Research

Latest:

Imaging the ultra-small with x-ray nano vision: first observation of 3D topological vortex in a single ferroelectric nanoparticle

Research Directions

Do you want to know about the read-head inside your computer works? Why are some butterflies blue, yellow and multi-colored? The arrangement of spins in magnetic nanostructures and how they fluctuate? How your TV remote control works? How is information stored and read in your “pen drives” and flash memory? The quantum fluctuations of atoms in single crystals of solid Helium at temperatures near absolute zero? Or the structure and dynamics of stripe domains and puddles in ferroelectric, ferromagnetic and superconducting phases? These topics and more are investigated by the Fohtung Group, primarily using the interaction of radiation (photons, neutrons, and ions) with condensed matter systems. The focus of our research is the study of the structure and dynamics of condensed matter.

We investigate the quantum interaction between electromagnetic radiation (photons and neutrons) with the condensed and soft matter at the nanoscale. To this end, we are interested in developing novel optical metrology (lens-less microscopy, coherent diffraction imaging with photons at synchrotrons and XFELs) and neutron scattering techniques to probe spatial and temporal dynamics due to competing charge-, spin-, orbital and lattice degree of freedoms at the nanoscale. Applications of our research include Magneto-electric based nano-mechanical oscillators, sensing, quantum photonics, Magneto-electric conversion of optical energy to electricity, transparent ceramics and biomimetics.  We also focus on training the next generation of innovative students/scientists. We aim to foster an environment that is conducive to learning, creativity, and personal development.

Our group is broadly engaged in the study of the structure and dynamics of condensed matter using the techniques of optical, x-ray and neutron scattering. For this purpose, we use in-house x-ray sources and diffractometers, Optical Metrology, while most of our studies require an intensity that can only be provided at synchrotron sources and XFELs

  1. Coherent Diffractive Imaging
  2. Nanoscale 3D Mapping of Topological Defects
  3. Ptychographic Imaging of Bio-tissues and Cancer cells
  4. Interfacial Magnetism

We investigate the quantum interaction between electromagnetic radiation (photons and neutrons) with the condensed and soft matter at the nanoscale. To this end, we are interested in developing novel optical metrology (lens-less microscopy, coherent diffraction imaging with photons at synchrotrons and XFELs) and neutron scattering techniques to probe spatial and temporal dynamics due to competing charge-, spin-, orbital and lattice degree of freedoms at the nanoscale. Applications of our research include Magneto-electric based nano-mechanical oscillators, sensing, quantum photonics, Magneto-electric conversion of optical energy to electricity, transparent ceramics and biomimetics.  We also focus on training the next generation of innovative students/scientists. We aim to foster an environment that is conducive to learning, creativity, and personal development.

Our group is broadly engaged in the study of the structure and dynamics of condensed matter using the techniques of optical, x-ray and neutron scattering. For this purpose, we use in-house x-ray sources and diffractometers, Optical Metrology, while most of our studies require an intensity that can only be provided at synchrotron sources and XFELs

Towards quantum biomemetics  and bio-inspired quantum devices

For decades, humans have mimicked nature for inspiration to create or optimize devices and machines, as well as industrial fabrication, characterization processes and strategies. Besides studying quantized plasma oscillations (plasmons) in metallic nanostructures and at interfaces between metals and dielectrics, we dwell into a branch of science which designs materials and machines inspired in the structure and function of biological systems. Since nature possesses a diversity of photonic structures that have been fine-tuned over millions of years by trial and error; we study a range of bio-inspired features such as structural color vs pigmentary color with the aim of  building new photonic devices.

A Conventional broadband reflection microscope with different angles of illumination (a) is used to image the bright metallic blue colored structures of the Morpho Marcus butterfly wing with illumination from all angles (b) and to show the directional dependency of the reflected light with blue wavelength synthesized in a pseudo-color image (c) where blue channel of the 2pm illumination image is assigned to a blue channel of the synthesized image, blue channel of the 8pm illumination image is assigned to red channel of the synthesized image and blue channel of 5pm illumination image is assigned to the green channel of the synthesized image. This is a direct evidence of structural coloring.

Spintronics

As an alternative to electronic charge, the storage and transport of electronic spin in semiconductor devices – “spintronics”, may revolutionize the electronic device industry, with spin based transistors , opto-electronic devices, and memory. Moreover, the ability to preserve coherent spin states in conventional semiconductors and quantum dots may eventually enable quantum computing in the solid state.

Magnetoelectrics

In the so-called intrinsic multiferroics (those that naturally combine magnetic and electric order parameters), the magnetoelectric coupling is however often weak and heterogeneous multiferroics (composite materials) are currently developed to optimize order parameter coupling. Magnetic and ferroelectric materials are thus artificially combined and two main paths are followed to achieve the magnetoelectric coupling: either strains or modulation of charge carrier density. Several works have been recently reported on strains-driven magnetoelectric coupling, generally achieved when combining magnetostrictive and piezoelectric materials and on the control of ferromagnetic properties achieved through the modulation of the carrier density by applying a gate voltage. In this domain, the effects on ferromagnetic semiconductors have been intensively investigated because its ferromagnetic properties are function of carrier concentration; But electric field control of coercivity, anisotropy, Curie temperature in ferromagnetic metallic layers have also been recently reported. Complex magnetic oxides are other interesting candidates for charge-driven magnetoelectric coupling because of high sensitivity of strongly correlated magnetic systems to competing electronic ground states. Charge plays especially a prominent role in double exchange, hopping and orbital overlap. A large charge-driven magnetoelectric coupling effect has thus been reported in Sr-doped lanthanum manganite (LSMO)/ferroelectric (PZT) composite structure.

We aim at taking an alternative path to multiferroics by designing a piezoelectric-ferromagnetic-semiconductor heterostructures or nanoscale devices in which the applied electric field induces strain field that propagates into the ferromagnetic material and alters the its magnetization.
In magneto-electronic materials such as multiferroics an external electric field displaces ions from their equilibrium positions, which alters the magnetostatic and exchange interactions yielding magnetoelectric coupling. There has been extensive effort to realize multiferroic materials in thin films and nanostructures. In ferromagnetic/piezoelectric composite structures (which is the focus of this proposal), electric field induced strain in the piezoelectric material can alter the magnetization of the ferromagnetic material due to magnetostrictive effects. Recent theoretical calculations suggest that in ferroelectric insulator/ferromagnetic heterostructures that ferroelectric displacements of the interfacial atoms may be reversed by electric fields, significantly altering the interfacial moment or anisotropy. More generally magneto-electric effects may be a common feature of interfaces between dielectrics and metals. Understanding and control of these effects may ultimately lead to new memory and spintronics logic elements.

Interfacial magnetism

Here, we utilize polarized neutron reflectometry (PNR) in consort with ab initio based density functional theory (DFT) calculations to study magnetoelectric coupling at the interface of a ferroelectric PbZr0.2Ti0.8O3 (PZT) and magnetic La0.67Sr0.33MnO3 (LSMO) heterostructure grown on a Nb-doped SrTiO3 (001) substrate. Functional device working conditions are mimicked by gating the heterostructure with a Pt top electrode to apply an external electric field, which alters the magnitude and switches the direction of the ferroelectric (FE) polarization, across the PZT layer. PNR results show that the gated PZT/LSMO exhibits interfacial magnetic phase modulation attributed to ferromagnetic (FM) to A-antiferromagnetic (A-AF) phase transitions resulting from hole accumulation. When the net FE polarization points towards the interface (positive), the interface doesn’t undergo a magnetic phase transition and retains its global FM ordered state. In addition to changes in the interfacial magnetic ordering, the global magnetization of LSMO increases while switching the polarization from positive to negative and decreases vice versa. DFT calculations indicate that this enhanced magnetization also correlates with an out of plane tensile strain, whereas the suppressed magnetization for positive polarization is attributed to out of plane compressive strain. These calculations also show the coexistence of FM and A-AF phases at zero out of plane strain. Charge modulations throughout the LSMO layer appear to be unaffected by strain, suggesting that these charge mediated effects do not significantly change the global magnetization. Our PNR results and DFT calculations are in consort to verify that the interfacial magnetic modulations are due to co-action of strain and charge mediated effects with the strain and charge effects dominant at different length scale.