The Néel temperature (TN) of (Mg1–X,FeX)O and its compositional dependence have been studied with a superconducting quantum interference device (SQUID) in the temperature range 5–300 K at constant magnetic field of 1000 Oe. We determined the magnetic phase diagram at ambient pressure. As the TN constantly increased with iron content, we found a kink in the paramagnetic-antiferromagnetic boundary at around X = 0.5, which suggests a possible change of the magnetic structure in the antiferromagnetic state. We propose that the magnetic boundary ends at X = 0.12(1) and 0 K. Our results show that the pressure of the rhombohedral distortions of (Mg,Fe)O can be very sensitive to Fe content if the structure distortion is correlated to magnetic ordering.

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