Measuring a peak shift in propagating spin-wave spectroscopy

Student thesis: Master

Abstract

The main data storage technology, the hard disk drive, is slow, energy consuming and vulnerable as it requires moving parts for reading an writing information on a rotating magnetic disk. Novel data storage technologies have been proposed that solve these problems by shifting magnetic domains or magnetic skyrmions in which the data is stored along a magnetic nanowire instead of physically moving the information. An essential ingredient for moving the domain walls or skyrmions along this nanowire is the interfacial Dzyaloshinsky-Moriya interaction (i-DMI). To use the i-DMI in these novel technologies it has to be investigated and tailored, and an easy and reliable manor of measuring the i-DMI is required for doing so. We investigate propagating spin-wave spectroscopy as a technique for measuring the i-DMI which causes a shift in the resonance fields of spin-wave propagating in opposite directions. In this thesis we have designed and optimized devices that can be used for propagating spin-wave spectroscopy, and developed a model to relate measurements with these devices to spin-wave theory. Optimizations included matching the impedances of the device and selecting the insulating layer. These devices excite spin-waves in a magnetic strip using microwave frequency currents through a spin-wave antenna and detect them via induction with a second identical antenna. Because this technique is able to measure spin-waves propagating in opposite directions it should be able to measure a shift in the resonance eld of the two spin-waves that is related to the i-DMI. With these devices first a Ta/Py/Al system was investigated in order to explore the measurement technique as in this system no i-DMI is expected. Indeed, no eld shift was found between the oppositely propagating spin-waves which agrees with the absence of an i-DMI. From these measurements we learned how to correctly interpret spin-wave spectroscopy measurements and that our devices yield plausible results. Finally the measurement technique was used on Pt/Co/Ir samples in which a strong i-DMI is expected. In these measurements we indeed found a field shift of ∼ +1.7mT between the two oppositely propagating spin-waves for spin-waves with wave number 6.59µm−1 at a frequency of 16GHz. However, it has an opposite sign and a larger magnitude than the field shift of ∼ −1.35mT that was expected from literature. Further investigations into the wave number dependency of the shift showed that the direction of the dependency agrees with a shift due to i-DMI but the dependency was significantly stronger than expected. Moreover, a large positive wave number independent offset was observed. This behaviour could not be explained by the i-DMI and has not yet been observed. Other mechanisms that are known in literature to cause a peak shift such as the difference in interfacial anisotropy between the two interfaces of the ferromagnetic layer, or the coupling of the spin-waves to a metallic substrate could not be used to explain the observed behaviour. An additional mechanism is required to understand the measured eld shift. New insights into the origin of the unexpected behaviour might be gained by varying the ferromagnetic layer thickness or inverting the stack in order to separate the individual contributions to the eld shift.
Date of AwardFeb 2018
Original languageEnglish
SupervisorJ. Lucassen (Supervisor 1) & Henk J.M. Swagten (Supervisor 2)

Keywords

  • Spin-Waves
  • DMI
  • Nano-magnetism

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