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Unikandanunni, Vivek

Publications and source records attributed to Unikandanunni, Vivek.

5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Ultrafast Terahertz Field Control of the Emergent Magnetic and Electronic Interactions at Oxide Interfaces

Ultrafast electric-field control of emergent electronic and magnetic states at oxide interfaces offers exciting prospects for the development of the next generation of energy-efficient devices. Here, it is demonstrated that the electronic structure and emergent ferromagnetic interfacial state in epitaxial LaNiO3/CaMnO3 superlattices can be effectively controlled using intense, single-cycle THz electric-field pulses. A suite of advanced X-ray spectroscopic techniques is employed to measure a detailed magneto-optical profile and the thickness of the ferromagnetic interfacial layer. Then, a combination of time-resolved and temperature-dependent optical measurements is used to disentangle several correlated electronic and magnetic processes driven by ultrafast, high-field THz pulses. Sub-picosecond non-equilibrium Joule heating of the electronic system is observed, ultrafast demagnetization of the ferromagnetic interfacial layer, and slower dynamics indicative of a change in the magnetic state of the superlattice due to the transfer of spin-angular momentum to the lattice. These findings suggest a promising avenue for the efficient control of 2D ferromagnetic states at oxide interfaces using ultrafast electric-field pulses.

X-ray spectroscopy and scattering↗
DOE OSTI2024

Characterizing the nonequilibrium response of FeRh thin films using time-domain thermoreflectance

Time-domain thermoreflectance (TDTR) characterization of FeRh throughout its first-order antiferromagnetic (AF) to ferromagnetic (FM) transition shows that the transient reflectance ΔR⁡(t)/R strongly depends on the magnetic order of the sample. Using TDTR, which uses optical pulses to induce small temperature excursions, we have found that Δ⁢R⁡(t)/R of the AF phase exhibits a large negative response, while the response of the FM phase is positive. This magnetic phase sensitivity has allowed us to study the transient response of both the AF and FM phases to the pump-pulse excitation and the mixed phase of the material. These results are significant since the ultrafast properties of antiferromagnetic materials and mixed antiferromagnetic and ferromagnetic materials are difficult to detect using other conventional techniques. We have found that the AF phase exhibits a strong subpicosecond decaying signal not observed in the FM phase. The magnetic phase dependence of the sign of Δ⁢R⁡(t)/R is qualitatively explained using the results of ab initio density functional theory calculations. Using the two-temperature model, we found that the change in the thermalization time across the transition is caused by differences in both the electronic heat capacity and the electron-phonon coupling factor of the AF and FM phases. The electron-phonon coupling constant in the AF phase is also determined using the two-temperature model conducted using the ntmpy code package. For the FM phase, we provide boundaries for the magnitude of the electron-phonon coupling factor for the FM phase. Finally, these results indicate that TDTR can be used to study the transient properties of magnetic materials that are otherwise challenging to probe.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Anisotropic ultrafast spin dynamics in epitaxial cobalt

We investigate the ultrafast spin dynamics in an epitaxial hcp($1\bar100$) cobalt thin film. By performing pump–probe magneto-optical measurements with the magnetization along either the easy or hard magnetic axis, we determine the demagnetization and recovery time for the two axes. We observe an average of 33% slower dynamics along the easy magnetization axis, which we attribute to magneto-crystalline anisotropy of the electron–phonon coupling, supported by our ab initio calculations. This points toward an unambiguous and previously undisclosed role of anisotropic electron–lattice coupling in ultrafast magnetism.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗