Experimental determination of the electrical resistivity of iron at Earth’s core conditions

Abstract

Earth continuously generates a dipole magnetic field in its convecting liquid outer core by a self-sustained dynamo action. Metallic iron is a dominant component of the outer core, so its electrical and thermal conductivity controls the dynamics and thermal evolution of Earth’s core1. However, in spite of extensive research, the transport properties of iron under core conditions are still controversial2,3,4,5,6,7,8,9. Since free electrons are a primary carrier of both electric current and heat, the electron scattering mechanism in iron under high pressure and temperature holds the key to understanding the transport properties of planetary cores. Here we measure the electrical resistivity (the reciprocal of electrical conductivity) of iron at the high temperatures (up to 4,500 kelvin) and pressures (megabars) of Earth’s core in a laser-heated diamond-anvil cell. The value measured for the resistivity of iron is even lower than the value extrapolated from high-pressure, low-temperature data using the Bloch–Grüneisen law, which considers only the electron–phonon scattering. This shows that the iron resistivity is strongly suppressed by the resistivity saturation effect at high temperatures. The low electrical resistivity of iron indicates the high thermal conductivity of Earth’s core, suggesting rapid core cooling and a young inner core less than 0.7 billion years old10. Therefore, an abrupt increase in palaeomagnetic field intensity around 1.3 billion years ago11 may not be related to the birth of the inner core.

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Figure 1: Change in electrical resistivity of iron upon phase transitions.
Figure 2: Change in the resistivity of ε iron with increasing temperature.
Figure 3: Iron resistivity at 212 GPa and high temperatures.

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Acknowledgements

We thank H. Gomi for discussions and assistance with experiments. H. Ichikawa helped with the temperature distribution calculations. High-pressure experiments were conducted at BL10XU, SPring-8 (proposal numbers 2012B1131, 2012B1212, 2013B0080, 2014A0080, and 2014B0080).

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Authors

Contributions

K.O. designed the project. Y.K., K.H., K.S. and Y.O. supported the experiments. The manuscript was written by K.O. and K.H. and reviewed by all authors.

Corresponding author

Correspondence to Kenji Ohta.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Images of iron sample and sample configuration for electrical resistance measurements in a DAC.

a, A composite of an iron sample and electrodes shaped by a focused ion beam. b, c, Photomicrographs of a sample chamber viewed through a diamond anvil at 115 GPa and 300 K (b) and 3,700 K (c). The four-probe method was used for electrical resistance measurements. At each set of pressure and temperature conditions, we measured the voltage difference between two potential leads (V+ and V) twice, when electric current passed through the sample from a positive current (I+) lead to a negative current (I) lead and in the opposite direction. These two voltage values were averaged to eliminate thermal voltage, and the resistance is calculated from Ohm’s law.

Extended Data Figure 2 XRD patterns of iron samples at high pressures and temperatures.

a, Data collected at 26 GPa (see Fig. 1a for resistivity measurement), showing the diffraction peaks of ε Fe, γ Fe, and Al2O3 pressure medium (Cor.). Liquid Fe is indicated by a diffuse signal at 2θ = 10° to 14°. b, ε Fe at 140 GPa (Fig. 2e). Part of the SiO2 glass pressure medium crystallized into a CaCl2-type phase (labelled as CS) when thermal annealing occurred at 80 GPa.

Extended Data Figure 3 Pressure and temperature conditions of electrical resistivity measurements of iron.

Circles show the conditions of laser-heated DAC experiments (Figs 1 and 2b–f), and squares indicate those for measurements in a muffle furnace up to 450 K (Fig. 2a). Phase boundaries are from the literature42,43. The symbol colours in this figure correspond to those in Figs 1 and 2. Error bars, 1σ.

Extended Data Figure 4 Electrical resistivity of ε iron at high pressure and 300 K.

Bold black and blue curves show the results of previous DAC experiments by Gomi et al.7 and Seagle et al.8, respectively, with uncertainties shown by bands. The red curve connecting crosses is from theoretical calculations4. All these results are consistent with each other above ~80 GPa. For comparison, the resistivity of ε iron at 1 bar deduced from the measurements of hexagonal close-packed (hcp) Fe–Os alloy44 and at low pressures in a DAC34 are also shown.

Extended Data Figure 5 Temperature maps of the iron sample and electrodes in a laser-heated DAC at 115 GPa and 4,500 K.

Al2O3 is the pressure medium. The top panel shows the temperature map viewed along the compression axis. The bottom panel shows the temperature map of the cross-section of a sample chamber and a gasket. The maximum temperature difference in the area for resistance measurement is 200 K, smaller than the uncertainty in the temperature determination.

Extended Data Table 1 Saturation resistivity ρsat of ε iron at high pressures

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Ohta, K., Kuwayama, Y., Hirose, K. et al. Experimental determination of the electrical resistivity of iron at Earth’s core conditions. Nature 534, 95–98 (2016). https://doi.org/10.1038/nature17957

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