7.1. Conclusions
This work succeeded in the development of a process workflow for the fabrication of magnetic point contact nano-oscillators, featuring optical access to the magnetic layer in the close vicinity of the point contact. The mask design and fabrication process optimization for the simultaneous realization of four different device structures that enable consistent point contact nano-oscillator characterization using both electrical and optical techniques required a substantial amount of effort and time.
The observed anomalous behavior of point contact resistance and magnetoresistance figures as a function of point contact size was considered. Supported by finite element simulations, this behavior was explained by the presence of intricate current distributions in the device electrodes that lead to the observation of a parasitic, partly magnetoresistive series resistance. Based on these observations, a model was constructed that provides a close fit with experimental data, considering both CIP and CPP contributions to the magnetoresistance. Moreover, the physical values of the CIP and CPP magnetoresistance values associated with the electrode and intrinsic point contact resistances could be estimated. Concerning the device optimization process, the observations also provided some guidelines to improve device performance and measurement accuracy, among which the most important is the further downscaling of the point contact size through improvement of the lithographic definition and etching process.
A similar conclusion follows from the microwave measurements that were performed on the point contact devices. These measurements showed that upon the application of a direct current and out-of-plane fields, low frequency oscillations were preferably induced which are assumed to be due to the gyrotropic motion of a vortex core under the point contact area. Only further downscaling of the point contact will reduce the Oersted field associated with the direct current which acts as a restoring force for the vortex state and enable the observation of high frequency dynamics. Furthermore, the downscaling of point contact size in both cases also reduces the current necessary to obtain the required high current density which facilitates the observation of spin torque oscillations.
One of the primary goals of this thesis was to provide point contact nano-oscillator devices featuring optical access to the extended magnetic layer around the active point contact area, enabling the study of spin wave emission with the optical Brillouin light scattering technique, which uses a focused laser spot to probe the free magnetic layer
around the point contact. It has been shown that these devices have been successfully fabricated and that spin wave emission was indeed observed in a limited number of BLS experiments so far. Although further device optimization is required to enable the study of direct current spin torque induced spin wave emission, the observation of RF excited spin waves in the vicinity of the point contact revealed a number of interesting phenomena, some of which require substantial further analysis to enable definitive conclusions. However, the direct observation of spin wave emission around a point contact does indicate the feasibility of these spin waves as a coupling mechanism for nano-oscillator synchronization.
Finally, micro-magnetic simulations were performed to gain more insight into the magnetization dynamics associated with spin wave emission in a common magnetic layer. The dispersion relations of the magnetostatic modes for an in-plane field were successfully simulated in a reduced one-dimensional simulation geometry. It was also shown that the interplay between the point contact size and the magnetostatic modes can generate strong localization of the dynamics inside the point contact area, with minimal emission into the extended thin film. This is explained by the formation of standing wave patterns in the point contact area, for specific frequencies which are not harmonically related due to the dispersive nature of the magnetostatic modes.
Moreover, since thicker films were observed to facilitate the long range propagation of magnetostatic modes, increasing the free layer film thickness in fabricated devices may relax the dimension constraints on the BLS tips. Comparison of the one-dimensional simulations with a more realistic two-one-dimensional setting revealed good wave vector matching with the one-dimensional and analytical case for small to intermediate wave vectors. This indicates the validity of the one-dimensional simplification in this range, at least concerning the obtained wave vectors. Similar to the one-dimensional case, mode localization can occur in a square point contact in the two-dimensional case. However, due to the different dispersion relations for the modes that propagate along and perpendicularly to the external in-plane field, the associated frequencies leading to confinement differ for these modes. Consequently, confinement generally occurs in only one direction and the spatial spin wave radiation pattern becomes largely anisotropic. Interestingly, the direction of radiation inside the device may be tuned by an externally applied field and the frequency of excitation of an RF source. An interesting experiment would be to verify this behavior in square elements. Additionally, anisotropic power emission may have applications in the minimization of power losses in one-dimensional arrays of coupled, phase-synchronized oscillators.
7.2. Outlook
Although this work has provided spin torque nano-oscillator devices which have been shown to meet the compatibility requirement with optical BLS measurements, the generation of direct spin current induced high frequency dynamics could not yet be obtained. However, some important conclusions can be drawn from the measurements that address the further optimization of the device fabrication process. Therefore, further device optimization is mainly concerned with the downscaling of the present point contact sizes. Not only will this help in avoiding the low frequency vortex mode oscillations in favor of high frequency dynamics, it will also improve current confinement and thus reduce the absolute current required to induce these high frequency dynamics, while at the same relaxing any further design constraints on the BLS tips. In order to obtain smaller point contact sizes, either the current BHF wet etching method should be re(de)fined, e.g. through the use of a thinner resist layer, or alternative definition and etching methods should be considered, which use different resist systems or even completely different etching methods, such as chemically assisted plasma etching. Furthermore, because the BLS tips seem to break down for relatively low currents, the next design iteration could consider making these tips considerably wider. However, much information is contained in the rich magnetization dynamics very close to the point contact, so the downscaling of the point contact itself rather than upscaling the tips seems favorable if the possibility of probing the magnetization dynamics in the close vicinity of the point contact is to be retained. It is hoped that after these further optimizations, high frequency dynamics will become accessible in both the RF and BLS experiments and this for reduced currents. As stated in the BLS results section, the generation of spin waves due to an RF AC current (or in the future: a DC spin current) is an extremely complicated process involving many device and measurement setup parameters. For example, the masked roles of the DC generated Oersted field and the spin torque action should be unraveled. These actions should also be fully incorporated in continued simulations which move from the rather artificial local field generation of the magnetization dynamics towards a setting that incorporates the spin torque effect as well as the Oersted field associated with the DC spin current (and maybe, in a later stadium, even temperature). Additionally, these simulations should eventually be quantitatively validated with experiment. In a larger context, it is hoped that further progress will be made towards the complete understanding of the phenomenon of point contact oscillator mode locking through spin wave coupling, which could eventually find applications in novel devices containing arrays of nonlinear coupled point contact spin torque oscillators.
8. Acknowledgements
This thesis would not have been what it is without the continued support of a few particular persons that I would like to direct a special thank you to. First of all, I would like to thank Liesbet Lagae for offering me the chance to participate in the interesting research on spin torque driven magnetic oscillators which is currently an active area in the spintronics group of IMEC. Furthermore, enormous appreciation goes to Maarten van Kampen for his continued support, supervision and advice in gaining insight in both the hands-on experimental intricacies of microstructure fabrication as in the physics associated with magnetic point contact oscillator devices.
Maarten, as well as Liesbet and also Wim van Roy have continuously tried to push my thinking and actions a step further and helped me to see the real points of interest in the research of point contact nano-oscillators and spintronics in general. Although these were the persons I had the most interaction with, there are many others that in some way contributed to this work. Generally speaking, I have found the atmosphere at IMEC being one characterized by both strong collegial empathy and a collective feeling for progress. Not a single time a question remained unanswered or someone neglected to give a helping hand, even if not asked for, in the fabrication, characterization and discussion of the results associated with this research. For example, regarding the clean room work, the assistance of Erwin Vandenplas and Stijn De Jonge (training and ellipsometry), helpful tips of Peter Hooylaerts, and e-beam exposures by Jos Moonens (e-e-beam) were greatly appreciated. Moreover, both inside and outside the clean room, my days were filled with many interesting, sometimes inspiring and often amusing conversations with numerous group members including my office-between-cupboard colleagues Reinier Vanheertum and Pieter Neutens, but also Sven Cornelissen, Koen Weerts, Pol Van Dorpe, Iwijn De Vlaminck, Koen Cox, Koen Vervaeke, Celso Cavaco, Zhen Li, Kristof Lodewijks, Jan Mol, Martin Jouk, Chengxun Liu, Swaroop Ganguly, Willem van de Graaf and some non group members I have learned to know, including Gert Claes, Peter Vicca, Robert Muller, John Viaene, Antoine Pacco and many others who I would like to direct a non-research thank you to. I would also especially like to thank Helmut Schultheiß from the University of Kaiserslautern for his cooperation concerning the optical Brillouin light scattering experiments of which some results were presented in this thesis. Both Helmut and Bert Koopmans helped me to better understand the concept of spin waves in microstructure devices. Last but not least I would like to thank Henk Swagten of the Physics of Nanostructures Group of the Eindhoven University of Technology for his supervision and follow-up of my progress during this thesis. After all, if it was not for him, my interest in spintronics might never even have been triggered. A final thank you goes to my parents, who have been supporting me on many fields throughout the duration of my entire study, including this thesis.
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