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360° angle sensor using spin valve materials with SAF structure

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4 Author(s)
Dexin Wang ; NVE Corp., Eden Prairie, MN, USA ; Brown, J. ; Hazelton, T. ; Daughton, Jim

Microchips of 360° angle sensors using spin valve materials were designed, fabricated, and tested with excellent performance. The spin valve material used for the angle sensor has a structure of Ta-NiFeCo-CoFe-Cu-CoFe-Ru-CoFe-CrMnPt. External magnetic fields have little torque on the CoFe-Ru-CoFe structure up to at least 500Oe, which is the highest value available during the test. This is due to the strong antiparallel exchange coupling between the two CoFe sublayers of the same thickness via a thin layer of Ru and the resulting zero net magnetic moment. There is a sharp switching of the free layer with a low coercivity of 4 Oe along the easy axis. Both the high standoff field and small coercivity ensure that the sensor operates properly with large tolerance in mechanical assembly. The angle sensor is used stationary in combination with a disc-shaped permanent magnet attached to a rotating shaft near the sensor. The permanent magnet is magnetized in-plane, thus creating a field that is rotating with the shaft. The magnetic field from the permanent magnet forces the free layer magnetization to follow the field and rotate with it. With a fixed reference layer magnetization and an in-phase following of the free layer magnetization, the magnetoresistance is a simple cosine function of the angle between the rotating permanent magnet and the stationary sensor. A special Wheatstone-bridge with four spin valve resistors is used to compensate the thermal drift expected in application environments. One half bridge has a 90° phase delay from the other, resulting in a cosine and a sine function, in combination to uniquely determine any angular relationship between the permanent magnet and the sensor between 0 to 360°.

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Magnetics, IEEE Transactions on  (Volume:41 ,  Issue: 10 )