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The mechanism of oscillation or magnetization rotation of a field generation layer (FGL) in a spin torque oscillator (STO) under the large gap field between main pole and trailing shield for microwave assisted magnetic recording (MAMR) was investigated using an analytical model. The following results were obtained. The oscillation frequency at the maximum magnetic field output is decided by the vertical component of the magnetic field from the main pole. When there is a strong external magnetic field, it is difficult to restrain the FGL magnetization in the plane of FGL and the oscillation becomes unstable. A FGL with zero (or positive) anisotropy will not be suitable for use in the strong external magnetic field because it will not oscillate properly. It can be expected that the spin torque oscillator with a planar FGL with negative perpendicular magnetocrystalline anisotropic material is likely to be suitable for MAMR.