Highly Sensitive Detection of Weak Low Frequency Magnetic Fields Using Single Nanoscale Orthogonal MgO Magnetic Tunnel Junctions under a Large Bias Field

Abstract

Ultrasensitive magnetic field sensors at low frequencies envisaged for applications on biosensors require the detection of superparamagnetic nanoparticles as biomarkers and weak biomagnetic fields. However, superparamagnetic nanoparticles can be magnetized and detected in the presence of a relatively large external magnetic field in which other low field sensors and flux concentrators cannot be used since those are saturated in the bias field. To overcome this problem, we made submicron-sized orthogonal magnetic tunnel junctions with an appropriate perpendicular anisotropy free layer. By applying a perpendicular external magnetic field, the magnetization in the magnetic free layer is brought into an unstable state. As a result, relatively high field sensitivity of 1.3%/Oe is achieved in the external field of about 975 Oe for the 300 nm magnetic tunnel junction. By tilting the angle of the bias field of 10 degrees from film normal, the maximum sensitivity of 4.5%/Oe was obtained at 295 Oe. We also investigated the low-frequency noise generated by the sensor for their capability to detect weak low-frequency magnetic signals. Noise equivalent signal of the device is estimated to be about 235 nT/√Hz from 1/f noise measurements which is suitable for single superparamagnetic nanoparticles detection.