MgO-based Magnetoresistive Biosensor for Magnetically labeled Cells Detection.

Since the discovery of the giant magnetoresistance (GMR), many spintronic devices have been developed and used in various applications such as information storage and automotive industry. Nowadays, increasing research in the field of spintronics and its application in the development of magnetoresistive (MR) biomolecular and biomedical platforms is giving rise to a new family of biomedical sensors [1]–[3]. Magnetic tunnel junctions (MTJ), based on MgO barriers, are promising magnetic field sensor solutions in the framework of electronic components integration and miniaturization. MgO-based MTJs show superior sensitivity for the detection of small magnetic fields needed in many industrial and biomedical applications. MgO-based MR sensors have been integrated for biological applications, such as biochips. The concept, explained in [4] and [5], relies on the capability of the sensor for detection of the fringe field generated by magnetized nano/microparticles attached to biomolecules. In this work, we aim to implement MgO-based MR biosensors for measurement of the flux of magnetically labeled cells. As a representative schematic, the biochip in figure 1.a shows different components of the MR biosensor. Figure 1.b illustrates the concept with superparamagnetic beads. As shown, a magnetic bead above the sensor will be magnetized by the magnetic field generated by the current in the gold strip. The stray field of the bead can be sensed by the magnetic field sensor, if the magnetic bead is within its sensing space. When a larger number of magnetic beads labeling the cells are mobilized inside the micro-tube, a larger signal will be observed. We should mention that the manipulation of these particles and biomolecules requires handling fluidic samples. Moreover, the labeling particles should be handled under minimum aggregation, preferably in a paramagnetic state. We designed and fabricated MgO-based MR sensors presented in figure 2.a. Each sensor consists of 1200 elliptic 16*8 mm 2 pillars in series. MTJ multilayer films were deposited using a magnetron sputtering system (Singulus Rotaris) on thermally oxidized Si wafers. The MTJ stack used in this study had the following layer structure: (thicknesses in nanometers) Si/SiO2/(3)Ru /(8) Ta /(3)Ru(8) Ta /(3) Ru /(8)MnIr $_{20} /(2.3)$ Co 70 Fe $_{30} /(0.85)$ Ru / (2.4) Co 60 Fe 20B20 ferromagnetic pinned layer)/ (1.53) MgO / (1.45) Co 60 Fe 20B20 magnetic free layer)/(3)Ru /(8) Ta. MTJ stack was patterned into micron-sized elliptical devices using standard optical lithography and ion milling. A 150-nm-thick gold layer was deposited over the junction area and patterned into low-resistance electrical contacts for each MTJ. After patterning, the samples were annealed at $360 ^{circ}\mathrm {C}$ for 2 h at $1.10 ^{-6}$ Torr in an applied field of 8 kOe. The magnetoresistance properties of the MR sensors were measured at room temperature in air by a conventional DC four-probe method and current driven Helmholtz coils controlled with LabView. Figure 2.d shows the transfer curve of one of MR sensor. The results prove that the proposed MR sensor has great sensitivity and has linear response in the range of [-5 Oe -5 Oe]. In this work we propose a new design for MgO-based MTJ magnetoresistive biosensor and demonstrate its functionality for detection of magnetically labelled cells. More experiments are in progress to fully optimize and characterize the proposed device.