We have fabricated and characterized a low-noise direct-coupled magnetometer based on a 100 pHYBa2Cu3O7−xdc superconducting quantum interference device (SQUID) on a 10 mm×10 mm SrTiO3bicrystal substrate with 30° misorientation angle. The thin films were deposited by hollow cathode discharge sputtering and patterned using conventional photolithography and Ar ion beam etching. The SQUID magnetometer was operated using direct-coupled flux-locked-loop electronics with bias reversal. The sensor had a usable voltage swing of 39 &mgr;V and a white magnetic field noise of 32fTHz−1/2with a 1/fcorner at 2 Hz, including electronics and environmental noise. The voltage versus flux(V−&Fgr;)characteristic showed a pronounced distortion on the negative slope. Numerical simulations were performed to explain the distortedV−&Fgr;characteristic. Measurements of magnetocardiograms demonstrate the suitability of this sensor for biomagnetic applications. ©1998 American Institute of Physics.