Towards the imaging of spin-wave mixing
Imaging and characterisation of backward volume spin-waves using boron vacancy centres
E.A. Nielen (TU Delft - Applied Sciences)
Y.C. Doedes – Mentor (TU Delft - Applied Sciences)
T. van der Sar – Graduation committee member (TU Delft - Applied Sciences)
Sander Otte – Graduation committee member (TU Delft - Applied Sciences)
M. Rossi – Graduation committee member (TU Delft - Applied Sciences)
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Abstract
Spin waves are collective excitations of the magnetisation in a magnetic material and are a promising candidate for future information-processing technologies. To understand and exploit spin-wave devices, it is essential to be able to detect and image spin-waves with high spatial resolution. In this thesis, boron-vacancy (V−B) centres in hexagonal boron nitride were used to image spin-waves in a Permalloy film. The long-term goal is the spatial imaging of spin-wave mixing signals, where two spin-waves interact nonlinearly to generate a signal at the difference frequency.
To support these future experiments, magnet calibration procedures were developed to provide accurate control over both the strength and direction of the applied magnetic field. By introducing a corkscrew calibration trajectory and improved fitting procedures, the calibration time was reduced by approximately a factor of fifty, from around a weekend to about one hour. In addition, a simulation package with graphical user interfaces in both MATLAB and Python was developed, combining colour-centre dynamics and spin-wave dispersion calculations to rapidly determine suitable measurement conditions.
Backward volume spin-waves were systematically excited and imaged for different magnetic fields and excitation frequencies. Although clear changes in spin-wave activity were observed, the measured patterns appeared strongly scattered and did not resemble well-defined plane waves, making them unsuitable as reference material for future spin-wave mixing experiments. Furthermore, neither Fourier-transform nor power-spectral-density analysis yielded a reliable reconstruction of the backward volume dispersion. However, the inverse participation ratio proved to be a useful measure for quantifying changes in spin-wave activity and showed qualitative agreement with the expected dispersion.
Future work should therefore focus on Damon–Eshbach spin-waves, which exhibit more well-defined wavefronts and are expected to provide better reference material for spin-wave mixing measurements. Together, the magnet calibration procedures, simulation tools, and measurements presented in this thesis provide an important foundation for future spatial imaging of spin-wave mixing using colour-centre magnetometry.