Voltage Control of Magnetic Properties in Ferromagnetic Thin Films

Low-power voltage control of magnetic properties of ferromagnetic thin films has long been of interest for many technical applications including magnetic random access memory (MRAM) devices, high-frequency tuneable magnetic devices and magnetic sensors. This research explores novel routes to further develop theses devices. This thesis presents a study of the variation of magnetic properties of permalloy (Ni80Fe20) and Ni thin films when part of an ionic liquid cell via a voltage induced oxidation thinning/thickening of the magnetic film. The cells consisted of a thin ferromagnetic film (<50 nm), the ionic liquid 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTSFI) and an indium-tin oxide (ITO) coated glass slide. Magneto-optical Kerr effect (MOKE) magnetometry and vibrating sample magnetometry (VSM) demonstrated drops in coercivity and magnetisation by more than 50%, upon application of low voltages (<4 V), which were partially recovered upon reversal of the voltage polarity. X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry studies showed a voltage dependent oxidation of the thin films and gave the electrochemical window of the cell to be approximately 4.2 V, with redox peaks at ± 2.5 V. Ferromagnetic resonance (FMR) spectroscopy demonstrated thickness dependent changes in magnetisation of 5 nm, 10 nm and 50 nm permalloy films, while changes in surface anisotropy and Gilbert damping constant were also observed. This work hopes to further the understanding of the mechanisms involved in the voltage control of magnetic properties in thin films and advance the technology for future applications.