Magnetic polaron imaging boosts possibility of magnetically charged devices

The possibility of devices based on magnetic, rather than electric, charges is a step closer after researchers imaged a magnetic polaron for the first time.

Aalto University and Lawrence Berkeley National Laboratory researchers were able to image the polaron by designing an artificial magnetic material. They demonstrated that polaron formation also occurs in a system of magnetic charges, and not just in a system of electric charges.

Polarons are an example of emergent phenomena known to occur in condensed matter physics. For instance, an electron moving across a crystal lattice displaces the surrounding ions, together creating an effective quasiparticle, a polaron, which has an energy and mass that differs from that of a bare electron. Polarons have a profound effect on electronic transport in materials.

Artificial spin ice systems are metamaterials that consist of lithographically patterned nanomagnets in an ordered two-dimensional geometry. The individual magnetic building blocks of a spin ice lattice interact with each other via dipolar magnetic fields.

The researchers used material design as a tool to create a new artificial spin ice, the dipolar dice lattice.

“Designing the correct two-dimensional lattice geometry made it possible to create and observe the decay of magnetic polarons in real time,” said postdoctoral researcher Alan Farhan from Lawrence Berkeley National Laboratory.

“We introduced the dipolar dice lattice because it offers a high degree of frustration, meaning that competing magnetic interactions cannot be satisfied simultaneously. Like all systems in nature, the dipolar dice lattice aims to relax and settle into a low-energy state. As a result, whenever magnetic charge excitations emerge over time, they tend to get screened by opposite magnetic charges from the environment.”

The researchers at Berkeley used photoemission electron microscopy, or PEEM, to make the observations. This technique images the direction of magnetisation in individual nanomagnets. With the magnetic moments thermally fluctuating, the creation and decay of magnetic polarons could be imaged in real space and time. Postdoctoral researcher Charlotte Peterson and Professor Mikko Alava at Aalto University (Finland) performed simulations, which confirmed the rich thermodynamic behaviour of the spin ice system.

“The experiments also demonstrate that magnetic excitations can be engineered at will by a clever choice of lattice geometry and the size and shape of individual nanomagnets. Thus, artificial spin ice is a prime example of a designer material. Instead of accepting what nature offers, it is now possible to assemble new materials from known building blocks with purposefully designed functionalities,” said Professor Sebastiaan van Dijken from Aalto University.

“This concept, which goes well beyond magnetic metamaterials, is only just emerging and will dramatically shape the frontier of materials research in the next decade.”