Sub-nano-tesla in-plane vector magnetometer employing single magnetoresistor

This work demonstrates that the difficulties, which are associated with employment of magnetoresistors (MRs): residual magnetization in the ferromagnetic material if the MR is employed with no ac bias; Barkhausen noise when ac bias is employed; 1/f low-frequency noise of the MR, and also that of the readout electronics; noise introduced by the ac bias sources; strong dependence of the magnetoresistance on temperature; influence of the measured field direction; nonlinearity of MR transfer characteristic, etc., can be successfully overcome by providing a single MR with a special mode of ac excitation. We show both analytically and experimentally that such a mode can be realized by exciting a single MR by sufficiently strong magnetic field, which rotates in the MR´s plane. We found that special simple and explicit solutions of the Stoner-Wohlfarth equation can be used in this case to closely evaluate the MR output. A remarkable outcome of our investigation is that the resulted rotation of the MR thin film magnetization becomes uniform when the excitation field is elliptically polarized. The two orthogonal components of an external in-plane dc or low-frequency measured magnetic field disturb the uniform rotation of the magnetization in different instances of time. This magnetization behavior enables simultaneous but still separate measurements of both external field components by synchronously detecting the phase deviations related to the corresponding instances of time at the MR output