Epsilon iron as a spin-smectic state

Abstract

Using x-ray emission spectroscopy, we find appreciable local magnetic moments until 30-40 GPa in the high-pressure phase of iron, however no magnetic order is detected with neutron powder diffraction down to 1.8 K contrary to previous predictions. Our first-principles calculations reveal a "spinsmectic" state lower in energy than previous results. This state forms antiferromagnetic bilayers separated by null spin bilayers, which allows a complete relaxation of the inherent frustration of antiferromagnetism on a hexagonal close-packed lattice. The magnetic bilayers are likely orientationally disordered, owing to the soft interlayer excitations and the neardegeneracy with other smectic phases. This possible lack of long-range correlation agrees with the null results from neutron powder diffraction. An orientationally-disordered, spinsmectic state resolves previously perceived contradictions in high pressure iron and could be integral to explaining its puzzling superconductivity.Iron is well-known since antiquity for its unique magnetic properties and continues to captivate scientists to this day. The study of iron and its alloys has many applications, including steel production and geophysics. Regarding the latter, the application of hydrostatic pressure induces a phase transition from the body-centered cubic (bcc) structure of α-iron to the hexagonal close-packed (hcp) structure of ε-iron (Fig. 1). Iron is being studied at increasingly high pressures and temperatures, since it and its alloys compose the majority of the Earth's core (1). Nonetheless, the relatively low-pressure, low-temperature region of ε-iron has remained a mystery for decades. The ferromagnetism (fm) found in α-iron disappears during the α-ε transition (2-4), however the magnetic state of 1 B.W.L and T.G contributed equally to this work 2 Corresponding author -matteo.dastuto@neel.cnrs.fr -Institut Néel CNRS -25, av des Martyrs -38042 Grenoble cedex 9; tel: (+33)(0)4 76 88 12 84 arXiv:1903.04792v2 [cond-mat.mtrl-sci]

The AiiDA-Spirit Plugin for Automated Spin-Dynamics Simulations and Multi-Scale Modeling Based on First-Principles Calculations

“…Incorporating a change of the spin moment with the spin-spiral wavevector might further improve our agreement to the earlier ab initio results of Knöpfle et al (Knöpfle et al, 2000) and Sjöstedt and Nordström (Sjöstedt and Nordström, 2002) and also the experimental spin-spiral wavevector (Tsunoda, 1989). The need to include spin moment change is also observed in the high-pressure ε-phase of iron (Lebert et al, 2019). Furthermore, including higher order exchange interactions (K i,j,k,l ≠ 0) could also be important.…”

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). However, we now think it is more likely that the remanent magnetization originates from nonconverted bcc‐Fe that goes undetected by XAFS, Mössbauer, XRD, etc.…”

Electronic correlations in dense iron: from moderate pressure to Earth’s core conditions

“…X-ray emission spectroscopy of Refs. [62,63] detected a magnetic signal, but it is not clear whether its origin is a static order or rapidly fluctuating local moments. The same very recent work [63] proposed a quasi-2d order of alternating AFM and "magnetically dead" layers for -Fe but their neutron diffraction measurements failed to detect any magnetic order in this phase down to 1.8 K at 20 GPa.…”

“…[62,63] detected a magnetic signal, but it is not clear whether its origin is a static order or rapidly fluctuating local moments. The same very recent work [63] proposed a quasi-2d order of alternating AFM and "magnetically dead" layers for -Fe but their neutron diffraction measurements failed to detect any magnetic order in this phase down to 1.8 K at 20 GPa. The presence of a highfrequency satellite of the E 2g Raman mode in -Fe [64] was initially ascribed to a splitting of this mode in the AFM-ordered phase [60]; the satellite peak is, however, found to disappear at low temperatures, i. e., where the AFM state is supposed to be stable [65].…”