Thermally Assisted Magnetic Tunneling Junction for Biosensing Applications

We present a concept of the thermally assisted magnetic tunneling junction (MTJ)-based biosensor. The sensor consists of an MTJ, an isolation layer, and a gold coating layer right underneath a biocoating layer. During the sensing phase, a current pulse heats the top free ferromagnet of the MTJ stack above its blocking temperature. The top magnet loses its magnetization at this point. During the cool down period, the top free magnet picks up its magnetization orientation based on the external magnetic field. A vertical excitation field is applied to excite the superparamagnetic biolabels during the sensing phase. If there is no magnetic biolabel captured by the sensing cell, the magnetization in MTJ top free magnetic layer will be programmed by the fringe field from the bottom pinned magnet and will be anti-parallel to the latter. If there are one or more magnetic labels captured by the sensing cell, the superparamagnetic nanoparticle(s) will generate fringe field. The top magnet follows the orientation of the in-plane fringe field component, which is parallel to the pinned bottom magnet. The sensing result can be read out by measuring the tunneling resistance of the MTJ cell. The proposed biosensor design can achieve high sensitivity in three different ways: 1) the magnetic sensing and electronic read-out operations are decoupled in time domain; 2) the binary readout increases the signal to noise ratio significantly; and 3) thermally assisted magnetic sensing decouples the sensor sensitivity from the magnetic stiffness of the sensing layer. Our simulation results demonstrate sensitivity sufficient for single nanoparticle detection. The thermal analysis indicates that the temperature on the top surface of gold layer can be maintained below 50 degC in order to avoid damage to the biocoating layer.