166
5 Compositionally Modulated and Multilayered Deposits
the tuning of the coercive force of the layer stack for achieving a material with
soft magnetic properties not available in the bulk form. This was demonstrated for
an amorphous/crystalline soft magnetic multilayer film in which both layers are
composed of Ni–Co–B alloys with different composition [127]. The layer deposited
with a low-current density resulted in an amorphous deposit with about 6 wt.% boron
content, while the layer deposited with high-current density was crystalline with 1%
B content (in parallel, the Ni:Co ratio in the alloy also changed). The layers could
be resolved with TEM down to about a 25 nm bilayer thickness, and the lowest
coercivity achieved was less than 0.5 Oe with a square-shaped hysteresis loop. The
study of the electrodeposited FM/FM system of Fe–Co alloys showed [128] that the
deposition with two-pulse plating results in a compositionally modulated alloy even
if the nominal layer thicknesses are below 1 nm. Here, both the lattice constant of the
deposit and its saturation magnetization were a function of the layer thicknesses but
did not depend simply on the composition. It was achieved only by the composition
modulation at the nanoscale that the system approached the maximum saturation
magnetization as a function of the Fe:Co ratio.
The other basic type of magnetic multilayers are composed of alternating
ferromagnetic/non-magnetic layers (which will be called hereinafter as FM/NM
multilayers). It was shown in the previous chapters on mechanical and corrosion
properties that the thickness of the layers was orders of magnitude larger than the
atomic plane distance in the crystals. For such layers, the relative fluctuation of the
layer thickness (the mean deviation of the thickness divided by the mean layer thickness) was not crucial, and the desired properties could be achieved with a relative
thickness fluctuation up to 10–20%. With the FM/NM multilayers, we arrive at a
material family in which the layer thickness fluctuation should be suppressed as
much as possible because the desired sample properties can be achieved mostly with
1–3 nm layer thicknesses. The magnetic or magnetoresistance properties are based
on the so-called coupling of the neighbouring magnetic layers, which means that
the relative direction of the magnetization of the neighbouring layers can be aligned
in either parallel or antiparallel direction. Moreover, the parallel/antiparallel configuration can manifest itself in an oscillating manner where the coupling direction
is inverted with a ~0.5 nm change in the NM layer thickness. Therefore, the layer
thicknesses should be as even as possible with the least possible interface roughness
and component intermixing, especially when magnetoresistance properties are in the
focus of research.
For the ideal composition of FM/NM multilayers, the concentration of the FM
ions has to be high, typically 0.5–2.3 M, while the NM metal ion concentration is
at most a few tens of mM. The magnetic component is mostly Ni and/or Co, with
Fe as a possible alloying element beside the previous ones. The current during the
deposition of the FM metal is so high that the NM metal impurity is negligible at
least concerning its impact on the magnetic properties. An NM metal mole fraction
in the FM layer between 0.5 and 5 at.% is customary. However, the NM metal has
to be as pure as possible in order to avoid a direct magnetic coupling between the
neighbouring magnetic layers. The layer thicknesses in the relevant studies vary
between 0.5 and 8 nm.
5 Compositionally Modulated and Multilayered Deposits
the tuning of the coercive force of the layer stack for achieving a material with
soft magnetic properties not available in the bulk form. This was demonstrated for
an amorphous/crystalline soft magnetic multilayer film in which both layers are
composed of Ni–Co–B alloys with different composition [127]. The layer deposited
with a low-current density resulted in an amorphous deposit with about 6 wt.% boron
content, while the layer deposited with high-current density was crystalline with 1%
B content (in parallel, the Ni:Co ratio in the alloy also changed). The layers could
be resolved with TEM down to about a 25 nm bilayer thickness, and the lowest
coercivity achieved was less than 0.5 Oe with a square-shaped hysteresis loop. The
study of the electrodeposited FM/FM system of Fe–Co alloys showed [128] that the
deposition with two-pulse plating results in a compositionally modulated alloy even
if the nominal layer thicknesses are below 1 nm. Here, both the lattice constant of the
deposit and its saturation magnetization were a function of the layer thicknesses but
did not depend simply on the composition. It was achieved only by the composition
modulation at the nanoscale that the system approached the maximum saturation
magnetization as a function of the Fe:Co ratio.
The other basic type of magnetic multilayers are composed of alternating
ferromagnetic/non-magnetic layers (which will be called hereinafter as FM/NM
multilayers). It was shown in the previous chapters on mechanical and corrosion
properties that the thickness of the layers was orders of magnitude larger than the
atomic plane distance in the crystals. For such layers, the relative fluctuation of the
layer thickness (the mean deviation of the thickness divided by the mean layer thickness) was not crucial, and the desired properties could be achieved with a relative
thickness fluctuation up to 10–20%. With the FM/NM multilayers, we arrive at a
material family in which the layer thickness fluctuation should be suppressed as
much as possible because the desired sample properties can be achieved mostly with
1–3 nm layer thicknesses. The magnetic or magnetoresistance properties are based
on the so-called coupling of the neighbouring magnetic layers, which means that
the relative direction of the magnetization of the neighbouring layers can be aligned
in either parallel or antiparallel direction. Moreover, the parallel/antiparallel configuration can manifest itself in an oscillating manner where the coupling direction
is inverted with a ~0.5 nm change in the NM layer thickness. Therefore, the layer
thicknesses should be as even as possible with the least possible interface roughness
and component intermixing, especially when magnetoresistance properties are in the
focus of research.
For the ideal composition of FM/NM multilayers, the concentration of the FM
ions has to be high, typically 0.5–2.3 M, while the NM metal ion concentration is
at most a few tens of mM. The magnetic component is mostly Ni and/or Co, with
Fe as a possible alloying element beside the previous ones. The current during the
deposition of the FM metal is so high that the NM metal impurity is negligible at
least concerning its impact on the magnetic properties. An NM metal mole fraction
in the FM layer between 0.5 and 5 at.% is customary. However, the NM metal has
to be as pure as possible in order to avoid a direct magnetic coupling between the
neighbouring magnetic layers. The layer thicknesses in the relevant studies vary
between 0.5 and 8 nm.