A spin-torque oscillator (STO) has a single free ferromagnetic layer that forms part of both a giant magnetoresistance (GMR) structure with a nonmagnetic conductive spacer layer and a tunneling magnetoresistance (TMR) structure with a tunnel barrier layer. The STO has three electrical terminals that connect to electrical circuitry that provides a spin-torque excitation current through the conductive spacer layer and a lesser sense current through the tunnel barrier layer. When the STO is used as a magnetic field sensor, the excitation current causes the magnetization of the free layer to oscillate at a fixed base frequency in the absence of an external magnetic field. A detector coupled to the sense current detects shifts in the free layer magnetization oscillation frequency from the base frequency in response to external magnetic fields.