Abstract: The present invention provides a method and apparatus for utilizing magnetoresistance devices for the measurement of weak magnetic fields. An oscillating excitation magnetic field is applied to a magnetoresistive (MR) sensing element such that the MR element is driven into one or both of two antiparallel saturation states. The amplitude of the excitation field is large enough to reverse the magnetization of the soft layer during each cycle. In one embodiment, the MR element is provided with a current, and a voltage proportional to the resistance is measured. Components of the voltage signal at multiples of the excitation frequency are then proportional to the environmental magnetic field. In one embodiment, an MR element having a resistance-versus-field transfer function that is symmetric (e.g., an anisotropic MR element) is used; while in another embodiment, an MR element having a resistance-versus-field transfer function that is asymmetric (e.g., a spin-valve MR element) is used.

Abstract: The present invention provides a method and apparatus for utilizing spin valve magnetoresistance devices for the measurement of weak magnetic fields. The magnetoresistive element consists of a pinned ferromagnetic layer and a soft ferromagnetic layer separated by a thin spacer layer. The pinned layer may be pinned by high intrinsic coercivity, or by a neighboring antiferromagnet or high coercivity ferromagnet layer. An oscillating magnetic field is applied to the device. The amplitude of the excitation field is large enough to reverse the magnetization of the soft layer during each cycle, but small enough that the magnetization direction of the pinned layer is not much affected. In one embodiment, the applied field is applied using a current strip deposited onto the top of the other layers, so that the entire device can be produced on a single chip.