1. By Jonathan Wood
Electric-field control of magnetic
thin film properties using ionic
liquid electric double layers
Jonathan Wood1, Martin Grell2 & Dan A. Allwood1
1Maerials Science & Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD
2 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
References
1. 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