Proton Transport in Hydrated GdOx Measured with GdCo Magnetic Sensing Layer

Abstract

Solid state ionic materials such as metal oxide ceramics are of interest due to the wide variety of properties that can be controlled through voltage control of ion concentration and transport in these materials, e.g. electrical resistivity, optical properties, and magnetic properties. Ion movement through a solid oxide electrolyte film can be controlled very simply by electrical gating of the oxide, i.e. applying a voltage across the film. The ease of processing and CMOS compatibility of metal oxides makes them strong candidate materials for next-generation devices for information storage and computing, sensing, energy generation (fuel cells), and energy storage (batteries).Hydrogen ions (protons) take the form of (OH)O • defects in oxides and move by hopping between oxygen sites (Grotthuss mechanism); their small size allows for high conductivity.8 Protonic conduction in oxides has been studied in solid oxide fuel cells (SOFCs)1; proton incorporation has also been shown to change the optical4, electrical2,3, and magnetic properties of the oxide5, or a combination of multiple properties e.g. through inducing phase changes in strontium cobalt oxide (SCO)6. However, current work focuses on materials that require high-temperature processing and operation (>200C, or up to 600-1000C for SOFCs), and bulk or microscale films. This work aims to take solid oxide protonic devices down to the ultrathin (10-100nm) scale, to introduce simpler fabrication techniques (sputtering), and to achieve room-temperature operation of ionic devices. We investigate gadolinium oxide (GdOx) as a model material due to its hygroscopic nature, which allows for fast room-temperature proton conduction.7,8 Using variations of a Pt/Co/GdOx/Au device structure, a gate voltage of 2-5V is applied, initiating a hydrolysis reaction at the top electrode and allowing water to be incorporated into the film. The electric field within the GdOx drives protons to the bottom interface, where it can diffuse into the adjacent magnetic layer and affect a variety of properties. Thin film ferromagnetic materials such as cobalt have been shown to undergo dramatic changes in their magnetic anisotropy with voltage gating, due to the incorporation and removal of hydrogen at the interface with the electrolyte and within the magnetic layer.6 Control of the compensation temperature of a thin film ferrimagnet and control of exchange bias in an antiferromagnet have also been demonstrated with this magneto-ionic proton pump mechanism.9,10 Using a gadolinium cobalt (GdCo) magnetic layer inserted into the MOM device stack to detect the presence of hydrogen, we have developed a sensor for use in a novel time-domain proton transport measurement. We apply short voltage pulses to the device and use magneto-optic Kerr effect (MOKE) to observe the resulting changes in the compensation temperature and magnetic state of the sensor layer. This can be used as a more direct measure of the presence of protons at the bottom interface, as compared to frequency domain measurements such as electrochemical impedance spectroscopy (EIS). We also show that humidity affects the proton transport in the oxide layer, using neutron and x-ray reflectometry to observe the growth of a gadolinium hydroxide (Gd(OH)3) layer with high hydrogen concentration upon exposing the film to a high humidity environment.11 References Q. Minh, M. B. Mogensen. Reversible Solid Oxide Fuel Cell Technology for Green Fuel and Power Production. Electrochem. Soc. Interface, 55–62 (2013).Huang et al. Three-terminal resistive switch based on metal/metal oxide redox reactions, Scientific Reports, 7 (2017).Messerschmitt, F. Kubicek, M., Rupp, J. L. M. How Does Moisture Affect the Physical Property of Memristance for Anionic–Electronic Resistive Switching Memories?, Advanced Functional Materials, 25, 5117-5125 (2015).Huang, M., Jun Tan, A., Büttner, F. et al. Voltage-gated optics and plasmonics enabled by solid-state proton pumping. Nat Commun 10, 5030 (2019).Tan et al. Magneto-ionic control of magnetism using a solid-state proton pump, Nature Materials, 17 (2018).Lu, N. et al. Electric-field control of tri-state phase transformation with a selective dual-ion switch, Nature 546, 124–128, (2017)Norbya, T. Proton conduction in oxides, Solid State lonics, 40/41, 857-862 (1990).Larring and Norby. Protons in rare earth oxides, Solid State Ionics, 77 (1995).Huang, M., Hasan, M.U., Klyukin, K. et al. Voltage control of ferrimagnetic order and voltage-assisted writing of ferrimagnetic spin textures. Nanotechnol. 16, 981–988 (2021).Zehner, J. et al. Magnetoionic control of perpendicular exchange bias. Phys. Rev. Materials 5, L061401 (2021).Tan, A.J., Huang, M., Sheffels, S., et al. Hydration of gadolinium oxide (GdOx) and its effect on voltage-induced Co oxidation in a Pt/Co/GdOx/Au heterostructure. Phys. Rev. Materials 3, 064408 (2019). Figure 1