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A new method is proposed to measure the magnetic hysteresis as a function of temperature. It is based on the spectral decomposition of magnetization and measurement of different harmonics using the harmonic-detection function of a lock-in amplifier. Applying this method, a measuring system of the BH loop was designed and fabricated. This system gives spectra of magnetic flux density B driven by an ac-magnetic field h. The BH loops are reconstructed from the spectra of the B. The spectra and BH loops were measured for a NiZn ferrite toroid at various temperatures. When the temperature increases from the room temperature, the higher harmonics of the spectra increase. Approaching the Curie temperature TC from below, magnetic loss and nonlinearity of the BH loops increase, and the loop shows the saturation just below TC.