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Measure ultra low magnetic fields using GMR spin-valves in combination with magnetic sweeping field with and without locking technique.

IP.com Disclosure Number: IPCOM000031601D
Original Publication Date: 2004-Sep-30
Included in the Prior Art Database: 2004-Sep-30
Document File: 4 page(s) / 20K

Publishing Venue

IBM

Abstract

This invention combines a GMR-Spin valve bridge with a sweeping coil, made by thin film technique, and an electronic circuit to build a miniaturized high sensitive magnetic sensor. A sensitivity of 10µV/V/µT without flux guides and 200µV/V/µT with flux guides at a measurement range of 10000 µT and a signal to noise ration up to 0.02 µT is possible. This performance reaches the performance of flux gates however the device is very small. If the thin film technique is combined with semiconductor processes the integration level will be extremely high and a very small device with very high sensitivity to magnetic fields is available.

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1. Functional principle

A Wheatstone bridge, as shown in Fig.1, is build out of four magnetic sensitive resistors. E.g. GMR spin valves, AMR or TMR sensor elements. Additionally to the magnetic sensitive resistors flux guides and / or shields can be added to improve the performance of the bridge.

Important is that the bridge has a transfer curve with odd symmetry .

    This bridge is placed into a sweeping coil as shown in Fig. 2 and connected to an integrator. Here the integrator in Fig. 2 is a simple RC-element. The RC-element can be replaced by any other kind of electronic device with the ability to do integration, like band-pass filter, lock-in amplifier etc.

    The sweeping coil produces a symmetric field relative to the transfer curve . If no external field is applied, the output voltage of the device is zero because the integration of an odd function over symmetric boundaries results always in zero.During one sweeping period the area of negative output is equal to the area of positive output. Hence the integration over one or more full periods results in an output of zero voltage.

    If an external magnetic field is applied to the sensor device the integration boundaries are shifted relative to the transfer curve. That means the integration boundaries are not symmetric to the transfer curve, anymore . The result is that the integration over one or more full periods results in a none zero output .

    As long as the sweeping field drives the sensor into saturation the output is proportionally to the external magnetic field. That means the linear range of the device is almost equal to the range of the sweeping field.

    Under this condition any hysteresis of the transfer curve of the Wheatstone bridge (Fig.1) does not affect the output because the integration during one full period is done over one full major hysteresis curve. Hence the influence of any hysteresis is always constant independent of the external magnetic field . The sweeping field is created in that way that the device output is zero at zero external magnetic field. That means that the device is calibrated in that way that any influence of the hysteresis is eliminated.

    Also any strong magnetic field that is applied to the sensor at any time does not create a permanent magnetization in the magnetic sensitive layers because of the demagnetizing behavior of the sweeping process.

    If the transfer curve of the bride and the magnetic field created...