Our group focuses on the functional properties of nanoscale materials and nanoscale phenomena. While this is an extremely broad identification, it accurately captures the breadth of our research ambitions. We have previously worked on materials for hard-drives, spintronics, motors and generators, biomedical applications, engineering applications, catalysts, thermoelectrics, optically controlled magnetism, and memristors/magneto-ionics, among others. A list of our current research topics (as of October 2019) is listed below:
Collaborators: Prof. Eric Fullerton (UCSD), Dr. Lisa Debeer-Schmitt (ORNL), Dr. Sergio Montoya (NAVWAR), Dr. Julie Borchers (NIST), Dr. Alexander Grutter (NIST)
Magnetic skyrmions are chiral magnetic textures, in-which the magnetic moments wrap into a continuous co-planar loop, with magnetization at the core and perimeter of the loop oriented in opposite out-of-plane orientations. This unusual magnetic structure gives the skyrmion a topological protection, meaning it cannot be continuously deformed into any other shape (which belongs to a different topologically class), giving it robust stability against certain types of defects. The small size (typically <100 nm) and stability against defects make the skyrmion attractive for next generation spin-based logic and data technologies (e.g. spintronics). The problem with skyrmions is that they tend to be stable only in a narrow parameter space well below room temperature and always with an external magnetic field. The topological protection also give the skyrmion quasi-particle properties as-well, which are of interest from a fundamental science perspective.
Our lab has worked closely with Prof. Fullerton, Dr. Debeer-Schmitt and Dr. Montoya to discover new hybrid skyrmion structures which are stable at room temperature and over a range of magnetic fields, including zero field! In this work and others we have developed methodologies to order magnetic skyrmions into large lattices. Our lab has recently used small angle neutron scattering to investigate the high-speed dynamics (~2 Ghz) and low-speed lattice formation dynamics (~1 Hz) in skyrmions. We have also been successful at grazing-incidence SANS measurements of skyrmions, allowing us to directly determine the depth profile.
We have been awarded a DOE Career Award to pursue this research (06/2020)
Voltage Control of Materials Using Ionic Migration
Collaborators: Dr. Alexander Grutter (NIST), Dr. Julie Borchers (NIST), Prof. Kai Liu (Georgetown),
Controlling the properties of materials with voltage offers a paradigm-shift in technology development. At the flip of a switch, we can have on-demand magnets, electronics, mirrors/windows, thermal insulators/conductors. This premise could be used, for example, to store data for computation devices, or as optical or thermal isolators. Most attempts to achieve this level of control have relied on designing the electronic structure of materials, which tends to have small, volatile effects. We propose that controlling ion distributions instead offers low-power, non-volatile control that occurs on a much larger scale. Our group has focused on the control of magnetism, electrical resistance, structure, and superconductivity using voltage-controlled ionic migration. Leveraging the element-specific sensitivity of X-ray spectroscopy, and the depth-resolved sensitivity of neutron reflectometry, we are able to map where the oxygen ions move and they effect other elements within the material. Using ion migration, we have demonstrated the ability to inject or remove oxygen ions, promoting or suppressing the materials within metals and oxides.
Functional High Entropy Materials
Collaborators: Prof. Seungha Shin (UTK), Prof. Philip Rack (UTK/ORNL), Prof. Peter Liaw (UTK), Prof. Veerle Keppens (UTK), Dr. Lisa Debeer-Schmitt (ORNL), Dr. Nicholas Butch (NIST/UMD), Dr. Julie Borchers (NIST), Prof. Hugh Metal (UTK)
High entropy alloys and oxides (HEA/HEO) are materials comprised of five or more elements randomly decorating a single crystal lattice. The large distribution in atomic mass, size, electronegativity, spin, and inter-atomic coupling cause significant changes to the local electronic and structural environment, which critically impacts the properties of the material. As a result of the 'chaotic' distributions within the material, developing a direct understanding of these materials is challenging, however, simple considerations about the interactions and their length scales quickly reveals that there is interesting science in these materials. Indeed this has been borne-out in several experiments, revealing fascinating phenomena in magnetism, thermoelectricity and superconductivity. Here at the University of Tennessee, HEA/Os have been investigated for structural applications for quite some time; we have some of the world’s experts on these materials. Our lab hopes to leverage this expertise through collaborative projects to accelerate the investigation into these materials from a functional perspective. In less than a year, we have discovered exciting new materials, including high-anisotropy ferromagnets and superconductors with high critical fields. We are also working on developing high- throughput, machine learning based techniques to navigate the vast HEA/O parameter space. These are exciting new directions for our lab and will be constantly changing as new discoveries are made! Keep an eye out for our work!
Planned Future Directions
Starting at UTK in August 2018, we are in a state that we can still define our lab's identity and are flexible on our research! Enabled by generous equipment donations, loans and transfers we are rich in equipment and hence opportunity. In the near future we are working on assembling a quantum materials growth cluster, allowing us to grow thin-films of materials that are traditionally not compatible or have a tendency to cross-contaminate. This new instrument will give us opportunities to study new materials that have never before been fabricated. We plan to leverage this instrument to investigate quantum phenomena, most notably at interfaces with topological insulators.
In another exciting direction, we hope to make low-density materials with integrated functionalities, in the form of nanofoam structures. In this work, we will electrodeposit nanowires, which will be woven into a bird's nest like structure. These structures have been shown to possess densities of <0.1% bulk, with surface area to volume ratios of >106.
Unique Lab Capabilities
Prof. Gilbert spent four years at the NIST Center for Neutron Research, performing neutron scattering experiments on magnetic materials. While his focus was on thin-films probed with neutron reflectometry, Prof. Gilbert has also performed SANS, triple-axis diffraction, and time-of-flight spectroscopy. Neutron scattering provides a unique insight into material properties, probing the nuclear and magnetic structure inside materials, including atomic-resolution at burried interfaces.
This is a video from the NIST Center for Neutron Research descibing the use of neutrons in scientific invrestigations
Here is a video highlighting neutron research at the High Flux Isotope Reactor at Oak Ridge National Laboratory.
Here is a video highlighting neutron research at the Spallation Neutron Source at Oak Ridge National Laboratory.
Here is a video narrated by Sir Patrick Stuart highlighting neutron research at the European Spallation Source.