Cryogenic Amplification of Magnetoresistance in Magnetic Tunnel Junctions with the Aid of SiGe-HBT*

Cryogenic amplification has been used for qubit readout, single electron transistor operation, and charge sensing with quantum point contacts; however, this useful tool has been missing from electron transport measurements in the broad field of magnetism. Silicon-Germanium Heterojunction Bipolar Transistors were used for cryogenic preamplification of nanoscale magnetic tunnel junctions at 8 K. The preamplification system increased the signal-to-noise ratio of the tunneling magnetoresistance signal by a factor of 6.62. The current gain provided by the preamplification also allowed for a higher bandwidth measurement limited by the external transimpedance amplifier, rather than cryogenic wiring. The high impedance (≈1 MΩ) of the junctions allow for easy migration to study the magnetodynamics of more interesting samples.