Tunable Bias Magnetic Field of Nano-Granular TMR Sensor Using FePt Film Magnet

The nano-granular in-gap tunneling magnetoresistance (TMR) sensors with FePt film bias magnet were developed. It is confirmed that FePt films possessed polycrystalline structure and large maximum energy product [(BH)
_max
] of 10 MGOe. The large bias magnetic fields above 20% of anisotropic magnetic field were obtained for magnetic sensors of several sensitivities by using 1 μm of thickness of FePt film. Finite-element method (FEM) calculation revealed that bias magnetic field was increased with decreasing the length of soft magnetic yoke, because of the increase permeance of magnetic circuit. The FEM calculation also revealed that the strength of bias field can be tuned by changing the dimension of film magnet. These results indicated that the suitable bias magnetic field can be applied for each nano-granular in-gap TMR sensors with several sensitivities and dynamic ranges by adjusting the shape of film magnet.