
_Magnetization reversal behavior of ferromagnetic_
_thin films and nano-structures_
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In general terms, this thesis studies the magnetization reversal behaviour of
ferromagnetic thin films and nano-structures. In one core part of the thesis,
the
influence of crystallographic alignment, materials composition and thickness
onto the
magnetic properties of Co and Co alloy thin films has been studied in detail.
Hereby,
an epitaxially grown 30 nm thick (1010) Co thin film, which has in-plane
uniaxial
magnetic anisotropy, has been utilized as a reference structure due to its
simple and
well-understood behaviour. In order to modify the crystallographic alignment
in a
continuous and controlled fashion, a method to interrupt the epitaxy has been
developed and applied. The magnetic properties of these crystallographically
modified samples have been analysed by magneto-optical Kerr effect
magnetometry
and microscopy. Hereby, it was observed that while in samples with good
crystallographic alignment the magnetization reversal is still simple and
dominated by
uniform magnetization states, non-uniform intermediate stable or meta-stable
states
emerge in the case of sufficiently disordered samples, even though macroscopic
uniaxial anisotropy is still maintained. Furthermore, in samples with partial
crystallographic alignment, an anomaly has been found, in which conventional
hard
axis behaviour disappears in a very narrow range of applied field directions.
In such
samples, a frustrated magnetic state occurs when the magnetic field is applied
along
the hard axis, which arises from the competition between ferromagnetic
exchange and
locally misaligned uniaxial anisotropies of adjacent grains. The existence of
such a
frustrated state along the nominal hard axis has been theoretically explained
in the
framework of a two-grain model and has been experimentally corroborated by
microscopic imaging. As a function of thickness, epitaxial Co films exhibit a
slight
variation of their magnetization properties, which is triggered by and
consistent with a
crystallographic strain release upon increasing the film thickness.
For the study of magnetic alloys, CoRu films of different compositions have
also been grown epitaxially with (1010) crystallographic orientation. Hereby,
it has
been necessary to modify the epitaxial growth sequence and incorporate an
individualized template for each magnetic alloy concentration, in order to
fabricate
alloy films of comparable crystal quality. For the so-prepared samples, key
magnetic
properties have been measured, such as, Curie temperature, saturation
magnetization
and anisotropy constants as a function of composition. It has been found that
the
magneto-crystalline anisotropy constant shows a complex and non-monotonous
behaviour as a function of Ru content in the alloy, while saturation
magnetization and
Curie temperature show a more expected monotonic decrease with Ru
concentration.
In addition to the material oriented studies, Co-films have also been utilized
to
experimentally investigate the dynamic phase transition and specifically, the
influence
of a constant bias field onto the phase diagram and phase stability. These
experimental studies have been furthermore complemented by theoretical
calculations
based upon the Kinetic Ising model in mean field approximation. Also here, the
role
of the constant bias field has been analysed, which was identified as the
conjugated
field of the dynamic order parameter.
Finally, the benefits that the magneto-optical Kerr effect microscope offers
for
the study of individual structures, even nano-scale structures, has been
demonstrated
and analysed. Specifically, single cycle hysteresis loop measurements of 30 nm
wide
Co wires, fabricated by means of focused electron beam induced deposited, have
been
demonstrated. Also, 100 nm wide wires of only 0.8 nm Co layer thickness (as
part of
a Pt/Co/Pt-trilayer structure) have been measured with the Kerr microscope
with
excellent signal to noise ratio, allowing the distinction of uniform vs. non-
uniform
reversal. These specific Pt/Co/Pt nanostructures have been successfully
fabricated,
maintaining their original perpendicular anisotropy, by means of focussed ion
beam
induced layer intermixing. For this purpose, a special TiN hard mask had to be
developed, which acts as a sacrificial layer, and opens up a new and promising
pathway for directly nano-structuring multilayer materials by means of focused
ion
beam exposure without damaging their magnetic properties in a wide surface
area.
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