Research
In our research, we use micromagnetic simulations, Monte Carlo simulations, as well as analytical modeling to predict how magnetic materials and nanostructures respond to external stimuli and to interpret experimental observations of complex magnetic states. Magnetic materials play a key role in a wide range of applications, such as energy, biomedicine, and computing, and deciphering their internal dynamics and magnetic fields enables us to drive these technologies further.
Ultrafast Magnetization Dynamics
In nanoscale ferromagnetic materials, the competition between different components of the internal magnetic field leads to the formation of localized magnetization whirls, as seen on the right here. These whirls have special properties that drive the magnetization to unwind with very high speed thus enabling ultrafast switching. In turn, the rapidly changing nano-magnetic fields generate secondary electromagnetic pulses with a duration of just a few picoseconds. These are promising phenomena for energy conversion applications, such as in nano-antennas and logic devices. Click on the image for the full story.
Controlling Magnetism with Mechanical Strain
The link between magnetism and mechanics has been studied since the discoveries of Joule and Villari. When stretching a ferromagnetic material, the distances between the atoms change, and these small changes can have drastic effects on the internal magnetic fields.
Recently, we were able to observe this magnetoelastic phenomenon with unprecedented resolution. Using powerful electron microscopes that can image the internal magnetic fields, we studied a nickel sample. What we observed is that the configuration of the magnetization changes drastically, forming an ordered array of equally sized domains, as seen in the figure here. Then, using large-scale computer simulations we were able to compare experiment and theory one-to-one and interpret the experimental observations and quantify the internal magnetic structure of the sample. These observations shed new light to the connection between structure and magnetism and will enable the further development of magnetostrictive technologies for energy harvesting and sensing applications. Click on the image for the full story on Nature Communications.
Deciphering Magnetism of Earth & Space Materials
Metallic meteorites, which cool down over billions of years, are a fascinating celestial laboratory of metallurgy. Iron-Nickel alloys have a phase diagram that contains several intermetallic phases. Of special interest is the 50-50 alloy of Fe and Ni with a tetragonal crystal structure, called tetrataenite, because it has a strong internal anisotropic magnetic field, making it promising for permanent magnet applications without relying on rare-earth elements. This phase is only found in meteorites because it is inaccessible in the laboratory. Studying meteorites allows us to examine and interpret the formation and stability of these important alloys to understand their magnetism. Through the use of powerful electron microscopes, tomography, and computer modeling, we were able to discover a new hidden intermetallic phase that comes along tetrataenite in the form of ultra-small particles. This opened new paths to study these promising systems and helped in obtaining a better understanding of their internal magnetic fields. Click on the image for the full story on ACS Nano Letters.
Magnetic Nanoparticles
When the dimensions of solids become reduced to the nanoscale, they often exhibit properties that do not exist in their bulk form. With growing capability of controlled synthesis and characterization, there have been significant advances in our understanding of the magnetic properties of nanoparticle systems. Iron-oxide nanoparticles have been attracting increasing interest due to their potential applications in biomedicine and other important fields.
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Meanwhile, the building blocks of such systems, have been around for millions of years in natural systems. One prime example are magnetotactic bacteria. These are organisms that biomineralize iron from their environment and form iron-oxide (or sulfide) nanoparticles along a cytoskeletal filament. The linear arrangement of these nanoscopic magnets forms a dipole with significant magnetic moment, and this serves as a compass that enables bacteria to navigate along Earth's magnetic field lines. Over the past few years, we have worked alongside geologists and microbiologists to investigate the magnetic properties of these bacteria.
By combining ferromagnetic resonance spectroscopy and theoretical modeling, we have been able to quantify the magnetic dipole field of the linear chains and provide tools to detect and characterize different strains of the bacteria based on their spectroscopic signatures. Importantly, the physical properties that govern the magnetism in bacteria are the same in any other setting, and therefore these are excellent testbeds for a wide range of nanotechnological studies with important potential applications. See, for exampe, our recent article in the Journal of Applied Physics.