Transcript No Slide Title

AGU 3-7.12.2012 , San Francisco
Control ID: 1465159
Poster：PP31B-2037
Magnetization of iron under pressure up to 21 GPa
Qingguo Wei1, Stuart A. Gilder1
1
Department of Earth and Environmental Sciences , Geophysics, Ludwig-Maximilians-Universität, Munich, D-80333, [email protected]
The isothermal remanent magnetization (SIRM) of iron was
Fe
measured up to 21 GPa, by combining non-magnetic moissanite
anvil cell technology with a high-precision, three-axis SQUID
Experimental:
Bcc iron- Ferromagnetic
Hcp iron- ?
Fcc iron- ?
Moissanite
anvil
Re gasket
Moissanite anvil cell (Fig. 2):
(super conducting quantum interference device) magnetometer.
Samples: (1) pure iron powder (average particle size is 45 μm in
At room temperature, the remanent magnetization of iron at 21
diameter) and (2) pure iron foil.
GPa is higher than at ambient (initial) pressure.
Sample chamber:150-200 μm in diameter × 100 μm in height;
BeCu support
rings
Pressure medium: (1) iron powder mixed with silica gel and (2) iron
Introduction:
foil sandwiched between layers of sodium chloride.
Iron is one of the most abundant elements in the universe, and main
Pressure gauge: ruby fluorescence spectroscopy (Mao et al. 1986);
constituent of planetary cores (Fig. 1). The question remains whether
Pressure gradients are about 2 GPa from sample center to edge at 20
iron can in the inner cores of the terrestrial planets can interact with the
GPa.
Hole drilled
in Re gasket
Sample loaded
in hole
dynamo process. Although studied for decades, the magnetism of iron
under pressure is still poorly understood.
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.
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
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).
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.
Conclusions:
1. High saturation remanent magnetization of iron up to 21 GPa in a
non-magnetic moissanite anvil cell, indicates ferromagnetic of hcp-
SIRM
iron.
2. Saturation remanent magnetization of iron increases with pressure
Fig.3 X-ray measurements for iron powder and iron foil. Mo source.
up to 17 GPa, and gets to highest when pressure is released to 8Three-axis SQUID magnetometer (Fig. 4 left) resolution < 1*1012 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.
10GPa.
3. Our result is contradict with the results that get from Mössbauer
Hcr
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.
Ferromagnetic Hcp-iron
Fig.4 Three-axis SQUID magnetometer (left) and measurements of empty
moissanite anvil cell at room temperature(right).
Fig.5 Isothermal remanent magnetization initially starting from back-field saturation
remanence for iron powder (up) and iron foil (down).
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).
spectroscopy. We are still working on it, and trying to find why.
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.
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.