Exploring The Fabrication Techniques And Applications In Spin Orbit Torque Magnetic Tunnel Junctions

In recent years,there has been significant interest in studying the application of spin orbit torque effects in various fields,such as information storage,logical computing,and probabilistic computing.Spin orbit torque magnetic tunnel junctions are promising spintronic devices that have practical usages,but their fabrication process is complex,resulting in low yields and high costs.Therefore,studying the efficient fabrication of the spin orbit torque magnetic tunnel junctions,particularly small-sized junction devices,is essential for the lateral practical applications.This dissertation discusses the laboratory fabrication process of hundred-nanometer scale spin orbit torque magnetic tunnel junctions and analyzes the current process’s common problems and limitations.The study focuses on two types of applications,namely the spin logic and the true random number generator,in the self-designed and fabricated spin orbit torque magnetic tunnel junctions.The primary research results include:（1）We prepared and optimized type-Y spin orbit torque magnetic tunnel junctions films with different layers for device processing using sputtering and heat treatment.After researching and comparing,we confirmed a process route that took into account efficiency and device performance.The final fabricated device had a hundred-nanometer scale（elliptical major axis 200 nm and minor axis 50 nm）,tunneling magnetoresistance ratio>100%,critical inversion current density of 10¹¹ A/m² at a pulse width of 6 ns,and write times>10¹²which was in line with the advanced performances of other groups.（2）We investigated the effect of heat treatment on the performance of magnetic tunnel junction devices that resulted from the mixing of heavy metal writing lines and magnetic free layers.We used both multi-layer magnetic interdiffusion and single-layer magnetic interdiffusion to enhance this type of mixing.The results showed that the tunneling magnetoresistance ratio in devices with magnetic interdiffusion was generally higher than that in samples without interdiffusion.In the multilayer magnetic interdiffusion research,we prepared micron-level junction devices,and the average value for 49 inserted devices was 193%,while the average value for 43 non-inserted devices was 157%.In the single-layer magnetic interdiffusion study,we prepared the samples with magnetic interdiffusion layers inserted at different positions of the writing line,and the average tunneling magnetoresistance ratio of them was 179%,higher than145%of those without interdiffusion.The critical switching current density of the devices with insertion positions closer to the free layer was 18%lower than that of devices with no insertion layer and the insertion position farthest away from the free layer.Using electron microscopic characterization and second harmonic testing,we found that the increase in the tunneling magnetoresistance ratio was due to the improvement in the crystal quality of the Mg O film,while the decrease in the critical switching current density was more likely due to the introduction of extrinsic factors during the device fabrication process.Overall,the mixing of heavy metals and magnetic layers did not adversely affect the devices prepared in this study.（3）The research presented in this dissertation explores the use of a T-type magnetic structure film in a Hall bar device to demonstrate its adjustable switching polarity and switching ability without the assistance of an external magnetic field.In addition,the dissertation proposes a scheme for implementing XOR gates and successfully implements the five logic functions（AND,OR,NAND,NOR,and NOT）in this device.By combining the concept of logic function implementation in Hall bar devices with circuit design,the dissertation achieves successful implementation of AND,OR,NOT,NAND,NOR,XOR,and XNOR seven logic functions in the hundred-nanometer scale spin orbit torque magnetic tunnel junctions.（4）We studied the stochastic switching phenomenon in the spin orbit torque magnetic tunnel junctions and fabricated those devices with micrometer and hundred-nanometer scales experimentally.Due to the multi domain state effects caused by the large size of the micrometer-scale devices,they were not suitable for developing random number generators.In the experiments after developing the hundred-nanometer scale devices,we successfully achieved a clearly distinguishable probability adjustable stochastic switching between high and low resistance states.We used the sigmoid function to fit the probability and excitation signal,and obtained a complete probability regulation curve.Using this type of random number generator,we generated a random number sequence with a length of 33736 and passed 12 random number quality tests from SP 800-22.To further study the stochastic switching effect driven by spin-orbit torques,we introduced a thermal noise term into the magnetic torque-driven spin dynamics equation containing the spin orbit torque term.We reproduced the process of stochastic switching through simulation and confirmed that one of the sources of stochastic switching driven by the spin orbit torque was caused by the thermal noise,providing a theoretical basis for optimizing the performance of the spin orbit torque magnetic tunnel junction random number generator.