November 07, 2022
Minneapolis, United States
Single Molecular Magnet (SMM) Producing Photovoltaic Effect(PV) on Magnetic Tunnel Junction
Spin based photovoltaic (PV) effect is novel research in energy harvesting area.Recently, spin-PV effect was demonstrated on magnetic tunnel junction-based molecular spintronics (MTJMSD).This prior work mainly focused on NiFe/AlOx/NiFe MTJ1.To advance this field there is a critical need of investigating a wide range of magnetic materials.MgO insulator and CoFeB electrode-based MTJS are well established for showing high TMR ratio and heavily studied for computer memory operation.However,they have never been integrated into the molecular spintronics study.This study made effort to develop a fabrication method by which CoFeB/MgO/CoFeB MTJ could be produced in the cross-junction form to facilitate transport and electrical measurements.To do so we had to overcome numerous fabrication challenges.First, we investigated the air stability of CoFeB using reflectance study as a function of temperature.We observed that CoFeB was stable in air up to ~100C.To produce stable and low leakage current tunnel junctions we iteratively investigated the role of multiple bottom electrode fabrication factors through Taguchi Design of experiment.We produced bottom electrode in such a way that thin films have optimum tapered edge geometry and ~0.15 nm Rq roughness.To produce exposed side edges for molecule spin channel attachment we deposited MgO and top electrode through photolithorgraphically produced cavity perpendicular to the bottom electrode. To convert MTJ into MTJMSD we electrochemically bridged the SMM molecules across MgO and conducted current-voltage study before and after molecule spin channel creation.Interestingly, SMM produced current suppression at room temperature by ~200% at 300mV. Also, we observed PV effect. This MTJ produced 0.08 open circuit voltage and 5.19E-9 saturation current under 1sun(1000 mW/cm2) light intensity. To investigate the mechanism, we perform KPAFM. The difference between electrodes before and after SMM bridging was 33 and 150 mV, respectively. Higher difference in KPAFM voltage signifies a large difference in electrode resistivity. KPAFM also suggests the observed PV effect is mainly from the impacted ferromagnetic electrode region and not due to the interaction of light at molecular channels.
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