Low-frequency noise study of magnetic tunnel junctions and their applications on biomarker immunoassay

• Main Authors: Lei, Zhiqiang, 雷志强
• Other Authors: Pong, PWT
• Language: English
• Published: The University of Hong Kong (Pokfulam, Hong Kong) 2014
• Subjects: Magnetoresistance; Immunoassay
• Online Access: http://hdl.handle.net/10722/193413

The noise performances of Al2O3-based magnetic tunnel junctions (MTJs) in the low-frequency regime were systematically studied. Conetic alloy Ni77Fe14Cu5Mo4 was deposited as both the MTJ pinned layer and free layer due to its superb magnetically soft properties. The Al2O3-MTJ sensors with Conetic alloy exhibited a linear field response with a relatively small easy-axis coercivity of 3 Oe. A tunneling magneto-resistance (TMR)of 9.5% and Hooge parameter of 3.82 × 〖10〗^(-7)μm^2 were achieved without any hard-axis bias field. The hysteresis was removed by applying an external magnetic field of 8 Oe along the hard axis and the sensor sensitivity was 0.4%/Oe within a linear region at room temperature. The relation between the Hooge parameter and hard-axis field was investigated and the result illustrated that the 1/f noise could be suppressed by an optimized hard-axis bias field. Additionally, a rotating magnetic field was employed to investigate the angular dependence of the MTJ low-frequency noise. Measurement results indicated that the Hooge parameter was angular-dependent and exhibited a linear relation with respect to the angular magneto-resistive susceptibility. Also, it could be inferred that the Hooge parameter possessed a higher value when the Conetic MTJs were in the region of antiparallel state. These studies showed that it was feasible to use Conetic alloy as the soft magnetic layer in MTJ sensor for its small coercivity, and a hard-axis bias field could be used to linearize the sensor response and suppress the 1/f noise induced by the magnetic fluctuation in the MTJ ferromagnetic layers. Besides the study of low-frequency noise in MTJs, the feasibility of biomarker immunoassay with MTJ sensors is investigated. Initially, with the assistance of Al2O3-MTJ sensors with Conetic alloy, 20-nm iron-oxide magnetic nanoparticles (MNPs) with three different concentrations were successfully detected by the shift of the TMR curve. The maximum resistance deviation was 0.16 Ω for an MNP concentration of 20.0 mg/mL. Based on this study, the detection of alpha-fetoprotein (AFP) labeled with 20-nm iron-oxide MNPs together with MTJ sensors by a sandwich-assay configuration was further investigated. AFP is a vital hepatic tumor biomarker and the detection of AFP has significant applications for clinical diagnostics and immunoassay for early-stage liver cancer indication. To further increase the sensitivity, MgO-MTJ sensors with a TMR of 122% and sensitivity of 0.95%/Oe were employed. The target AFP antigens of three concentrations were detected, and experimental data indicated that the resistance variation of the MTJ sensor increased with the AFP concentration ratio proportionally, which was consistent with previous studies. These results conclusively demonstrate that MTJ sensors together with MNPs are a promising bio-sensing platform for biomarker immunoassay. === published_or_final_version === Electrical and Electronic Engineering === Doctoral === Doctor of Philosophy