  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.
