174
5 Compositionally Modulated and Multilayered Deposits
presented in Chap. 4.3.3 for ultrathin magnetic layers. Instead, oxidation/reduction
cycles in magneto-ionic devices can be taken as a semi-bulk process.
The impact of the electrochemical treatment on the magnetization behaviour of
2.8 nm thick CoPt sample was studied in an aprotic medium (LiClO 4 in the equimolar
mixture of ethylene carbonate dimethyl carbonate) [182]. The role of the native oxide
reduction in the change of the magnetization properties was well evidenced. Although
the role of the double layer charging effect in the magnetic behaviour could be
excluded, no reversible magnetization change could be achieved in the non-reactive
medium applied in the potential range where Co did not dissolve selectively.
A reversible change in the magnetic properties was achieved for Fe/FeO x layer
pairs [183–185]. For this system, the controlled-potential electrochemical oxidation
of the Fe layer in a dilute KOH solution can lead to an oxide layer formation with
a well-defined thickness of about 3–4 nm, which is equivalent to the oxidation of
about 1.5 nm of metallic Fe due to the volume expansion accompanying the oxidation
[183]. The relative change in magnetization is proportional to the thickness ratio
of the remaining metallic Fe and the electrochemically produced oxide, and it is
straightforward that the effect measured increase with a decrease of the total layer
thickness. The change in magnetization measured as a function of the electrode
potential and the corresponding layer structure is shown in Fig. 5.16.
The great achievement related to the electrochemical oxidation/reduction of thin
Fe layers in dilute KOH solutions was that the redox transformation can take place
without either a morphology change in the layer of an iron loss, at least for the limited
cycle number (12) that was used for the demonstration of the reversibility effect. If
the Fe layer is produced by electrodeposition, an all-electrochemical device can be
constructed where the magnetization is directly governed by the sample voltage
[184]. However, if the underlayer exhibits perpendicular magnetic anisotropy (like
FePt as shonw in [182]), the electrochemical reduction/oxidation cycle results in
a transient between rigid magnet to exchange spring magnet, respectively, hence
opening the way for electrochemically driven spintronic devices.
5.4.7 Annealing Behaviour of Electrodeposited Multilayers
Electrodeposition is a non-equilibrium sample preparation method. Therefore, asreceived electroplated samples often exhibit a large structural fault density which
can be partly eliminated with annealing. Depending on the properties of the samples,
annealing effects show a great diversity and the optimum annealing temperature may
also vary from one sample to another.
Annealing at temperatures below 200 °C does not result in a significant long-range
atomic motion in the solid deposit but leads only to the relaxation of dislocations
and local stress. Depending on the nature of the samples, a moderate-temperature
annealing may also lead to the improvement of the sample properties. This can be
seen for Co/Cu multilayers with GMR properties where a gradual increase of the
magnetoresistance was found for annealing temperature up to 300 °C [186]. The
5 Compositionally Modulated and Multilayered Deposits
presented in Chap. 4.3.3 for ultrathin magnetic layers. Instead, oxidation/reduction
cycles in magneto-ionic devices can be taken as a semi-bulk process.
The impact of the electrochemical treatment on the magnetization behaviour of
2.8 nm thick CoPt sample was studied in an aprotic medium (LiClO 4 in the equimolar
mixture of ethylene carbonate dimethyl carbonate) [182]. The role of the native oxide
reduction in the change of the magnetization properties was well evidenced. Although
the role of the double layer charging effect in the magnetic behaviour could be
excluded, no reversible magnetization change could be achieved in the non-reactive
medium applied in the potential range where Co did not dissolve selectively.
A reversible change in the magnetic properties was achieved for Fe/FeO x layer
pairs [183–185]. For this system, the controlled-potential electrochemical oxidation
of the Fe layer in a dilute KOH solution can lead to an oxide layer formation with
a well-defined thickness of about 3–4 nm, which is equivalent to the oxidation of
about 1.5 nm of metallic Fe due to the volume expansion accompanying the oxidation
[183]. The relative change in magnetization is proportional to the thickness ratio
of the remaining metallic Fe and the electrochemically produced oxide, and it is
straightforward that the effect measured increase with a decrease of the total layer
thickness. The change in magnetization measured as a function of the electrode
potential and the corresponding layer structure is shown in Fig. 5.16.
The great achievement related to the electrochemical oxidation/reduction of thin
Fe layers in dilute KOH solutions was that the redox transformation can take place
without either a morphology change in the layer of an iron loss, at least for the limited
cycle number (12) that was used for the demonstration of the reversibility effect. If
the Fe layer is produced by electrodeposition, an all-electrochemical device can be
constructed where the magnetization is directly governed by the sample voltage
[184]. However, if the underlayer exhibits perpendicular magnetic anisotropy (like
FePt as shonw in [182]), the electrochemical reduction/oxidation cycle results in
a transient between rigid magnet to exchange spring magnet, respectively, hence
opening the way for electrochemically driven spintronic devices.
5.4.7 Annealing Behaviour of Electrodeposited Multilayers
Electrodeposition is a non-equilibrium sample preparation method. Therefore, asreceived electroplated samples often exhibit a large structural fault density which
can be partly eliminated with annealing. Depending on the properties of the samples,
annealing effects show a great diversity and the optimum annealing temperature may
also vary from one sample to another.
Annealing at temperatures below 200 °C does not result in a significant long-range
atomic motion in the solid deposit but leads only to the relaxation of dislocations
and local stress. Depending on the nature of the samples, a moderate-temperature
annealing may also lead to the improvement of the sample properties. This can be
seen for Co/Cu multilayers with GMR properties where a gradual increase of the
magnetoresistance was found for annealing temperature up to 300 °C [186]. The