Magneto-ionic effect in CoFeB thin films with in-plane and perpendicular to
plane magnetic anisotropy (Supplemental Material)
L. Baldrati,1,2 A. J. Tan,2 M. Mann,2 R. Bertacco 1,3, G. S. D. Beach2
1Dipartimento
2Department
di Fisica, Politecnico di Milano, Milano, 20133, Italy
of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139,
USA
3IFN-CNR,
Milano, 20133, Italy
Uniaxial in-plane magnetic anisotropy of as-grown CFB-Ta
In figure S1 we show the longitudinal MOKE data of a sub//Ta(26 nm)/Co0.6Fe0.2B0.2(1)/GdOx(4) thin films sample, before
patterning. The MOKE loops were acquired by a red laser beam at an angle of incidence of ~45° respect to the direction
normal to the plane of the sample, as a function of the in-plane angle. From the image an uniaxial in-plane anisotropy is
evident, as set by the small field applied during growth (~10 mT, in plane). No anisotropy was seen in the plane in
perpendicularly magnetized CFB-Pt samples, as expected. Note that this longitudinal MOKE setup does not have a
micrometric resolution, so that measurements on patterned capacitors in applied voltage as done by polar micro-MOKE are
not possible.
Figure S1: (a) Longitudinal MOKE hysteresis loops at an in-plane angle of 0° and 90° respect to the applied magnetic field
during growth. This magnetic field defines the easy axis of the as-grown sample. (b) Polar diagram of the remnant
magnetization MR/MS and normalized coercive field HC/HC,MAX determined by longitudinal MOKE, as a function of the in
plane angle. The maximum coercive field is μ0HC = 2.3 mT.
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Comparison between CFB-Pt and CFB-Ta stacks with identical CoFeB 0.9 nm nominal thickness
In Figure S2 we show selected μ-MOKE measurements in applied voltage of a sample of layout sub//Ta(4
nm)/Pt(10)/Co0.6Fe0.2B0.2(0.9)/GdOx(4). The as-grown CoFeB sample in a virgin state at 0.9 nm is in a multi-domain state, but
magneto-ionic effects are detected, resembling the case of the 0.7 nm sample, whose behavior is shown in Figure 2 of the
main text, thus signaling the persistence of the magneto-ionic effect at higher CoFeB thickness. These measurements further
allow a direct comparison to the CFB-Ta sample shown in Figure 3 of the main text, thanks to the identical CoFeB nominal
thickness of 0.9 nm. Note that the magnetization easy axis is perpendicular to the plane in the as-grown stack if Pt is used as a
buffer layer, while if Ta is used, the magnetization easy axis is in the plane. The bias necessary to probe magneto-ionic effect
in the case of CFB-Ta is higher, which is probably due to the fact that polar MOKE is less sensitive to in-plane magnetized
magnetic signal or to the fact that the layer in contact to the CoFeB (Pt or Ta) plays some role in the process.
Figure S2: Selected polar μ-MOKE loops from a sample of layout sub//Ta(4 nm)/Pt(10)/Co0.6Fe0.2B0.2(0.9)/GdOx(4). The
reorientation of the magnetic easy axis from in-plane magnetized to out-of-plane magnetized occurs also at this thickness.
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Multiple switching and deterioration
In figure S3 we show the switching on a sub//Ta(4 nm)/Pt(10)/Co0.6Fe0.2B0.2(0.7)/GdOx(4) specimen several times,
demonstrating the reversibility of the process. After the application of a high unipolar bias for a long time an irreversible
deterioration of the system occurs.
Figure S3: Evolution of the MR/MS ratio over time of a sample of layout sub//Ta(4 nm)/Pt(10)/Co0.6Fe0.2B0.2(0.7)/GdOx(4).
Low MR/MS values indicate the transition to in-plane magnetized easy axis, while if the ratio comes back to values near unity,
the magnetization easy axis has been reoriented perpendicularly to the plane of the sample. After 200 s, the deterioration of
the sample, due to the application of a high unipolar bias for a long time, can be seen. No further switching occurred from
that point on.
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Thermodynamic considerations on switching dynamics
In Figure 2a of the main text it is shown that when a positive voltage is applied, the evolution of the magnetic properties is
slower respect to a voltage of the same magnitude and opposite polarity. In Figure S4 the direction of movement of the ions
is depicted. Here we show by simple thermodynamic considerations that oxygen tends to bind to Gd in a closed system in
standard conditions, while if the gaseous oxygen in the air is considered, the thermodynamic stable state is reached when all
the layers are oxidized. Since from Table ST1 it is evident that Fe is the most favorable element other than Gd to be oxidized,
this will be the only one considered in the following analysis. Calculations including Co do not change the outcome.
Figure S4: direction of movement of the negatively charged oxygen ions if a positive or negative voltage is applied,
respectively.
Compound
Gd2O3
ΔHf0(kJ/mol)
-1819.6
Compound
FeO
Fe2O3
Fe3O4
ΔHf0(kJ/mol)
-272.0
-824.2
-1118.4
Compound
CoO
Co3O4
ΔHf0(kJ/mol)
-237.9
-891.0
Table ST1: Standard enthalpy of formation for Gd, Co and Fe oxides.
Closed system (no interactions with gaseous O2 in the air): if a negative voltage is applied the Fe is oxidized and the Gd
reduced at the interface (unfavorable)
9𝐹𝑒 + 4𝐺𝑑2 𝑂3 → 3𝐹𝑒3 𝑂4 + 8𝐺𝑑, ∆𝐻𝑓0 = 3923𝑘𝐽/(12 𝑚𝑜𝑙 𝑂)
If a positive voltage is applied the opposite happens (favorable)
3𝐹𝑒3 𝑂4 + 8𝐺𝑑 → 9𝐹𝑒 + 4𝐺𝑑2 𝑂3 , ∆𝐻𝑓0 = −3923𝑘𝐽/(12 𝑚𝑜𝑙 𝑂)
So, if the system is closed there is no thermodynamic reason why the application of a negative voltage provokes a faster
evolution than a positive one. However, there could be a kinetic reason: a negative voltage drives O2- ions in the CoFeB.
During this evolution, the oxidation front can move freely, since CoFeB in the starting state is not oxidized. If, on the
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contrary, a positive voltage is applied, the O2- ions at the CoFeB/GdOx interface have to meet a vacancy at the CoFeB/GdOx
interface, in order for the second reaction to occur, but the vacancy could take some time to diffuse and reach the ion.
Open system (interactions with O2 in the air): If a negative potential is applied the processes occurring are the oxidation of
Fe at the Fe/GdOx interface (it can be seen by the change in the MOKE hysteresis loop):
9𝐹𝑒 + 4𝐺𝑑2 𝑂3 → 3𝐹𝑒3 𝑂4 + 8𝐺𝑑, ∆𝐻𝑓0 =
3923𝑘𝐽
12 𝑚𝑜𝑙 𝑂
While at the surface Gd is oxidized by the oxygen in the air (it can not be seen by MOKE):
7276𝑘𝐽
12 𝑚𝑜𝑙 𝑂
8𝐺𝑑 + 6𝑂2 → 4𝐺𝑑2 𝑂3 , ∆𝐻𝑓0 = −
So that the whole process in this case is thermodynamically favorable
9𝐹𝑒 + 6𝑂2 → 3𝐹𝑒3 𝑂4 , ∆𝐻𝑓0 = −
3355.2𝑘𝐽
12 𝑚𝑜𝑙 𝑂
If, instead, a positive voltage is applied the reactions occurring are:
4𝐺𝑑2 𝑂3 → 8𝐺𝑑 + 6𝑂2 , ∆𝐻𝑓0 = +
7276𝑘𝐽
12 𝑚𝑜𝑙 𝑂
3𝐹𝑒3 𝑂4 + 8𝐺𝑑 → 9𝐹𝑒 + 4𝐺𝑑2 𝑂3 , ∆𝐻𝑓0 = −
3923𝑘𝐽
12 𝑚𝑜𝑙 𝑂
So that the whole process in this case is thermodynamically non favorable
3𝐹𝑒3 𝑂4 → 9𝐹𝑒 + 6𝑂2 , ∆𝐻𝑓0 = +3355.2𝑘𝐽/(12 𝑚𝑜𝑙 𝑂)
Using this second model the thermodynamic of the process when a negative voltage is applied is favorable. Anyway the same
kinetic considerations made before could apply in this second case as well.
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