Our research focuses on utilising either thin film growth techniques or application of external electric fields to control the magnetic or electronic properties of advanced functional materials. We are particularly interested in the electronic modifications in materials resulting from application of very large electric fields during liquid electrolyte gating and how to utilise these changes to make novel computing devices. We also study magnetic thin films for spintronic applications (such as magnetic data storage) where we use disorder and nanostructuring to control the properties. Representative examples of our work can be found below.
Liquid Electrolyte Gating to Realise Novel Materials Properties
“Distinct Electronic Structure of the Electrolyte-Gate Induced Conducting Phase in Vanadium Dioxide Revealed by High Energy Photoelectron Spectroscopy” J. Karel, et al., ACS Nano (2014)
VO2 is a promising material for future computing devices because it exhibits a transition from conducting to non-conducting behaviour (metal-insulator transition) near room temperature which is characterised by a change in resistivity of over five orders of magnitude. Perhaps more interestingly, it was found that liquid electrolyte gating leads to a suppression of this transition entirely, making the material conducting down to very low temperatures. This effect is also reversible. It was known that the technique induces changes in the structure and resistivity of the material, but the modifications in the electronic structure as a result of this procedure were not known. Our work found that a distinct electronic structure is produced. These results suggest liquid electrolyte gating as an extremely promising route to reversibly create new metastable states in materials for future computing devices.
(a) Experiment and device schematic. The gate electrode and the electrical contacts to the channel are formed from Au and shown as yellow. The VO2 channel is depicted in pink, and the semi-transparent turquoise droplet is the ionic liquid. The linearly p-polarized light impinges the sample at an incidence angle of 5º and 60º for a near-normal (θ =85º) and off-normal (θ =30º) photoemission measurements. (b) VB spectra at near-normal emission condition measured at 3.0 keV for VO2 in the rutile (350 K), gated (120 K) and monoclinic (120 K) states. Subfigures (c), (d) and (e) show the dependence of the VB spectra intensity on the light incidence for the three phases. Solid (dashed) lines represent the incidence angle of 5º (60º). Inset figures depict the relationship between orbital polarization and the electric field direction (E) of the light in each case.
“A Transparent Conducting Oxide Induced by Liquid Electrolyte Gating” Proceedings of the National Academy of Science C.E. ViolBarbosa, J. Karel, et al., (2016)
Highly conducting transparent oxides are essential in modern technologies, where optical transparency through a low resistance electrode is needed. These materials can be readily found in applications such as electronic displays, touchscreens and photovoltaics; they are also employed in energy-conserving windows to reflect the infrared spectrum. Our work discovered that highly conducting transparent oxide films can be formed by electrolyte gating thin films of tungsten trioxide, WO3, that are insulating as initially prepared. The results of this work point toward electrolyte gating of insulating oxides as a novel means of obtaining new classes of transparent conducting electrodes for touchscreens, electronic displays and photovoltaics.
WO3 thin film (a) optically transparent and insulating in the as-prepared state and (b) optically transparent and metallic after liquid electrolyte gating. Liquid electrolyte gating leads to a slight expansion of the material, which is exaggerated for clarity in (b).
Controlling Magnetic Thin Film Properties using Disorder and Nanostructuring
“Evidence for In-Plane Tetragonal c-axis in MnxGa1-x Thin Films using Transmission Electron Microscopy” J. Karel et. al., Scripta Materialia (2016), and “MnxGa1-x Nanodots with High Coercivity and Perpendicular Magnetic Anisotropy” J. Karel et. al., Applied Surface Science (2016)
Mn-Ga is an interesting material for future data storage devices, in part due to it exhibiting uniaxial magnetic anisotropy. This means that the magnetic moments point along a single crystallographic axis in the material. In thin film form, this property manifests itself as so called perpendicular magnetic anisotropy (PMA), which means the magnetic moments align perpendicular to the film plane. A magnetic data storage device with PMA is sought after since it increases device stability by reducing thermal magnetic fluctuations which can lead to data loss. Many researchers have found that Mn-Ga thin films exhibit a secondary magnetic phase, which is detrimental to the use of this material in data storage devices. The origin of this secondary phase was unclear, and our work found it originated from a fraction of the film with the crystallographic axis oriented in a different direction. We showed that this secondary magnetic contribution was reduced by modifying the thin film preparation conditions. We also showed that Mn-Ga nanodots could be prepared from these films using a low-cost self-assembly nanolithography procedure with polystyrene nanospheres; the dots still exhibited PMA after this procedure. These results suggest the material and fabrication technique can be used for preparation of nanostructured spintronic devices.
(a) Cross sectional HRTEM image from the x=0.75, 350oC sample. The inset shows a portion of the FFT in the film region. Diffraction spots for tetragonal Mn3Ga with 2 different orientations (200 and 004) are highlighted. (b) Color map showing the spatial distribution of the different crystallographic orientations highlighted in the FFT. The inset shows the film microstructure (c) HRTEM image and (d) corresponding FFT from the x=0.75, 300oC sample. The inset shows that the film forms faceted islands with a single out of plane orientation. [Figure from Karel, et al., Scripta Materialia (2016)
Schematic of nanolithography procedure and corresponding scanning electron microscope image. [Figure from Karel et al., Applied Surface Science (2016)]
“Using Structural Disorder to Enhance the Magnetism and Spin Polarisation in FexSi1-x Thin Films for Spintronics” J. Karel et al., Materials Research Express (2014)
In this work, structural disorder was utilised to improve the magnetic properties. That is, we evidenced an enhancement in the magnetic moment and spin polarisation by making the material amorphous (no long-range periodicity in the lattice). This result is particularly surprising when considered in the context of other amorphous transition metal alloys. It is the first time that an amorphous structure has actually lead to an enhancement in the magnetic moment and spin-polarisation, a fact which not only suggests the potential of this material as a spin injector but also that structural disorder could be used as a method to enhance the properties of other material systems.
Magnetization at 2 K versus Fe concentration for FexSi1-x amorphous and crystalline materials. Solid symbols are experimental data: amorphous films (red squares) and epitaxial films (blue circles). Open symbols are theory: amorphous (red stars), A2 (half filled black circle), B2 (blue triangle), D03 (blue square with cross). The red and blue dashed lines are a guide to the eye.