Knowledge of iron’s phase diagram under high-pressure (HP) and high-temperature (HT) conditions is not only of importance for geophysics but also as a model system for the theoretical understanding of 3d electronic properties. High-pressure magnetism in particular is believed to play a crucial role in the stability of iron’s structural phases and elastic properties. Theoretical improvements beyond the local-density approximation now make it possible to derive the correct magnetic ground state at ambient conditions. However, puzzling results recently observed under extreme conditions have perturbed the situation anew. Superconductivity was found in Fe along with ferromagnetism at low temperature and high pressure [1] and hints of antiferromagnetic (AF) coupling in -Fe, the supposedly nonmagnetic hcp HP phase. Even more poorly understood is the -Fe fcc phase stabilised at high temperature and high pressure. First-principles calculations favour a noncollinear magnetism in the form of spin spiral density waves [2] but results in the phase are extremely scarce because of the HP/HT experimental constraints. Here, we demonstrate the collapse of the short-range iron magnetic state in the HP/HT phases by X-ray emission spectroscopy (XES), a local probe of the 3d spin magnetic moment.

Fig. 37: Kß emission spectra of iron under high pressure and temperature in the , and phases: (a,c) full scale; (b,d) expanded intensity scale.

The spin sensitivity of XES arises from the core-hole/3d exchange interaction in the final state which splits the emission spectrum into a main peak and satellite. We focus on the Kβ emission line which shows the highest sensitivity to the spin state in a transition metal. The spectra were measured at beamline ID27 at 33 keV, simultaneously to X-ray diffraction, using the Si(531) analyser from ID16. The sample was loaded in a diamond-anvil cell together with Al2O3 powder both for thermal isolation and pressure-transmitting medium. HP/HT conditions were achieved in situ by a double-sided laser heating technique where an intense laser beam heats the sample inside the pressure cell. Figure 37 shows the evolution of the Kβ emission spectra under high pressure at 300 K and 1400 K.

In iron metal, the satellite is notably weaker than that in other iron compounds. Nevertheless, we clearly notice a decrease in the satellite intensity at high pressure which suggests a change in the iron spin state.

Fig. 38: Iron spin magnetic moment derived from the XES spectra (left scale, circles) and calculated (right scale, lines).

The magnetic information contained in the XES spectra can be extracted by means of integrated absolute difference (IAD), a quantity that is linearly related to the 3d spin moment. Figure 38 shows the pressure dependence of the spin state from the IAD analysis. At 300 K, the results bear out the magnetic collapse when iron enters the HP phase, as already known from XES and Mössbauer spectroscopy. Additionally, we observe non vanishing magnetism above the transition that decays at higher P. This agrees with recent theoretical model suggesting that the magnetic moment is not completely suppressed in -Fe but instead remains up to a fairly large pressure in presence of AF coupling. This is in contrast to the high-spin (HS) to low-spin (LS) transition usually evoked at the - structural change. The behaviour in -Fe at 1400 K is remarkably similar to that at 300 K, suggesting that the mechanism of the magnetic transition in the phase primarily involves a HS to LS transition, besides the occurrence of a spin spiral density wave. Comparing our results to the theoretical magnetic moment of iron computed in the spin spiral density wave model, we find a fair agreement at low pressure, while at high pressure a better match is found when considering the low q limit of the spin spiral density wave structure. The latter corresponds to a magnetic state with saturated ferromagnetism and a clearly defined HS-to-LS transition.

XES therefore appears as a powerful probe of the 3d magnetic properties under extreme conditions. In iron, the results show hints of non-vanishing magnetism in the -phase under pressure in agreement with predictions of non-collinear magnetism while in the phase, we observe a LS collinear structure at HP/HT. This substantially minimises the role of spin spiral density waves in the magnetic change at extreme conditions in the phase.

 

Principal publication and authors

J.-P. Rueff (a,b), M. Mezouar (c) and M. Acet (d), Phys. Rev. B. 78, 100405(R) (2008).
(a) Synchrotron SOLEIL, Gif sur Yvette (France)
(b) Laboratoire Chimie Physique - Matière et Rayonnement, CNRS - UPMC, Paris (France)
(c) ESRF
(d) Experimentalphysik, Universität Duisburg-Essen, Duisburg (Germany)

References

[1] K. Shimizu et al., Nature London 412, 316 (2001).
[2] S. Shallcross et al., Phys. Rev. B 73, 104443 (2006).