Intrinsic noise in magnetic film/planar coil sensors

Abstract

To enhance performances and approach the detection limits of a magnetometer, it is necessary to investigate its intrinsic noise. The intrinsic noise of magnetic film/planar coil sensors has been analyzed, where the sensors consist of magnetic films and planar coils. Two architectures of magnetic film/planar coil sensors are investigated: self-inductive and mutual inductive. The pickup coil is in an open circuit during the detection, there is no resistive noise source in a mutual inductive magnetic film/planar coil sensor, and its noise is lower than that of a self-inductive one. Irrespective of any architectures, the noise of magnetic film/planar coil sensors is closely related to their electric excitations. The excitation signal generates an ac field Hac inside the magnetic films. When the magnitude of the ac field Hacm is less than that of the anisotropic field HK in the magnetic film, the noise of a mutual inductive sensor can be reduced by increasing amplitude of excitation current. Until Hacm=HK, mutual inductive sensor obtains a minimum noise value. When Hacm>HK, the noise of mutual inductive sensor can be reduced by reducing the amplitude of excitation current. It is observed that the noise is strongly dependent on the excitation frequency. Under the excitation current of 1.5 mA at 100 kHz, the voltage noise spectral density of a mutual inductive magnetic film/planar coil sensor has a minimum value of 47.55 nV/√Hz @ 1 Hz, and the equivalent magnetic noise is 66 nT/√Hz @ 1 Hz. The voltage noise spectral density of a self-inductive magnetic film/planar coil sensor has a minimum value of 86.26 nV/√Hz @ 1 Hz, and the equivalent magnetic noise is 452.4 nT/√Hz @ 1 Hz.