Ultrasensitive magnetic sensor array based on magnetic tunnel junctions*

We have fabricated an ultrasensitive magnetic sensor array with high spatial and temporal resolution, based on magnetic tunnel junctions in vortex magnetization state (vortex MTJs). The sensor array consists of vortex MTJ elements of miniature size (diameter less than or equal to 5 μm) with small element separation (20 μm or less). We have also designed a full set of signal-conditioning circuits, where signal from each MTJ element is amplified by one low-noise preamplifier, and all amplified signals are read by a data-acquisition system simultaneously. The magnetic sensor array and the corresponding circuitry can be incorporated with Quantum Design® Physical Property Measurement System (PPMS), which provides the additional capability to set the measurement temperature from 4 K to 400 K. Under a uniform test field, all the MTJ sensors in a 3×3 array show consistent sensitivity across a broad frequency (1 Hz - 100 kHz) and temperature (4-300 K), which shows good homogeneity of the sensor array. The sensor array can be used with the stimulus field to measure magnetic susceptibility of magnetic materials, or without external field to measure correlation at various distances in magnetic materials.