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Radiofrequency heating of magnetic particles promises highly efficient and direct heating of catalytic reactors for coupling of low carbon electricity with energy intensive chemical transformations. In this work, a novel real‐time and in situ magnetometry method is developed to measure minor and major hysteresis loops of soft magnetic nanopowders. It is applied to measure the magnetic properties and hysteresis power absorption of magnetite and maghemite powders up to 500 °C. An arctangent model for saturation magnetization is adapted for minor hysteresis loops. It produces an excellent fit for hysteresis loop power across field strengths up to 18.5 kA m−1 and allows prediction of heating power, remanence, and susceptibility. Samples of magnetite and maghemite are shown to heat rapidly from room temperature at more than 25 °C s−1, with maghemite giving the strongest heating response. The peak heating power occurs at the transition beyond the ellipsoidal Rayleigh law region. These findings suggest that the properties of magnetic powders, coupled with variable magnetic field strengths and frequencies, can be tuned to optimize the heating power for a variety of applications. [ABSTRACT FROM AUTHOR]