Probing the Magnetodynamics of Magnetic Tunnel Junctions with the Aid of SiGe HBTs

High impedance (about 1 Megaohm) magnetic tunnel junctions (MTJs) are used to observe and record the magnetodynamics of the nanomagnets that form the junctions themselves. To counteract the bandwidth limitations caused by the high impedance of the junction and the parasitic capacitance intrinsic to any cryogenic system, silicon-germanium heterojunction bipolar transistors (SiGe HBTs) are used as cryogenic preamplifiers for the MTJs. The resulting measurement improvements include an increase in bandwidth by a factor of 3.89, an increase in signal-to-noise ratio by a factor of 6.62, and a gain of 7.75 of the TMR signal produced by the MTJ. The limitation to the measurement system was found to be from the external, room temperature electronics. Despite this limitation, these improvements allow for better time-resolved magnetodynamics measurements of the MTJs. These experiments pave the way for future cryogenic, magnetodynamics measurement improvements, and could even be useful in cryogenic memory applications.