Sub-pT magnetic field detection by tunnel magneto-resistive sensors

Abstract

We developed tunnel magneto-resistive (TMR) sensors based on magnetic tunnel junctions (MTJs) that are able to detect a weak, sub-pT, magnetic field at a low frequency. Small detectivities of 0.94 pT/Hz1/2 at 1 Hz and 0.05 pT/Hz1/2 at 1 kHz were achieved by lowering the resistance of MTJs and enhancement of the signal using a thick CoFeSiB layer and magnetic flux concentrators. We demonstrated real-time measurement of magnetocardiography (MCG) and nuclear magnetic resonance (NMR) of protons using developed sensors. This result shows that both MCG and NMR can be measured by the same measurement system with ultra-sensitive TMR sensors.

Development of Magnetocardiograph without Magnetically Shielded Room Using High-Detectivity TMR Sensors

“…In order to reduce the variation in detectivity, we aimed to uniformize the film thickness of each layer by oblique sputtering. The material for the free layers was Co 70.5 Fe 4.5 Si 15 B 10 with excellent flatness and a soft magnetic property [ 23 , 25 , 26 ], which has been reported to lead the small anisotropy field and high sensitivity. The free layers were microfabricated to be 600 × 82 μm with the longitudinal side along the sensing axis.…”

“…After the microfabrication, the wafers were first annealed at 325 °C in the external magnetic field of 1 T vertical to the sensing axis, followed by the second annealing at 225 °C with the field parallel to the sensing axis to rotate the easy axis of pinned layer by 90 degrees. By orthogonalizing the magnetic easy axes of the pinned and free layers, the transfer curve with the magnetic field along the hard axis of the free layers reflects the M - H curve for the hard direction of the free layers [ 23 , 24 , 25 , 26 ]. The first magnetic flux concentrators (MFCs) of permalloy were deposited to be 2.0 mm × 1.6 mm × 10 μm with the magnetic field of 100 mT vertical to the sensing axis.…”

High-Resolution Magnetoelectric Sensor and Low-Frequency Measurement Using Frequency Up-Conversion Technique

“…According to the working principle, magnetic sensors can be divided into the following types: magnetoresistance types (for example, anisotropic magnetoresistance, giant magnetoresistance, and tunneling magnetoresistance), magnetic flux types (fluxgate magnetometer, optic-pumping atomic magnetometer, proton precession magnetometer, and superconducting quantum interference devices), magnetic field types (giant magnetoimpedance and magnetoelectric coupling sensor), and so on. Most magnetoresistance sensors have relatively low magnetic field sensitivity; however, recently, the sensitivity of tunneling magnetoresistance sensors has been greatly improved, in the order of sub-pT [ 7 ]. Magnetic flux-type sensors exhibit very high magnetic field resolution, but the magnetic field resolution strongly depends on the dimension of the sensor.…”

Portable Magnetic Detectors Based on TMR Sensors

“…With the continuous development of magnetic sensors towards miniaturization, lightweight, and array, TMR, one of the most successful magnetic sensors in industrial applications in recent years, plays an increasingly important role in the magnetic sensor market [4]. Due to the small size of the sensing unit, high integration, flexible design, and other characteristics, TMR sensors are widely used in current detection [5,6], magnetic anomaly detection (MAD) [7][8][9], angular velocity meter [10][11][12], civil meter [13,14], eddy current nondestructive testing (NDT) [15][16][17][18][19][20], bio-magnetic signal measurement [21,22], and other aspects. MAD has been paid more attention to because of its large detection range, good penetrability, and high detection sensitivity.…”