Making order visible in crystals

Many advances in energy storage, superconductivity, and manufacturing rely on developing new materials with desirable properties. An international team has developed methods to better understand these structures, providing valuable tools for future research.

Dr. Michał Stękiel (l.) utilized instrument scientist Dr. Tobias Schrader’s (r.) X-ray diffractometer to complement neutron measurements on the crystal structure. © FRM II / TUM

Dr. Michał Stękiel (l.) utilized instrument scientist Dr. Tobias Schrader’s (r.) X-ray diffractometer to complement neutron measurements on the crystal structure. © FRM II / TUM

The crystal structure of a material is the arrangement of the atoms that make it up. The exact arrangement also determines the properties of the material. A simple example is carbon: it occurs in two crystalline forms: graphite and diamond. While diamonds are insulators, graphite can conduct electricity simply because of a different arrangement and bonding of the carbon atoms that make up its crystal lattice.

Tiny magnets in crystals?
The same principle applies to crystals with a more complex structure and bonding, for which the structure-property relationships are usually more complicated. This is the case for the group of materials to which the CePdAl₃ system belongs and which researchers from the MLZ and partner institutions have now investigated. The paper’s first author, Dr Michał Stękiel from Forschungszentrum Jülich at MLZ, reports: “As the technological demands are pushing us toward investigating complex types of magnetic order in crystals, we wanted to find out which structural characteristics allow CePdAl₃ to develop magnetic order in the first place.”

Magnetic order describes the relationship between the magnetic moments of individual atoms. In simple terms, the magnetic moments are treated as tiny magnets and can be thought of as compass needles that indicate the direction of the magnetic fields generated by individual atoms. In ferromagnetic materials, for example, they are aligned in the same direction; in many others, they are more complex or even chaotic.

Like neutron beam instruments, the X-ray diffractometer measures the scattering pattern of X-rays coming from the left on the detector on the right. © FRM II / TUM

Like neutron beam instruments, the X-ray diffractometer measures the scattering pattern of X-rays coming from the left on the detector on the right. © FRM II / TUM

Neutrons reveal magnetic order
Using neutrons at the diffractometers HeiDi and DNS at the MLZ and the time-of-flight spectrometer PANTHER at the French Institut Laue-Langevin (ILL), the researchers were able to determine the orientation of the magnetic moments of the atoms in the CePdAl3 crystals.
“We knew that tetragonal CePdAl₃ cannot develop uniform magnetic order. The magnetic structure is frustrated, and the atoms block each other to align their magnetic moments comfortably, so to speak,” explains Michał Stękiel. “However, with the transition to an orthorhombic structure, we realised that the magnetic moments are more comfortably aligned and show antiferromagnetic order.”

Progress in small steps
While these results seem very technical at first glance, Michał Stękiel continues: “In the course of this work, we have revisited the methods that we use to investigate the magnetic and electric fields in crystals. We have developed a new tool that enables us to better analyse the vast amounts of data we obtain through measurements on spectrometers.