Synthesis and Transfer of Freestanding Strain-Engineered Vertically Aligned Nanocomposite Thin Films

Abstract

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.

Figures (9)

• Figure 1: Growth and delamination of Co$_{0.5}$Ni$_{0.5}$-SrTiO$_3$ VAN membranes. (a) First growth step: pulsed laser deposition of homoepitaxial SrVO$_3$ and SrTiO$_3$ thin films on SrTiO$_3$(001) substrate. (b) Second growth step: sequential pulsed laser deposition of Co$_{0.5}$Ni$_{0.5}$-SrTiO$_3$ VAN on SrTiO$_3$/SrVO$_3$/SrTiO$_3$(001). (c) Cross-section transmission electron microscopy image of a Co$_{0.5}$Ni$_{0.5}$-SrTiO$_3$ VAN grown on SrTiO$_3$(001): the formation of Co$_{0.5}$Ni$_{0.5}$ nanopillars is evidenced by Moiré patterns. (d) Dissolution of the SrVO$_3$ sacrificial layer. (e) Removal of the substrate using a thermal release tape (TRT). (f) Transfer on Si$_3$N$_4$/Si membranes.
• Figure 2: Delaminated membranes. (a) Optical microscopy image of a partially covered Si$_3$N$_4$ membrane window. (b) Optical microscopy image of a fully covered Si$_3$N$_4$ membrane window. (c) AFM topography scan (30 $\mu$m $\times$ 30 $\mu$m) over the edge of a flake (a profile is shown in the inset) and (d) topography scan on top of a flake (5 $\mu$m $\times$ 5 $\mu$m).
• Figure 3: (a-b) X-ray absorption spectrum at the Ni (a) and Co (b) L$_{2,3}$ edges acquired in transmission for the reference metallic foils (black) and for the delaminated VAN sample (red). Red asterisks in the Co absorption spectrum highlight the contribution from minute Ba impurities. (c) Experimental configuration for XRMS measurements on delaminated VAN samples. (d-g) XRMS data gathered at the Ni L$_3$ edge with $\mu_0 H_{app}$=+200 mT : (d) scattering pattern acquired with circular left polarization and with (e) circular right polarization, (f) charge signal $I_c$, (g) magnetic signal $I_m$. (h-k) XRMS data obtained by tuning the x-ray energy to the Ni L$_3$ edge with $\mu_0 H_{app}$=$-$200 mT: (h) scattering pattern acquired with circular left polarization, (i) same with a circular right polarization, (j) charge signal $I_c$, (k) magnetic signal $I_m$. Scale bars in (d-k): intensity (arbitrary units).
• Figure 4: (a-c) Magneto-optical Kerr effect measurements of Co$_{x}$Ni$_{1-x}$-SrTiO$_3$ VAN membranes at room temperature, with the magnetic field $\mu_0 H$ applied out-of-plane, parallel to the axis of the nanopillars. (d) Coercive field as a function of the composition $x$ of Co$_{x}$Ni$_{1-x}$ nanopillars. Squares: values before delamination, triangles: values after delamination.
• Figure 5: X-ray diffraction measurements before and after delamination and transfer of Co$_{0.5}$Ni$_{0.5}$-SrTiO$_3$ VAN on Si$_3$N$_4$ membranes. Data were collected on a laboratory 5-circle diffractometer (Rigaku SmartLab) with Cu K$_{\alpha}$ radiation (wavelength of 1.54056 Å ). (a-b) SrTiO$_3$ matrix: out-of-plane 002 and in-plane 200 Bragg reflections after delamination. (c-d) Co$_{0.5}$Ni$_{0.5}$ nanopillars: out-of-plane 002 and in-plane 200 Bragg reflections after delamtination. (e) $c/a$ ratios of the matrix (green) and nanopillars (red) before (open symbols) and after (plain symbols) delamination, as determined by analysis of the XRD measurements. (f-g) Schematic illustrations of the VAN strain state before (f) and after (g) delamination.