A yoke includes a disk-like base end portion and a columnar center pole protruding from a center portion of the base end portion. An annular magnet is arranged on the base end portion. An annular top plate is arranged on the magnet so that an inner circumferential surface thereof is opposite to an outer circumferential surface of the center pole. A non-magnetic ring member is arranged on an inner circumferential surface side of the magnet, the non-magnetic ring member having a same inner circumferential diameter as an inner circumferential diameter of the top plate.
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June 18, 2014
February 23, 2016
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