The development of high-sensitivity magnetic field sensors at low frequencies and ambient temperatures is of great importance for many practical applications, where different aspects of the sensor performance need to be considered. In this paper, it is presented that by tuning magnetic nanostructures of the free layers in magnetic tunnel junctions, widedynamic-range or ultra-high-sensitivity tunneling magnetoresistive sensors can be obtained. Tunneling magnetoresistive sensors with a linear response from -75 mT to +75 mT are demonstrated. Also, it is demonstrated that an optimized ultra-high-sensitivity magnetic sensor with a sensitivity of 57,790 %mT can be achieved. This sensitivity is currently the highest among all magnetoresistive sensors that have been reported. The estimated noise of our magnetic sensor is 2.3 pTHz12 at 1 Hz and 190 fTHz12 at 100 Hz respectively. This tunneling magnetoresistive sensor dissipates only 25 μW of power when it operates under an applied voltage of 1 V at room temperature.