| Due to the high tunneling magnetoresistance, magnetic tunnel junction has now become the basic element of the magnetic sensor and magnetic random access memory. It is very significant to study the transport properties and noise characteristics, further to improve the signal noise ratio of magnetic tunnel junctions for its application in the field of industry. This paper is the study of transport and Noise properties of MgO-based double barrier magnetic tunnel junctions, and the contents mainly divided into the following four parts:1. Understanding the working principle and cooling processes of dilution refrigerator。The cool down principles of dilution refrigerator is to use superfluid He4 dilute superfluid He3. This theory was put forward in 1951, and achieved in 1965. Cooling processes:(1) Evacuate each chamber of dilution refrigerator;(2) Use the pulse tube compressor, the OVC of dilution refrigerator can cooled to 50 K and IVC to 3 K;(3) The circulation of He3 and He4 makes the temperature of the mixing chamber of dilution refrigerator cool down to 10 mK.2. Set up room temperature transport measurement platform, and carried on its calibration, and the preliminary test results based on it. We designed a four-pole vector magnet, which can generate magnetic field in plane in any direction and it is convenient to rotate magnetic field 360 degree. The power supply of the magnet can be controlled by LABVIEW program. For its calibration of magnetic field, we use a Gaussmeter. If the distance of pole-head is 70 mm, the maximum of magnetic field is 2100 Oe. If the distance is 40 mm, the maximum of magnetic field is 3400 Oe. In addition, the continuity of the magnetic field can be guaranteed in both cases, which can measure magnetoresistance curves and noise characteristics of magnetic tunnel junctions and magnetic thin films. Cooperate with existing Flow cryostat, the temperature of sample can realize from room temperature to liquid nitrogen and further to liquid helium temperature region. The tunneling magnetoresistance of double barrier magnetic tunnel junction increases as the temperature decreases, and decreases as the bias applied to junctions increases.3.1/f noise characteristics of double barrier magnetic tunnel junctions at room temperature.Low frequency noise has been investigated at room temperature for asymmetric double barrier magnetic tunnel junctions(DB磁性隧道结s),where the coupling between the top and middle CoFeB layers is antiferromagnetic with a 0.8 nm thin top MgO barrier of the CoFeB/MgO/CoFe/CoFeB/MgO/CoFeB DB磁性隧道结.At enough large bias,1/f noise dominates the voltage noise power spectra in the low frequency region,and is conventionally characterized by the Hooge parameterαmag.While the top and bottom ferromagnetic layers are?xed,and the middle free layer rotates from antiparallel to parallel state,αmag and magnetoresistance-sensitivity-product show a linear dependence,consistent with the quasi-equilibrium assumption and the?uctuation dissipation relation.With the magnetic?eld applied at different angles(θ)to the easy axis of the free layer,the linear dependence persists while the intercept of the linear?t ofαmag satis?es a cos(θ)dependence,similar to the angle dependence of the magnetoresistance,suggesting some intrinsic relation between magnetic losses and magnetoresistance.4. low temperature double barrier tunnel junction of zero bias magnetic anomaly and shot noise properties. The temperature of sample can be cool down to 3.6 K by using dilution refrigerator. In the region of low bias, we found that the magnetoresistance of double barrier tunnel junction appears a sharp peak, call it zero bias anomaly(ZBA), which can be attributed to electron-electron interaction(EEI). The shot noise of double barrier tunnel junction appear was carried on at different states. The results show that all Fano factors are 0.72, in other words, the electron tunneling in the junction is well suppressed, and it can be responsible for the EEI. |
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