Magnetic phase diagram of ${\varepsilon }^{\prime }$-FeH

Iron hydrides attract significant interest as candidates for the main constituents of the Earth's core in geophysics and planetary science. However, their basic physical properties are still not well known. Here, we combined high-pressure transport, synchrotron radiation Mössbauer, and Fe $K\beta$ x-ray emission spectroscopy measurements on ${\varepsilon }^{\prime }$-FeH to map out a detailed magnetic phase diagram of this hydride phase of iron. In contrast to our original expectations, we found two magnetic phase transitions at high pressure due to two inequivalent iron sites existing in the ${\varepsilon }^{\prime }$-FeH structure. Our results account for the previous large pressure difference on the loss of ferromagnetism between experiment and theoretical calculations. The discovery of an unexpected complex magnetic phase diagram in ${\varepsilon }^{\prime }$-FeH has implications for a better understanding of the magnetic and physical properties of the iron-hydrogen compounds, important for the conditions of planetary interiors.

Figure 1

The crystal structures of (a) $\varepsilon$-iron and (b) ${\varepsilon }^{\prime }$-FeH. The two crystallographically inequivalent iron sites are labeled as Fe1 and Fe2, respectively.

Figure 2

The temperature dependence of the resistance for ${\varepsilon }^{\prime }$-FeH under various pressures. (a) The resistance increases steeply after the reaction of iron with hydrogen above 4 GPa, and then it increases continuously with increasing pressure. (b) The resistance starts to decrease above 25 GPa, which is due to the magnetic transition. All the resistance curves show metallic behavior and no superconductivity is detected down to 2 K.

Figure 3

(a) Anomalous Hall effect as measured at 300 K, which confirms the ferromagnetism in ${\varepsilon }^{\prime }$-FeH. (b) Anomalous Hall resistivity ${\rho }^{\text{AH}}$ rapidly decreases around 25 GPa, which is consistent with the loss of ferromagnetism found previously [12].

Figure 4

(a)–(d) High-pressure synchrotron Mössbauer measurements of ${\varepsilon }^{\prime }$-FeH under various temperatures. The red lines are the fits. (e)–(h) The hyperfine magnetic fields at the Fe1 and the Fe2 sites are obtained from the fits. At all temperatures, the hyperfine magnetic fields drop above 26 GPa, however, the value of the hyperfine field at the Fe1 site stays at 15–20 T below 200 K, indicating the remaining magnetism at high pressure. The black and red lines are guides for the eye.

Figure 5

Magnetic phase diagram of ${\varepsilon }^{\prime }$-FeH. The weight of the ordered moments shows two sharp changes around 26 and 43 GPa, which are related to the two magnetic phase transitions. The deduced difference intensity from the x-ray emission spectroscopy measurements also shows an anomaly around 43 GPa, which indicates that the local magnetic moments are changed in the course of the magnetic phase transition.