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A detection scheme for both Brownian and Néel relaxation of magnetic nanoparticles (MNPs) is demonstrated by a mixing-frequency method in this paper. MNPs are driven into the saturation region by a low-frequency sinusoidal magnetic field. A high-frequency sinusoidal magnetic field is then applied to generate mixing-frequency signals that are highly specific to the Brownian relaxation of MNPs. These highly sensitive mixing-frequency signals from MNPs are picked up by a pair of balanced built-in detection coils. The relationship between MNPs' relaxation time and phase delays of the mixing-frequency signals behind the applied field is derived, and is experimentally verified. Magnetite MNPs with the core diameter of 35 nm are used for the measurement of Brownian relaxation, and Magnetite MNPs with the core diameter of 12 nm are used for the measurement of Néel relaxation. The results show that both Brownian and Néel relaxation depend on the magnetic offset field. This study provides an in-depth understanding of the relaxation mechanisms of MNPs.