A powerful tool to explore new magnetic states and quantum phenomena

Materials with unusual magnetic, superconducting and quantum properties representing new electronic states of matter offer huge potential for the information and energy technologies of the future. Understanding the underlying mechanisms is crucial for the development of applications, and neutrons play a key role: thanks to their spin, they behave like tiny magnets; having no electric charge, they probe atomic states like no other method.  Neutron scattering under extreme conditions of (high) pressure at (ultra-)low temperatures, despite being particularly challenging, has long been pursued.

A neutron diffraction experiment down to the unprecedented low temperatures of 160 mK simultaneously at a pressure of 20 GPa (nearly 200 000 times the atmospheric pressure on Earth) has been conducted at the ILL and is now published. This has been made possible due to novel developments in cryogenic and high-pressure techniques.

The setup consisted of a combination of two main elements: a so-called Paris-Edinburgh press (able to generate a force of 130 tons onto a sample of up to 50 mm³) hosted inside a cryogen-free dilution cryostat. As shown in the picture, the pressure cell is attached to the stage of the mixing chamber and placed inside a tight calorimeter filled with 10-30 mbar of helium gas to ensure thermalisation.

Measurements were carried out at the newly commissioned instrument XtremeD (built and operated at the ILL by Spanish research institutions as a Collaborating Research Group instrument) and concerned e-Fe, the high-pressure phase of elemental iron. One of the most abundant and stable elements in the universe, iron transforms under compression (at 14-20 GPa) from the cubic a-phase to the hexagonal lattice close-packed e-form, which is the major component of the Earth’s core. While e-Fe is generally believed to be non-magnetic, theory and indirect experimental findings suggest the existence of a remnant magnetic moment – and hence the possibility of magnetic ordering at sufficiently low temperatures.

This result does not necessarily exclude the existence of a remnant magnetic moment in e-iron, but considerably reduces possible scenarios: either it is really small (about 10 times lower that in ‘normal’ iron and 3-5 times smaller than suggested by theory and measurements), or magnetic order must be short-range (correlation lengths less than 10-20 Å, i.e. typically a few unit cell distances).

Most importantly, the techniques developed in this work open new experimental possibilities for the study of structures and excitations in condensed matter at extreme P/T conditions. Examples include the behaviour of magnetically frustrated systems which may order close to 0 K, the interplay between magnetism and superconductivity, and investigations of quantum critical points and other emergent quantum phenomena in condensed matter.

"Scientific progress is often limited by the tools at our disposal. Our newly engineered system makes it possible to study atomic structures at 20 GPa and 100 mK. This leap in instrumentation doesn't just advance technology — it accelerates discovery, offering our users a powerful new lens to explore the fundamental properties of matter under extreme conditions,” says Eddy Lelièvre-Berna, head of the Service for Advanced Neutron Environment at the ILL.