  Due to their compact size and exceptional sensitivity at room temperature,
magnetoresistance (MR) sensors have garnered considerable interest in numerous
fields, particularly in the detection of weak magnetic signals in biological
systems. The magnetrodes, integrating MR sensors with needle-shaped Si-based
substrates, are designed to be inserted into the brain for local magnetic field
detection. Although recent research has predominantly focused on giant
magnetoresistance (GMR) sensors, tunnel magnetoresistance (TMR) sensors exhibit
significantly higher sensitivity. In this study, we introduce TMR-based
magnetrodes featuring TMR sensors at both the tip and mid-section of the probe,
enabling detection of local magnetic fields at varied spatial positions. To
enhance detectivity, we have designed and fabricated magnetrodes with varied
aspect ratios of the free layer, incorporating diverse junction shapes,
quantities, and serial arrangements. Utilizing a custom-built magnetotransport
and noise measurement system for characterization, our TMR-based magnetrode
demonstrates a limit of detection (LOD) of 300pT/Hz1/2 at 1 kHz. This implies
that neuronal spikes can be distinguished with minimal averaging, thereby
facilitating the elucidation of their magnetic properties.
