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High-data-transfer-rate readers beyond 3 Gbit/s composed of spin-torque oscillators (STOs) are considered and the signal-to-noise ratios (SNRs) of the output signals under the thermal magnetization fluctuations are calculated by using the recent nonlinear theories. The “STO Reader” senses the media field as a modulation in the oscillation frequency, enabling high-data-transfer rates beyond the limit of ferromagnetic relaxation. The output (digital) signal is obtained by frequency modulation (FM) detection, which is commonly used in communication technologies. As the problem of rapid phase diffusion in nonlinear STOs caused by the thermal fluctuations is overcome by employing a delay detection method, the sufficiently large SNRs are expected even in nonlinear STOs less than 30×30 nm2 in size. The prompt response of the STO frequency (phase) to the media field and the high-data-transfer rate beyond 5 Gbit/s are shown by micromagnetic simulation. The frequency transition of STO in less than 1 ns under the pulse field is also confirmed by experiment.