Showing papers by "Stéphane Fusil published in 2021"

Voltage-Controlled Reconfigurable Magnonic Crystal at the Sub-micrometer Scale.

Abstract: Multiferroics offer an elegant means to implement voltage-control and on the fly reconfigurability in microscopic, nanoscaled systems based on ferromagnetic materials. These properties are particularly interesting for the field of magnonics, where spin waves are used to perform advanced logical or analogue functions. Recently, the emergence of nano-magnonics is expected to eventually lead to the large-scale integration of magnonic devices. However, a compact voltage-controlled, on demand reconfigurable magnonic system has yet to be shown. Here, we introduce the combination of multiferroics with ferromagnets in a fully epitaxial heterostructure to achieve such voltagecontrolled and reconfigurable magnonic systems. Imprinting a remnant electrical polarization in thin multiferroic BiFeO 3 with a periodicity of 500 nm yields a modulation of the effective magnetic field in the micron-scale, ferromagnetic La 2/3 Sr 1/3 MnO 3 magnonic waveguide. We evidence the magneto-electrical coupling by characterizing the spin wave propagation spectrum in this artificial, voltage induced, magnonic crystal and demonstrate the occurrence of a robust magnonic bandgap with > 20 dB rejection.

Surface and bulk ferroelectric phase transition in super-tetragonal BiFeO 3 thin films

Abstract: The temperature-dependent ferroelectric properties of super-tetragonal ${\mathrm{BiFeO}}_{3}$ are investigated using surface-sensitive low-energy electron microscopy (LEEM). We use epitaxial oxide ${\mathrm{BiFeO}}_{3}/{\mathrm{Ca}}_{0.96}{\mathrm{Ce}}_{0.04}{\mathrm{MnO}}_{3}$ bilayers grown by pulsed laser deposition on ${\mathrm{YAlO}}_{3}$ substrates. Ferroelectric, micrometer-scale domains are written by piezoresponse force microscopy and subsequently observed by LEEM from room temperature up to about 950 K. Kelvin probe force microscopy and LEEM spectroscopy reveal that the surface potential is efficiently (g50%) screened by adsorbates that are only released after annealing above 873 $\ifmmode\pm\else\textpm\fi{}$ 50 K in ultrahigh vacuum. The surface structure and chemistry of the ferroelectric thin films are analyzed using scanning transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy, discarding the occurrence of a putative ``skin layer'' effect. While its magnetic and structural transitions were reported in the literature, the true, ferroelectric Curie temperature of super-tetragonal ${\mathrm{BiFeO}}_{3}$ has not been determined so far. Here, we measure a Curie temperature of 930 $\ifmmode\pm\else\textpm\fi{}$ 30 K for the super-tetragonal ${\mathrm{BiFeO}}_{3}$ surface and corroborate it with volume-sensitive, temperature-dependent x-ray diffraction measurements. These results suggest that LEEM can be used as a powerful tool to probe surface charge and ferroelectric transitions in ultrathin films.

Patterning enhanced tetragonality in BiFeO3 thin films with effective negative pressure by helium implantation

Abstract: Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independently from the in-plane strain controlled by epitaxy. In particular, implantation by means of a helium microscope allows for local implantation and patterning down to the nanometer resolution, which is of interest for device applications. We present here a study of bismuth ferrite (BiFeO3) films where strain was patterned locally by helium implantation. Our combined Raman, XRD and TEM study shows that the implantation causes an elongation of the BiFeO3 unit cell and ultimately a transition towards the so-called super-tetragonal polymorph via states with mixed phases. In addition, TEM reveals the onset of amorphization at a threshold dose that does not seem to impede the overall increase in tetragonality. The phase transition from the R-like to T-like BiFeO3 appears as first-order in character, with regions of phase coexistence and abrupt changes in lattice parameters.

X-ray absorption and x-ray magnetic circular dichroism in bulk and thin films of ferrimagnetic GdTi O 3

Abstract: Perovskite rare-earth titanates are prototypical Mott insulators in which ${\mathrm{Ti}}^{3+}$ ions with $3{d}^{1}$ electronic configuration exhibit ferromagnetic or antiferromagnetic spin order, depending on the rare-earth size. This peculiar magnetic behavior has, however, been barely studied with element-specific probes, either in bulk or in thin films. The recent finding of fingerprints of ferromagnetism in two-dimensional electron gases at oxide interfaces involving rare-earth titanates has produced a surge of the interest in these complex materials. Harnessing the interfacial magnetic states in these heterostructures calls for a better understanding of their insufficiently explored magnetic states in bulk and especially in thin film form. In this paper, we combine high-resolution transmission electron microscopy with x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) to determine the structural, electronic, and magnetic structure of $\mathrm{GdTi}{\mathrm{O}}_{3}$ in bulk and thin film form. In both cases, we find that the sample surface is strongly overoxidized but a few nm below, Ti is mostly 3+ and shows a large XMCD. We provide evidence for the ferrimagnetic nature of $\mathrm{GdTi}{\mathrm{O}}_{3}$ with antialigned Gd and Ti sublattices and show that, just as in antiferromagnetic $\mathrm{LaTi}{\mathrm{O}}_{3}$ or ferromagnetic $\mathrm{YTi}{\mathrm{O}}_{3}$, Ti carries no orbital moment.

Patterning enhanced tetragonality in BiFeO3 thin films with effective negative pressure by helium implantation

Abstract: Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independently from the in-plane strain controlled by epitaxy In particular, implantation by means of a helium microscope allows for local implantation and patterning down to the nanometer resolution, which is of interest for device applications We present here a study of bismuth ferrite (${\mathrm{BiFeO}}_{3}$) films where strain was patterned locally by helium implantation Our combined Raman, x-ray diffraction, and transmission electron microscopy (TEM) study shows that the implantation causes an elongation of the ${\mathrm{BiFeO}}_{3}$ unit cell and ultimately a transition towards the so-called supertetragonal polymorph via states with mixed phases In addition, TEM reveals the onset of amorphization at a threshold dose that does not seem to impede the overall increase in tetragonality The phase transition from the R-like to T-like ${\mathrm{BiFeO}}_{3}$ appears as first-order in character, with regions of phase coexistence and abrupt changes in lattice parameters