5.4 Properties of Electrodeposited CMAs
173
In the above equation, GMR SPM is a constant and the MR FM (H) function saturates
at magnetic field where ferromagnets are technically saturated (usually at H < 2
kOe).
Where magnetization was also measured in parallel to the magnetoresistance, it
was shown that the SPM particle size obtained from the two measurements are the
same. The significance of this decomposition procedure is that an estimate can be
obtained for the size of the small segregated magnetic domains (SPM particles),
and the partial FM magnetoresistance contribution of the sample can be obtained
as a “residue”; i.e., the difference of the MR curve measured and that calculated as
the SPM contribution. Hence, the FM part can be calculated, although there is no
theoretical function shape available for the FM part of the MR curves. The FM/SPM
ratio is often calculated and is taken as an indicator of the continuity and perfectness
of the FM layer [38, 40, 61, 89, 154, 176, 180].
It is to be emphasized that magnetization is a volume-proportional (extensive)
quantity, while magnetoresistance depends much on the relative position of all
magnetic entities in the system. Therefore, large magnetoresistance values are not
always obtained when the deposition parameters are tuned properly in accord with
the principles described in Sect. 5.3.1. The saturation magnetization value can be
large for samples in which the displacement of the magnetic layer can be identified
from the deposition conditions. Also, the surface roughness of the deposit was found
to play a key role in both the magnetoresistance ratio and in the superparamagnetic
contribution, maximizing the GMR effect at an NM layer deposition potential where
neither the composition nor the layer thickness ratio are predictable from the pulse
parameters [39]. Such effects make the electrodeposited multilayers with GMR very
difficult to design.
5.4.6 Magneto-ionics
Magneto-ionics is a new field of materials science where the ion migration is used
to tune the magnetic property of a material, mostly that of a thin layer. The typical
implementation of a magneto-ionic device includes a metal/metal-oxide interface
(like Co/GdO x [181]) across which oxide ions can migrate and modify both the
resistance and the magnetic properties of the layered system. Since both the resistivity
and the magnetization can be easily tuned with the electric charge passed, magnetoionic devices may constitute a new class of nonvolatile memories.
The electrochemical approach to magneto-ionics [182–185] includes a thin ferromagnetic layer which can be reversibly oxidized and reduced with classical means
of electrochemical polarization in a three-electrode cell. Here, the starting material
is not multilayered but a bilayer structure is obtained with the device operation. The
magnetic layer oxidized has to be thin (i.e., a few nanometres) so that a little charge
passed the system can achieve a significant variation in the magnetic properties. The
resulting oxide is not restricted so a surface atomic layer, which means that magnetoionics differ from double layer- or adsorption-induced magnetization modulation as
173
In the above equation, GMR SPM is a constant and the MR FM (H) function saturates
at magnetic field where ferromagnets are technically saturated (usually at H < 2
kOe).
Where magnetization was also measured in parallel to the magnetoresistance, it
was shown that the SPM particle size obtained from the two measurements are the
same. The significance of this decomposition procedure is that an estimate can be
obtained for the size of the small segregated magnetic domains (SPM particles),
and the partial FM magnetoresistance contribution of the sample can be obtained
as a “residue”; i.e., the difference of the MR curve measured and that calculated as
the SPM contribution. Hence, the FM part can be calculated, although there is no
theoretical function shape available for the FM part of the MR curves. The FM/SPM
ratio is often calculated and is taken as an indicator of the continuity and perfectness
of the FM layer [38, 40, 61, 89, 154, 176, 180].
It is to be emphasized that magnetization is a volume-proportional (extensive)
quantity, while magnetoresistance depends much on the relative position of all
magnetic entities in the system. Therefore, large magnetoresistance values are not
always obtained when the deposition parameters are tuned properly in accord with
the principles described in Sect. 5.3.1. The saturation magnetization value can be
large for samples in which the displacement of the magnetic layer can be identified
from the deposition conditions. Also, the surface roughness of the deposit was found
to play a key role in both the magnetoresistance ratio and in the superparamagnetic
contribution, maximizing the GMR effect at an NM layer deposition potential where
neither the composition nor the layer thickness ratio are predictable from the pulse
parameters [39]. Such effects make the electrodeposited multilayers with GMR very
difficult to design.
5.4.6 Magneto-ionics
Magneto-ionics is a new field of materials science where the ion migration is used
to tune the magnetic property of a material, mostly that of a thin layer. The typical
implementation of a magneto-ionic device includes a metal/metal-oxide interface
(like Co/GdO x [181]) across which oxide ions can migrate and modify both the
resistance and the magnetic properties of the layered system. Since both the resistivity
and the magnetization can be easily tuned with the electric charge passed, magnetoionic devices may constitute a new class of nonvolatile memories.
The electrochemical approach to magneto-ionics [182–185] includes a thin ferromagnetic layer which can be reversibly oxidized and reduced with classical means
of electrochemical polarization in a three-electrode cell. Here, the starting material
is not multilayered but a bilayer structure is obtained with the device operation. The
magnetic layer oxidized has to be thin (i.e., a few nanometres) so that a little charge
passed the system can achieve a significant variation in the magnetic properties. The
resulting oxide is not restricted so a surface atomic layer, which means that magnetoionics differ from double layer- or adsorption-induced magnetization modulation as