4.3 Non-UPD Deposition of Ultrathin Metallic Layers
121
electrodes were usually placed in side compartments of the cell, hence providing
access to the working electrode for the magnetic observation.
Substrates in these experiments are either single crystals (Cu or Au) or highly
textured noble metal surfaces (like sputtered gold with a dominant (111) orientation). When gold was used as substrate, it was found that the appropriate surface
reconstruction is indispensable for the bulk deposition of the metals of interest [174,
180, 196, 197]. When a capping layer was applied for protecting the magnetic layer
(i.e., for an ex situ study after completing the electrodeposition), a flow cell arrangement is necessary [200]. The construction principles of such cells are quite identical
to those listed in Sect. 4.1.
The magnetic behaviour of ultrathin magnetic layers is generally very similar
to their counterparts produced with high-vacuum processes, while the experimental
setup costs much less. An advantage of the electrochemical experiment as opposed
to high-vacuum ones is that the deposition and the dissolution both can be carried out
in situ, and hence, the growth and the dissolution can be compared [188]. Electrodeposition exhibits a variable that is mostly very restricted in high-vacuum devices;
namely, the deposition rate which is controlled through the overpotential of the
deposition. As it will be discussed later, it has a strong impact on the magnetization
behaviour of ultrathin layers.
An important factor of interest in the ultrathin magnetic layers is the out-of-plane
magnetization. For a planar magnetic object, the minimization of the magnetostatic
energy favours the in-plane magnetization. However, this is not true for an ultrathin
layer that mostly exhibits a threshold thickess of a few atomic layer below which
the magnetization is perpendicular to the layer plane. This effect is highly sought
in magnetic data storage where the miniaturization of the bit areas requires the
occurrence of perpendicular magnetization.
Since magnetization stems from the exchange coupling of the electron spins in a
particular layer, the surface state of this layer is of high importance. The situation
is similar to the ratio of volume and surface energy in any other nanosystem. As
the surface interaction can influence the electron density at the boundary layer, the
occurrence of either the magnetization itself or its direction depends on the atomic
environment of both the bottom (substrate side) and the top (solution or covering layer
side) of the deposit. A valid comparison of the results obtained from magnetization
measurement is possible only if these factors are taken into account, as shown in Table
4.4 that summarizes the features of ultrathin electrodeposited layers by focusing on
their magnetization. It cannot be stressed enough that once a magnetic layer is in
contact with an electrolyte solution, the adsorbed anions and/or the oxidation state
of the surface is also an important determining factor of the overall magnetization
process. An example for the turnover of the spontaneous magnetization direction
as a function of the chemical environment of ultrathin magnetic layers is shown in
Fig. 4.16.
Among electrodeposited magnetic metals, cobalt gained an outstanding interest.
This is because Co has two crystalline forms, hcp (thermodynamically stable bulk
form at room temperature) and fcc (stable as a bulk form above 422 °C). Electrodeposition can be driven so that either of the Co crystalline form is obtained, and
121
electrodes were usually placed in side compartments of the cell, hence providing
access to the working electrode for the magnetic observation.
Substrates in these experiments are either single crystals (Cu or Au) or highly
textured noble metal surfaces (like sputtered gold with a dominant (111) orientation). When gold was used as substrate, it was found that the appropriate surface
reconstruction is indispensable for the bulk deposition of the metals of interest [174,
180, 196, 197]. When a capping layer was applied for protecting the magnetic layer
(i.e., for an ex situ study after completing the electrodeposition), a flow cell arrangement is necessary [200]. The construction principles of such cells are quite identical
to those listed in Sect. 4.1.
The magnetic behaviour of ultrathin magnetic layers is generally very similar
to their counterparts produced with high-vacuum processes, while the experimental
setup costs much less. An advantage of the electrochemical experiment as opposed
to high-vacuum ones is that the deposition and the dissolution both can be carried out
in situ, and hence, the growth and the dissolution can be compared [188]. Electrodeposition exhibits a variable that is mostly very restricted in high-vacuum devices;
namely, the deposition rate which is controlled through the overpotential of the
deposition. As it will be discussed later, it has a strong impact on the magnetization
behaviour of ultrathin layers.
An important factor of interest in the ultrathin magnetic layers is the out-of-plane
magnetization. For a planar magnetic object, the minimization of the magnetostatic
energy favours the in-plane magnetization. However, this is not true for an ultrathin
layer that mostly exhibits a threshold thickess of a few atomic layer below which
the magnetization is perpendicular to the layer plane. This effect is highly sought
in magnetic data storage where the miniaturization of the bit areas requires the
occurrence of perpendicular magnetization.
Since magnetization stems from the exchange coupling of the electron spins in a
particular layer, the surface state of this layer is of high importance. The situation
is similar to the ratio of volume and surface energy in any other nanosystem. As
the surface interaction can influence the electron density at the boundary layer, the
occurrence of either the magnetization itself or its direction depends on the atomic
environment of both the bottom (substrate side) and the top (solution or covering layer
side) of the deposit. A valid comparison of the results obtained from magnetization
measurement is possible only if these factors are taken into account, as shown in Table
4.4 that summarizes the features of ultrathin electrodeposited layers by focusing on
their magnetization. It cannot be stressed enough that once a magnetic layer is in
contact with an electrolyte solution, the adsorbed anions and/or the oxidation state
of the surface is also an important determining factor of the overall magnetization
process. An example for the turnover of the spontaneous magnetization direction
as a function of the chemical environment of ultrathin magnetic layers is shown in
Fig. 4.16.
Among electrodeposited magnetic metals, cobalt gained an outstanding interest.
This is because Co has two crystalline forms, hcp (thermodynamically stable bulk
form at room temperature) and fcc (stable as a bulk form above 422 °C). Electrodeposition can be driven so that either of the Co crystalline form is obtained, and