Energy Storage via Topological Spin Textures

We formulate an energy-storage concept based on the free energy associated with metastable magnetic configurations. Despite the active magnetic region of the battery being electrically insulating, it can sustain effective hydrodynamics of spin textures, whose conservation law is governed by topology. To illustrate the key physics and potential functionality, we focus here on the simplest quasi-one-dimensional case of planar winding of the magnetic order parameter. The energy is stored in the metastable winding number, which can be injected electrically by an appropriately tailored spin torque. Because of the nonvolatility and the endurance of magnetic systems, the injected energy can be stored essentially indefinitely, with the topological charge cycles that do not degrade over time.

Figure 1

A schematic of a quasi-one-dimensional spin spiral of length $L$ that stores energy density $\propto ({\partial }_x\phi {)}^2$, where $\phi$ is the order-parameter angle within the magnetic easy plane. The winding density $\rho \equiv -{\partial }_x\phi$ of the easy-($yz$)-plane directional order parameter is associated with a collective metastable spin current $I_s$ flowing along the loop. This spin current is loaded by applying a spin torque $\mathbf{\tau }$ at the ends of the loop, utilizing the spin Hall effect in metallic wires (red) deposited on top. Externally applied electrical current $I$ is thus used to load energy into the battery. This energy is eventually released by exerting a reciprocal spin-motive force onto an external spintronic circuit. The blue gate controls magnetic properties underneath it, allowing one to locally interrupt the collective spin current on demand, by turning the gate voltage $V_g$ on or off.