Acknowledgements: Our research is sponsored by DFG SPP planet magnetisms project G1712 7/1.

1. SIRM AGU 3-7.12.2012 , San Francisco Control ID: 1465159 Poster：PP31B-2037 Magnetization of iron under pressure up to 21 GPa Moissanite anvil Re gasket Hcr BeCu support rings Hole drilled in Re gasket Sample loaded in hole Qingguo Wei1, Stuart A. Gilder1 1 Department of Earth and Environmental Sciences , Geophysics, Ludwig-Maximilians-Universität, Munich, D-80333, Qingguo.Wei@Geophysik.uni-muenchen.de Experimental: Moissanite anvil cell (Fig. 2): Samples: (1) pure iron powder (average particle size is 45 μm in diameter) and (2) pure iron foil. Sample chamber:150-200 μm in diameter × 100 μm in height; Pressure medium: (1) iron powder mixed with silica gel and (2) iron foil sandwiched between layers of sodium chloride. Pressure gauge: ruby fluorescence spectroscopy (Mao et al. 1986); Pressure gradients are about 2 GPa from sample center to edge at 20 GPa. The isothermal remanent magnetization (SIRM) of iron was measured up to 21 GPa, by combining non-magnetic moissanite anvil cell technology with a high-precision, three-axis SQUID (super conducting quantum interference device) magnetometer. At room temperature, the remanent magnetization of iron at 21 GPa is higher than at ambient (initial) pressure. Fe Bcc iron- Ferromagnetic Hcp iron- ? Fcc iron- ? Introduction: Iron is one of the most abundant elements in the universe, and main constituent of planetary cores (Fig. 1). The question remains whether iron can in the inner cores of the terrestrial planets can interact with the dynamo process. Although studied for decades, the magnetism of iron under pressure is still poorly understood. Fig.1 Phase diagram of iron with solid cores’ P-T of Earth, Venus, Mars, Mercury ,the Moon and Ganymede. Fig.2 Moissanite anvil cell and iron powder sample loading Fig. 3 X-ray diffraction measurements at room temperature and pressure show that the iron powder and iron foil used in our experiments are pure iron metal with a body center cubic structure. Results: Fig. 5 shows the results of back field magnetization acquisition for iron powder and iron foil as a function of pressure. Fig. 6 summarizes the SIRM (saturation isothermal remanent magnetization ) moment with respect to pressure. Compared to starting (ambient) conditions, SIRMs increase with pressure up to about 17 GPa and remain elevated when pressure is released to 8-10 GPa (Fig. 6) for iron powder and iron foil. At 21 GPa, iron is still ferromagnetic with 6-13 times SIRM magnetization compared to that of iron at ambient condition. Which means that hcp- iron is ferromagnetic up to 21 GPa. Possible reasons for Mössbauer spectroscopy fail to detect peaks of ferromagnetic hcp- iron: Mössbauer spectroscopy is sensitive to the direction of hyperfine magnetic field. Mössbauer spectroscopy studies on hcp-iron found no hyperfine splitting, that can signal the presence of magnetic moments, it may caused by reason that the magnetic moment of hcp-iron in diamond anvil cell is perpendicular to the cell’s main stress axis, which also perpendicular to the incident radiation source (Gilder and Le Goff, 2008). Fig.4 Three-axis SQUID magnetometer (left) and measurements of empty moissanite anvil cell at room temperature(right). Conclusions: High saturation remanent magnetization of iron up to 21 GPa in a non-magnetic moissanite anvil cell, indicates ferromagnetic of hcp-iron. Saturation remanent magnetization of iron increases with pressure up to 17 GPa, and gets to highest when pressure is released to 8-10GPa. Our result is contradict with the results that get from Mössbauer spectroscopy. We are still working on it, and trying to find why. Fig.3 X-ray measurements for iron powder and iron foil. Mo source. Three-axis SQUID magnetometer (Fig. 4 left) resolution < 1*10-12 Am2; 370 mT magnetized, empty MAC ~ 7*10-9 Am2 (constant with pressure, Fig. 4 right); 370 mT magnetized, iron sample in cell > 8*10-9 Am2. Fig.6 Summary of the high pressure results: saturation isothermal remanent magnetization (SIRM) normalized to the initial (non-pressurized) SIRM as a function of pressure. Arrows represents experimental progress for pressure steps. References: Gilder S and M. Le Goff. 2008. Systematic pressure enhancement of titanomagnetite magnetization. Geophys. Res. Lett, 35, L10302. Mao H K, Xu J, and Bell P M. 1986. Calibration of ruby pressure gauge to 800 kbar under quasi- hydrostatic conditions. Journal of Geophysical Research, 91: 4673-4676. Williamson D L, Bukshpan S, Ingalls R. 1972. Search for Magnetic ordering in hcp iron. Phys. Rev B: 6, 4194-4206. Ferromagnetic Hcp-iron Hydrostatic compression experiments at room temperature, iron transforms from body-centered cubic (bcc) structure to hexagonal-close-packed (hcp) phase at 13 GPa, due to the phase drag effect, fully transformation finished at about 16.5 GPa, and 8 GPa for the reverse transformation. In our experiments, it is concluded that hcp- iron is ferromagnetic up to 21 GPa. This conclusion is contradict with the non-magnetic hcp-iron conclusion from Mössbauer spectroscopy researches (Williamson et al., 1972). Acknowledgements: Our research is sponsored by DFG SPP planet magnetisms project G1712 7/1. Special thanks are given to Prof. Dr. Wolfgang. Schmahl and Dr. Bernd Maier for helping in X-ray diffraction measurement. Fig.5 Isothermal remanent magnetization initially starting from back-field saturation remanence for iron powder (up) and iron foil (down).