By Jonathan Wood

Electric-field control of magnetic thin film properties using ionic

liquid electric double layersJonathan Wood1, Martin Grell2 & Dan A. Allwood1

1Maerials Science & Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD2 Department of Physics & Astronomy, University of Sheffield, Hounsfield Road, Sheffield, S3 7RH

Introduction• We are studying the use of high electric-

fields of 40 GVm-1 to control and change magnetic properties of thin films

• We used electric double layers formed using ionic liquids to create E-fields (see fig. 1 a) & b)

• Ionic liquids are salts in their liquid states which have large electrochemical windows, high polarisability and in some cases a good stability in air1

• Thin films of permalloy, Ni80Fe20 were

covered with ionic liquid, EMITSFI, which has an electrochemical window of 4.2V and displays good air stability2

Fig. 1a) Schematic displaying electric double layer at the interface between cathode and ionic liquid. b) E-field equation, E, e-field, V, voltage and d is the separation between opposite charges

Experimental• 5nm permalloy films thermally

evaporated onto Si substrates, approximately 1 cm x 2 cm

• EMITSFI (fig. 2a) was deposited on the permalloy between Kapton tape strips

• An electronically conductive, transparent ITO coated glass layer was placed on top of the system (fig. 2b)

• Voltage was applied to the permalloy and ITO, generating an E-field perpendicular to the permalloy plane

• Magnetic properties were measured using MOKE & VSM

• Ionic liquid properties were measured using cyclic voltammetry of permalloy on Cu disks

• XPS analysed film surface properties

a)

b)

Fig. 2a) EMITSFI b) Schematic of experimental cell, H - magnetic field, KT- Kapton tape, IL- ionic liquid, ITO- indium tin oxide and red line- MOKE laser light

MOKE Measurementsa)

b)

MOKE• Magneto-optic Kerr

effect magnetometry measurements during voltage application

• 54% drop in coercivity between 0V and 4V (fig. 4)

• No change after voltage removal implies non-volatility Fig. 4 MOKE loops with increasing voltage to permalloy

electrode

VSM• Vibrating sample

magnetometry after exposure to increasing voltages

• 54% reduction in magnetic moment from 0V to 4V (fig. 5)

• Decreasing coercivity with increased voltage applied to permalloy electrode

Cyclic Voltammetry• The TFSI-/permalloy+

interface shows electric double layer characteristics from 0 to +2.5V (fig. 3)

• A large peak in current at +2.75 V could be due to oxidation at the interface or a chemical reaction

• The peak in forward sweep is repeatable, indicating reversibility

Fig. 3 Cyclic voltammogram for permalloy- EMITSFI-permalloy cell with voltages ranging from -0.2V to +4.5V

Conclusion• Cyclic voltammetry and XPS data shows increased voltage exposure increases

permalloy oxidation

• MOKE and VSM show this oxidation reduces coercivity and magnetic moment

References1. R. Hagiwara & Y.Ito, “Room temperature ionic liquids of alkylimidazolium cations and fluoroanions”, Journal of

Fluorine Chemistry, 2000, 105, 221-227

2. M. Hayyan et al., “Investigating the electrochemical windows of ionic liquids”, Journal of Industrial and Engineering Chemistry, 2013, 19, 106-112.

Fig. 5 VSM loops with increasing voltage to permalloyy electrode

XPS-Fe• X-ray photoelectron

spectroscopy after exposure to increasing voltages (fig. 6)

• Decrease in the metal peak after 2V exposure

• Fe2p1/2 oxide signal increased after 2V exposure

• Fe2p3/2 oxide peak broadens indicating further oxidation

Fig. 6 XPS spectra for Fe energy range with increasing voltage

Fig. 7 XPS spectra for Ni energy range with increasing voltage

XPS-Ni• Ni metal peaks strongly

decrease with voltage (fig. 7)

• Increase in oxide peaks, unclear which oxides present, NiO, Ni(OH)2 or both

• Largest change in Ni peak occurs after 2V

• In agreement with cyclic voltammetry data (fig. 3)

Future Work• Negative voltage ranges to investigate possible reversal in magnetic changes

• Other materials including aligned permalloy, CoFe and FePt (out-of-plane) and multilayers