Japanese scientists may have found a surprisingly simple way to predict the behavior of some unusual materials based on the size of their crystal structure.
In their latest study, they claim that this structural measurement can predict magnetic ground states in a family of unusual materials more consistently than the electron counting method that scientists have traditionally relied on.
This could make the search for unusual magnetic materials less of a guessing game.
The challenge so far has been that quasicrystals and related materials have extremely complex atom arrangements, making it difficult to find a single rule that applies to different chemical compositions.
“Quasicrystals are among the most unique structured materials discovered to date and are expected to exhibit novel magnetic states and quantum phenomena not found in ordinary crystals. Until now, there has been no consistent guideline for systematically exploring these novel phenomena in quasicrystals and their proximate crystals,” said Farid Labib, lead author of the study and assistant professor at Tokyo University of Science.
The new study suggests that the size of a crystal could be the missing common measure when researchers search for unconventional magnetism.
Look beyond the electron count
The researchers focused on Tsai-type approximant crystals. These materials mimic key structural features of quasicrystals while exhibiting a repeating crystal structure that can be studied experimentally.
Their atoms form nested clusters, with rare earth elements such as terbium, dysprosium and holmium occupying positions that carry magnetic moments.
Researchers have often used valence electron concentration, or the number of valence electrons per atom, to classify magnetic behavior.
However, the magnetic limits associated with this measurement shift with the composition of the rare earth element and alloy, making it less reliable as a universal guide.
The researchers therefore synthesized Au-Al and Au-Ga-based 1/1 approximant crystals containing terbium, dysprosium and holmium and then compared their structures and magnetic properties. They also combined their measurements with data from previously reported Tsai-type compounds to see whether the pattern held across different compositions.
They found an almost monotonic inverse relationship between the electron concentration and the lattice parameter – the dimensions of the crystal’s repeating unit. This allowed them to rearrange the materials based on lattice size rather than chemical composition.
Three magnetic states separated by crystal size
The distinction turned out to be remarkably clear. Crystals with lattice parameters above about 14.72 Å (14.72 x 10).-10 m) developed a swirling antiferromagnetic state in which magnetic moments form an ordered pattern with opposite contributions.
Between 14.62 and 14.72 Å, they showed a swirling ferromagnetic state with a net magnetization. Below about 14.62 Å, the materials enter a spinglass state in which magnetic moments are frozen in a disordered arrangement.
The swirling magnetic states in these non-Heisenberg compounds arise in part because the local atomic environment generates a strong electric crystal field. This field favors certain alignments of the rare earths’ magnetic moments, influencing how they ultimately arrange themselves.
The magnetic moments also interact indirectly through electrons moving through the material, linking the atomic structure to the way these moments arrange themselves.
A structural roadmap for new materials
The important point is not simply that grid size correlates with magnetism. The study establishes specific structural thresholds that place different compounds into a common magnetic phase diagram.
This makes the lattice parameter an experimentally accessible starting point for deciding which materials can host a desired magnetic ground state.
“The unified magnetic phase diagram constructed in this study can serve as a practical guide for the systematic exploration of new magnetic quasicrystals and approximant crystals that exhibit novel magnetic orders and quantum phenomena,” said Kazuhiro Nawa, co-author of the study and a researcher at Tohoku University.
“It can also serve as a guide for the development of new magnetic materials with targeted magnetic ground states and open new opportunities for the discovery of unconventional magnetism in quasi-periodic and complex intermetallic systems,” Nawa added.
The finding does not make the electron concentration irrelevant. Rather, it suggests that the lattice parameter may be a more consistent guide for Tsai-type compounds, while its utility in other magnetic materials remains untested.
It remains unclear why exactly the lattice parameter works so well for these different compounds. Researchers also need to determine whether the same relationship holds true for real quasicrystals.
Despite these limitations, the study actually offers a more systematic way to explore complex alloys and design new quantum materials.
The study is published in Journal of the American Chemical Society.
