Ultrasensitive Magnetometers Based on Rotational Magnetic Excitation

Three new types of fluxgate magnetometers are presented in this paper, able to monitor the three components of the ambient field, all of them based on the principle of rotational excitation field. The first type is based on YttriumIron Garnet (YIG) single crystal film, magnetized with rotational field on its plane, where the 2 nd , 4 th and 6 th harmonics offer the three components of the ambient field with sensitivity better than 1 pT at 0.2 Hz, its size being 25 cm 3 . The second type is based on permalloy film, where the rotational excitation field on its plane offers change of magnetoresistance with sensitivity better than 10 pT at 1 Hz, uncertainty of 1 ppm and size ~ 8 cm 3 . The third type, is based on amorphous film, where the rotation field mode offer sensitivity better than 100 pT at 1 Hz, uncertainty of 10 ppm and size ~ 10 mm 3 . Introduction Current technology of magnetic field sensors allows their use in applications such as mining activities, near surface monitoring, automotive & industrial engineering applications etc. The expansion of their use is prohibited by conflicting factors of sensitivity, size and cost. Moreover, three-dimensional magnetic field sensors with sensitivity better than 1 pT in the bandwidth of 1 Hz, integrated in a relatively small size, are yet to be developed. The state of the art in magnetic field measurements, being also the primary standard in field measurement, is the superconducting quantum interference devices (SQUID) which are based on the Josephson effect-junction. Although, in principle, SQUIDs are able to measure the magnetic quantum, their sensitivity is limited by the sensor electronics and becomes finally of the order of 0.1 – 1 pTat a frequency of 0.2 – 0.5 Hz. The need of cryogenic operating conditions increases costs and operational properties of SQUID sensors.The second best type of magnetic field sensors is the proton magnetometer. These sensors are based on the Zeeman effect, controlled by the magnetic field. The sensitivity of these devices is of the order of 1 pTat 1 Hz. These devices can operate in normal temperature conditions but their size is rather large. Fluxgate magnetometers use the non-linearity of the magnetization loop of soft magnetic materials [1-3]. Ideally this loop should be a un-hysteretic ramp, flattening at the saturation or anisotropy fields. Excitation and search coils are used to operate fluxgates, while hybrid systems, including conductive means in either excitation or signal detection, have also been designed. The signal processing may include monitoring of the second and higher harmonics of the excitation frequency, or phase shift measurements. Signal conditioning can be either open loop output detection or closed loop current correction feedback. Fluxgates measuremagnetic fields much weaker and slower than the excitation field. An important factor lowering the fluxgate sensitivity is Barkhausen noise of active magnetic medium. The sensitivity of the fluxgate to an external magnetic field is dependent on the sensitivity, hysteresis and non-linearity of the magnetic susceptibility of its sensing core. Typical sensitivity of fluxgates is 10-100 pTHz -1 .