Study On MgO Magnetic Tunnel Junction Based Magnetic Sensor And Spin Injection Into SiC

The MgO barrier based magnetic tunnel junction is one of the core components of spintronics.It is widely used in magnetic read heads of hard disk,magnetic random access memories and magnetic sensors due to its large magnetoresistance effect at room temperature,which greatly promoted the development of spintronics.The MgO based tunneling magnetoresistive sensor has excellent applications prospects in automotive electronics,industrial measurement,electronic compass and biological detection because of its high sensitivity,low noise and low power consumption.How to further improve the sensitivity of the sensor and reduce the noise is the key to realize the weak magnetic field detection.For this reason,we have systematically studied the magnetic tunnel junction sensor based on MgO barrier.In addition,the foundation of the booming electronic information industry today is the semiconductor technology.However,in the semiconductor technology,only the charge property of electron is utilized.If the spin property of electron can be utilized,it will undoubtedly have a profound impact on the development of the electronic information industry.The successful realization of spin injection into semiconductor is a prerequisite for utilizing the property of electron spin.For this reason,we have studied the spin injection of electrical methods into SiC materials.The main research results of this thesis are as follows:(1)In optimizing the magnetic tunnel junction sensor,we use a soft magnetic material with a smaller magnetic anisotropy field as the free layer to improve the sensitivity.The magnetic tunnel junctions based on the composite free layer of CoFeSiB and NiFe soft magnetic materials are prepared respectively.We obtained TMR value up to 200%,and the magnetic tunnel junction sensors were fabricated by means of two-step annealing process.By comparing the results of sensors based on different materials,we found that the sensitivity of the CoFeSiB composite free layer sensor is higher.In order to reduce the 1/f noise in the sensor,we use a large tunnel junction area,but this also makes the top free layer become multi-domain state,showing Barkhausen noise during transport.To solve this problem,we adopted a bias field to keep the free layer as single-domain state,which is at the expense of sensor sensitivity.After selecting the appropriate bias magnetic field to obtain the best performance of sensor,we obtained the sensitivities of the CoFeSiB and NiFe composite free layer sensors of 3.9%/Oe and2.5%/Oe,respectively.By simulating the commonly used Wheatstone full-bridge structure in the industry,the voltage sensitivities were 17 mV/V/Oe and 9.4 mV/V/Oe,respectively,and the linear range were±15 Oe and±25 Oe.Finally,the field noise were 4.5 nT/?Hz and 12.8 nT/?Hz at 10 Hz,which indicates that the magneitc tunnel junction based magnetic sensor of the CoFeSiB composite free layer has more obvious advantages in weak magnetic field detection.(2)The double pinned structure can adjust the sensitivity and linear range of the magnetic tunnel junction sensor through changing the film structure.Thus,the parameters of different application scenarios can be fully satisfied.We first prepared the magnetic tunnel junction sensors based on MgO barrier of double pinned structure,and also connected the sensor units in series and parallel at the chip level and integrated the magnetic flux concentrator.The series-parallel technique can reduce 1/f noise,and the mangetic flux concentrator increases the sensitivity by amplifying the extermal magnetic.Among the four sensors prepared,the sensitivities of the sensors only using the series-parallel technique were 1.85%/Oe and 2.5%/Oe,and the voltage sensitivities were 10.7 mV/V/Oe and 15 mV/V/Oe,respectively.The noise at 10 Hz under zero magnetic field were 3.3 nT/?Hz and 2.5 nT/?Hz,respectively.In addition,the sensors used the series-parallel technique and integrated with magnetic flux concentrator have greatly improved sensitivity to 103%/Oe and 130%/Oe,and the voltage sensitivity were460 mV/V/Oe and 650 mV/V/Oe,respectively.The noise at 10 Hz under zero magnetic field were as low as 82 pT/?Hz and 16 pT/?Hz.At last,we performed a total ionizing dose test on one of the self-packaging sensors.The experimental results show that when the cumulative dose reaches 300 krad(Si),the sensitivity and noise of the sensor are not significantly changed,showing good performance on anti-irradiation properties which make it a good application prospect in the space field.(3)We realized the electrical spin injection of SiC materials through the Schottky barrier.First,we used the magnetron sputtering to prepare the Schottky junction of n~+-SiC/CoFeB and measured the basic properties,and the barrier width was about 3.5 nm.For the deviation of the I-V curve from the ideal curve under negative bias,we believe that there are three transport channels:the hot electron emission channel,the direct tunneling channel,and the indirect tunneling channel via the interface defect state.And in the above Schottky junction,we observed both the Hanle effect and the inverse Hanle effect,indicating that the spin injection was successfully achieved.Next,we fit the results under different biases and derive the spin relaxation time.In order to explain the anomalous dependence of the spin relaxation time with the bias voltage,we have established a transport model with no interface defect state and a transport model with interface defect state.The former indicates that the spin relaxation time should be a constant and does not vary with the bias voltage.The latter shows that the indirect tunneling process is dominated under low negative bias,and the spin relaxation time of electron is close to the relaxation time of the defect.As the bias voltage increases further,the direct tunneling contribution increases so that the spin relaxation time is close to the spin relaxation time of the bulk SiC.Finally,we study the relationship between spin relaxation time and temperature.Combined with the results of bias voltage,we can see that the direct tunneling channel and the indirect tunneling channel via the interface defect state can be switched by changing the bias voltage and temperature and eventually affect the effective spin relaxation time.Finally,we find that the spin relaxation time in SiC is about 300 ps,and the spin relaxation time of the interface defect state is about 1 ns.