Permanent magnet magnetic circuit

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PROBLEM TO BE SOLVED : To reduce a leakage magnetic field without increasing a volume of yoke in a permanent magnet magnetic circuit by cooling the yoke at a cooling rate within a predetermined scope after holding it within a scope of a predetermined temperature for a predetermined time. SOLUTION : A basic structure of this permanent magnet magnetic circuit is a general magnet opposing type magnetic circuit in which it is formed by upper and lower flat plates 1, 2 of yoke and columns 3, 4 connecting them, rare earth permanent magnets 5, 6 and magnetic shunt plates 7, 8 are arranged in gaps, and gradient coils 9, 10 are fixed in the magnetic shunt plates 7, 8. Yoke is heat-treated. Here, it is important to improve magnetic characteristic deterioration of yoke due to residual distortion generated at the time of manufacture and machining, in particular, a magnetic hold force and a magnetic permeability. In the heat treatment of yoke, it is held in a scope of temperature of 600 deg.C or more and 900 deg.C or less for more than five minutes and is cooled in a furnace. Preferably, the yoke is held for about one hour after it reaches this temperature and then is cooled at a rate of 0.5 deg.C/min or more and 10 deg.C/min or less on the average.