The Spindy project implements gate-tunable devices based on two distinct concepts. The first approach focuses on controlling spin-orbit torque (SOT) efficiency at the interface between a metal and a ferromagnet. This is achieved by tuning orbital currents in light transition metals through controlled oxidation and reduction processes. To investigate this, we conducted a detailed characterization of spin and orbital torques in bilayers composed of copper (Cu) and ferromagnets such as cobalt (Co) and permalloy (Py). Our results revealed that both the magnitude and sign of the damping-like torque strongly depend on the oxidation state of Cu and the specific ferromagnet used. Building on these findings, we developed solid-state gated devices in which Cu can be reversibly oxidized by driving oxygen ions across a gadolinium oxide gate. Using these devices, we demonstrated that the amplitude of the damping-like torque can be modulated by the applied gate voltage. The second approach involves tailoring the magnetic properties of the rare-earth-free metallic ferrimagnet Mn₄N. After optimizing the growth of epitaxial Mn₄N thin films with perpendicular magnetic anisotropy, bulk-like magnetic saturation and good magnetotransport properties, we showed that the coercivity of these films can be tuned by reversibly modifying their nitrogen content through voltage-driven nitrogen migration. This is accomplished in solid-state gated devices by interfacing Mn₄N with nitrogen-affine metals such as vanadium (V) or tantalum (Ta). Our experiments demonstrate that Mn₄N can be switched between a low-coercivity state, where magnetization can be electrically reversed using low current, and a high-coercivity state, suitable for robust magnetic information storage. In such devices, when the magnetization vector is set with a low field in the low coercivity state, its orientation retained after gating to the high coercivity state. This represents a proof-of-principle energy efficient tunable device. We also demonstrate that the magnetization of Mn₄N can be electrically switched using an applied current. However, the switching is only partial, as the current required to reverse the magnetization throughout the entire film thickness generates excessive heat, risking device failure. Notably, the switching efficiency improves after gating, highlighting the potential of these devices as a promising platform for fully electrical reading and writing of magnetic information.
