High‐Pressure Phase Transition of Iron: A Combined Magnetic Remanence and Mössbauer Study

Abstract

We measured Mössbauer spectra and the acquisition of saturation isothermal remanent magnetization in alternating steps on the same sample of polycrystalline, multidiron metal powder in a diamond anvil cell across the body centered cubic (bcc) to hexagonal closed packed (hcp) phase transition at room temperature up to 19.2 GPa. Within the bcc stability field indicated by the presence of magnetic hyperfine splitting, saturation remanent magnetization and sextet area were well correlated during compression and decompression. The areas and dips of the outer (first and sixth) and middle (second and fifth) components of the sextet changed in relative proportion as a function of pressure, which was attributed to rotation of the magnetization direction perpendicular to the gamma‐ray source. Sextet peaks disappeared above ∼15 GPa, yet magnetic remanence persisted. Magnetic remanence intensity divided by the fractional area of the sextet, taken to represent bcc Fe, attained maxima at pressures near the boundaries of the hysteretic transition, which we attribute to strain‐related magnetostriction effects associated with a distorted bcc‐hcp phase. Magnetic remanence observed within the hcp stability field, as defined by the absence of sextet peaks, could be due to a previously described, distorted bcc‐hcp phase whose hyperfine field was below detection limits of Mössbauer spectroscopy. Our study suggests that distorted bcc‐hcp Fe holds magnetic remanence and leaves open the possibility that this phase carries magnetic remanence into the pressure range where only pure hcp Fe is considered stable.

Magnetism of Body‐Centered Cubic Fe‐Ni Alloys Under Pressure: Strain‐Enhanced Ferromagnetism at the Phase Transitions

“…We previously argued that the persistence of magnetism at pressures >18 GPa was due to a distorted hcp phase (Wei & Gilder, 2013; Wei et al., 2017). Indeed a recent combined X‐ray emission spectroscopy and neutron diffraction study is consistent with this conclusion (Lebert et al., 2019).…”

“…This also explains why the SIRM method is a robust indicator of the magnetic state of the samples, since the external field is applied parallel to the long axis ( x – y ) dimension of the sample chamber, where 0.37 T fields were more than enough to achieve full saturation (Figures 2 and 4a), and the remanent magnetization is measured in the same direction (Wei & Gilder, 2013). Lack of saturating fields can likely explain why Mössbauer spectra fail to resolve persistent ferromagnetism in Fe 100 Ni 0 and Fe 92 Ni 08 above the bcc‐out to hcp‐in transition (Cort et al., 1982; Nasu et al., 2002; Papandrew et al., 2006; Taylor et al., 1991), whereas SIRM is still measurable (Wei et al., 2017).…”

“…The discrepancy among the experimental and theoretical results leaves open the magnetic state of hcp Fe. This is why Wei et al (2017) measured Mossbauer spectra and the stepwise acquisition of saturation isothermal remanent magnetization (SIRM) on the same sample of polycrystalline, multidomain iron metal powder in a diamond anvil cell across the bcc to hcp phase transition at room temperature. A good correlation was found between the area of the Mossbauer sextet components and SIRM within the bcc stability field.…”

“…remanent magnetization is measured in the same direction (Wei & Gilder 2013). Lack of saturating fields can likely explain why Mössbauer spectra fail to resolve persistent ferromagnetism in Fe100Ni0 and Fe92Ni08 above the bcc-out to hcp-in transition (Cort et al, 1982;Taylor et al, 1991;Nasu et al, 2002;Papandrew et al, 2006) whereas SIRM is still measurable (Wei et al, 2017).…”

Microstructural effects and mechanism of bcc-hcp-bcc transformations in polycrystalline iron

“…As pressure is changed, the transformation occurs rapidly [3], suggesting that it is nondiffusive and martensitic. The structural transition is also associated with changes in magnetic ordering [4,5]. Consequently, the αtransformation in Fe has been the topic of numerous studies in static highpressure experiments [4][5][6][7][8][9][10][11][12][13][14], dynamic loading experiments [15][16][17][18][19], and numerical models [20][21][22][23][24][25][26].…”