatterning via self-assembly has gained considerable interest as an alternative to lithography-based techniques for nanostructure fabrication. We propose a procedure for producing highly ordered arrays of uniform metallic nanostructures based exclusively on three subsequent self-assembly processes [1]: crystal surface reconstruction, copolymer microphase separation, and metal diffusion on chemically heterogeneous surfaces. The versatile approach allows for preparing nanostructures with scalable sizes and in a variety of shapes and materials. With this high-throughput technique, nanopatterns covering areas of several square centimeters can be fabricated easily. We present results of in-situ structural and magnetic investigations of Fe nanodot arrays during formation by grazing incidence small angle X-ray scattering [2] and nuclear resonant scattering of synchrotron radiation [3], examining the dependence of the nanodot shape on deposition conditions and observing the evolution of magnetic moment dynamics during nanodot growth [4]. Possible applications of self-assembled nanopatterns could range from high-density magnetic data storage to catalysis or sensing based on surface plasmon resonance. [1] D. Erb, K. Schlage, R. Röhlsberger, Science Advances 1 (2015) e1500751 [2] G. Renaud, R. Lazzari, and F. Leroy, Surface Science Reports 64 (2009) 255 [3] E. Gerdau and H. de Waard (eds.), Hyperfine Int. 123-124 (1999) [4] D. Erb, Ph.D. thesis, University of Hamburg (2015)