  Magnetic sensing is present in our everyday interactions with consumer
electronics, and also demonstrates potential for measurement of extremely weak
biomagnetic fields, such as those of the heart and brain. In this work, we
leverage the many benefits of the micro-electromechanical systems (MEMS)
devices to fabricate a small, low power, inexpensive sensor whose resolution is
in the range of weak biomagnetic fields. The sensor works at room temperature,
and is suitable for consumer electronics integration. At present, such
biomagnetic fields can only be measured by expensive mechanisms such as optical
pumping and superconducting quantum interference devices (SQUIDs). Thus, our
sensor suggests the opening of a large phase space for medical and consumer
applications. The prototype fabrication is achieved by assembling
micro-objects, including a permanent micromagnet, onto a post-release
commercial MEMS accelerometer. With this system, we demonstrate a room
temperature MEMS magnetometer, whose design is only sensitive to gradient
magnetic fields and is generally insensitive to the Earth's uniform field. In
air, the sensor's response is linear with a resolution of 1.1 nT cm-1 and spans
over 3 decades of dynamic range to 4.6 {\mu}T cm-1. In 1 mTorr vacuum with 20
dB magnetic shielding, the sensor achieved 100 pT cm-1 resolution at resonance.
The theoretical floor of this design is 110 fT cm-1 Hz-1/2 with a resolution of
13 fT cm-1, thus these devices hold promise for both magnetocardiography (MCG)
and magnetoencephalography (MEG) applications.
