arXiv subjects

Varun Kumar Kushwaha

Publications and source records attributed to Varun Kumar Kushwaha.

2 recordsLinked to original sources

arXiv2025

Non-equilibrium Magnon Engineering Enabling Significant Thermal Transport Modulation

Thermal conductivity, a fundamental parameter characterizing thermal transport in solids, is typically determined by electron and phonon transport. Although other transport properties including electrical conductivity and thermoelectric conversion coefficients have material-specific values, it is known that thermal conductivity can be modulated artificially via phonon engineering techniques. Here, we demonstrate another way of artificially modulating the heat conduction in solids: magnonic thermal transport engineering. The time-domain thermoreflectance measurements using ferromagnetic metal/insulator junction systems reveal that the thermal conductivity of the ferromagnetic metals and interfacial thermal conductance vary significantly depending on the spatial distribution of nonequilibrium spin currents. Systematic measurements of the thermal transport properties with changing the boundary conditions for spin currents show that the observed thermal transport modulation stems from magnon origin. This observation unveils that magnons significantly contribute to the heat conduction even in ferromagnetic metals at room temperature, upsetting the conventional wisdom that the thermal conductivity mediated by magnons is very small in metals except at low temperatures. The magnonic thermal transport engineering offers a new principle and method for active thermal management.

cond-mat.mtrl-sci↗

arXiv2023

Observation of charge-to-spin conversion with giant efficiency at Ni$_{0.8}$Fe$_{0.2}$/Bi$_{2}$WO$_{6}$ interface

Magnetization switching using spin-orbit torque offers a promising route to developing non-volatile memory technologies. The prerequisite, however, is the charge-to-spin current conversion, which has been achieved traditionally by harnessing the spin-orbit interaction in heavy metals, topological insulators, and heterointerfaces hosting a high-mobility two-dimensional electron gas. Here, we report the observation of charge-to-spin current conversion at the interface between ferromagnetic Ni$_{0.8}$Fe$_{0.2}$ and ferroelectric Bi$_{2}$WO$_{6}$ thin films. The resulting spin-orbit torque consists of damping-like and field-like components, and the estimated efficiency amounts to about 0.48 $\pm$ 0.02, which translates to 0.96 $\pm$ 0.04 nm$^{-1}$ in terms of interfacial efficiency. These numbers are comparable to contemporary spintronic materials exhibiting giant spin-orbit torque efficiency. We suggest that the Rashba Edelstein effect underpins the charge-to-spin current conversion on the interface side of Ni$_{0.8}$Fe$_{0.2}$. Further, we provide an intuitive explanation for the giant efficiency in terms of the spin-orbit proximity effect, which is enabled by orbital hybridization between W and Ni (Fe) atoms across the interface. Our work highlights that Aurivillius compounds are a potential addition to the emerging transition metal oxide-based spin-orbit materials.

cond-mat.mtrl-sci↗