Z. Q. Lei

Liver cancer immunoassay with magnetic nanoparticles and MgO-based magnetic tunnel junction sensors

We have demonstrated the detection of alpha-fetoprotein (AFP) labeled with magnetic nanoparticles (MNPs) using MgO-based magnetic tunnel junction (MTJ) sensors. AFP is an important hepatic tumor biomarker and the detection of AFP has significant applications for clinical diagnostics and immunoassay for early-stage liver cancer indications. In this work, MgO-based MTJ sensors and 20-nm iron-oxide magnetic nanoparticles (MNPs) were used for detecting AFP antigens by a sandwich-assay configuration. The MTJ sensors with a sensing area of 4 × 2 μm2 possess tunneling magnetoresistance (TMR) of 122% and sensitivity of 0.95%/Oe at room temperature. The target AFP antigens of three concentrations were successfully detected, and the experimental data indicate that the resistance variations of the MTJ sensor increased with the AFP concentration ratios proportionally. These results demonstrate that MgO-based MTJ sensors together with MNPs are a promising biosensing platform for liver cancer immunoassay.

Review of Noise Sources in Magnetic Tunnel Junction Sensors

Noise problem limits the sensitivity of magnetic tunnel junction (MTJ) sensors for ultra-low magnetic field applications. Noise analysis not only helps in finding ways to eliminate noise disturbances but also essential for understanding the electronic and magnetic properties of MTJs. These approaches provide insight for optimizing the design of MTJ sensors before fabrication. This paper reviews the noise sources in MTJ sensors reported in recent years. Both the origins and mathematical derivations of the noise sources are presented, illustrating how different factors affecting the performance of MTJ sensors. A brief outlook of challenges in the future is also given.

Detection of 10-nm Superparamagnetic Iron Oxide Nanoparticles Using Exchange-Biased GMR Sensors in Wheatstone Bridge

We demonstrated the use of exchange-biased giant magnetoresistance (GMR) sensors in Wheatstone bridge for the detection of 10-nm superparamagnetic iron oxide nanoparticles (SPIONs). The SPIONs were synthesized via coprecipitation method, exhibiting a superparamagnetic behavior with saturation magnetization of 57 emu/g. The output voltage signal of the Wheatstone bridge exhibits log-linear function of the concentration of SPIONs (from 10 ng/ml to 0.1 mg/ml), making the sensors suitable for use as a SPION concentration detector. Thus the combination of 10 nm SPIONs and the exchange-biased GMR sensors has potential to be used in the bio-detection applications where ultra-small bio-labels are needed.

Magnetic Tunnel Junction Sensors With Conetic Alloy

Al2O3 magnetic tunneling junction (MTJ) sensors were fabricated with Conetic alloy Ni77Fe14Cu5Mo4 deposited as the free layer and pinned layer for its soft magnetic properties. It was observed that the Al2O3 MTJ sensors with Conetic exhibited relatively small easy-axis coercivity. Tunneling magnetoresistance (TMR) and noise measurements were carried out to characterize the sensors. TMR of 9.5% and Hooge parameter of 3.825 × 10-7 μm2 were achieved without any hard-axis field. Hard-axis bias field was applied to eliminate the hysteresis and improve the linear field response of the MTJ sensor. The hysteresis was removed by applying an external magnetic field along the hard axis at 8 Oe and the sensor sensitivity was 0.4 %/Oe within a linear region at room temperature. The relationship between the Hooge parameter and hard-axis field was also investigated and the result demonstrated that the 1/f noise can be suppressed by an optimized hard-axis bias field. This work shows that it is feasible to use Conetic alloy as the soft magnetic layers in MTJ sensors for its small coercivity, and a hard-axis bias field can be used to linearize the sensor response and suppress the 1/f noise.

Detection of Iron–Oxide Magnetic Nanoparticles Using Magnetic Tunnel Junction Sensors With Conetic Alloy

We demonstrated the detection of 20-nm iron-oxide magnetic nanoparticles (MNPs) using Al2 O3 magnetic tunnel junction sensors (MTJs) with Conetic alloy. Conetic alloy Ni77Fe14Cu5Mo4 was deposited as the MTJ free layer and pinned layer due to its magnetically soft properties. The magnetoresistance (MR) curves of MTJs with Conetic alloy showed tunneling magnetoresistance of 8.0% with small hysteresis and high linearity in the sensing region, after applying an external magnetic field of 14 Oe along the hard axis. The sensitivity of the MTJ sensors with Conetic alloy was determined to be 0.3%/Oe within a linear region at room temperature. The MNPs of three different concentrations were successfully detected by the shifts of the MR loops of the MTJs, and it was observed that the resistance deviations of the MTJ sensors increased with the logarithm of MNP concentrations. The maximum resistance deviation was 0.16 Ω for an MNP concentration of 20.0 mg/mL. MTJ sensors, together with MNPs, are a promising platform for future biosensor applications, and this paper shows that Conetic alloy is feasible for improving the performance of this platform.

Angular Dependence of Low-Frequency Noise in Al

We demonstrated the noise performances of Al2O3-based magnetic tunnel junction sensors (MTJs) in the low-frequency regimes. Conetic alloy Ni77Fe14Cu5Mo4 was deposited as both the MTJ pinned layer and free layer because of its superb magnetically soft properties. A rotating magnetic field was employed to investigate the angular dependence of the MTJ low-frequency noise. Hooge parameter was applied for parameterizing the low-frequency noise. The measurement results demonstrate that the Hooge parameters are angular-dependent and they exhibit a linear relation with respect to the angular magnetoresistive susceptibility. It can be also observed that the Hooge parameters possess a higher value when the Conetic MTJs are in the region of antiparallel state. These results indicate that the magnetic fluctuations in the ferromagnetic layers contribute to the low-frequency noise level in Conetic MTJ sensors.

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To enhance energy efficiency and reduce light pollution of overnight road lighting in suburban traffic, we propose a novel green transport system based on giant magnetoresistive (GMR) sensors. The basic principle is to detect the perturbation to the earth magnetic field by a ferrous vehicle with GMR sensors. This system can switch on the road lighting to full illumination gradually before the motor vehicle arrives and dim it out after the vehicle leaves without the driver noticing. Based on a sparse suburban road in the countryside of Hong Kong, a demonstration model was constructed to illustrate its feasibility. GMR sensors and the associated electrical energy control components including signal processors, relays, and dimmers were integrated into a complete system. The experimental result indicates that the sensing principle is feasible and the whole system can function together coherently to achieve over 90% energy saving. Such system can be scaled up to be implemented in real road conditions.

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A magnetically shielded setup was developed for characterizing magnetoresistance (MR) and noise properties of magnetic tunneling junction (MTJ) sensors. A mu-metal shielding is installed to avoid the interference of external magnetic disturbance. Both MR curves and noise power spectra of MTJ sensors can be obtained for further data analysis. Moreover, a hard-axis magnetic field can be applied to eliminate the hysteresis and the linear field response of MTJ sensors can be measured. The preliminary measurement results on MTJ sensors are presented to illustrate the characterization capabilities of this setup.