Magnetic tunnel junctions (MTJs) which comprise of a free magnetic layer (whose magnetization orientation can be manipulated) and a fixed magnetic layer (whose magnetization is pinned by anisotropy forces) that are separated by a thin insulating tunnel barrier have featured in many previous Pick of the Week articles. Most of the time, these MTJs are studied in Pick of the Week for reasons relating to their application in magnetoresistive random access memory (MRAM) or as sensors in hard disk drive read heads. This week in Pick of the Week, MTJs are discussed in the context of microwave detectors. The paper of interest is “High sensitivity microwave detection using a magnetic tunnel junction in the absence of an external applied magnetic field” in Applied Physics Letters by Gui et al., which demonstrates that the performance of MTJ microwave sensors can be enhanced by applying a DC bias current across the device.
Microwave: quick and easy
One of the fundamental principles of electromagnetism is that when a charge is accelerated an electromagnetic wave is produced. By the same token if any electromagnetic wave is incident on a conducting material a current will flow. The induced current is an alternating one owing to the wave like nature of the indent electromagnetic radiation, and its frequency is given by the frequency (or energy) of the electromagnetic wave. It is therefore obvious that electromagnetic radiation can be sensed by measuring the current it induces in conducting media.
There are currently three main approaches for creating microwave electromagnetic radiation detectors that all rely on the extremely basic principle above, but with the differences being how the induced AC current from the microwaves is turned into a useful and clear signal. By a long way the most established type of microwave detectors are those based upon semiconductor Schottky diodes, while integrated photonic microwave sensors are also in development. The focus here is on spintronic microwave sensors that utilise MTJs. These MTJ microwave sensors have two key potential advantages over their Schottky diode counterparts. Firstly, in MTJ microwave sensors the signal to noise improves as the size of the magnets within the device reduce, and this allows for MTJ sensors to be much smaller than Schottky diode ones. The benefit of having a small microwave sensor is that there will be less perturbation of the microwave field that is being sensed, i.e. less destructive sensing. Secondly, the maximum signal of Schottky diode sensors is limited by thermal voltages that give large zero-bias junction resistance. In MTJ structures such thermal voltages are not such a critical problem.
Spin diode another day
The spintronic MTJ microwave sensor is based upon the spin diode effect which was first documented 10 years ago. The basic principle is as follows. When an AC current is induced in an MTJ, charge carriers flow alternatively from the fixed layer to the free layer and vice-versa. For a given charge flow direction, say for example for charge flow from the fixed to free layer, the charge carriers will become spin-polarised by the fixed layer and then tunnel to the free layer where they can torque its magnetization due to the usual spin-transfer toque mechanism which has been well written about in Pick of the Week before. The torque between the spin-polarised carriers in the free layer and its magnetic moments causes the local moments to precess with a given frequency. For a DC current the moments would have sufficient time to precess and then be damped into a new steady state orientation. However, for an AC current thit is not necessarily the case. The spin diode effect is such that when the frequency of the AC current matches the characteristic frequency of the precession of the moments a resonance state is achieved. In this resonance state, for current in one direction the orientation of the free layer magnetization moves closer to that of the fixed layer and so the resistance across the MTJ decreases due to the tunnel magnetoresistance effect (TMR), and vice versa for current flow in the opposite direction. As both the size of current and resistance of the MTJ vary with time, a DC voltage appears across the device that depends on the magnitude of the induced AC current and the MTJ resistance characteristics. For a microwave current induced by microwave radiation, measuring the resultant DC voltage across the MTJ can therefore give a measure of the microwave current.
As would be expected, the spin diode effect (namely the size of the induced DC voltage across the MTJ) gets weaker the further away the microwave radiation frequency is from the resonant frequency is from the resonant frequency of the free layer. In the lab this problem can be circumvented by applying an external magnetic field to alter the resonant frequency of a free layer, but for device applications this is not a favourable approach – neither Schottky nor integrated photonic microwave sensors require an external field for their operation. It is therefore important to try and enhance the spin diode signal away from the resonance condition, and this is what the work of Gui and colleagues achieve by applying a DC bias current across the MTJ. Gui studies a standard CoFeB based MTJ which is exposed to microwave radiation. Using a lock-in technique, Gui separates voltages across the MTJ which arise from an additional DC current he applies through the device from those which arise due to the spin diode effect. It is found that the microwave sensor detectivity, which is defined as the ratio between the produced DC voltage across the MTJ divided by the average microwave power absorbed by the MTJ, is increased from 40mV/W at zero applied DC current bias to a maximum of around 350mV/W by applying a -14μA bias current, nearly a 9 times increase. While this finding indicates a pathway for enhancing external-field free MTJ microwave sensors, they are still some way off outperforming their Schottky counterparts which can boast a sensor detectivity of tens of thousands of mV/W. Gui presents a phenomenological model that captures the essence of the enhancement of the MTJ sensors with applied DC current, and (in a hand waving way) ascribes the enhancement to an increased non-linear photoresistance of the device with applied current. It is also suggested that the sensor detectivity can be significantly improved to 5000mV/W by materials optimisation.
Microwaveable magnetic memory?
Perhaps the most interesting development from Gui’s paper is the possible application of its findings beyond the scope of a simple microwave detector. Gui shows that the DC voltage generated across the MTJ depends on the relative orientations of the free and fixed layers (recall that the AC current only causes the free layer magnetization to precess slightly away from equilibrium as opposed to full switching). For the case of no current DC current applied across the device, the difference between the voltage across the MTJ for the case of the free and fixed layers being parallel and antiparallel is 20% when defined by a TMR-style ratio. For comparison the TMR here is 60%. However, when the DC bias current is applied, this difference between parallel and antiparallel configurations becomes 10,000%, much greater than the TMR. Furthermore, if an appropriate DC current bias is applied, the sign of the voltage across the device can switch with free layer magnetization orientation, offering functionality beyond what is possible from TMR. These findings suggest a novel method for reading MTJ memory cells, though the additional circuitry that would be required to generate simultaneous AC and DC currents within the MTJ may limit its applicability to MRAM.
Gui, Y., Xiao, Y., Bai, L., Hemour, S., Zhao, Y., Houssameddine, D., Wu, K., Guo, H., & Hu, C. (2015). High sensitivity microwave detection using a magnetic tunnel junction in the absence of an external applied magnetic field Applied Physics Letters, 106 (15) DOI: 10.1063/1.4918677