Research On Fabrication And Performance Of Magnetic Tunnel-junction Microwave Detector

Spin torque microwave detector based on spin of electrons has advantages of adjustable operating frequency,small size(nanoscale),anti-radiation,compatible with semiconductor technology,and so on.It has important application prospects in the fields of wireless communication,Internet of Things and microwave energy harvesting in the future.In order to solve the problems of low sensitivity and narrow working frequency of spin torque microwave detector,the fabrication and performances of a new type of microwave detector based on magnetic tunnel junctions are studied in this thesis.The main research results are summarized as follows:(1)Research on the design,preparation and microwave detection performances of the high-sensitivity microwave detector based on magnetic tunnel junctions.By optimizing the interfacial perpendicular magnetic anisotropy between the CoFeB free layer and the MgO barrier layer,a magnetic tunnel junction microwave detector with ultra-high detection sensitivity under zero magnetic field is obtained.Under a certain DC bias and a microwave signal,the microwave detection sensitivity of the device is 2.1 × 105 V/W,which is the maximum reported in the experiment.Furthermore,combined with theoretical analysis,the physical mechanism of high sensitivity originates from injection locking.Under DC bias,microwave emission of device is excited due to spin transfer torque effect.When the microwave emission is injection locking with an external microwave signal,the rectifier voltage generated by the device reaches the highest,which significantly improves the detection sensitivity of the device.(2)Magnetic tunnel junction devices with the MgO capping layer are designed and fabricated.The influence of MgO capping layer thickness on microwave detection performance is investigated.With the increase of MgO capping layer thickness from 0.5 nm to 0.9 nm,the average switching current density decreases by a factor of 2,and the microwave detection sensitivity increases by a factor of 2.Further analysis shows that the increase of sensitivity and the decrease of average switching current density can attribute to the reduction of effective demagnetization and damping factor caused by the increase of MgO capping layer thickness.(3)By designing and fabricating the devices with large aspect ratio and size,the device exhibits high microwave detection sensitivity(8000 V/W)at two frequencies(1.92 GHz and 3.56 GHz)under certain bias.Further analysis shows that the phenomenon of dual frequency high sensitivity microwave detection results from the coexistence of different magnetization oscillation modes in the free layer and injection locking.(4)Research on the broadband microwave detection performances of the magnetic tunnel junction microwave detector.By changing the thickness of CoFeB free layer in the magnetic tunnel junction to adjust the interfacial perpendicular magnetic anisotropy between the free layer and the barrier layer,the influence of interface effect on the microwave detection performance of the magnetic tunnel junction microwave detector is explored.The experimental results show that when the perpendicular magnetic anisotropy field of free layer is strong,the microwave detection bandwidth of the device can reach up to 3 GHz under the external magnetic fields.When the perpendicular magnetic anisotropy field is weak,the device can work under zero magnetic field,and the detection bandwidth of the device can reach 1.8 GHz.The microwave to DC energy conversion efficiency of the optimized detector is about 2%,which is expected to provide microwave wireless power for portable electronics and sensors in the future.