Abstract: An ultra-low noise sensor for magnetic fields comprises a mechanically resonant structure having a magnetized proof mass. The displacement of the proof mass due to a magnetic field provides a high resolution and highly amplified measurement of magnetic field fluctuations near the resonance frequency. A flux modulator may be used with the resonant structure to amplify magnetic fluctuations in a non-resonant frequency band. The resonant structure, combined with a high resolution readout device and a frequency-compensating numerical processor, can amplify magnetic fluctuations in a broad range of frequencies. A solenoid coil surrounding the resonant structure may be used to null the quasi-static earth's magnetic field and thereby increase the dynamic range of the sensor. Cryogenically cooling the resonant structure can improve the resolution of the sensor. A magnetometer that embodies features of the present invention is miniaturized and has improved amplification and resolution at room temperature.