Ultra-sensitive solid-state magnetic sensors are in strong demand in many applications where currently available sensors are inadequate. We have used high performance magnetic tunneling junction (MTJ) sensors and pushed the magnetic sensing limit to a high level. We have incorporated double-staged magnetic flux concentrators, one on the MTJ chip level and the other on a more macroscopic level, to amplify the external field of interest. With this approach and undergoing a process of optimization on the flux concentrators, we have increased the sensitivity of the MTJ sensor by a large factor of 517 to 775.4%/Oe in terms of magnetoresistance response. The coercivity of the sensor is only 0.12 Oe. We have achieved a detectable field limit of 30 pT/Hz⎯⎯⎯⎯⎯√ at 10 kHz. We have presented the noise spectrum and the sensitivity spectrum up to a maximum frequency of 100 kHz.