Multiferroics are materials that are simultaneously ferromagnetic and ferroelectric. While their very existence presents an intriguing case study in fundamental materials science, the prospect of controlling magnetism using electric fields and vice versa offers potential applications in new types of electronic memory devices, switches, and sensors.
As a graduate student, I studied a variety of unconventional ferroelectric and multiferroic materials with the goal of understanding the atomistic mechanism that underlies them, and using this to reveal new electric and magnetic switching pathways.
Experimental evidence for the mechanism that stabilizes polar metals
‘Polar metals’ simultaneously exhibit two seemingly incompatible properties - long-range dipolar order, and metallicity, which is expected to screen dipolar interactions. This naïve expectation is brought into question by the very existence of robust polar metals. Here, studying the polar metal LiOsO3 using nonlinear optical polarimetry, we find experimental evidence for a proposed mechanism that stabilizes them - the free electrons responsible for metallicity and the atomic displacements responsible for polar order are on different atoms, and energetically separated from each other.
(left) Polar plot of the nonlinear optical response of LiOsO3. (right) The extracted nonlinear susceptibility along the polar direction d33, which is an order of magnitude smaller than in isostructural insulating materials.
H. Padmanabhan, Y. Park, D. Puggioni, Y. Yuan, Y. Cao, L. Gasparov, Y. Shi, J. Chakhalian, J. M. Rondinelli, V. Gopalan. Applied Physics Letters 113 (12), 122906 (2018)
Symmetry-based exploration of magnetoelectric switching pathways
Multiferroics offer the intriguing prospect of switching the magnetization using an electric field. Whether or not this is possible is determined by the specific atomic switching pathway that is most energetically favorable. Employing a new symmetry-based approach in density functional theory calculations, we explore the energy landscape of magnetoelectric switching pathways in the multiferroic BiFeO3. We find that there are two competing minimum energy pathways - one in which switching the electric field switches the magnetism, and another in which it does not, a finding that helps rationalize previous experimental results.
The minimum energy magnetoelectric switching pathways in BiFeO3 constrained by different dynamical symmetries, obtained using density functional theory calculations.
H. Padmanabhan, J. M. Munro, I. Dabo, V. Gopalan. Annual Review of Materials Research 50, 255 (2020)
J. M. Munro, H. Akamatsu, H. Padmanabhan, V. S. Liu, Y. Shi, L-Q. Chen, B. K. VanLeeuwen, I. Dabo, V. Gopalan. Physical Review B 98 (8), 085107 (2018)