Magnetic signal scan imaging system based on giant magnetoimpedance (GMI) differential sensor

TL;DR

This work tackles the challenge of weak-field magnetic imaging with high spatial resolution without cryogenics or shielding. It introduces a differential GMI-based scanning imager using a matched pair of GMI sensors to suppress common-mode noise. Key results show a sensitivity of about 186,790 V/T, with noise densities as low as 9.2 pT/√Hz in shielded and 46 pT/√Hz in unshielded environments, and a spatial resolution better than 200 μm; banknote ink imaging and gradient-field reference tests validate performance. The approach offers a practical, room-temperature alternative to SQUID-based systems, with potential impact across biomagnetism, materials science, and nanomagnetic detection.

Abstract

This paper presents the design and implementation of a magnetic signal scanning and imaging system based on the giant magnetoimpedance (GMI) effect. The system employs a pair of performance-matched GMI sensing elements configured as a differential probe structure. Through co-optimized low-noise electronic and probe design, the system effectively suppresses both intrinsic sensor common-mode drift and external environmental magnetic noise, enabling high signal-to-noise ratio detection of nono-tesla to micro-tesla-level magnetic signals without magnetic shielding. Experimental results demonstrate that the differential system achieves significantly lower noise spectral density in unshielded environments compared to conventional GMI sensors (\SI{46}{pT}/$\sqrt{\text{Hz}}$ versus \SI{286}{pT}/$\sqrt{\text{Hz}}$ at \SI{1}{Hz}), with a sensitivity of 186,790 V/T and spatial resolution better than 200 micrometers. The system's excellent performance in weak magnetic field detection and spatial resolution was verified through scanning experiments of magnetic ink on US banknotes and magnetic reference samples. Compared to SQUID scanning systems, which requiring liquid helium cooling, this system based on the GMI effect offers advantages of room-temperature operation, compact structure, and low cost. Relative to conventional single-element GMI microscopes, it achieves significant improvements in signal-to-noise ratio and environmental adaptability. This research provides a practical solution for high-resolution magnetic field imaging at room temperature with broad application potential in materials magnetism, biomagnetic imaging, and nanomagnetic detection.

Figures (6)

• Figure 1: (a) Schematic diagram of the main components of the GMI-based differential magnetic signal scanning imaging system. The GMI differential magnetic sensor is mounted on a Z-axis displacement platform, while the sample is placed on an X-Y platform. The system operates without cooling or magnetic shielding. The three-axis displacement stage measures 46.0 cm in width, 39.0 cm in length, and 35.4 cm in height. (b) Configuration of the GMI differential magnetic sensor unit, showing the sensing probe mounted on a Z-axis displacement platform. (c) Schematic of the computer-controlled operation for the GMI differential magnetic scanning system. During magnetic field acquisition, the displacement stage remains stationary to minimize vibration-induced noise from the stepper motor.
• Figure 2: (a) Schematic of the three-dimensional model of the GMI differential magnetic sensing probe, mounted on a signal processing PCB. The PCB measures 9.5 cm in length and 7.0 cm in width, and the magnetic sensing probe is 1.5 cm in length. (b) Schematic of the signal processing circuit for the GMI differential magnetic sensor used in the scanning imaging system. (c) Structural diagram of the GMI differential magnetic sensing probe. The probe comprises an FeCoSiB amorphous wire with a diameter of 120 and a length of 10 mm, along with two sets of coils, each 5.0 mm in length. The two coil sets are used to detect magnetic field variations on the surface of the amorphous wire and to provide both additive and negative feedback signals.
• Figure 3: (a) Sensitivity characterization of the conventional GMI sensor. Under an external magnetic field sweeping from -46.8µ T to +46.8, the sensor output ranges from –4.917 V to +4.706 V. (b) Sensitivity characterization of the GMI differential magnetic sensor. Under the same magnetic field sweep from -46.8µ T to +46.8, the sensor output varies between –9.00 V and +8.48 V.
• Figure 4: (a) Magnetic noise spectra of the GMI differential magnetic sensor and the conventional GMI magnetic sensor measured in a magnetically shielded environment. The measured noise spectral densities at 1 Hz are 9.2pT/$\sqrt{\text{Hz}}$ for the differential sensor and 26.8pT/$\sqrt{\text{Hz}}$ for the conventional sensor. (b) Magnetic noise spectra of both sensors acquired under unshielded conditions. The corresponding noise spectral densities at 1 Hz are 46pT/$\sqrt{\text{Hz}}$ (differential sensor) and 286pT/$\sqrt{\text{Hz}}$ (conventional sensor).
• Figure 5: Magnetic field image of a United States one-dollar note section acquired at a working distance of 150. The banknote was previously magnetized by a 4.0 T vertically upward magnetic field. The grayscale represents magnetic field intensities ranging from approximately 26.77 nT (downward-pointing, white) to 1553 nT (upward-pointing, black).