Page 26 - ILL Annual Report 2019

MAGNETISM
Blair W. Lebert. Canadian and French
University of Toronto, Canada
‘I am post-doctoral researcher working on quantum spin liquids, thermoelectric materials and high-temperature superconductors using X-ray and neutron scattering. I received my PhD from Sorbonne Université, where I studied
studied the magnetism of Fe and FeSe under pressure using neutron diffraction at the ILL.’
Epsilon iron as a spin-smectic state
High-intensity two-axis diffractometer with variable resolution D20
Our comprehensive experimental and theoretical approach delivers new insight into the controversial magnetic state of
the high-pressure polymorph of iron. Neutron diffraction measurements on D20 played a key role. Using record-setting high-pressure/low-temperature conditions, we found no long-range magnetic order despite previous predictions. A novel ‘spin-smectic’ magnetic state, which corresponds with current experimental observations, is proposed.
AUTHORS
B.W. Lebert (University of Toronto, Canada)
T. Gorni (Université Paris Sciences et Lettres, Paris, France)
M. d'Astuto (UGA, Grenoble Université, France)
M. Casula and S. Klotz (CNRS and Sorbonne Université, Paris, France) Th. C. Hansen (ILL)
ARTICLE FROM
Proc. Natl. Acad. Sci. USA (2019)—doi: 10.1073/ pnas.1904575116t
REFERENCES
[1] G. Steinle-Neumann, L. Stixrude and R.E. Cohen, Proc. Natl. Acad. Sci. USA 101 (2004) 33
[2] S. Klotz, Th. Strässle, B. Lebert, M. d'Astuto and Th. Hansen, High Pressure Res. 36:1 (2016) 73
[3] A. Papandrew et al., Phys. Rev. Lett. 97 (2006) 087202
Iron is the prototypical ferromagnet and has long fascinated humankind. In the last few decades, it has been extensively studied under pressure, since it is the primary element of the Earth’s core. The high-pressure polymorph
of iron, the so-called epsilon iron phase, is formed above 15 GPa (150 000 times atmospheric pressure). Above
this pressure, iron transforms from its ambient, body-centred cubic (bcc) crystal structure to a hexagonal close-packed (hcp) crystal structure (figure 1). This structural transition is accompanied by a loss of ferromagnetism; however, the magnetism of epsilon iron is unknown and controversial due to seemingly paradoxical results from theory and experiment. Here, we propose a novel, ‘spin-smectic’, state that is compatible with previous studies as well as with our current X-ray and neutron scattering studies.
First, we used an X-ray scattering technique called X-ray emission spectroscopy, which can measure local magnetic moments, i.e. the magnetism of individual atoms. We confirmed the existence of magnetic moments in epsilon iron and measured their pressure and temperature
Figure 1
Schematic phase diagram of iron. The ambient, ferromagnetic, body-centred cubic phase (bcc–fm) is shown in purple. The high-pressure polymorph epsilon iron, which forms a hexagonal close-packed (hcp) structure, is shown in blue/green for magnetic phases (hcp–m)
and white for non-magnetic phases (hcp–nm). We measured the crossover from hcp–m to hcp–nm using X-ray emission spectroscopy. Our calculations predict that the disordered moments (blue) form a spin-smectic state (green) below a critical temperature Tm, which may be related to ε-iron’s superconductivity (Tc dome shown).
ANNUAL REPORT 2019