Influence of Mg component and pressure on magnetic ordering of (Mg,Fe)O

(Mg,Fe)O is the second most abundant phase in the Earth's lower mantle. Knowing the fundamental properties of (Mg,Fe)O is important to understand the Earth's materials. The relation between rhombohedral distortion and antiferromagnetic ordering in FeO have been discussed, as an end member of (Mg,Fe)O. However, it does not fully understand for (Mg,Fe)O yet because its structural transition pressure is influenced by Mg component and stress condition [e.g. Zhuravlev et al., 2007]. We should also account compositional dependence on Néel temperature [Woods and Fine, 1969; Fujii et al., 2011] and spin glass-like behavior [Abbas and Hicks, 1990; Jing et al., 1993]. In this study, we investigated the compositional dependence of magnetic ordering at ambient pressure, and pressure dependence of Néel temperature up to 1.27 GPa. Superconducting Quantum Interference Device (SQUID, MPMS-7, Quantum design) was used for all magnetic measurements. We measured the temperature dependence of susceptibilities cooling with in zero field (ZFC) and in a field (FC) at ambient pressure and observed splitting between susceptibility of ZFC and FC. We confirmed that the smaller compositional dependence on Néel temperature in (Mg_1-XFe_X)O (X ≤ 0.60) [Fujii et al., 2011] may be explained by spin-glass transition. The transition temperature of (Mg_0.40Fe_0.60)O is 82 K and is close to 76 K that observed in (Mg_0.23Fe_0.77)_0.92O [Abbas and Hicks, 1990]. The spin-glass like behavior does not affect on the Néel temperature of (Mg_1-XFe_X)O (X ≥ 0.70) suggested by previous work [Fujii et al., 2011] because spin-glass transition temperature would not exceed 82 K. We also measured the pressure dependence of Néel temperature of (Mg_0.30Fe_0.70)O, (Mg_0.25Fe_0.75)O and (Mg_0.20Fe_0.80)O using a piston cylinder type pressure cell. We calibrated pressure by superconducting transition of tin. Néel temperature of (Mg,Fe)O were increased with increasing pressure except for some measurements under non-hydrostatic condition. Pressure dependence of Néel temperature of (Mg_0.30Fe_0.70)O, (Mg_0.25Fe_0.75)O and (Mg_0.20Fe_0.80)O were determined as 4.6 ± 0.4 K/GPa, 2.7 ± 0.3 K/GPa and; 4.1 ± 0.3 K/GPa, respectively, in linear approximation. They are well consistent with the rhombohedral distortion determined by X-ray diffraction studies in hydrostatic condition [Lin et al., 2003; Zhuravlev et al, 2010] for X ≥ 0.75. It implies the rhombohedral distortion of iron-rich (Mg,Fe)O at room temperature was induced by antiferromagnetic ordering .