120
4 Ultrathin Layers
can be observed when the exchange coupling interaction is capable of ordering the
spins of the electrons in the metal in a parallel manner. It is obvious that there must
be a size limit where the exchange interaction in the growing phase can give rise to a
long-range magnetic order. This is particularly true for ultrathin magnetic layers [187]
where differences in thickness at the sub-monolayer scale can fundamentally influence the magnetic behaviour. The magnetization occurring during the deposition of
the ultrathin layers is by far too small for a vibrational magnetometer. Therefore, early
in situ studies are based on the observation of the magneto-optical Kerr effect or alternating field gradient magnetometry. Experimental setups are available in a number
of works [186, 188–191]. Results of the early studies were summarized by Allongue
and co-workers in various reviews [192, 193]. Recently, in situ magnetic observation
of ultrathin electrodeposited films was made possible in a SQUID magnetometer
device [194].
If an in situ magnetic measurement is carried out, the electrochemical cell has to be
placed into the gap of an electromagnet. This means that the gap of the electromagnet
has to be larger than for a usual measurement of a tiny sample, which limits the
maximum field available. In any case, the maximum available field has to be larger
than the saturation field of the samples. A field of a few hundred oersteds proved to be
sufficient for the observation of square-shaped magnetization curves [186, 191, 195],
but a few kOe maximum field is also available with a suitable configuration [174, 179,
196, 197]. The external magnetic field has to be adjusted as in-plane (longitudinal
arrangement) and perpendicular-to-plane (polar arrangement) with respect to the
substrate, which usually means a horizontal and a vertical direction of the magnetic
field, respectively. As a special example for in situ observation of magnetism, a
Mössbauer spectroscopy setup has also been implemented [198].
Magnetic properties of the ultrathin layers are studied with solutions in which
the concentration of the metal ions is typically a few mM (but at most 40 mM).
The supporting electrolytes used were typically sulphate salts or boric acid, the pH
being slightly acidic only (pH = 3…5). The comparison of the deposits obtained
from acidic and alkaline solutions was demonstrated for Co deposition [199], and
the role of the deposition conditions in the magnetic behaviour was found to be
crucial (see later). The impact of additives has not been tested in magnetizationoriented experiments. Since an interference of the hydrogen evolution can always
occur during the deposition of the magnetic metals due to their negative deposition potential, the equivalent thickness of the deposit cannot be determined from
the cathodic charge. Instead, the equivalent layer thickness is determined after the
magnetic measurement from the anodic charge consumed for the layer dissolution
[195, 196], occasionally corrected for the background current obtained with blank
solution [188]. The accuracy of the determination of the equivalent layer thickness
was estimated to be 0.02–0.1 monolayers (see [188] and [196], respectively). For
eliminating the negative impact of the hydrogen evolution, some attempt was made
to apply non-aqueous solvent already in the pioneering work of the field [195], but
it did not gain a further interest and later works used aqueous systems only. Due to
the small charge required for the deposition of the thin layers, the geometry of the
electrochemical cell is not of importance, and hence, the counter and the reference
4 Ultrathin Layers
can be observed when the exchange coupling interaction is capable of ordering the
spins of the electrons in the metal in a parallel manner. It is obvious that there must
be a size limit where the exchange interaction in the growing phase can give rise to a
long-range magnetic order. This is particularly true for ultrathin magnetic layers [187]
where differences in thickness at the sub-monolayer scale can fundamentally influence the magnetic behaviour. The magnetization occurring during the deposition of
the ultrathin layers is by far too small for a vibrational magnetometer. Therefore, early
in situ studies are based on the observation of the magneto-optical Kerr effect or alternating field gradient magnetometry. Experimental setups are available in a number
of works [186, 188–191]. Results of the early studies were summarized by Allongue
and co-workers in various reviews [192, 193]. Recently, in situ magnetic observation
of ultrathin electrodeposited films was made possible in a SQUID magnetometer
device [194].
If an in situ magnetic measurement is carried out, the electrochemical cell has to be
placed into the gap of an electromagnet. This means that the gap of the electromagnet
has to be larger than for a usual measurement of a tiny sample, which limits the
maximum field available. In any case, the maximum available field has to be larger
than the saturation field of the samples. A field of a few hundred oersteds proved to be
sufficient for the observation of square-shaped magnetization curves [186, 191, 195],
but a few kOe maximum field is also available with a suitable configuration [174, 179,
196, 197]. The external magnetic field has to be adjusted as in-plane (longitudinal
arrangement) and perpendicular-to-plane (polar arrangement) with respect to the
substrate, which usually means a horizontal and a vertical direction of the magnetic
field, respectively. As a special example for in situ observation of magnetism, a
Mössbauer spectroscopy setup has also been implemented [198].
Magnetic properties of the ultrathin layers are studied with solutions in which
the concentration of the metal ions is typically a few mM (but at most 40 mM).
The supporting electrolytes used were typically sulphate salts or boric acid, the pH
being slightly acidic only (pH = 3…5). The comparison of the deposits obtained
from acidic and alkaline solutions was demonstrated for Co deposition [199], and
the role of the deposition conditions in the magnetic behaviour was found to be
crucial (see later). The impact of additives has not been tested in magnetizationoriented experiments. Since an interference of the hydrogen evolution can always
occur during the deposition of the magnetic metals due to their negative deposition potential, the equivalent thickness of the deposit cannot be determined from
the cathodic charge. Instead, the equivalent layer thickness is determined after the
magnetic measurement from the anodic charge consumed for the layer dissolution
[195, 196], occasionally corrected for the background current obtained with blank
solution [188]. The accuracy of the determination of the equivalent layer thickness
was estimated to be 0.02–0.1 monolayers (see [188] and [196], respectively). For
eliminating the negative impact of the hydrogen evolution, some attempt was made
to apply non-aqueous solvent already in the pioneering work of the field [195], but
it did not gain a further interest and later works used aqueous systems only. Due to
the small charge required for the deposition of the thin layers, the geometry of the
electrochemical cell is not of importance, and hence, the counter and the reference