Frequency-assisted current-distribution effect in magnetic tunnel junction

Magnetoimpedance effects were studied in a series of magnetic tunnel junctions by means of the 4-terminal measurement. Magnetic tunnel junctions (MTJ) of layered structure NiFe/CoFe/Al/sub 2/O/sub 3//CoFe were prepared in a magnetron sputtering system with base pressure better than 3/spl times/10/sup -8/ torr. The study of magnetoimpedance effects were carried out with Angilent 4284A precision LCR meter at room temperature. The values of the real-part impedance were found to change their signs as the sensing frequency increased. At certain critical sensing frequency, a giant percentage change under sweeping magnetic field was obtained. This abnormal behavior is explained by the picture of frequency-assisted inhomogeneous current distribution effect, in which the effective tunnel resistance is tuned at the variation of sensing frequency.