Discipline: Technology and Engineering
Marzieh Savadkoohi - University of the District of Columbia
Co-Author(s): Eva Mutunga, Bishnu Dahal, Pawan Tyagi
Molecular spintronic is an emerging interdisciplinary field that links spintronic with molecular quantum electronics. Using both principles, one can manipulate charges and spins in electronic devices such as Magnetic Tunnel junctions (MTJs). Molecule-incorporated MTJs show intriguing phenomena such as complete current suppression, negative differential conductance [1-3], and switchability with light known as the photovoltaic (P-V) effect . Spin-based P-V effect is a new phenomenon that can produce voltage difference along with photon excited electrons. This study investigates a cross-junction-shaped magnetic tunnel junction molecular spintronic (MTJMSD) device that shows a measurable P-V effect upon magnetic molecules (i.e., SMM) attachments to the exposed edges. MTJMSD fabrication process consists of two-step i) photolithography to make the first electrode, alignment and second electrode followed by ii) thin film deposition after each step. To optimize our lithography process, we used the Taguchi design of experiment (DOE) and tested combinations of nine different parameters (e.g., spin speed, undercut time, exposure time, plasma cleaning, etc.,) with various intensities. The optimal conditions suggested by Taguchi software were used again to make new samples. The optimization step resulted in having a bottom electrode with a smoother surface and edges which minimized the chance of having short circuits through tunnel barrier. It also led to having a stable and consistent tunneling behavior in most of the cross-junctions. The compatibility of molecules with MTJMSD’s ferromagnetic (FM) layers (CoFeB) and tunnel barrier (MgO) was tested prior to device fabrication via magnetoresistance (VSM) measurements. We also performed a UV reflectance study to ensure that the observed P-V effect is not due to FM layers oxidation during the baking and lithography stage. MTJMSD showed noticeable different magnetic properties in cross-junctions and electrodes in the presence of light compared to the dark condition. Light absorbance of molecular-based MTJ was also tested with a solar simulator tool at different light intensities. Molecular-related variation of FM electrodes’ magnetic properties was measured with MFM and Kelvin-probe microscopy (KPFM). According to the KPFM scans, FM electrodes showed a 100 to 150 mV difference upon application of molecules. The susceptibility of MTJMSD to light intensity could be also explained by FMR results in which we observed some level of difference in signals pre versus post molecules incorporation.
Funder Acknowledgement(s): This work is supported by:National Science Foundation-CREST Award (Contract # HRD- 1914751)and Department of Energy/ National Nuclear Security Agency (DE-FOA-0003945).
Faculty Advisor: Dr. Pawan Tyagi, email@example.com
Role: I did the entire experimental research consisting of device fabrication, characterization, optimization, and measurements.