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High impedance (about 1 Megaohm) magnetic tunnel junctions (MTJs) are used toobserve and record the magnetodynamics of the nanomagnets that form thejunctions themselves. To counteract the bandwidth limitations caused by thehigh impedance of the junction and the parasitic capacitance intrinsic to anycryogenic system, silicon-germanium heterojunction bipolar transistors (SiGeHBTs) are used as cryogenic preamplifiers for the MTJs. The resultingmeasurement 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 ofthe TMR signal produced by the MTJ. The limitation to the measurement systemwas found to be from the external, room temperature electronics. Despite thislimitation, these improvements allow for better time-resolved magnetodynamicsmeasurements of the MTJs. These experiments pave the way for future cryogenic,magnetodynamics measurement improvements, and could even be useful in cryogenicmemory applications.

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