The development of freestanding oxide thin films enables the design of unconventional heterostructures with enhanced functionalities. Here, we explore the fabrication of membranes consisting of dense arrays of ultrathin CoxNi1-x nanowires embedded in a dielectric matrix. We present a lift-off process to create and transfer these membranes while preserving the structural and chemical integrity of the nanopillars. The large axial deformation of the metallic nanostructures is maintained, and their magnetic properties remain largely unaltered after substrate removal. This robustness enables the integration of such strain-engineered, vertically aligned nanocomposites into flexible spintronic and magnetic memory platforms.

The recent development of freestanding oxide thin films opens up exciting opportunities for the design of novel heterostructures with enhanced functionalities. Here, we explore the fabrication of membranes consisting of dense arrays of ultrathin CoxNi1-x nanowires epitaxially embedded in a SrTiO3 matrix. Through combined x-ray absorption spectroscopy, x-ray resonant magnetic scattering, x-ray diffraction and magnetooptical experiments, we show how a SrVO3-mediated lift-off process can be used to create and transfer these membranes while simultaneously preserving the structural and chemical integrity of the self-assembled, metallic CoxNi1-x nanopillars. With this approach, the large axial deformation of the embedded nanostructures is kept intact and, as a direct consequence, the magnetic properties of the nano-composite thin films remain largely unaltered after substrate removal. Our findings thus highlight a novel route for the synthesis of freestanding, strain-engineered vertically aligned heterostructures and pave the way for their future integration into spintronic and optomagnetic devices.

We present a sequential growth scheme based on pulsed laser deposition, which yields dense arrays of ultrathin, match-shaped Au/CoNi nanopillars, vertically embedded in SrTiO3 thin films. Analysis of the magnetic properties of these nanocomposites reveals a pronounced out-of-plane anisotropy. We show that the latter not only results from the peculiar nanoarchitecture of the hybrid films but is further enhanced by strong magneto-structural coupling of the wires to the surrounding matrix. Finally, we provide a detailed overview of the optical response of these vertical nanostructures. Combining ellipsometry measurements with finite-difference time-domain simulations allows us to assess the potential of our self-assembly approach, as well as its possible shortcomings, for producing hybrid thin films with well-tailored magneto-plasmonic properties.

Ultrathin CoxNi1-x alloy nanowires vertically embedded in SrTiO3/SrTiO3 (001) thin films were grown using a self-assembly approach based on sequential pulsed laser deposition. Due to vertical epitaxial coupling of the metallic and oxide phases, a large average tensile strain of up to 4% arises within the nanowires, which is evidenced using a combination of x-ray diffraction and transmission electron microscopy. Macroscopic magnetometry experiments are used to demonstrate that this huge deformation allows us to enhance the uniaxial anisotropy of the nanowires, leading to saturation field in excess of 1 T in the hard direction, large coercive field at low temperature along the easy axis, and to a blocking temperature exceeding 600 K in the case of nanowires with a diameter of 5 nm and 78% Co content. These data are complemented with angular dependent x-ray magnetic circular dichroism measurements at the Co and Ni L-2,L-3 edges. The value of the magnetic moment was extracted from these measurements by applying sum rules and the anisotropy of the orbital moment was investigated.

It is shown that information on the spatial correlation of nano-objects embedded in a crystalline matrix can be retrieved by analysing the X-ray scattering around the Bragg reflections of the host matrix. Data are reported for vertically aligned Ni and CoNi alloy nanowires (NWs) in an SrTiO3 matrix. When the Bragg condition is fulfilled for the matrix and not for the NWs, the latter can be approximated by voids, and the scattering around the matrix reflections contains information on the self-correlation of the NWs (i.e. on their diameter d) and on the correlation between NWs (interdistance D). Nondestructive synchrotron X-ray diffraction data provide information on these values averaged over large areas, complementing local transmission electron microscopy observations. The measurements show that off-Bragg scattering around the matrix reflections can be exploited to study the spatial correlation and morphology of embedded nano-objects, independently of their crystallinity or strain or the presence of defects.

Self-assembled vertically aligned nanocomposites (VANs) have recently emerged as a novel playground for strain engineering of physical properties in nanostructures. In contrast to thin films obtained by classical planar heteroepitaxy, VANs consist of two (or more) intertwined phases, coupled along vertical interfaces. Their unique nanoarchitecture, which can be tuned by choosing appropriate growth conditions, results in deformations that cannot be easily attained in traditional flat geometries. In this article, we show how nanometer-sized acicular inclusions of magnetic 3d metals in various oxide host matrices can be obtained via sequential pulsed laser deposition. We discuss the distinct sources of magnetic anisotropy in such metal–oxide VANs and demonstrate how to use strain to accurately control the magnetic properties of the nanocomposites. We finally present possible extensions of this approach to more than one embedded metallic phase and sketch some of the remaining challenges that must be overcome to create novel functional nanoarchitectures.

Journal Article Electronic Structure and Chemistry of Nanomaterials Embedded in a Matrix Using Atomically Resolved Near-edge Structures: The Example of Ferromagnetic Ni Nanowires Grown in SrTiO3 Get access Matthieu Bugnet, Matthieu Bugnet SuperSTEM Laboratory and School of Physics and School of Chemical and Process Engineering - University of Leeds and CNRS MATEIS, VIlleurbanne, Rhone-Alpes, France Search for other works by this author on: Oxford Academic Google Scholar Quentin Ramasse, Quentin Ramasse SuperSTEM Laboratory and University of Leeds, Daresbury, England, United Kingdom Search for other works by this author on: Oxford Academic Google Scholar Dominique Demaille, Dominique Demaille Sorbonne Université - CNRS, Paris, Ile-de-France, France Search for other works by this author on: Oxford Academic Google Scholar Marcel Hennes, Marcel Hennes Sorbonne Université - CNRS, Paris, Ile-de-France, France Search for other works by this author on: Oxford Academic Google Scholar Xiaorong Weng, Xiaorong Weng Sorbonne Université - CNRS, Paris, Ile-de-France, France Search for other works by this author on: Oxford Academic Google Scholar Yunlin Zheng, Yunlin Zheng Sorbonne Université - CNRS, Paris, Ile-de-France, France Search for other works by this author on: Oxford Academic Google Scholar Franck Vidal, Franck Vidal Sorbonne Université - CNRS, Paris, Ile-de-France, France Search for other works by this author on: Oxford Academic Google Scholar Guillaume Radtke Guillaume Radtke Sorbonne Université - CNRS, Paris, Ile-de-France, France Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 26, Issue S2, 1 August 2020, Pages 1070–1072, https://doi.org/10.1017/S1431927620016864 Published: 01 August 2020

Unveiling complex structural rearrangements and novel strain relaxation mechanisms in vertically aligned nanocomposites.

Manganese arsenide layers epitaxially grown on GaAs(001) are known to feature a temperature dependent self-assembled microstructure of ordered stripes, alternating the ferromagnetic α and paramagnetic β phases. The surface dipolar fields generated by the α/β stripes have been used for achieving temperature controlled magnetization switching of a ferromagnetic overlayer. For this kind of application, it is advantageous to minimize the MnAs layer thickness. In this work we investigate, using x-ray scattering techniques, the presence of the ordered microstructure as a function of the MnAs layer thickness and we identify a minimum value of ∼40 nm for the formation of ordered α/β stripes in MnAs/GaAs(001). These results have an impact for envisaging magnetization-switching applications that rely on the control of the temperature—or laser-driven surface dipolar fields in MnAs-based devices.

The growth of three dimensional spintronic structures has been proposed as a promising route to overcome the limits of current data storage technologies. Among the potential candidates, selfassembled ferromagnetic nanowires (NWs) embedded in an insulating matrix have recently attracted considerable attention (e.g. Ni in SrTiO3, see Fig. 1). This interest is mainly motivated by the possibility to tune the magnetic anisotropy either through the control of the chemical composition of the NWs or through the presence of interfacially-induced strain giving rise to magneto-elastic anisotropy [1]. In particular, the detection of Ni oxidation, which would be highly detrimental, via the potential presence of a NiO layer between the NW and the matrix, is of interest. This is achieved by investigating the NiL23 near-edge structures (ELNES), which show substantial differences between metallic Ni and Ni [2], and the O-K and Ti-L23 ELNES from the matrix to the Ni:SrTiO3 interface [3]. In this work, the local chemistry of the Ni:SrTiO3 interface is probed at the atomic scale in the aberration-corrected scanning transmission electron microscope (STEM). High energy resolution for ELNES (~160 meV), and sub-Å spatial resolution were used in a monochromated Cs-corrected Nion HERMES, operating at 60 kV.

Vertically aligned nanocomposites (VANs) of metal/oxide type have recently emerged as a novel class of heterostructures with great scientific and technological potential in the fields of nanomagnetism, multiferroism, and catalysis. One of the salient features of these hybrid materials is their huge vertical metal/oxide interface, which plays a key role in determining the final magnetic and/or transport properties of the composite structure. However, in contrast to their well-studied planar counterparts, detailed information on the structural features of vertical interfaces encountered in VANs is scarce. In this work, high resolution scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) are used to provide an element selective atomic-scale analysis of the interface in a composite consisting of ultrathin, self-assembled Ni nanowires, vertically epitaxied in a SrTiO3/SrTiO3(001) matrix. Spectroscopic EELS measurements evidence rather sharp interfaces (6-7 angstrom) with the creation of metallic Ni. Ti bonds and the absence of nickel oxide formation is confirmed by X-ray absorption spectroscopy measurements. The presence of these well-defined phase boundaries, combined with a large lattice mismatch between the oxide and metallic species, gives rise to pronounced magnetoelastic effects. Self-assembled columnar Ni:SrTiO3 composites thus appear as ideal model systems to explore vertical strain engineering in metal/oxide nanostructures.

We study the optical and magneto-optical properties of low-concentration arrays of ultrathin nickel nanowires by means of Brillouin spectroscopy of thermal magnons. Brillouin spectroscopy in such quasitransparent magneto-optical structures is dominated by the Bragg phase synchronism mechanism. At variance with ultrathin cobalt nanowire arrays, the Stokes/anti-Stokes scattering pattern is practically symmetrical. This feature is attributed to their peculiar optical properties.

We investigate the orthorhombic distortion and the structural dynamics of epitaxial MnAs layers on GaAs(001) using static and time-resolved x-ray diffraction. Laser-induced intensity oscillations of Bragg reflections allow us to identify the optical phonon associated with orthorhombic distortion and to follow its softening along the path towards an undistorted phase of hexagonal symmetry. The frequency of this mode falls in the THz range, in agreement with recent calculations. Incomplete softening suggests that the beta - gamma transformation deviates from a purely second-order displacive transition.

The potential of the two-color mode implemented at the FERMI free-electron laser (FEL) source for pumping and probing selectively different atomic species has been demonstrated by time-resolved scattering experiments with permalloy (FeNi alloy) and NiFe2O4 samples. We monitored the ultra-fast demagnetization of Ni induced by the pump FEL pulse, by tuning the linearly-polarized FEL probe pulse to the Ni-3p resonance and measuring the scattered intensity in the transverse magneto-optical Kerr effect geometry. The measurements were performed by varying the intensity of the FEL pump pulse, tuning its wavelength to and off of the Fe-3p resonance, and by spanning the FEL probe pulse delays across the 300–900 fs range. The obtained results have evidenced that for the case of NiFe2O4, there is a sensible difference in the magnetic response at the Ni site when the pump pulse causes electronic excitations at the Fe site.

We have investigated by time-resolved X-ray diffraction the manganese arsenide structure dynamics in MnAs/GaAs (001) epitaxial films, when the α → β magnetostructural phase transition is triggered by an ultra-fast optical laser pulse. Understanding the photo-induced phase transition dynamics is relevant for applications of MnAs/GaAs (001) as a magnetically active template in laser-induced magnetization switching processes. The main goal of our experiment is to determine the timescales of the structural changes that are likely to play a role in laser-driven magnetization reversal.

It was shown recently that the Fe magnetization reversal in the Fe/MnAs/GaAs(001) epitaxial system, attained by temperature control of the regular stripe pattern of the MnAs α- and β-phases, can also be driven by an ultrashort optical laser pulse. In the present time-resolved scattering experiment, we address the dynamics of the MnAs α-β self-organized stripe pattern induced by a 100 fs optical laser pulse, using as a probe the XUV radiation from the FERMI free-electron laser. We observe a loss in the diffraction intensity from the ordered α-β stripes that occurs at two characteristic timescales in the range of ~10−12 and ~10−10 s. We associate the first intensity drop with ultrafast electron-lattice energy exchange processes within the laser-MnAs interaction volume and the second with thermal diffusion towards the MnAs/GaAs interface. With the support of model calculations, the observed dynamics are interpreted in terms of the formation of a laterally homogeneous MnAs overlayer, the thickness of which evolves in time, correlating the MnAs microstructure dynamics with the Fe magnetization response.

The advent of free-electron laser (FEL) sources delivering two synchronized pulses of different wavelengths (or colours) has made available a whole range of novel pump–probe experiments. This communication describes a major step forward using a new configuration of the FERMI FEL-seeded source to deliver two pulses with different wavelengths, each tunable independently over a broad spectral range with adjustable time delay. The FEL scheme makes use of two seed laser beams of different wavelengths and of a split radiator section to generate two extreme ultraviolet pulses from distinct portions of the same electron bunch. The tunability range of this new two-colour source meets the requirements of double-resonant FEL pump/FEL probe time-resolved studies. We demonstrate its performance in a proof-of-principle magnetic scattering experiment in Fe–Ni compounds, by tuning the FEL wavelengths to the Fe and Ni 3 p resonances.

We studied the temperature and field dependence of the magnetization in a Co/FeRh/MgO(001) film patterned into a matrix of sub-mu m sized rectangles, using element selective resonant scattering of polarized soft x-rays. We show that it is possible to reverse partially the magnetization of the Co layer in a thermal cycle that crosses the FeRh antiferromagnetic to ferromagnetic transition. Our results support interest in patterned Co/FeRh films and their potential for achieving temperature induced magnetization switching.

Strain engineering is a powerful tool to tailor the physical properties of materials coherently stacked in an epitaxial heterostructure. Such an approach, applied to the mature field of planar heteroepitaxy, has yielded a variety of new phenomena and devices. Recently, heteroepitaxial vertically aligned nanocomposites have emerged as alternatives to planar structures. Owing to the peculiar geometry of such nanoarchitectures, efficient strain control can be achieved, opening the way to novel functionalities. In this paper, we report a very large tensile axial strain in epitaxial transition metal nanowires embedded in an oxide matrix. We show that axial strains in excess of 1.5% can be sustained over a large thickness (a few hundred nanometers) in epitaxial nanowires having ultrasmall diameters (∼3–6 nm). The axial strain depends on the diameter of the nanowires, reflecting its epitaxial nature and the balance of interface and elastic energies. Furthermore, it is experimentally shown that such strain is metastable, in agreement with the calculations performed in the framework of the Frenkel-Kontorova model. The diameter dependence and metastability provide effective ways to control the strain, an appealing feature for the design of functional nanoarchitectures.