Breaking the ice: Scientists use attracting particles to form an anti-spin ice

In a groundbreaking study published in Nature Communications, scientists have successfully created an "anti-spin ice" – a system that operates in opposition to conventional spin ices. The research, led by co-author Pietro Tierno from the University of Barcelona, sheds light on the intriguing behavior of magnetic systems that mimic the rules of water ice.

Spin ice systems are characterized by the "ice rule," which states that each oxygen atom should keep two hydrogen atoms close and two farther away. This rule leads to a disordered arrangement of hydrogen atoms, resulting in geometric frustration. In artificial spin ices, particles are arranged in arrays or colloids, following the ice rule. However, the researchers decided to explore what happens when these particles exhibit attractive interactions instead of repulsive ones.

When the particles attract each other, they tend to cluster around certain vertices of the lattice and repel others. This behavior contradicts the ice rule, which seeks to minimize local topological charges. In the newly formed anti-spin ice, vertices with all four particles nearby would have a high positive charge (+4), while vertices with all four particles farther away would have a high negative charge (−4).

These configurations are forbidden by the ice rule, leading to a system that maximizes local topological charges instead.

To create this unique system, the researchers used polystyrene particles embedded with iron oxide grains, which become magnetic when subjected to an external magnetic field. The particles were placed in a pentaheptite lattice, consisting of five- and seven-sided loops. In previous magnetic colloidal ices, particles repelled each other when the field was applied along the surface.

However, by rapidly rotating the magnetic field in the plane of the surface, the researchers were able to make the particles attract each other, resulting in the formation of anti-spin ice.

The study's findings reveal that the ice rule, which has been the organizing principle behind most artificial spin ice research, plays a central role in governing the low-energy states of frustrated systems. By altering the sign of the pair interaction between particles, the researchers have opened up new possibilities for studying geometric frustration in engineered systems.