Robust and high sensitivity biosensor using injection locked spin torque nano-oscillators

Abstract

Magnetic nano-particles have been used in biological assays for bio-sensing applications [1]. A wide variety of biological species — such as cells, proteins, antibodies, pathogens, toxins, and DNA — can be labeled by attaching them to magnetic nano-beads (fig. 1). Magnetic nano-particles are non-toxic, stable, non-reactive (in most biological assays) and scalable, and hence, can be highly useful for developing cost-effective, high-sensitivity and reliable point-of-care bio-sensing techniques. Field induced magneto-resistive signal produced by nano-scale magnetic tunnel junctions (MTJ) can offer a sensitive and scalable mechanism for detecting the magnetic nano-tags [1]. An MTJ can be operated as Spin Torque Nano Oscillator (STNO) which constitutes of a thin insulating oxide sandwiched between two ferromagnetic layers (Fig. 1). The ferromagnetic layers have two stable spin-polarization states, depending upon magnetic anisotropy. The magnetization of one of the layers is fixed, while that of the other (free-layer) can be influenced by a charge current passing through the device or by an applied magnetic field. The high-polarity fixed magnetic-layer spin-polarizes the electrons constituting the charge-current, which in turn exert spin transfer torque (STT) on the free layer. STT results in precession of the effective spin moment of the free layer away from the original magnetization axis, while an inherent inertial damping torque tries to restore it along the original state. [2] A static magnetic field can be used to obtain sustained spin-precession of the free layer at an angle φ, at which the STT and the damping torque balance out each other. (Fig. 2a) The dynamics of the free layer is governed by the Landau-Lifshitz-Gilbert Slonczewski Equation. [2]