Conclusions
In this thesis, the magnetization dynamics due to a spin polarized current and propagating spin waves in magnetic nanowire and circular disc are investigated by experiments as well as by micromagnetic simulations. The experimental work have been performed by using a homodyne detection scheme with a cryogenic sys-tem to control the sample sys-temperature. The micromagnetic simulations have been done by using a object-oriented micromagnetic framework explained in Chapter 2.5. This thesis is mainly consists of two parts. In the first part, the magnetic vortex core dynamics excited by the spin polarized microwave currents in a mag-netic disc are investigated. The interaction between propagating spin waves and magnetic domain walls are studied in the second part.
A homodyne detection scheme is a versatile tool to investigate the magnetic vortex core dynamics. The homodyne signal can be used to identify the full mag-netic state of the vortex structure (polarity and chirality) using a specific geometry.
To identify this vortex state, the 1 µm Py disc with a notch to break symmetry is used. Since the homodyne signal is very sensitive to the phase shift between the microwave current excitation and the magnetoresistance response. The polarity of the vortex core is determined from the phase difference between two polarities
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and the chirality is also determined by using an symmetry disc structure. From the field and angle dependence measurements, the unexpected higher resonance frequencies, which indicate the strong pinning effect are observed at the certain magnetic field and angle. To confirm the pinning effect, homodyne signals are measured with different temperatures and injected microwave powers. Combined with the trajectory of the vortex core calculated by micromagnetic simulations, a two dimensional pinning map is drawn. Since the phase shift give us informa-tion about the driving force of the vortex core, the Oersted field contribuinforma-tion is distinguished by analyzing the magnetic field and angle dependent phase shift measurements. In our case, the Oersted field contribution is about 75 % of the force exerted on the vortex core gyration.
The vortex core dynamics with the large displacement of the vortex core due to the in-plane external field is observed. Due to the strong shape anisotropy and the combination of the spin torque effect the Oersted field effect make the nonlin-ear trajectory of the vortex core. This nonlinnonlin-ear dynamics are obtained from the position dependent homodyne detection measurements on a circular disc without symmetry breaking. The amplitude of the homodyne signal increases when the vortex core is closed to the electric contacts since the Oertsted field effect is from the inhomogeneous current distribution due to the thickness differences between the disc and electric contacts. Moreover, the homodyne signal is averaged out when the vortex core gyration is nonlinear. This nonlinear regime is measured the frequency sweep homodyne detection measurement.
All information (resonance frequency, the amplitude of the vortex core gyra-tion, and the phase shift) are obtained from the fitting of the homodyne signal.
The analytical expression of the vortex core motion is shown in Chapter 3.4.
Since the Oerted field contribution is dominant in our case, the modified Thiele’s equation including a spin transfer torque term and Oersted field term is explained.
The interaction between propagating spin waves and the magnetic domain walls are numerically investigated. First, the doamin wall velocities as a function of the spin wave frequency is calculated. At the certain frequency, a significant domain wall motion against the spin wave propagation due to the magnetostatic energy gradient is observed. Second, there are two different mechanisms to move and depin the domain wall. One is the momentum transfer from spin waves to
the domain wall accompanied strong spin wave reflection. The other mechanism is the collective domain wall oscillation mediated by the internal modes of the domain wall at the certain spin wave frequencies.
As the other spin wave generation method, the oscillating Oersted field in a magnetic nanowire is numerically investigated. For the case of that the domain wall is magnetized along the nanowire, a propagating spin wave is not observed.
However, the nonlinear dynamics at the certain spin wave frequency is obtained.
For the case of that the domain wall is magnetized perpendicular to the naowire due to the strong applied field, coherent propagating spin waves are observed.
At the high frequency range, one is observed that the spin wave propagating is allowed for only one direction.
Outlook
From the experience and the results obtained during this work, many other ex-periments promise exciting results in future.
The main key issue of the domain wall motion is to measure the nonadiabaticity β. As shown in Chapter 3.4, the homodyne signal depends on the nonadiabaticity β. Thus the effect of the nonadiabatic spin transfer torque effect induced vortex core dynamics in magnetic discs or ellipse are able to determine the β value. In order to reduce the Oersted field effect from the inhomogeneous current distribu-tion, an elliptic magnetic disc is suggested.
The vortex core position dependent homodyne detection allows us to scan the pinning site in a magnetic disc. Our efficient method of characterization in con-junction with structural investigation such as high resolution electron microscopy will open a path to the identification of the defect types and resulting materials improvement for devices with reliable dynamical switching.
Spin wave assisted domain wall motion generates new concept for race track memory [Park04, PHT08] without any Joule heating. However, a key issue is to generate strong propagating spin waves. In this point of view, the spin torque
oscillator and the point contact device are suggested as the sources of the spin waves. These can be investigated by using OOMMF.
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Pursuing a Ph.D research is a both painful and enjoyable experience. It is like a climbing mountain with admirable collaborators. Many people have supported this work in many different ways and I would like to give my sincere thanks to all these people here.
My first thank goes to Prof. Dr. Mathias Kläui and Prof. Dr. Ulrich Rüdi-ger, who accepted me into the research group and supervise my all research works.
They have offered me a Ph.D position and gave me a lot of scientific opportunities.
I specially appreciate for Prof. Dr. Mathias Kläui who has always encouraged me and gave me a helping hand whenever I need his help. When I was confused due to many scientific problems, he did not hesitate to make me on the right direction.
I have learned a lot from him. I could not imagine that I finished my Ph.D thesis without his help.
Special thank is given to Dr. Olivier Boulle. When I started my Ph.D, I had many difficulties since I have changed my research field from the theoretical physics to the experimental solid state physics. At that time, he has taught me step by step for everything from the basic measurement techniques to the very sensitive microwave techniques. I never forget his help and careful advices.
I also really appreciate Prof. You at Inha University in South Korea. He had accepted me as a research assistance before I started my Ph.D. He gave me a big
107
chance what I was able to change my research field successfully. Therefore, I could start Ph.D research without any problem. Until now, he always give a lot of fruit-ful advices whenever I need it. I would like to say that I so admire him. Morover, when he has visited to Konstanz, we had a good discussion and enjoyed Konstanz.
Special thank is also given to Prof. Luis Lopez-Diaz at the University of Sala-manca in Spain. As my second supervisor, he has invited me and supervised to develop the micromagnetic simulation codes for spin wave project. It was a great chance for my Ph.D. I never forget the time when I was in Salamanca (09.2009 ∼ 10.2009) as my secondment. I also appreciate Dr. Eduardo Martinez who helped
Special thank is also given to Prof. Luis Lopez-Diaz at the University of Sala-manca in Spain. As my second supervisor, he has invited me and supervised to develop the micromagnetic simulation codes for spin wave project. It was a great chance for my Ph.D. I never forget the time when I was in Salamanca (09.2009 ∼ 10.2009) as my secondment. I also appreciate Dr. Eduardo Martinez who helped