Surface Electron Spin Resonance Study on Ruby Crystal Using Evanescent Microwave Microscopy Techniques

As the silicon technology approaches to its physical limit, the future electronic devices will depend on behaviors of a few electrons. This study is to explore the possibility of detecting a single electron spin transition by using nondestructive evanescent microwave microscopy (EMM) techniques. To enhance the RF magnetic field and minimize the dielectric losses, the sample is placed at the center of the conventional electron spin resonance (ESR) microwave cavity that does not have scanning image capabilities. In this paper, a magnetic dipole probe (MDP) is presented that not only has the advantages of the microwave cavity, but is also capable of surface scanning at high speeds. At present, the minimum detectable electron spin transitions are 20 $\thinspace$000 on the ruby crystal ( ${\hbox {Cr}}^{3+}$ doped in ${\hbox {Al}}_{2}{\hbox {O}}_{3}$) surface, whereas the commercially available ESR microwave cavity has a resolution of $10^{6}$ minimum detectable spins limit. Three ESR energy absorption spikes were detected at 3.77 and 3.73 GHz with the ruby crystal placed inside and outside of the MDP conductor loop, respectively. The measured ESR energy absorption spectra are consistent with theoretical analysis and the conventional ESR experimental results. The current MDP sensor has a 500-$\mu{\hbox {m}}$ spatial resolution with a 1-mm radius conductor loop made by 150- $\mu{\hbox {m}}$ copper wires. The nondestructive and noninvasive natures of the EMM microscopy are suitable for many biomedical applications, such as DNA sequencing, Alzheimer, and other biological tissue studies. Future efforts will be focused on integration of the MDP on the atomic force microscopy with carbon-nanotube bridges.