The recent increase in connectivity of the modern world has left us dependent on the battery life of our personal electronics, forcing us to keep a watchful eye on the icon in the corner of our screen as it ticks toward 0%. Enormous amounts of time and effort have been dedicated to discovering new battery materials and improving existing ones that pack larger energy-storage capacities into smaller spaces. However, making our electronics more energy efficient may be complementary extending the lifetime of our electronics.
As reported in Nature Communications (doi:10.1038/ncomms12264), a research team led by Dustin Gilbert and Alexander Grutter from the National Institute of Standards and Technology (NIST), have implemented a recently demonstrated “magneto-ionic” approach in a push toward ultralow power electronics. Their approach utilizes electric fields to alter the chemical and magnetic makeup of materials, and opens pathways to nonvolatile memory and logic devices that potentially require much less power to operate.
Gilbert says, “In classical electronics you’re relying on the charge of an electron; as the electron moves through your material, scattering produces heat. In this [new approach] there’s essentially no movement of the electrons; you’re applying a voltage only and no real current. The voltage drags oxygen from the oxide into the neighboring metallic material, changing its magnetic properties.”
Illustration of oxygen migration mechanism: (a) as-grown film, (b,c) positive electrical bias, and (d,e) negative electrical bias. AlOx (red), GdOx (green), metallic ferromagnetic Co (light blue), insulating non-FM Co (dark blue), and interstitial oxygen (orange). Credit: Nature Communications.
The researchers grew thin-film heterostructures with special consideration given to ensuring clean, well-defined interfaces between the AlOx, GdOx, and Co layers. Utilizing a powerful technique called polarized neutron reflectometry (PNR), the researchers were able to probe the chemical and magnetic profile of the sampl e as a function of depth. PNR measurements revealed diffusional migration of oxygen from the AlOx and GdOx oxide layers into the Co layer when the films were heated to 230°C. The migration of oxygen ions was demonstrated to be significantly promoted by the application of an electric field through the film thickness. Furthermore, the researchers found that the process of cobalt oxidation and oxygen migration was semi-reversible. By simply reversing the polarity of the field, oxygen at the GdOx/Co interface returned to the oxide layers, but the oxygen buried deeper in the Co layer remained in place.
The researchers asserted that the films must be broken up into individual grains acting as if isolated, rather than as a continuous film. Oxygen that is loosely bound to the surface of the oxide grains can be pulled away much easier and diffuse readily along the surface of the Co grains. However, diffusion would be somewhat slower into the core of the Co grains producing a slow transition from a metallic to insulating state with an accompanying change in the magnetic properties. When the field is reversed the oxygen on the surface of the Co grains can diffuse back to the oxide layer, but due to an electric screening effect, the oxygen in the core of the Co grains remains trapped. This remaining oxide acts as a secondary magnetic phase and an irreversible change in the overall magnetic properties.
Until now this effect has only been seen in ultrathin films; this new work points to its application in bulk materials. Using this reversible migration of ions as a way to control physical properties in materials holds enormous potential for improving the energy efficiency of countless types of devices. Gilbert envisions this process enabling “control over essentially every physical property of our material … this work is just the first step.”
