Tunnel magneto-resistance (TMR) effect can be applied to magnetic field sensors because this effect is resistance change of magnetic tunnel junctions (MTJs) by external magnetic field. We have to develop the MTJs with a very high sensitivity (=TMR/2Hk, Hk: magnetic anisotropy field) above 100%/Oe to detect a small bio-magnetic field. In this work, MTJs using CoFeSiB electrode with a low Hk were fabricated. A very high sensitivity of 115%/Oe was achieved by optimization of the CoFeSiB thickness. This sensitivity is the highest value among single MTJ devices and the MTJs with such a high sensitivity can be applied to bio-magnetic field sensor devices.
