Our group investigates new physical phenomena the emerge in nanoscale systems, at surfaces and interfaces of different materials. Our overarching goal is to develop fundamental understanding of the effects of confinement, interfaces, the resulting emerging interactions, and strongly nonequilibrium physical states that become possible to achieve only at nanoscale, and use this knowledge to develop nanoscale devices with new functionalities. Our laboratory is located in rooms W104, W106, and E108 of the Math and Science Building on the beautiful campus of Emory University.
“Non-classical spin transfer effects in an antiferromagnet”, PRL 2021. We utilize simulations of electron scattering by a chain of quantum spins, to demonstrate large contributions to the spin transfer effect in antiferromagnets (AFs) that cannot be accounted for by the existing semiclassical models of magnetism. AFs are attractive for the development of novel nanomagnetic devices, since their vanishing magnetization and high characteristic dynamical frequencies may enable fast and low-power device operation. Spin transfer effect – the transfer of angular momentum from the conduction electrons to the magnetic system – plays a central role in such devices, enabling electronically-driven rotation of magnetic order. Our results demonstrate the possibility to achieve a substantial enhancement of spin transfer efficiency by taking advantage of non-classical effects, and suggest the possibility to utilize spin transfer to control purely quantum magnetic systems such as spin liquids.
“Brillouin Light Scattering of Spin Waves Inaccessible with Free-Space Light”, Phys. Rev. Research (2020). Inelastic light scattering techniques, such as Brillouin light spectroscopy (BLS), provide efficient tools for studies of elementary excitations, such as phonons or magnons, in a variety of materials. However, the small momentum of light limits the accessible excitations to the center of the Brillouin zone. We utilize a metallic nanoantenna fabricated on the archetypal ferrimagnet yttrium iron garnet to demonstrate the possibility of Brillouin light scattering from large-wavevector, high-frequency spin wave excitations that are inaccessible with free-space light. The antenna facilitates sub-diffraction confinement of electromagnetic field, which enhances the local field intensity and generates momentum components significantly larger than those of free-space light. The approach we have demonstrated for spin waves can be generalized to other types of excitations and light scattering techniques. It provides access to short-wavelength excitations that can be important for the development of fast nanoscale devices, where such excitations are generally expected to play an important role.
“Energy and momentum conservation in spin transfer”, PRB 2020. The advent of spin transfer effect - the transfer of angular momentum from the current-carrying conduction electrons to magnetization in magnetic nanostructures – has transformed our understanding of magnetism, and led to the development of novel magnetoelectronic nanodevices. This effect is presently understood as a consequence of angular momentum conservation, but the role of the other two fundamental conservation laws, of linear momentum and energy - has remained a puzzle for over 20 years. We utilized quantum simulations to demonstrate that the laws of energy and momentum conservation substantially constrain the dynamical magnetization states induced by ST, as well as the resulting orbital and spin dynamics of electrons. Our findings provide a conceptually new framework for the analysis of ST, scattering of electrons by magnetic materials, and other related magnetoelectronic phenomena. These findings may provide a route for the development of novel laser-like magnetic nanodevices, where specific magnetic modes singled out by energy and momentum conservation are excited by spin transfer.
“Observation of Anomalous Non-Ohmic Transport in Current-Driven Nanostructures”, PRX 2020. The idea of Joule heating - an increase of temperature due to the electrical current flowing through materials or devices - has been a paradigm in condensed matter physics for almost two centuries. At the microscopic level, current-driven electron scattering on impurities and phonons results in phonon generation. The resulting phonon distribution may not be adequately described by a temperature as implied by the Joule's heating law. We have demonstrated that the current-driven phonon distribution in a variety of metallic microstructures is qualitatively different from that expected for Joule heating, as manifested by a weakly-singular linear dependence of resistance on current. Our results suggest that it may be possible to optimize thermal dissipation in nanodevices beyond the limits set by the Joule heating law.
Spin liquid in a thin-film ferromagnet/antiferromagnet bilayer
Among the general themes in modern science are the new fundamental phenomena and functionalities that emerge in "designer" materials and structures. Is it possible to form a new magnetic state of matter by putting together two materials exhibiting well-known magnetic phases, such as a ferromagnet (F) and an antiferromagnet (AF)? We gave a positive answer to this question in a series of papers, PRB 2015, PRB 2016, and PRB 2018, where we utilized time-domain measurements of magnetization aging to demonstrate that a sufficiently thin AF film can turn into a magnetic glass. In our latest work, published in JMMM 2019, we utilized variable-temperature, variable frequency transverse magnetic susceptibility measurement developed in our lab [example in the Figure on the left], to demonstrate that the magnetization of AF undergoes rapid freezing with decreasing temperature. The magnetic viscousity increases by four orders of magnitude within 20K temperature range, as expected for the magnetic glass transition. The newly identified magnetic phase may become particularly useful for the development of neuromorphic magnetic devices, since it is expected to stabilize a multitude of magnetization states in nanomagnets.