Functional modification in ultrathin films: from metastable magnets to molecular materials [Elektronische Ressource] / vorgelegt von Xiangdong Liu

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• Physik

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• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2004
• Nombre de lectures: 13
• Langue: English
• Poids de l'ouvrage: 8 Mo

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Functional Modification in Ultrathin Films:
from Metastable Magnets to Molecular
Materials
Von der Fakultat¨ fur¨ Mathematik, Informatik und
Naturwissenschaften der Rheinisch Westfalischen¨
Technischen Hochschule Aachen zur Erlangung des
akademischen Grades eines Doktors der Naturwissenschaften
genehmigte Dissertation
vorgelegt von
Xiangdong Liu
aus Lianyuan/P. R. China
Berichter: Prof. Matthias Wuttig
¨Prof. Stefan Blugel
Tag der mundlichen¨ Prufung:¨ 18. Mai 2004
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online
verfugbar¨ .2Abstract
Ultrathin films have become an area at the frontier of materials science due to their
novel properties and new applications. In particular, ultrathin magnetic films and
ultrathin organic films are drawing more and more attention. The former is associ
ated with a series of new phenomena such as giant magntoresistence and oscillatory
exchange coupling which enable fabrication of new devices and applications, while
the interest in the later is ignited by the promise prospect of molecular optoelec
tronics and molecular electronics. To make a full use of these films, it is important
to learn how to manipulate their growth and how to tailor their properties. The aim
of this work was to explore methods efficient for functional modification of these
films.
(Fe, Ni) bilayers with different individual thickness and different deposition
sequences have been investigated experimentally with special attention to the tem
perature dependence of their magnetic parameters. It is unequivocally found that in
(Fe, Ni) bilayers the spin reorientation transition shifts to larger Ni film thicknesses
compared with Ni/Cu(100) films. This result is explained by an enhanced demag
netization field when the two magnetic layers with unequal magnetization are put
together and possibly by a Fe/Ni in plane interface anisotropy. The non monotonic
temperature dependence of the coercivity observed in the bilayers strongly suggests
the existence of an in plane anisotropy at the Fe/Ni interface. Magnetic live lay
ers of Fe have been found at the Fe/Ni interface. The magnetic structure of the
surface layer of 9 ML Fe on Ni on Cu(100) is closely related to the thickness of
the underlying Ni film. A magnetic live layer with Curie temperature around 230
K is observed when the thickness of Ni layer is chosen to be 10 ML, while it is
absent when the Ni thickness increases to 15 ML. The structural relaxation of the
Ni layers with thickness is thought to be responsible for the observations. These
results provide a further evidence for a sensitive correlation between the structure
and magnetism in fcc Fe. The single domain state is not stable for the (Fe, Ni)
bilayer with small perpendicular anisotropy. With the withdrawal of the external
field, the single domain state gradually relaxes to a multi domain state, resulting in
a decay of remanent magnetization with time. The relaxation behavior disappears
iii
when the perpendicular anisotropy is large.
An exchange biasing in ultrathin Fe films on Ni/Cu(100) has been observed
when the (2£1) and (4£1) phase coexist. This biasing could be attributed to the
coexistence of AFM and FM domains as observed previously in granular systems.
However, with all the direct experimental result objecting the existence of a AFM
phase at the studied temperature, a new model is proposed to account for this un
usual biased coupling phenomenon. It is based on the assumption that the strong
biquadratic exchange coupling results in an orthogonal coupling between the (2£1)
Fe domains and their underlying Ni layer.
By measuring and analyzing the hysteresis loops of Ni/Cu(100) films with
several thicknesses at different temperatures, the magnetization reversal mecha
nisms have also been investigated for Ni/Cu(100) films with perpendicular mag
netic anisotropy. The magnetization reversal procedure consists of the nucleation
of the reversed domains and the following motion of the domain wall. Which pro
cess is the dominant mechanism depends on the temperature and the film thickness.
For thick films with large perpendicular anisotropy, the nucleation field, which is
correlated to the anisotropy, is greater than the pinning field of most pinning cen
ters. The magnetization reversal is dominated by the nucleation and the reversal
procedure can be described by the nucleation followed by the viscous motion of the
wall without obstacles, resulting in a high squareness in the shape of the hystere
sis loops. With decreasing film thickness, the effective anisotropy field becomes
comparable with the effective pinning field. Then the role of temperature
important. At high temperature, the motion of the domain wall is thermally as
sisted and thus easy, so the nucleation of the reversed is still the dominant
process. At low temperature the pinning effect of the wall motion is visible. There
fore the shape of the hysteresis loop changes from rectangular at high temperature
to inclined and round at low temperature. In this case the adsorbed residual gases
have a pronounced effect on the reversal procedure acting as pinning centers. The
high stability of the magnetic properties of Ni/Cu(100) films with perpendicular
anisotropy upon multiple magnetization reversals has been measured and is con
firmed by a theoretical analysis.
AFM and x ray diffraction have been used to investigate the growth behavior
of perylene films on a (111) oriented polycrystalline Au substrate, as well as on
the same substrate but additionally coated with a self assembled monolayer (SAM)
of 1 Octadecanethiol molecules. It has been found that the perylene molecules
have a smaller diffusion coefficient on the SAM than on the Au surface. An
additional self assembled monolayer of 1 Octadecanethiol molecules on an Au
substrate greatly modifies the properties of the subsequently vacuum deposited
perylene films. The grain size becomes smaller and a strong c axis texture is in
troduced. Both factors tend to reduce the roughness of the perylene films. TheseABSTRACT iii
effects are attributed to the changes in the perylene molecule substrate interaction
by the additional self assembled monolayer.
To further reveal the unique growth behavior of molecular crystals, perylene
have been evaporated on a oil film. The growth speed along the crystallographic
[100] axis is found to be dramatically different from that along its opposite [ 100].
The ratio of two growth speed is around 2.3, and does not show an obvious de
pendence on supersaturation. This asymmetric growth is believed to has a steric
origin, which is special for organic crystals. An organic molecule, which is the
unit building block of a organic crystal, consists of a number of atoms and shows
a finite size and specific shape. These features complicate the growth behavior
of organic crystals. For the growth of perylene crystals on an oil film, the steric
environment for a perylene molecule to be incorporate to a crystal is different for
[100] and [ 100], resulting in a difference in the growth speed. Interesting growth
patterns have also been observed for perylene crystals grown on a oil film. Though
it is preliminary, our study indicates that the stress field at the growth interface and
especially around a step has significant effects on the growth behavior.Acknowledgements
This work would not have been possible without the contributions from many peo
ple. Therefore, I am deeply indebted to them all.
First, I would like to express my sincere thanks to my supervisor, Prof. Dr.
Matthias Wuttig for his support, guidance, suggestions, and encouragement during
the whole period of this work, as well as a lot of warm hearted help which were
often crucial to a foreigner.
I wish to express my sincere gratitude to Prof. Dr. H. Ibach for being my host
for part of this work.
I wish to express my sincere gratitude to Mrs. J. Elbert. I can always get her
help when I am in difficulty.
I would like to thank Prof. Dr. S. Blugel¨ for the enlightening discussion on
magnetism, and thank Prof. Dr. Th. Michely for many helpful discussions about
film growth.
I would like to thank Josef Larscheid, Michael Huppertz, U. Linke, Stephan
Hermes, R. Siedling and Oliver Lehmann for their technical support.
During the last five years I have got help from many of my colleagues. I would
like to thank Volker Weidenhof, Ines´ Friedrich, Henri Hansen, Carsten Busse,
Selvaraj Venkataraj, Hansjog¨ Weis, Christian Dahmen, Wolfgang Kalb, Martin
Reufer, Fengming Pan and all the members in I. Physikalisches Institut der RWTH
Aachen for their help and friendship.
Finally, I would like to thank my wife, Lin Zhang. With her love, I have always
been in an agreeable mood for work.
ivContents
Abstract i
Acknowledgements iv
Introduction to this thesis 1
I Magnetic properties of ultrathin Fe and Ni films and their bi
layer on Cu/(100) 3
1 Magnetic Metal Films: Theoretical Basis 4
1.1 Growth of Ultrathin Metal Films . . . . . . . . . . . . . . . . . . 4<