| Summary: | 碩士 === 國立臺灣大學 === 物理研究所 === 93 === The electronic properties of the spin dependent tunnel junction (STJ) are usually measured by direct current (DC) source. In our measurement, the MTJs CoFe/Al2O3/CoFe/NiFe was measured by a four-probe method at room temperature in the frequency range from 40 Hz to 110 MHz, with the external magnetic field ranging in strength up to 500 Oe. At high frequency, the capacitance will play an important role to affect the measured result. Generally speaking, the capacitance is determined by the geometry and materials. But We observed that the effective capacitance changes with the application of an external magnetic field. The magnetocapacitance ratio can get up to 15%. At high frequency, The real part of impedance was found to change from positive to negative. The phenomenon is similar to the inhomogeneous current distribution effect that derived from the electrode resistances comparable to or higher than the junction resistance. The MTJs with a tunnel barrier formed by O2 plasma. We control the time of O2 plasma sputerring in forming the tunnel barrier is effective for achieving a different junction resistance and for causing the oxidation degree of the bottom electrode during plasma oxidation in MTJs. By the way to caculated the critical frequency that the real part cahege sign. At this critical frequency, the real part of magetoimpedance (TM-Re) have a huge increase.
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